Victor Chung
email: victor.chung@ens.psl.eu

Julie Grèzes

Elisabeth Pacherie

Collective emotion: a framework for experimental research

Research on collective emotion spans social sciences, psychology and philosophy. There are detailed case studies and diverse theories of collective emotion. However, experimental evidence regarding the universal characteristics, antecedents and consequences of collective emotion remains sparse. Moreover, current research mainly relies on emotion self-reports, accounting for the subjective experience of collective emotion and ignoring their cognitive and physiological bases. In response to these challenges, we argue for experimental research on collective emotion. We start with an overview of theoretical frameworks to identify a set of three characteristics of collective emotion. Based on research in cognitive and affective sciences, we then examine the corresponding candidate mechanisms. Finally, we highlight outstanding questions, review experimental evidence, and suggest ideas for future experimental research.

Introduction

Background. Crowds cheer in stadiums, friends and relatives meet for celebrations, and online communities thrive on their members hating or loving together [START_REF] Turner | The sociology of emotions[END_REF]. These examples are typical case studies of collective emotion, which is the topic of a growing literature in social sciences, psychology and philosophy (von Scheve & Salmela, 2014). Research on collective emotion investigates the idea that emotions can be shared with other individuals, and that shared emotions have specific characteristics. In affective sciences, the development of this idea is also related to the theories of human cognition that stress the socially extended and distributed nature of mental processes [START_REF] Gallagher | The socially extended mind[END_REF], including emotions [START_REF] Krueger | Extended emotions[END_REF][START_REF] Krueger | Varieties of extended emotions[END_REF][START_REF] Slaby | Emotions and the extended mind[END_REF][START_REF] Huebner | Genuinely collective emotions[END_REF]. Furthermore, research on collective emotion fits into the recent development of integrative research programs in neuroscience and psychology, which span multiple levels of analysis to elucidate the bases of collective cognition [START_REF] Vlasceanu | Cognition in a social context: A social-interactionist approach to emergent phenomena[END_REF][START_REF] Sliwa | Toward collective animal neuroscience[END_REF].

Objectives. Our primary goal is to address two main challenges for research on collective emotion moving forward. First, there is still little consen-sus regarding the definition of collective emotion. Available theories rely on different conceptual and methodological frameworks that target different aspects of emotion (e.g., expression, subjective conscious experience) at different levels of analysis (e.g., individuals, dyads, crowds). As a consequence, these theories often focus on different facets of collective emotion in isolation. Previous literature reviews have offered comprehensive accounts of available theories [START_REF] Salmela | Shared emotions[END_REF][START_REF] Salmela | The Functions of Collective Emotions in Social Groups[END_REF], highlighted their differences and discussed potential mechanisms [START_REF] Pacherie | Collective phenomenology[END_REF][START_REF] Von Scheve | Towards a theory of collective emotions[END_REF]. More research is now needed to understand how these theories relate to each other and, in particular, the extent to which they make compatible or conflicting empirical predictions. The second challenge stems from the bias of current research towards the phenomenology of collective emotion and, methodologically, its reliance on the subjective experience reported by individuals. In comparison, emotion is thought to involve multiple processes besides subjective experiences, such as evaluations, physiological responses, expressions and action tendencies [START_REF] Moors | Theories of emotion causation: A review[END_REF]. In other words, there is a gap between current research on collective emotion and contemporary affective sciences, which comes with the risk of overlooking non-conscious aspects of collective emotion (see [START_REF] Lamm | Insights into collective emotions from the social neuroscience of empathy[END_REF] for a similar argument). In response to these two challenges, we argue for developing experimental research on collective emotion. This approach opens new perspectives for identifying the mechanisms of collective emotion and testing empirical predictions of existing theories, thereby providing new, critical material for discussions.

Scope. The following discussion relies on a few assumptions. First, we are not talking about collective emotion to imply that social entities can experience emotions in the same way as individuals. Therefore, we leave aside the discussions on the nature of group agents and the possibility of group consciousness [START_REF] Pacherie | Collective phenomenology[END_REF][START_REF] List | What is it Like to be a Group Agent?[END_REF]. Second, we focus our discussion on collective emotions, which are relatively short-term acute responses to specific objects or events, as opposed to other types of affects such as collective moods. Third, we do not assume a priori that collective emotion only occurs in large groups as opposed to small groups and dyads, and during in-person physical gatherings (e.g., marches) as opposed to technologically mediated interactions (e.g., online discussions). However, we distinguish between collective emotion and related phenomena that do not involve influence between individuals. As such, we provisionally define collective emotion as a pattern of emotions that emerges within a group from common and reciprocal influence between individuals. Table 1 provides definitions that clarify the distinctions between collective emotion and related concepts mentioned in the text.

Plan. The first section presents an overview of recent theories that emphasize different aspects of collective emotion. Our goal is not to provide a systematic review of the theoretical literature, but to introduce the reader to different theoretical approaches. In the second section, we then derive a set of three characteristics of collective emotion (emotion alignment, feeling of togetherness, mutual awareness) from the theories. These characteristics provide a simple reference frame highlighting the similarities and differences between theoretical approaches. In the third section, we turn to research in affective and cognitive sciences to identify the candidate mechanisms underlying these three characteristics. Finally, in the fourth section, we review previous and on-going experimental research on collective emotion, and we outline promising directions for future research.

Theories of collective emotion

Theories of collective emotion can be divided into three main approaches focusing on different facets of the phenomenon: the aggregation of emotional responses across individuals, the subjective feelings re-lated to experiencing collective emotion, and the similarity and reciprocity of emotional dispositions across individuals.

Emotional responses

Some theories of collective emotion in social sciences focus on the properties that emerge from the aggregation of emotional responses across individuals. The aggregation of emotions at group-level is "the sum of its parts" (Barsade & Gibson, 1998, p.88), and it can be apprehended as the mean intensity or the variability of emotional responses within the group. Goldenberg et al. (2020) gave several examples of group-level patterns that characterize collective emotion: emotional amplification, an increase or decrease in the average magnitude of emotional responses (Fig. 1a); emotional convergence or polarization, a decrease in the variability of emotional responses within the group (Fig. 1b) or within subgroups (Fig. 1c); emotional synchronization, a decrease in the temporal variability of emotional responses (Fig. 1d); and emotional cascading, which involves delayed synchronization (Fig. 1e). The aggregative theories of collective emotion contend that these emotion patterns result from connections between individuals, but they disagree on the type of patterns and the type of connections that define collective emotion.

For instance, von Scheve and Ismer (2013) defined collective emotion as the "synchronous convergence in affective responding across individuals towards a specific event or object" (p. 411). According to this definition, collective emotion corresponds to the convergence and synchronization of emotional responses across individuals who are connected, at the minimum, through common influence. As per this definition, interaction between individuals is not necessary: for example, the converging emotions of remote individuals who attend the same TV broadcast count as collective. In comparison, Goldenberg et al. (2020) defined collective emotions as "macrolevel phenomena that emerge from emotional dynamics among individuals who are responding to the same situation" (p. 2). This latter definition does not presuppose synchrony or convergence, as other types of group-level patterns could emerge during collective emotion (e.g., emotional cascading). Moreover, the notion of "dynamics among individuals" suggests that reciprocal influence, in addition to common influence, is necessary for collective emotion.

The latter definition sets a distinction between, on the one hand, group-based emotion and, on the other hand, collective emotion as a type of group-shared emotion [START_REF] Menges | Group emotions: Cutting the Gordian knots concerning terms, levels of analysis, and processes[END_REF]. Group-based emotion occurs when individuals self-categorize as members of a social group and adopt the group's stance [START_REF] De Rivera | Emotional climate, human security, and cultures of peace[END_REF] emotional atmosphere daffective phenomenon manifesting spatially and unconsciously perceived by individuals (see [START_REF] Trigg | The role of atmosphere in shared emotion[END_REF] emotion alignment dynamic and reciprocal adjustment between individuals' emotions, which can result in the synchronization and convergence of their emotions emotion contagion a mechanistic explanation of emotion alignment between an emoter and an observer, which involves different steps for the observer: perception of emotion expression, mental simulation of perceived emotion, unintentional mimicry of perceived emotion expression, experience of the mimicked emotion (see [START_REF] Hatfield | Primitive emotional contagion[END_REF] when responding to an emotional event [START_REF] Mackie | Group-based emotion in group processes and intergroup relations[END_REF]. Group-based emotion predicts that members of a common social group will experience similar emotions in response to a group-relevant situation (e.g., the joy of political partisans in response to their party's success). However, it requires neither that co-present and interacting individuals reciprocally influence each other, nor that individuals be concurrently responding to a common emotion-elicitor (but see "Social identification" for a more nuanced picture). In contrast, group-shared emotion requires "synchrony of attention among group members to the emotion-eliciting stimuli, and some level of social interaction among group members" [START_REF] Menges | Group emotions: Cutting the Gordian knots concerning terms, levels of analysis, and processes[END_REF]p. 851). It is worth mentioning that group-based and group-shared emotions are not mutually exclusive, and that collective emotion in the strong sense may result from their combination. In summary, aggregative theories of collective emotion emphasize group-level emotion patterns, but these theories do not necessarily presuppose that collective emotion has a specific experiential quality.

Subjective experiences

In contrast, phenomenological theories in philosophy define collective emotion as a specific type of subjective experience. These definitions draw from the concept of we-mode experience inspired by Tuomela's terminology (2007). The core idea is that a collective emotion is felt as our emotion rather than my emotion, although it remains debatable what exactly is meant by the first-person plural. [START_REF] Salmela | Two types of togetherness in shared emotions [and many other collectively intentional states[END_REF] distinguished two possible interpretations in terms of togetherness. According to the first interpretation, togetherness corresponds to the pleasant feeling of closeness and rapport with other individuals. According to the second interpretation, togetherness corresponds to "awareness that there are particular others with whom one is experiencing the emotion" (Salmela, 2022, p. 53).

