life. She provides transnational examples. It is, in this regard, a compelling observation for both academic scholarship of activism and grassroots political mobilization. SJ, SB 1: How does decolonial queerness/sexualities speak to strands of decolonial theorizing which range today from decolonizing university initiatives to incisive critiques of Eurocentrism? To which contemporary issues is it most relevant in terms of political praxis? We are thinking of transformative politics.

PB: Certainly, today decolonial queerness/sexualities is in conversation with many strands of non-queer and non-feminist decolonial theorizing. These polylogues are unfolding around questions and practices of decolonizing the university and of ongoing critiques of eurocentrism but also of US-centrism. Several of us within and outside the Decolonizing Sexualities Network have written collectively about our work as queers of colour to decolonize the university [START_REF] Bacchetta | On the Need to Claim (Physical) QTBIPOC Spaces[END_REF][START_REF] Bacchetta | Queer of Color Space-Making in and Beyond the Academic Industrial Complex[END_REF]. Decolonial queerness/sexualities as an area of critical analysis and political practice informs many other issues, theorizations, and both oppositional and non-oppositional resistance against the colonialism and coloniality that currently characterize the planet: settler, administrative, military, commercial, internal, external, and combinations of all of these. Decolonial queerness/sexualities, despite the different names in disparate sites across the planet, is a necessary dimension to transformative politics.

I think of decolonial queerness/sexualities as a large rubric that can include many disparate situated theoretical-interpretative sites from which liberationoriented subjects can critique relations of power and their operations, reimagine the world otherwise, and act. Decolonial queerness/sexualities is a dynamic assemblage that connects disparate fruitful analytics and practices. It has room for global northern revolutionary queer of colour analytics and for revolutionary queer epistemologies of the global South(s). It includes strands such as radical decolonial anti-capitalist queer-of-colour critiques from the global North(s), as well as the communitarian feminism as articulated by indigenous collectivities, about which Julieta Paredes, the out lesbian of the Aymara people of Bolivia in Abya Yala (or what colonizers called Latin America), and others, write. It also includes the decolonial thought of queer intellectuals and activists in anticolonial-occupation liberation struggles of our times.

Decolonial queerness/sexualities has thoroughly transformative potentiality when it implicates all relations of power, and is tied up with every dimension of the most subaltern of queer and non-queer life, of humans, all beingsbecoming and the planet. It needs to be concerned with all registers: the epistemological, economic, social, cultural, affective, corporeal, subjective, symbolic, energetic-spiritual, or other. For instance, in the realm of epistemology, decolonial queerness/sexualities can offer a place from which to perform a subaltern, and possibly a subalternative, critique of dominant analytics and modes and tools of knowledge production. It can reveal dominant presuppositions, categories, concept-terms, logics, and conclusions that otherwise might be taken for granted, that saturate our contexts and that become automatisms. In doing this work of disclosure, of laying bare, it can open the way for thinking, feeling, acting politically and living otherwise, in a spirit of liberation-orientation.

Decolonial queerness/sexualities ventures beyond colonialism-and-raceamnesiac feminist and queer theory, i.e. the white queer theory that dominates in the global North(s). I define white queer theory, of course, not in essentialist, morphological terms according to the racial positionality or body-representation of the person creating it, but instead in epistemological terms according to its approach, presuppositions, categories, logic and conclusions. White theory can be characterized primarily by what it neglects. It is colonialand-race-amnesiac. It "forgets" to address colonialism, racism, Islamophobia, capitalism, class relations and other relations of power, and thereby reproduces them inside the theory itself. Here the term white signals a kind of blank space. It is not an empty space but rather a space of the direct reproduction of relations of power.

Unfortunately, white theory, colonialism-and-race-amnesiac feminist and queer theory, from the US also circulates in the global South(s) among some elite sectors of queers. There is a uni-directionality and unevenness of ideological flows from the global North(s) to the global South(s). These theories often make sense to a certain relatively dominant sector of feminists and queers in the global South(s) because like their relatively dominant global northern counterparts they do not necessarily consider the most subaltern conditions and subjects, or subaltern and subalternative theories and practices, in their own contexts. There, too, white feminist and queer theory sustains and reinforces the dominant order by default, as it consolidates and inadvertently reproduces the bulk of the contextual relations of power in both the sending and receiving sites. In contrast, an example of decolonial queerness/sexualities epistemological work is that of the queer working-class Chicana writer and academic Gloria Anzaldúa. Her writing has informed the work of Walter Mignolo, among other decolonial theorists who make no claims to queerness and he/they also cite her. Anzaldúa rewrote the genealogy of present-day Chicanas (and Chicanos and Chicanx) in Borderlands/La Frontera [START_REF] Anzaldúa | Borderlands/La Frontera: The New Mestiza[END_REF][START_REF] Anzaldúa | Borderlands/La Frontera: The New Mestiza[END_REF][START_REF] Anzaldúa | Border Arte: Neplanta, el Lugar de la Frontera[END_REF]. She proposed the notion of Borderland/borderlands to open up and resignify the notion of the border to mean both the geographical space and the cultural, symbolic, internal and spiritual space. She reimagined the subject beyond the neoliberal idea of the bounded-individual-with-choice, via the (indigenous) Nahuatl notion of the subject in fusionality with other subjects in community.

Another example of insights that decolonial queerness/sexualities provides concerns the economic and social organization of societies. If we can consider the planet's various systematic arrangements of sexism and queerphobia as co-productions that are co-constituted in and by multiplicities of relations of power (colonialism and coloniality, race, capitalism, etc.), we can better understand their commonalities and differences (Bacchetta 2015, forthcoming). For instance, not all systems of sexism, obligatory heterosexuality and queerphobia are organized as patriarchy or the rule of the father. Instead, some are organized as fraternarchy (rule of the brothers, as in racial-democratic sexist regimes from the Greek States to the United States), or filiarchy (rule of the sons, as the Hindu Right that is currently in power in India that self-designates its membership as "sons of the Bharatmata"), or other. And within and beyond each of these overarching arrangements there are different modalities, such as matriliny, patriliny, matrilocality, patrilocality, etc. Hence, it is imperative to note that any globalizing notion of patriarchy has severe limitations.

To think about different kinds of sexist, heterosexist and queerphobic orders together, to put into relief some of their differences and similarities, I proposed the notion of misogynarchies as a large rubric. I think of misogynarchies as co-productions, that is, as co-constituted not with gender and sexuality alone but rather as composed with gender and sexuality and all other contextual relations of power even if articulated primarily as relations of gender and sexuality. Thus, for example, in the United States fraternarchy is white supremacist even when some people of colour as individuals hold a dominant positionality therein. White supremacy is structural and self-perpetuating. Undoing it implies a radical dismantling of the entire system, including the position of the President, and life needs to be reorganized otherwise. In India, filiarchy is supremacist along religion and caste lines, even when Hindu nationalists recruit some Muslims, Dalits or Adivasis into their leadership. Every misogynarchy formation is at the same time