The first interpretation is best developed in accounts of collective emotion in terms of collective effervescence [START_REF] Collins | Interaction ritual chains and collective effervescence[END_REF][START_REF] Páez | Collective emotional gatherings: their impact upon identity fusion, shared beliefs, and social integration[END_REF]. In his neo-Durkheimian conceptualization of collective emotion, Collins (2014) described collective effervescence as "the rhythmic entrainment of all participants into a mood that feels stronger than any of them individually, and carries them along as if under a force from outside" (p. 299). More recently, [START_REF] Rimé | Why We Gather: A New Look, Empirically Documented, at Émile Durkheim's Theory of Collective Assemblies and Collective Effervescence[END_REF] described collective effervescence as the "high-intensity experience of self-transformation" (p. 18). The authors highlighted two aspects of this subjective experience: the experience of amplified emotions leading to feelings of empowerment, and the experience of perceived similarity with other individuals leading to feelings of closeness and rapport with these individuals. This definition partly overlaps with the definition of "kama muta", which is a specific emotion involving the positive experience of feeling moved and connected to others, as well as physiological responses such as goose bumps [START_REF] Fiske | Kama muta: Discovering the connecting emotion[END_REF].

The second interpretation corresponds to accounts of collective emotion as a subjective emotional experience from the second-person perspective. According to these theories, two conditions are necessary for collective emotion to occur: reciprocal otherawareness and integration [START_REF] León | Emotional sharing and the extended mind[END_REF][START_REF] Zahavi | You, me, and we: The sharing of emotional experiences[END_REF][START_REF] Thonhauser | Shared emotions: a Steinian proposal[END_REF]. First, collective emotion requires that several individuals experience emotions and that these individuals be reciprocally aware of each other (i.e., they are aware of being attended by the other individual). Hence, this condition sets a distinction between collective emotion (i.e., reciprocally engaging from the second-person perspective) and other types of socio-emotional experiences such as vicarious feelings (i.e., feeling the emotion of another individual as one's own, from the first-person perspective) and empathetic concern (i.e., understanding the emotion of another individual without necessarily feeling it, from the third-person perspective) [START_REF] Szanto | Introduction: empathy, shared emotions, and social identity[END_REF][START_REF] Zahavi | Empathy is not sharing: Perspectives from phenomenology and developmental psychology[END_REF]. In other words, collective emotion requires preserving the perceived distinction between the self and other individuals. Second, collective emotion requires integration or reciprocal identification so that individuals perceive each other as similar-yet-distinct individuals. This specific condition sets a distinction between collective emotion and cases in which individuals are reciprocally aware of each other but experience antagonistic emotions (e.g., those of a perpetrator and its victim). Thonhauser (2022) translated these two requirements in terms of "self-organizing systems". He defined collective emotion as "a complex system of socially distributed components based on dynamical self-organization through social interaction". This definition, by stressing the importance of social interaction, adopts the aforementioned distinction between collective emotion and group-based emotion. Overall, phenomenological theories do not consider that emotion patterns, such as fine-grained similarity in subjective emotional experiences, are defining features of collective emotion. For example, individuals may feel their emotion as collective, while experiencing dissimilar but compatible and complementary emotions (e.g., anger and sadness in response to the failure of a joint activity).

Emotional dispositions

In comparison to phenomenological theories, analytical theories build upon a definition of emotion in terms of concerns (e.g., desires, aversions, interests, attachments). These concerns contribute to "patterns of emotional dispositions" [START_REF] Salmela | Two types of togetherness in shared emotions [and many other collectively intentional states[END_REF]) that determine how individuals appraise and respond to a situation. Indeed, appraisal theories [START_REF] Moors | Appraisal Theories of Emotion: State of the Art and Future Development[END_REF] consider that emotional responses result from the evaluation, often unconscious, of a situation. Accordingly, collective emotion is a matter of holding similar concerns, to which individuals are committed as members of a common group [START_REF] Helm | Emotional communities of respect[END_REF]. These common concerns then lead to similar appraisals and emotional responses [START_REF] Salmela | Shared emotions[END_REF][START_REF] Salmela | The Functions of Collective Emotions in Social Groups[END_REF][START_REF] Salmela | Collective emotions and joint action[END_REF]. On this view, collective emotion follows from public and joint, as opposed to private and overlapping, commitments to common concerns. For example, the emotions of football supporters are collective, insofar as their social identities commit them to share similar concerns for their favourite team, to appraise the game's events accordingly, and therefore to show emotional responses of the same type.

Following this idea, Salmela and Nagatsu (2016) listed the defining features of collective emotion: "x and y experience an emotion of the same type with similar (1) evaluative content and (2) affective experience"; and "x and y are mutually aware [thereof]" (p. 36). In other words, individuals are aware that they experience similar emotions, that other individuals are aware of this fact, and that they all are aware of their respective awareness of this fact. This definition shares with phenomenological theories the idea that collective emotion entails awareness of self and others. Yet, there is a disagreement on what individuals are thought to be aware of. On the one hand, individuals are "mutually aware that others are feeling the same" [START_REF] Salmela | Collective emotions and joint action[END_REF]. On the other hand, individuals are mutually aware of being co-present and taking part in a collective emotional episode, without necessarily being aware of the similarity of their emotions [START_REF] Thonhauser | Shared emotions: a Steinian proposal[END_REF][START_REF] León | Emotional sharing and the extended mind[END_REF]. In both cases, mutual awareness does not necessarily imply an affective experience such as felt togetherness. According to [START_REF] Salmela | Two types of togetherness in shared emotions [and many other collectively intentional states[END_REF], the experiential quality of collective emotion derives from mutual awareness and varies as a function of individuals' commitments to their concerns (see Interim conclusion). Moreover, unlike the aforementioned phenomenological theories, Salmela and Nagatsu defined collective emotion as a pattern emerging from both emotional dispositions and emotional responses, including expressions, physiology and action tendencies, besides subjective feelings.

Interim conclusion

In summary, there are both similarities and differences between theories of collective emotion. The list of debated issues includes: whether patterns of emotional responses, emotional dispositions or both are defining features of collective emotion; whether collective emotion has a distinct experiential quality; whether it requires awareness of and identification with other individuals ("top-down" processes) or can emerge from unconscious emotional interactions ("bottom-up" processes); what individuals are aware of during collective emotion; and the extent to which emotional responses should exhibit finedgrained similarity. However, some of these debates may reflect the existence of different types of collective emotion rather than incompability of theories. Indeed, it has been argued that collective emotion is a continuum [START_REF] Salmela | Shared emotions[END_REF][START_REF] Salmela | The Functions of Collective Emotions in Social Groups[END_REF] and that there are different types of collective emotion [START_REF] Thonhauser | Towards a Taxonomy of Collective Emotions[END_REF]. Salmela distinguished three types of collective emotion: weakly, moderately and strongly collective emotions 1 . In short, the more emotional dispositions reflect commitments that are conjoint and public (i.e., expected), as opposed to unilateral and private, the more emotions are collective. In comparison, Thonhauser (2022) distinguished five types of collective emotion: fusion, segregation, matching, contagion and sharing. Each type represents a combination of distinct features 2 along a continuum 3 . Both typologies valuably underscore the complementarity between different theories of collective emotion, although they come with their own acknowledged limitations. The typology in terms of emotional dispositions accounts for the "intentional background" of collective emotion (Salmela, 2014, p. 171), hence, it insists on the antecedents of collective emotion. By comparison, considering the dynamics of emotional responses unfolding during a collective emotional episode could provide a different, complementary typology 4 . As such, Thonhauser's typology provides a valuable framework, but it primarily focuses on collective emotion experience. In contrast, we believe 1 Weakly collective emotions refer to overlapping concerns with private commitments, moderately collective emotions refer to social concerns with private commitments and strongly collective emotion refer to social concerns with explicit joint commitment between individuals. See [START_REF] Salmela | Shared emotions[END_REF] for examples. 2 The four characteristics are: collective evaluative perspective, collective appraisal, sense of togetherness, self and other awareness. For example, "fusion" amounts to similar emotional dispositions, with individuals similarly appraising the situation from the group's stance and experiencing a feeling of togetherness, but without them being reciprocally aware of each other and the collective nature of their emotions. In comparison, "sharing", or collective emotion in the strongest sense, amounts to all four characteristics at once. 3 Thonhauser gave the example of the momentary precedence of togetherness over self and other awareness in "fusion". This type of collective emotion does not imply the total absence of self and other awareness, nor prevents the possibility to shift to another type of collective emotion, such as "sharing", when partaking individuals reflect on their own experience. 4 Salmela (2014, p. 172) suggested that, similarly to the collectiveness of emotional dispositions (i.e., concerns), the collectiveness of emotional responses varies along a continuum according to their degree of synchronization.

that collective emotion involves not only subjective experiences but also unconscious affective processes (see "Mechanisms of collective emotion") that are relevant for establishing a typology that makes empirical predictions. In this regard, considering other facets of emotional responses seems an essential step towards elucidating how the different families of theories relate to each other. For this purpose, we now turn to the characteristics and mechanisms of collective emotion. In the following section, we derive from the theories of collective emotion three characteristics that clarify the similarities and differences between conceptual frameworks (Table 2).

Characteristics of collective emotion

The theories described in the previous section suggest that collective emotion can be defined by combinations of three basic characteristics: emotion alignment, feeling of togetherness and mutual awareness.

Emotion alignment

A recurring idea in theories of collective emotion is that individuals reciprocally influence each other so that they have, to some extent, similar emotions. In other words, collective emotion is characterized by a dynamic process that results in patterns of similar intertwined emotions. We define this process as emotion alignment, namely, "the dynamic and reciprocal adjustment of the components of a system for its coordinated functioning" (Dumas & Fairhurst, 2021, p. 3), where the system components are the emotions of individuals. Following our working definition of collective emotion, emotion alignment implies coupling, or reciprocal interaction between individuals.

Emotion alignment can result in different types of emotion patterns, notably: the convergence of emotions, or increasing similarity in emotion between individuals [START_REF] Anderson | The Emotional Convergence Hypothesis: Implications for Individuals, Relationships, and Cultures[END_REF][START_REF] Dezecache | Emotional convergence: A case of contagion[END_REF], and the synchronization of emotions, or instantaneous or delayed co-variation of emotions [START_REF] Wood | Forms and Functions of Affective Synchrony[END_REF][START_REF] Butler | Temporal Interpersonal Emotion Systems: The "TIES" That Form Relationships[END_REF]. Convergence corresponds to the consequence of alignment on the form and content of emotions, whereas synchrony corresponds to the consequence of alignment on the time course of emotions. Overall, theoreties debate whether collective emotion is defined by alignment of emotional dispositions, emotional responses, or both. It is also worth noting that some theories do not subscribe to the view that collective emotion implies convergence. For instance, reciprocal influence between individuals can lead to the synchronization of emotional responses without clear convergence patterns over the course of the emotional episode.

Feeling of togetherness

Another characteristic of collective emotion is its distinct experiential quality: individuals not only feel as other individuals do, but with others, which translates into a feeling of togetherness. Here, we describe this feeling as the positively valenced subjective experience of increased similarity between the self and other individuals. This definition highlights the potential compatibility between accounts of collective effervescence and phenomenological integration. However, both approaches may differ in their conception of the relation between the feeling of togetherness and the other characteristics of collective emotion.

The question is whether the feeling of togetherness arises from awareness of emotion alignment with other individuals. Furthermore, theories of collective emotion do not all agree that the feeling of togetherness is a defining characteristic of collective emotion: it is sometimes argued that being aware of emotion alignment without having this feeling is sufficient.

Mutual awareness

Phenomenological and analytical theories of collective emotion posit that individuals taking part in the collective emotion are mutually aware of each other.

Here, we describe mutual awareness as a form of mutual co-representation, meaning that individuals represent both their own mental states and the mental states of the individuals with whom they interact. Co-representation, initially discussed in the context of cooperative joint action [START_REF] Sebanz | Representing others' actions: just like one's own?[END_REF], has two properties worth mentioning. First, it does not necessarily imply reflective awareness of the mental states of other individuals. Second, successful co-representation entails distinguishing between the self and other individuals so that individuals do not mistake the representations of other individuals for their own. In other words, co-representing individuals need not feel the emotion of the co-represented individual.

Interim conclusion

Emotion alignment, feeling of togetherness and mutual awareness are building blocks of theories of collective emotion. These three characteristics compose a reference frame clarifying the similarities and differences between definitions of collective emotion (Table 2). Specifically, theories emphasize different characteristics and make different hypotheses about the relations between these characteristics. In the next section, we turn to research on emotion and social cognition to review candidate mechanisms for each characteristic.

Mechanisms of collective emotion

There are different possible mechanisms underlying emotion alignment, feeling of togetherness and mutual awareness. In this section, we briefly describe these mechanisms and how they relate to each other.

Emotion alignment

Social identification. To begin with, individuals taking part in a collective emotional episode may belong to a common social group (i.e., family, activity group, organization)5 . Social groups are characterized by values, beliefs, concerns, as well as norms about the emotions that group members are expected to feel [START_REF] Hochschild | Emotion Work, Feeling Rules, and Social Structure[END_REF] and display [START_REF] Ekman | The Repertoire or Nonverbal Behavior: Categories, Origins, Usage and Coding[END_REF] in a given context. These characteristics are part of the culture and knowledge of social groups [START_REF] Von Scheve | Towards a theory of collective emotions[END_REF][START_REF] Illouz | Emotions and Cultural Theory[END_REF], and they shape the individual's emotional dispositions.

In particular, group membership predicts group identification, meaning that individuals subjectively experience that they are similar to exemplar tokens of group members and entertain the desire to belong to that group [START_REF] Taipale | The Structure of Group Identification[END_REF]. Individuals who identify to a common group hold similar concerns related to the group culture, they appraise events that are relevant to their group from this group's stance, and they experience similar emotions on behalf of the group. This process, which corresponds to the convergence of emotions across group members, leads to what we described earlier as group-based emotion [START_REF] Mackie | Group-based emotion in group processes and intergroup relations[END_REF]. Although group-based emotion does not necessarily involve interaction between individuals, social interactions can promote social identification in specific ways. In particular, the culture of social groups includes recurring social interactions that correspond to specific places, events, practices and norms, such as religious offices or military ceremonies. These collective affective niches are designed to trigger group identification and compliance to the group's emotional norms, giving rise to collective emotion as a result of joint success or failure in doing so [START_REF] Nagatsu | Interpersonal and collective affective niche construction: Empirical and normative perspectives on social media[END_REF]. In this respect, there may be differences between acquainted group members (e.g., spouses, friends), who are already aligned in terms of emotional dispositions, and unacquainted group members (e.g., music fans, political partisans), for which engaging in a joint activity is key for social identification [START_REF] Salmela | Two types of togetherness in shared emotions [and many other collectively intentional states[END_REF]. Crucially, individuals are committed to conform to the group's emotional norms, even without awareness of this normative influence [START_REF] Cialdini | Social influence: compliance and conformity[END_REF].

Social appraisal. Unlike group-based appraisal, social appraisal relies on the influence of other individuals on how one appraises a situation [START_REF] Parkinson | Intragroup Emotion Convergence: Beyond Contagion and Social Appraisal[END_REF]. Indeed, emotional appraisal is a dynamic process, and it is permeable to contextual social information: one's emotion is continuously shaped by the emotions of other individuals [START_REF] Manstead | Social appraisal[END_REF]. For instance, one may realize that one's emotion is incongruent with those expressed by other individuals, which leads to revising one's appraisal of the situation. Yet, social appraisal does not necessarily imply consciously perceiving the emotions of other individuals. In addition, the relation between social appraisal and emotion alignment depends on identification with other individuals: individuals are more likely to align via social appraisal with their fellow group members than with individuals from an out-group. Therefore, social appraisal could explain how the perception of the emotions and social identities of other individuals interplay to cause collective emotion [START_REF] Bruder | Social appraisal as a cause of collective emotions[END_REF].

Emotion regulation. Social identification and social appraisal suggest that individuals invest resources in intrapersonal emotion regulation to remedy the discrepancy between their own emotion and the emotions perceived to be the group's norm. Emotion regulation is not necessarily a conscious process, and it can manifest in different strategies corresponding to, e.g., changing one's appraisal of the situation (reappraisal), changing one's attentional focus (distraction) or altering one's physiology through drugs or relaxation exercises [START_REF] Mcrae | Emotion regulation[END_REF]. In the context of social interaction, individuals may also engage in interpersonal emotion regulation or coregulation. This process corresponds to the dynamic alignment between interacting individuals towards a stable emotional state [START_REF] Butler | Emotional Coregulation in Close Relationships[END_REF][START_REF] Zaki | Interpersonal emotion regulation[END_REF]. Previous research argued that emotion co-regulation, such as between infants and their caregivers, is the core mechanism of collective emotion [START_REF] Krueger | The affective 'we': self-regulation and shared emotions[END_REF].

Emotion matching and imitation. Another category of explanations describes emotion alignment as the by-product of the perception-action matching system. This cognitive system is responsible for the effortless and unconscious mental simulation of others' actions during social interactions [START_REF] Preston | Empathy: Its ultimate and proximate bases[END_REF]. For instance, perceiving someone laughing would activate motor and premotor areas responsible for laughter (but see [START_REF] Caruana | Cracking the laugh code: laughter through the lens of biology, psychology and neuroscience[END_REF] for a more nuanced picture). According to the Primary Emotion Contagion model [START_REF] Hatfield | Primitive emotional contagion[END_REF], the perception and simulation of emotional expressions leads to their imitation: individuals tend to unintentionally and automatically mimic the emotional facial expressions and bodily postures of other individuals (e.g., reciprocated smiles during a conversation). Mimicry of emotion expressions then leads individuals to experience the mimicked emotions through afferent feedback. This mechanism, labelled "emotion contagion", has been presented as a precursor of collective emotion [START_REF] Hatfield | Emotional contagion as a precursor to collective emotions[END_REF]. However, it has been argued that mimicry is not automatic and reflects a communicative action based on knowledge about the emotions of other individuals [START_REF] Hess | Do we mimic what we see or what we know[END_REF]. For example, previous research found that participants spontaneously mimicked happiness displays of other individuals, yet mimicked sadness displays only when the expresser was a fellow group member [START_REF] Bourgeois | The impact of social context on mimicry[END_REF]. This finding has been discussed as evidence that emotion alignment relies on contextual interpretation of the expresser's emotional expression, similar to social appraisal [START_REF] Fischer | Mimicking emotions[END_REF]. Consequently, other researchers have distinguished between emotion contagion and empathy, the latter involving reflective processes (i.e., awareness that one's emotion results from the perception of another individual's emotion) that regulate emotion alignment [START_REF] Lamm | Insights into collective emotions from the social neuroscience of empathy[END_REF].

Interpersonal entrainment. An alternative explanation of emotion alignment highlights the role of orientation calibration [START_REF] Parkinson | Intragroup Emotion Convergence: Beyond Contagion and Social Appraisal[END_REF] or interpersonal entrainment [START_REF] Pacherie | Collective phenomenology[END_REF]. During a social interaction, individuals unintentionally and unconsciously adjust their gaze direction and coordinate their bodily movements to those of other individuals, orienting their attention and behaviour in socially relevant ways. For instance, visual attention is oriented in priority to socially relevant signals such as emotions, and it is influenced by information about what other individuals attend to (e.g., eye gaze), which can lead individuals to mutually adjust their attentional focus [START_REF] Brosch | Neurocognitive mechanisms of attentional prioritization in social interaction[END_REF]. This mechanism is not limited a priori to visual attention and also applies to auditory attention (see also "Social and joint attention" below). Crucially, interpersonal entrainment reflects a spontaneous tendency for alignment, and it does not require experiencing emotions nor perceiving the emotions of other individuals in the first place.

Relations between mechanisms. The potential mechanisms of emotion alignment are not mutually exclusive. As an illustration, social identification increases the saliency of co-present individuals as fellow group members, which facilitates social appraisal, unconscious emotion co-regulation and mimicry-based imitation, besides dispositions for interpersonal entrainment. However, specific contexts can recruit these mechanisms independently. For example, interpersonal entrainment does not require perceiving the emotios of other individuals. Moreover, the abovementioned mechanisms do sometimes reflect competing explanations of emotion alignment. A debate concerns the extent to which emotion alignment results from automatic processes (i.e., emotion contagion) or from context-dependent inferential processes (i.e., social appraisal) that vary with the type of emotion and relationship between individuals (see [START_REF] Wróbel | Broadening the Perspective on Emotional Contagion and Emotional Mimicry: The Correction Hypothesis[END_REF] for a discussion).

Feeling of togetherness

Social identification. We described the potential role of social identification in emotion alignment. Here, we consider social identification as a potential mechanism of felt togetherness. It has been argued that the feeling of oneness with the group is the result of identity fusion between the personal and social identities of individuals [START_REF] Swann | When group membership gets personal: A theory of identity fusion[END_REF]. Fusion occurs when individuals "view themselves through their group membership ("My group membership is a crucial part of who I am")" and when they "perceive the group through their personal self ("I am an important part of the group")" (p. 442). Crucially, identity fusion does not require perception and awareness of the presence of other individuals, nor that individuals engage in a joint activity. A different account of the feeling of togetherness argues that social identification in crowds is conceptually different from self-categorization involved in groupbased emotion and identity fusion [START_REF] Neville | The experience of collective participation: Shared identity, relatedness and emotionality[END_REF]. During collective gatherings, crowd members not only identify with a common group but also perceive other co-present individuals as fellow group members, leading to the emergence of a shared identity. Here, social identification is the consequence of a joint activity between co-present individuals and varies as a function of the saliency of the shared identity. The authors proposed that shared identity can generate a positive feeling of empowerment, which arises from the possibility of acting in unison with other group member to attain shared goals.

Social and joint attention. An alternative explanation of the feeling of togetherness emphasizes the role of attention in collective emotion. As mentioned before, individuals tend to orient their attention to individuals expressing emotions, and they tend to adjust their attentional focus to those of other individuals. This entrainment mechanism predicts different types of social attention. [START_REF] Siposova | A new look at joint attention and common knowledge[END_REF] distinguished between forms of social attention that have an individualistic nature and forms of social attentions that are relational and bidirectional (joint attention properly speaking). While simply attending to other individuals is an individualistic form of attention (i.e., monitoring others from a third-person perspective), joint attention proper is relational: individuals simultaneously attend to the same event, as well as to each other attending to the event and to other individuals. Accordingly, there are two candidate mechanistic pathways for the feeling of togeth-erness. On the one hand, the feeling of togetherness results from an individualistic form of social attention (monitoring of other individuals) leading to the belief that one's emotion align with those of other individuals. On the other hand, it results from joint attention leading to mutual or shared knowledge (mutual awareness) among individuals that they are part of a bidirectional relationship [START_REF] Siposova | A new look at joint attention and common knowledge[END_REF].

Relations between mechanisms. Social identification and social attention are conceptually distinct mechanisms, but they likely interplay. In particular, monitoring attention leading to perceived alignment or joint attention leading to mutual awareness could explain why individuals identify with each other as fellow group members. Both categories of mechanisms are mentioned in research on collective effervescence. According to [START_REF] Rimé | Why We Gather: A New Look, Empirically Documented, at Émile Durkheim's Theory of Collective Assemblies and Collective Effervescence[END_REF], the feeling of togetherness experienced during collective effervescence can arise at multiple stages of the process: as the result of social identification leading to the realization of a shared identity with other individuals, as the result of perceived alignment of attention and emotional responses during the event, and finally as the result of emotion amplification leading to empowerment and reinforcement of shared identity in response to the event.

Mutual awareness

Joint attention. In paradigmatic examples of collective emotion, such as audience members watching a sports game together [START_REF] Thonhauser | Emotional sharing in football audiences[END_REF] or listening to music together [START_REF] Krueger | Varieties of extended emotions[END_REF], individuals not only reciprocally adjust and orient their attention to the same emotion-eliciting event, but they also engage in forms of social attention that sustain mutual awareness. In their typology of attention, [START_REF] Siposova | A new look at joint attention and common knowledge[END_REF] further distinguished between two forms of joint attention: mutual and shared. Both forms of joint attention involve individuals being aware -in a second-person relation to each other -that they are attending to the same thing with each other. The main difference between these two forms lies in the absence (mutual attention) or presence (shared attention) of intentional communication about the object of attention. It is worth noting that joint attention does not presuppose that individuals actually orient their attention to, and perceive, the emotions of other individuals.

Emotion recognition. Another interpretation of mutual awareness during collective emotion holds that individuals are mutually aware of emotion alignment, which we defined as emotion co-representation. The conjunction of emotion alignment and awareness thereof is characteristic of empathy, presented as a mechanism of collective emotion [START_REF] Lamm | Insights into collective emotions from the social neuroscience of empathy[END_REF]. In particular, empathy implies inferences of the emotions of other individuals. Empathic accuracy [START_REF] Ickes | Empathic accuracy: Judging thoughts and feelings[END_REF] measures the ability to infer the thoughts and feelings of another individual, and emotional aperture [START_REF] Sanchez-Burks | Emotional aperture and strategic change: The accurate recognition of collective emotions[END_REF] measures the ability to infer the emotional composition of a group (e.g., the emotion most expressed by group members). Empathic accuracy and emotional aperture generally presuppose that inferences are based on the emotions expressed by faces, postures, gestures such as clapping, and sounds such as cheers and groans. In contrast, when information about the emotions of other individuals is lacking, individuals may instead base their inferences on the belief that their experience is shared [START_REF] Smith | Representation and Incorporation of Close Others' Responses: The RICOR Model of Social Influence[END_REF]. In these contexts, mutual awareness of emotion alignment can be explained by the combination of social projection and self-stereotyping [START_REF] Cho | I'm like you and you're like me: Social projection and self-stereotyping both help explain self-other correspondence[END_REF]: the former refers to attributing mental states that are similar to one's own to other individuals, whereas the latter refers to adopting mental states that congruent with one's beliefs about the group one identifies with.

Interim conclusion

The foregoing review of candidate mechanisms of collective emotion is not exhaustive. Yet, it already suggests that these mechanisms operate at different levels (intrapersonal, interpersonal and group-level) and reflect different types of processes (automatic, context-dependent). Crucially, these mechanisms are the hypothetical universal building blocks that support the emergence of collective emotion across cultures. Moreover, making hypotheses about the mechanisms of collective emotion hints at relevant methods to collect experimental data.

Experimental evidence and future research

Based on this review of theories, characteristics and mechanisms of collective emotion, we now turn to outstanding questions, available experimental evidence and promising directions for future research.

In this section, we focus on experimental research, and we will not cover the full range of empirical research that includes rich case studies of collective pride and shame (e.g., [START_REF] Sullivan | Understanding collective pride and group identity[END_REF][START_REF] Sullivan | Collective emotions in celebratory, competitive, and conflictual contexts: exploring the dynamic relations between group-based and collective pride and shame[END_REF][START_REF] Pettigrove | Shame: A case study of collective emotion[END_REF].

Antecedents of collective emotion

What is the relation between collective emotion and other affective phenomena such as emotional climates (de Rivera [START_REF] De Rivera | Emotional climate, human security, and cultures of peace[END_REF]) and emotional atmospheres [START_REF] Trigg | The role of atmosphere in shared emotion[END_REF]? Emotional climate refers to the prototypical emotions that characterize relationships in a social group (e.g., fear in a dictatorship), whereas emotional atmosphere refers to the mood that unintentionally spatially manifests as a response to a specific event (e.g., amusement in the theatre at a comic show). Emotional climates and atmospheres highlight the existence of an affective background for collective emotion. This background includes, e.g., emotional dispositions underlying relations between individuals and collective moods corresponding to affective niches [START_REF] Nagatsu | Interpersonal and collective affective niche construction: Empirical and normative perspectives on social media[END_REF]. Emotional atmospheres have been described as bases for collective emotion, as they establish a "common ground between people" that facilitates mutual awareness and feelings of togetherness [START_REF] Trigg | The role of atmosphere in shared emotion[END_REF]. Remarkably, the analysis of large datasets corresponding to the expressions of emotions in online communications clarifies how collective emotion emerges from background collective moods. Indeed, there is now reliable evidence that emotion alignment occurs through interactions on online social networks (Goldenberg & Gross, 2020), which results in collective cyberemotion that do not involve face-to-face interactions [START_REF] Holyst | Cyberemotions: Collective emotions in cyberspace[END_REF]. Research on frequency patterns of collective cyberemotions revealed the existence of periodic collective emotional episodes. For example, Sano and colleagues (2019) analysed Japanese blog articles over ten years, which revealed a difference between collective responses to transient events (e.g., the 2011 T ōhoku earthquake) and periodic events (e.g., Valentine's Day). Studies of collective emotion on online social media also shed light on the temporal dynamics of collective emotional episodes across different cultures (e.g., Covid-19;[START_REF] Metzler | Collective emotions during the COVID-19 outbreak[END_REF][START_REF] Wu | Classification of endogenous and exogenous bursts in collective emotions based on Weibo comments during COVID-19[END_REF]. We hypothesize that collective moods and collective memories [START_REF] Hirst | Collective memory from a psychological perspective[END_REF]

Consequences of collective emotion

What are the consequences of collective emotion? Has collective emotion a function? Social bonding. Ritualistic accounts of social interactions and collective gatherings suggest that collective emotion promotes social bonding [START_REF] Collins | Interaction ritual chains and collective effervescence[END_REF][START_REF] Páez | Collective emotional gatherings: their impact upon identity fusion, shared beliefs, and social integration[END_REF]. Following the pioneering work of Durkheim on rituals (1912Durkheim on rituals ( /1915)), collective gatherings such as protests, religious marches and music festivals have been described as rituals eliciting collective effervescence via social identification, interpersonal entrainment, joint attention, emotion amplification, alignment with other group members, and awareness thereof [START_REF] Hobson | The psychology of rituals: An integrative review and process-based framework[END_REF]Rosner & Meher, 2014;[START_REF] Knottnerus | Collective events, rituals, and emotions[END_REF]. A recent metaanalysis found that collective effervescence, commonly measured as perceived emotional synchrony [START_REF] Wlodarczyk | Perceived Emotional Synchrony in Collective Gatherings: Validation of a Short Scale and Proposition of an Integrative Measure[END_REF], predicted social integration, empowerment and adhesion to social values [START_REF] Pizarro | Emotional processes, collective behavior, and social movements: A meta-analytic review of collective effervescence outcomes during collective gatherings and demonstrations[END_REF]. Previous studies in the laboratory and in the field shed light on the neurochemical pathways underlying social bonding in ritual contexts (Charles et al., 2020). Crucially, this social bonding effect is not limited to physical gatherings and positive events: similar to research on rituals, [START_REF] Garcia | Collective emotions and social resilience in the digital traces after a terrorist attack[END_REF] observed a burst in the number of negative emotional communications on Twitter after the Paris terrorist attacks of November 2015. The analysis of tweets revealed that participating in the episode of emotion alignment on Twitter after the attacks positively related to expressing solidarity in subsequent messages up to a month after. Lastly, the consequences of collective emotion may not only extend across time but also beyond the group of participants. For instance, the self-transformative experience of participants in a secular mass gathering predicted their willingness to invest time to help socially distant strangers [START_REF] Yudkin | Prosocial correlates of transformative experiences at secular multi-day mass gatherings[END_REF]. Overall, social rituals are ubiquitous [START_REF] Whitehouse | The ritual animal: Imitation and cohesion in the evolution of social complexity[END_REF], and researchers found that experiences of collective effervescence are frequent in everyday settings [START_REF] Gabriel | Creating the sacred from the profane: Collective effervescence and everyday activities[END_REF]. Accordingly, collective emotion, even of negative valence, may increase the motivation for subsequent social interactions by inducing "emotional energy" [START_REF] Collins | Interaction ritual chains and collective effervescence[END_REF]. This process would explain how micro-level smallscale interactions translates, at macro-level, into social groups exhibiting solidarity and shared identity. For example, it has been argued that collective emotion contributes to national identification [START_REF] Beyer | The social consequences of collective emotions: National identification, solidarity and out-group derogation[END_REF][START_REF] Sullivan | Collective emotions: A case study of South African pride, euphoria and unity in the context of the 2010 FIFA World Cup[END_REF]. This functional approach is also central in Salmela's typology based on shared concerns, with stronger types of collective emotion predicting social bonding to a greater extent than weaker types of collective emotion [START_REF] Salmela | Shared emotions[END_REF][START_REF] Salmela | The Functions of Collective Emotions in Social Groups[END_REF].

Collective action. Emotion is both a cause and a reason for action [START_REF] Pacherie | Emotion and Action[END_REF], hence, it has been proposed that collective emotion promotes collective action (e.g., [START_REF] Michael | Shared emotions and joint action[END_REF][START_REF] Salmela | Collective emotions and joint action[END_REF]. First, collective emotion would act as a prerequisite, motivating participants to form collectives to act together [START_REF] Jasper | Emotions, sociology, and protest[END_REF]. This effect is exemplified in social movements [START_REF] Van Ness | Emotions in social movements[END_REF], and it matches evidence-based models of collective action. A meta-analysis found that groupbased emotion mediated the relationship between social identification and collective action [START_REF] Van Zomeren | Toward an integrative social identity model of collective action: a quantitative research synthesis of three socio-psychological perspectives[END_REF]. Conversely, an experimental study inducing group-based emotions revealed that participants were more willing to engage in collective action with fellow group members when they had perceived that their own group-based anger was congruent with the emotions of fellow group members [START_REF] Livingstone | We feel, therefore we are: Emotion as a basis for self-categorization and social action[END_REF]. Moreover, willingness to engage in collective action was higher when the anger of ingroup members contrasted with the indifference of out-group members. In other words, collective action may follow not only from self-categorisation, which characterizes group-based emotion, but also from perceived emotion alignment with fellow group members, which characterizes collective emotion. Second, collective emotion would act as a facilitator of coordinated action. [START_REF] Salmela | Collective emotions and joint action[END_REF] argued that constitutive aspects of collective emotion (e.g., joint attention, interpersonal synchrony, mutual awareness of shared concerns) are key processes to ensure successful joint action, especially impulsive joint actions engaged in without prior deliberation and planning. However, despite some evidence regarding the relation between emotion and joint action [START_REF] Beaurenaut | Action co-representation under threat: A Social Simon study[END_REF][START_REF] Bie Ńkiewicz | Impact of emotion-laden acoustic stimuli on group synchronisation performance[END_REF], the role of collective emotion in coordinating joint action remains experimentally under-explored.

Moving forward, future research should compare collective emotion to group-based emotion, which will help to elucidate their respective consequences on collective action and social bonding and to clarify the role of related mechanisms such as interpersonal synchrony [START_REF] Rennung | Prosocial Consequences of Interpersonal Synchrony: A Meta-Analysis[END_REF][START_REF] Mogan | To be in synchrony or not? A meta-analysis of synchrony's effects on behavior, perception, cognition and affect[END_REF]. Besides, mutual awareness of emotion alignment and feelings of togetherness among co-present individuals are potential triggers of collective action stemming from group-based emotion, and they may explain the long-lasting effects of collective emotion in extended social networks. Future research should also investigate the relations between typologies of collective emotion [START_REF] Salmela | Shared emotions[END_REF][START_REF] Thonhauser | Towards a Taxonomy of Collective Emotions[END_REF] and typologies of joint action and sense of agency [START_REF] Pacherie | How does it feel to act together?[END_REF][START_REF] Silver | Social Agency as a continuum[END_REF][START_REF] Loehr | The sense of agency in joint action: An integrative review[END_REF]. Does collective emotion, which involves feeling connected to other individuals, foster collective action, which is characterized by united agency (i.e., the sense of acting as one)? Lastly, previous research suggests that collective emotion can have negative outcomes, such as violence in intergroup conflict (e.g., [START_REF] Sullivan | Understanding collective pride and group identity[END_REF][START_REF] Beyer | The social consequences of collective emotions: National identification, solidarity and out-group derogation[END_REF], poor collective decision-making, and lack of creativity as consequences of group conformity and obedience [START_REF] Gelfand | The cultural evolutionary trade-off of ritualistic synchrony[END_REF]. These observations raise questions about the type of groups and interactions that favour positive over negative social consequences.

Group membership and affiliation

To what extent are common group membership and affiliation pre-requisites of collective emotion? Does collective emotion emerge more quickly, last longer, and feel stronger in groups in which common group membership is salient or explicit? Previous research found that the degree of social identification with a group predicted the similarity between the personal emotions of group members and the emotion they reported on behalf of their group [START_REF] Seger | Subtle activation of a social categorization triggers group-level emotions[END_REF]. There is also preliminary evidence that affiliation a pre-requisite for the alignment of physiological markers of emotion: in the context of a fire-walking ritual, [START_REF] Konvalinka | Synchronized arousal between performers and related spectators in a fire-walking ritual[END_REF] detected significant levels of cardiac synchrony between fire-walkers and audience members, but only when performers and spectators were related family members. In other words, interpersonal alignment of cardiac activity could relate to empathy driven by kinship. These findings are consistent with evidence that group membership modulates candidate mechanisms of collective emotion such as attentional orientation (e.g., [START_REF] Brosch | Neurocognitive mechanisms of attentional prioritization in social interaction[END_REF], emotion perception (e.g., [START_REF] Gamond | Minimal group membership biases early neural processing of emotional expressions[END_REF], behavioural mimicry (e.g., [START_REF] Bourgeois | The impact of social context on mimicry[END_REF] and mental simulation of other's emotions [START_REF] Eres | The influence of group membership on the neural correlates involved in empathy[END_REF]. Crucially, some of these studies suggest that minimal group membership (i.e., artificial, context-specific and experimentally-induced) is sufficient to generate these effects (e.g., [START_REF] Gamond | Minimal group membership biases early neural processing of emotional expressions[END_REF]. A promising direction consists of manipulating social identification in the context of shared emotional experiences, either explicitly by inducing minimal group membership [START_REF] Tajfel | Experiments in Intergroup Discrimination[END_REF], or implicitly by changing the salience of social identity (e.g., [START_REF] Kuppens | Social identity salience shapes group-based emotions through group-based appraisals[END_REF]. Future studies could then test whether group-level alignment of emotions (subjective experiences, physiological responses) vary as a function of social identification.

Group size

Are there typical features of groups who take part in collective emotions? In particular, to what ex-tent does group size modulate the mechanisms and characteristics of collective emotion? Assuming that rituals are paradigmatic examples of social gatherings inducing collective emotion, it is worth considering the distinction between two types of rituals: doctrinal rituals that are high-frequency, large-scale, low-arousal and foster group identification by way of routinized behaviours (related to shared culture and knowledge); and imagistic rituals that are lowfrequency, small-scale, highly arousing and lead to shared identity by way of intense emotional experiences [START_REF] Whitehouse | The ties that bind us: Ritual, fusion, and identification[END_REF][START_REF] Whitehouse | The ritual animal: Imitation and cohesion in the evolution of social complexity[END_REF]. This distinction hints at differences in smallscale as opposed to large-scale collective emotions. For example, in large groups, in which individuals only engage in direct and reciprocal interaction with a subset of the group, attention and coupling to a common third-party (e.g., speaker or performing artists on stage) could provide a basis for emotion alignment. Similarly, emotion perception and recognition may greatly differ in large as opposed to small groups. Research on emotional aperture found that participants asked to evaluate the emotional composition of a group tended to focus their attention on a few individuals [START_REF] Masuda | Placing the face in context: Cultural differences in the perception of facial emotion[END_REF], and in particular on those expressing more intense emotions, which in another study led participants to overestimate the group's average emotional intensity [START_REF] Goldenberg | The Crowd-Emotion-Amplification Effect[END_REF]. This finding suggests that in large groups, inaccuracy in emotion representation could actually contribute to the emergence of group-level emotion amplification. In sum, future research should investigate how the mechanisms and characteristics of collective emotion change with the size of the group. A potential benefit is clarifying the relations between theories of collective emotion that focus either on large-scale crowd events or on bidirectional dyadic relationships. Experimental studies could contrast dyads with groups of three and up to, e.g., six participants. Indeed, previous research indicated that human groups spontaneously split into subgroups of about three to four people in the context of conversations [START_REF] Dunbar | Size and structure of freely forming conversational groups[END_REF] and laughter [START_REF] Dezecache | Sharing the joke: the size of natural laughter groups[END_REF]. Based on cases studies of collective emotions (e.g., [START_REF] Sullivan | Collective emotions in celebratory, competitive, and conflictual contexts: exploring the dynamic relations between group-based and collective pride and shame[END_REF][START_REF] Sullivan | Collective emotions: A case study of South African pride, euphoria and unity in the context of the 2010 FIFA World Cup[END_REF], future experimental research could also investigate the influence of other group features such as its structure (e.g., reciprocity and strength of connections, presence of an out-group) and the balance between homogeneity and heterogeneity across individuals (e.g., diversity in the motives of participants, leaderfollower relationships).

Type of emotion

Does collective emotion preferentially occur in positive as opposed to negative contexts, or beyond certain intensity thresholds? In their meta-analysis, [START_REF] Pizarro | Emotional processes, collective behavior, and social movements: A meta-analytic review of collective effervescence outcomes during collective gatherings and demonstrations[END_REF] found that collective effervescence was associated with self-reported experiences of emotional arousal, positive emotional valence and perceived positive general mood -although the absence of a significant association with negative emotional valence could be explained by the small sample of intense negative events. In the context of cyberemotions, the analysis of over four million comments on social media indicated that the discussion length was predicted by intensity at the start of the discussion thread, suggesting that emotion intensity regardless of valence (positive or negative) modulates the duration of collective cyberemotion and the lifespan of the online group it binds together [START_REF] Chmiel | Collective emotions online and their influence on community life[END_REF]. Future research should combine dimensional measures of emotion (e.g., scales of valence and arousal; [START_REF] Bradley | Measuring emotion: the self-assessment manikin and the semantic differential[END_REF], discrete measures of emotion (e.g., [START_REF] Scherer | The GRID meets the Wheel: Assessing emotional feeling via selfreport[END_REF], and physiological measures (e.g., cardiac activity) to clarify whether some of the characteristics of collective emotion arise only in intense emotional contexts. Moreover, we speculate that specific types of emotions such as self-conscious emotions (e.g., pride, shame) relate to specific combinations of characteristics and mechanisms (e.g., emphasising mutual awareness).

Awareness of collective emotion

What is the evidence regarding the role of awareness of other individuals during collective emotion? Although the spontaneous verbal communication of one's emotions (or social sharing of emotion; [START_REF] Rimé | Emotion Elicits the Social Sharing of Emotion: Theory and Empirical Review[END_REF] is a strong candidate mechanism for emotion alignment during social interactions, previous research found that participants emotionally converged based on the visual perception of non-verbal facial cues and without explicit awareness of the perceived emotion [START_REF] Dezecache | Evidence for unintentional emotional contagion beyond dyads[END_REF]. Nonetheless, experimental research on shared experiences indicated that awareness that acquaintances are present and facing the same situation amplified subjective affective experiences, an effect increasing with affiliation between individuals [START_REF] Boothby | Psychological Distance Moderates the Amplification of Shared Experience[END_REF]. In the context of aesthetic experiences, participants simultaneously attending to positive or negative emotional stimuli felt more intense emotions than participants attending alone [START_REF] Shteynberg | Feeling more together: Group attention intensifies emotion[END_REF], and participants watching negative film clips on the same screen as opposed to different ones reported higher liking of and perceived similarity with other participants [START_REF] Rennung | Facing sorrow as a group unites. Facing sorrow in a group divides[END_REF]. These findings support the idea that joint attention leads to social bonding by establishing mutual awareness and felt togetherness, which ultimately reinforces the shared concerns that underlie emotional dispositions [START_REF] Shteynberg | A collective perspective: Shared attention and the mind[END_REF]. What is the relation between mutual awareness of emotion, reflected in empathic accuracy, and interpersonal alignment? Previous research found that empathic accuracy relies to some extent on alignment between the perceiver and the emoter via sensorimotor simulation [START_REF] Wood | Altering sensorimotor feedback disrupts visual discrimination of facial expressions[END_REF][START_REF] Paracampo | Sensorimotor network crucial for inferring amusement from smiles[END_REF] or interpersonal synchrony [START_REF] Levenson | Empathy: A physiological substrate[END_REF], although findings are mixed [START_REF] Mackes | Tracking emotions in the brain-revisiting the empathic accuracy task[END_REF][START_REF] Jospe | The contribution of linguistic and visual cues to physiological synchrony and empathic accuracy[END_REF][START_REF] Soto | Emotion recognition across cultures: The influence of ethnicity on empathic accuracy and physiological linkage[END_REF]. To our knowledge, research on empathic accuracy has to date only evaluated this cognitive ability in a unidirectional fashion (i.e., the ability of an observer to recognize the emotion of an emoter). Therefore, future research could investigate the relation between emotion alignment (e.g., as a group-level index of convergence or synchronization of emotional responses) and mutual awareness within the group (e.g., as a group-level average index of empathic accuracy).

Promising directions

Evidence regarding collective emotion mainly comes from research on collective gatherings and rituals, cyberemotions, shared experiences, and computational models of group behaviours. Research on rituals and cyberemotions provide valuable insights into some of the mechanisms and outcomes of collective emotion with a high degree of ecological validity. However, these studies typically rely on self-reports and personal communications (but see [START_REF] Konvalinka | Synchronized arousal between performers and related spectators in a fire-walking ritual[END_REF][START_REF] Liebst | Exploring the sources of collective effervescence: A multilevel study[END_REF] that are relatively silent about non-conscious aspects of collective emotion. In comparison, computational models of collective emotion draw from research on complex systems and its application to collective cognition [START_REF] Couzin | Collective cognition in animal groups[END_REF]. As such, models of collective emotion investigate the emergence of group-level emotion patterns through repeated local interactions between individuals. However, these models typically simulate the emergence of emotion patterns in physical crowds as the spread of a contagious disease (e.g., Gao & Liu, 2017;[START_REF] Fu | Simulation of emotional contagion using modified SIR model: A cellular automaton approach[END_REF], equating collective emotion with emotion alignment. There are also discrepancies between present experimental research and contemporary theories of collective emotion. In particular, there is relatively little investigation of mutual awareness and its relation to emotion alignment, and of the relation between collective emotion and joint action. Moreover, research mostly focuses on collective emotion in the context of verbal interactions, whereas our review of candidate mechanisms points to the role of non-verbal, unconscious processes.

Experimental studies in laboratory settings are particularly relevant to investigate the mechanisms and outcomes of collective emotion, while ensuring a fine-grained control over the social interaction, the type of group, and the type of emotion. A typical experimental protocol could include the induction of minimal group identities, the elicitation of grouprelevant emotions via joint activities (e.g., watching together emotion-inducing film clips), the manipulation of joint attention (e.g., [START_REF] Rennung | Facing sorrow as a group unites. Facing sorrow in a group divides[END_REF][START_REF] Bruder | Emotional signals in nonverbal interaction: Dyadic facilitation and convergence in expressions, appraisals, and feelings[END_REF], and the manipulation of communication modality between participants (e.g., [START_REF] Järvelä | Intragroup Emotions: Physiological Linkage and Social Presence[END_REF]. Emotion induction phases would be preceded and followed by experimental tasks capturing social bonding, action coordination and cooperation during interactions between participants. These studies could also benefit from methodological advances in research on interpersonal synchrony of behaviour [START_REF] Mogan | To be in synchrony or not? A meta-analysis of synchrony's effects on behavior, perception, cognition and affect[END_REF][START_REF] Rennung | Prosocial Consequences of Interpersonal Synchrony: A Meta-Analysis[END_REF] and physiology [START_REF] Chen | Physiological linkage during shared positive and shared negative emotion[END_REF][START_REF] Palumbo | Interpersonal autonomic physiology: A systematic review of the literature[END_REF][START_REF] Mayo | Interpersonal autonomic nervous system synchrony and its association to relationship and performance-a systematic review and meta-analysis[END_REF].

For example, experimenters could manipulate emotion alignment between co-participants and assess whether convergence and synchrony of emotions covary with felt togetherness, while controlling for affiliation and group bias. This kind of study would help to clarify the relation between different aspects of emotion alignment (convergence of self-reports, physiological synchrony) and their relation to other characteristics of collective emotion (feeling of togetherness, mutual awareness). Ultimately, these experiments would contribute to tackle theoretical debates, e.g., whether the convergence of emotions between individuals is necessary for subjectively experiencing collective emotion.

Furthermore, the validity of these studies in laboratory settings could then be tested using mobile devices and wearable physiological sensors in the context of large groups attending emotion-eliciting events, such as concerts or theatre plays (e.g., [START_REF] Chabin | Audience Interbrain Synchrony During Live Music Is Shaped by Both the Number of People Sharing Pleasure and the Strength of This Pleasure[END_REF][START_REF] Sun | Using Wearable Sensors to Measure Interpersonal Synchrony in Actors and Audience Members During a Live Theatre Performance[END_REF]. Besides providing increased ecological validity, these replications would enable testing hypotheses about the influence of group features such as its size and spatial arrangement. Naturalistic studies could also bridge the gap between experimental findings and qualitative research by targeting specific, discrete types of collective emotion (e.g., pride; see Sullivan, 2015 for a discussion).

Lastly, future research could combine experimental studies with computational modelling. New models of collective emotion could be developed based on insights from experimental research, and the validity of these models could then be tested on empirical datasets (e.g., [START_REF] Garcia | An Agent-Based Modeling Framework for Online Collective Emotions[END_REF]. Crucially, compu- The relations between characteristics, mechanisms, antecedents and consequences of collective emotion are represented by dark arrows and correspond to hypotheses for future experimental research. The gradient colours illustrate that collectiveness is a continuum that includes several types of collective emotion. tational models could generate new hypotheses for experimental studies, notably regarding the influence of group features (e.g., the connections between participants) and the temporal dynamics of collective emotion within the group. Conversely, experimental results could then be used to fine-tune models.

Conclusion

An ideal framework for experimental research would build from theories of collective emotion, previous empirical research on rituals, case studies of collective emotion, and methods in affective sciences and social cognition. Experimental research on collective emotion should therefore bring together philosophers, psychologists, cognitive and social scientists from the very first stage of evaluating key questions and designing studies accordingly. This joint undertaking is likely to generate new ideas to conceptually and methodologically approach collective emotion. As an illustration, we hypothesize that collective emotion preferentially emerges at the intersection between social groups (cognitive categories reflecting common emotional dispositions), social networks (empirical structures reflecting the strength and recurrence of connections between individuals) and interaction clusters (see van der Löwe & Parkinson, 2014 for a detailed discussion). The present paper proposed some bases for interdisciplinary collaboration.

We started with the observation that research on collective emotion, while spanning multiple academic fields, remains divided into specific research foci and partly limited to accounts of subjective, conscious experiences. Thus, we proposed to make sense of collective emotion as a universal, cross-cultural phenomenon that involves a core set of three characteristics (emotion alignment, mutual awareness, feeling of togetherness), each related to universal, evolved cognitive mechanisms. In other words, we adopted a functional stance that helps to elucidate the universal cognitive bases recruited in collective emotion across cultures. Based on this framework, we identified current theoretical debates, and we proposed a framework to generate and test empirical predictions with experimental methods (Fig. 2). We showed that experimental research on collective emotion is not only feasible, but also that it would be beneficial for theories of collective emotion.

Furthermore, we believe that bridging the gap between research on collective emotion and broader research in cognitive and affective sciences would be mutually beneficial, as it would contribute to the social turn to macro-level affective phenomena [START_REF] Van Kleef | Emotional collectives: How groups shape emotions and emotions shape groups[END_REF][START_REF] Keltner | Social functions of emotions at four levels of analysis[END_REF]. Experimental research on collective emotion is also an essential step to map the relations between different aspects of human collective cognition such as collective memory [START_REF] Hirst | Collective memory from a psychological perspective[END_REF], collective decisionmaking [START_REF] Tsarev | Phase transitions, collective emotions and decision-making problem in heterogeneous social systems[END_REF][START_REF] El Zein | Shared responsibility in collective decisions[END_REF] and collective intelligence [START_REF] Centola | The network science of collective intelligence[END_REF]). An integrative multi-disciplinary approach to collective phenomena is likely to elucidate their universal bases, while clarifying the existence of cultural specificities. Finally, putting empirical predictions to the test is crucial if we aim to understand the role of collective emotion in group behaviours related to a broad range of societal topics, such as social movements [START_REF] Dunbar | Black Lives and Black Research Matter: How our Collective Emotions Continue to Drive a Movement[END_REF], group conflicts [START_REF] Bar-Tal | Collective emotions in conflict situations: Societal implications[END_REF], responses to terrorist attacks [START_REF] Garcia | Collective emotions and social resilience in the digital traces after a terrorist attack[END_REF], pandemics [START_REF] Metzler | Collective emotions during the COVID-19 outbreak[END_REF] and climate change [START_REF] Fritsche | Collective climate action: When do people turn into collective environmental agents?[END_REF].

Figure 1 :

 1 Figure 1: Examples of emotion patterns emerging at group-level.

Figure 2 :

 2 Figure 2: A framework for experimental research on collective emotion.

Table 1 :

 1 Glossary.

	Concept	Definition
	collective emotion	interpersonal pattern of emotions emerging within a group from common and
		reciprocal influence between individuals
	group-based emotion	emotion caused by self-categorization as a member of a social group leading to
		appraise and respond to a group-relevant event from the group's stance (see
		Mackie & Smith, 2017)
	emotional climate	dominant enduring moods and feelings perceived and attributed by
		individuals to a group in a specific situation (see de Rivera &

Table 2 :

 2 Comparison between theories of collective emotion. Table representing our interpretation of the theories of collective emotion reviewed in the first section, based on the three characteristics defined in the second section. For each theory, the defining characteristics of collective emotion are marked by an asterisk. As shown in the table, there are differences both across theories and within research traditions and their respective focus. The view of León et al. implies some form of alignment, because there is an interdependence between the subjective emotional experiences of participants. However, this alignment primarily relates to reciprocal identification rather than dynamic coupling of emotional responses. We represent Thonhauser's view of collective emotion in the strongest sense, i.e., "sharing".

	Focus	Theory	Emotion	Mutual	Feeling of
			alignment	awareness	togetherness
	Emotional responses	von Scheve & Ismer (2013)	*	-	-
	Emotional responses	Goldenberg et al. (2020)	*	-	-
	Subjective experiences	León et al. (2017)	-	*	*
	Subjective experiences	Thonhauser (2022)	*	*	*
	Emotional dispositions	Salmela & Nagatsu (2016)	*	*	-
	Notes:

Author accepted manuscript -©CC-BY 4.0

Author accepted manuscript -©CC-BY 4.0Collective Emotion: A Framework for Experimental Research

Social groups can also emerge from social interactions -we further discuss this phenomenon and its relation to social identification in the third section.

Acknowledgements

We thank Michèle Chadwick and Rocco Mennella for their proofreading and feedback, and anonymous referees for their valuable suggestions on earlier versions of this manuscript. A CC-BY public copyright license has been applied by the authors to the present document and will be applied to all subsequent versions up to the Author Accepted Manuscript arising from this submission, in accordance with the funding grants' open access conditions.

Funding

This work was supported by the Institut National de la Santé et de la Recherche Médicale and the Agence Nationale de la Recherche, (grant numbers ANR-10-IDEX-0001-02, ANR-17-EURE-0017, ANR-20-CE28-00,0 ANR-23-CE28-0003).

Declaration of Conflicting Interests

The Authors declare that there is no conflict of interest with respect to the research, authorship, and/or publication of this article.

Author accepted manuscript -©CC-BY 4.0


1-What is a natural ecosystem? What are some examples? 2-What are biogeochemical cycles ? 3-Role of microorganisms on global biogeochemical cycles 4-Role of microorganisms in polluted environment

Environmental degradation is a major issue of our time. A basic environmental building block is the ecosystem.

An ecosystem is a combination of two words: "ecological" and "system." Together, they describe the collection of biotic and abiotic (living and nonliving) components and processes that comprise a defined subset of the biosphere. (The "biosphere" is the area of Earth that contains life, whether on the planet's surface or in the air.) Definition :

Natural vs. Artificial Ecosystems

Natural ecosystems may be terrestrial (such as a desert, forest, or meadow) or aquatic (a pond, river, or lake). A natural ecosystem is a biological environment that is found in nature (e.g. a forest) rather than created or altered by man (a farm). Humans have modified some ecosystems for their own benefit. These are artificial ecosystems. They can be terrestrial (crop fields and gardens) or aquatic (aquariums, dams, and manmade ponds).

Types of Natural Ecosystems Aquatic Ecosystems

Aquatic ecosystems cover 71% of the earth's surface. There are three different varieties, defined by the kind of water in which the system's organisms interact.

Freshwater: This type includes lakes, rivers, ponds, streams, and makes up the smallest percent of the earth's aquatic ecosystems.

Transitional

communities: These are places where freshwater and saltwater come together, such as estuaries.

Marine: More than 70% of the earth is covered by marine (also called saltwater) ecosystems. These include shorelines, coral reefs, and open ocean.

Terrestrial Ecosystems

The four terrestrial ecosystems are classified by the type of land or terrestrial area in which organisms interact.

Course Outline

The Carbon Cycle Carbon dioxide gas exists in the atmosphere and is dissolved in water. Photosynthesis converts carbon dioxide gas to organic carbon, and microbial respiration and decomposition cycles the organic carbon back into carbon dioxide gas and Methane. Long-term storage of organic carbon occurs when matter from living organisms is buried deep underground and becomes fossilized. Volcanic activity and, more recently, human emissions, bring this stored carbon back into the carbon cycle https://cnx.org/contents/ZdFkREJc@6/Biog eochemical-Cycles

Human effect on the Carbon Cycle

The Permafrost timebomb Permafrost is soil that has been frozen for at least two consecutive years.

It is found in large parts of the

Oil biodegradation

After an accidental oil spill into open water, the numbers of oil eating microbes increase. Oxygen concentration is high in sea water close to the surface, so microbes break down oil at relatively rapid rates using aerobic respiration.

Oil eating bacteria include Alcanivorax borkumensis, which breaks down straightchain and branched-chain hydrocarbons 1. As the oil is fragmented into droplets by wave action, Alcanivorax cluster on the surface of droplets forming a biofilm, and secrete biosurfactant which helps with ingesting the oil.

Aerobic and anaerobic biodegradation

•Aerobic •Oxygen is reduced to water and the organic molecules (e.g. petroleum, sugar) are oxidized •Anaerobic

•An inorganic compound is reduced and the organic molecules are oxidized (e.g. nitrate is reduced and sugar is oxidized) •NOTE: Many microbes can do both aerobic and anaerobic respiration; the process which produces the most ATP is used first!

  Forest: These ecosystems feature dense tree populations, and include boreal and tropical rain forests. Desert: Deserts receive less than 25 cm of rainfall per year.Grassland: These ecosystems include tropical savannas, temperate prairies, and arctic tundra.Biogeochemical cycles involve the fluxes of chemical elements among different parts of the Earth: from living to non-living, from atmosphere to land to sea, and from soils to plants. They are called "cycles" because matter is always conserved and because elements move to and from major pools via a variety of two-way fluxes, although some elements are stored in locations

Frozen in ice for millennia, this Siberian mummy is the best-preserved ancient horse ever found.

The Nitrogen Cycle

Nitrogen enters the living world from the atmosphere via nitrogen-fixing bacteria. This nitrogen and nitrogenous waste from animals is then processed back into gaseous nitrogen by marine and soil bacteria, which also supply terrestrial food webs with the organic nitrogen they need.

Eutrophication effect

Eutrophication is an enrichment of water by nutrient salts that causes structural changes to the ecosystem such as: increased production of algae and aquatic plants, depletion of fish species, general deterioration of water quality and other effects that reduce and preclude use Eutrophication affects 54% of Asian lakes, 53% of those in Europe, 48% of those in


What is pollution?

Presence of matter (gas, liquid, solid) or energy (heat, noise, radiation) whose nature, location, or quantity directly or indirectly alters characteristics or processes of any part of the environment, and causes (or has the potential to cause) damage to the condition, health, safety, or welfare of animals, humans, plants, or property.

POLLUTION

Chemical pollution is defined as the presence or increase in our environment of chemical pollutants that are not naturally present there or are found in amounts higher than their natural background values. Most of the chemicals that pollute the environment are man-made, resulted from the various activities in which toxic chemicals are used for various purposes.

Water pollution

Water pollution is the contamination of the water on Earth. It includes contamination by pollutants such as chemical, bacterial or particulate that reduces the purity of the water.

Oil seepage is one of the most common forms of pollution. Water pollution occurs in lakes, oceans, rivers and even underground reservoirs.

Water pollution is one of the most dangerous form of pollution as it decreases the amount of drinkable water that is available. It can also reduce the amount of water that can be used for irrigation as well as harm the wildlife that depend on the water.

Air pollution

Air pollution is the contamination of the natural air by mixing it with different pollutants such as harmful fumes and chemicals. This contamination can be caused by gases emitted by vehicles or from burning material or harmful fumes emitted as a byproduct of industries.

The higher the concentration of air pollutants, the harder it is to remove it effectively through natural cycles.

The higher concentrations also result in breathing problems for living things. Some effects include increase in smog, higher rain acidity, crop depletion from inadequate oxygen, and higher rates of asthma. According to experts, global warming is one of the biggest side effects of air pollution.

Land pollution

Soil pollution, also known as land pollution is the contamination of the soil or the land that prevents growth of natural life, which includes land used for cultivating, wildlife as well as habitation.

Common causes of soil pollution includes non-sustainable farming practices, hazardous wastage and seepage into the soil, mining as well as littering.

Soil pollution can result in reduced growth of agriculture as well as poisoning of the land and nearby water. When soils are repeatedly contaminated and accumulate large amounts of poisonous materials and chemicals, the soil reaches a point where it cannot support plant life. Soil pollutants interfere with soil chemistry, biology, and structure. When these changes occur, beneficial soil bacteria, soil microorganisms, soil nutrients, and soil chemical processes begin to deteriorate to an extent where they diminish soil fertility.

 Effect on biodiversity

Course Outline

How to define micro-pollutants?

The term "micropollutants" means organic or mineral substances whose toxic, persistent and bioaccumulative properties may have a negative effect on the environment and/or organisms even at low concentration.

They are present in many products that we consume daily (drugs, cosmetics, phytosanitary products, insecticides, etc.), at the home or in industry.

Progress in laboratory analysis is increasingly highlighting their presence in the aquatic environment at extremely low concentrations, in the order of one nanogram per litre or microgram per litre (hence the term micropollutants).

Some of these substances are liable to have potentially chronic direct or indirect effects on ecosystems (e.g. the feminisation of fish due to endoctrine-effect substances in the aquatic environment), and even on human health.

What are the micro-pollutants families?

Lead, cadmium, mercury, arsenic, antimony, radon, uranium





  About 40 % of deaths world wide are caused by water pollution  People are getting desease like Cholera, Typhoid, Diarrhoea…  Aquatic life gets disturbed  Water pollution affect the food web chain  Fresh water not available Soil pollution effect  Endangering Human Health  Air and Water Contamination  Effect on Plant Life

  Soil pollutant can be carcinogenic, intensifying the chances of developing cancer Polluted soil by natural means contributes to air contamination by discharging volatile compounds into the atmosphere.Soil pollution can also lead to water pollution if the toxic chemicals and materials like dangerous heavy metals leach into groundwater or contaminate storm water runoff, which reaches lakes, rivers, streams, or oceans Soil pollutants can also cause skin diseases, muscular blockage, and central nervous system disorders case of micro-pollutants

  Polycyclic aromatic hydrocarbons are organic compounds containing only carbon and hydrogen-that are composed of multiple aromatic rings PAHs are uncharged, non-polar molecules found in coal and in oil deposits. They are also produced by the incomplete combustion of organic matter (for example, in engines and incinerators or when biomass burns in forest fires). Some carcinogenic PAHs are genotoxic and induce mutations that initiate cancer; others are not genotoxic and instead affect cancer promotion or progression An adduct formed between a DNA strand and an epoxide derived from a benzo[a]pyrene molecule (center); such adducts may interfere with normal DNA replication. compounds containing the bisphenyl structure with different numbers (one to ten) and arrangements of chlorine atom attached PCBs were widely used as dielectric and coolant fluid, in transformers, capacitors PCBs are persistents in environnent (non biodegradable, hydrophobic…) PCBs are carcinogenic Dredging of PCBs contaminated sediments PolyBrominated Diphenyl Ethers (PBDE) Polybrominated diphenyl ethers or PBDEs, are organobromine compounds that are used as flame retardant. The family of PBDEs consists of 209 possible substances, which are called congeners. PBDEs have been used in a wide array of productsshare the environmental long life and bioaccumulation properties with PCBs PBDEs can cause cancer in people Time required for half of a quantity or concentration of a pollutant to disappear from the biotope or a contaminated organism photooxidation, thermooxidation ...) leading to the more or less complete disappearance of a molecule. Degradation Biological degradation carried out by bacteria, fungi and obtaining metabolites of lower molecular weight. Biodegradation Primary biodegradation = partial attack of molecules; may result in more persitant, more bioavailable, and sometimes more toxic metabolites than the initial molecule Ultimate biodegradation: complete degradation of the molecule; leads to CO 2 , CH 4 , water, mineral elements.

1-What is pollution? 2-Different types of pollution 3-Sources of pollution 4-Effects of pollution 5-The case of micro-pollutants Course Outline

Air pollution effect

Smog

Respiratory problems This value makes it possible to apprehend the hydrophilic or hydrophobic nature of a molecule.

Greenhouse effect Acid Rain

Indeed, if the Log Kow is positive and very high, it expresses the fact that the molecule considered is much more soluble in octanol than in water, which reflects its lipophilic or hydrophobic character, and vice versa.

Relationship between polarity and bioaccumulation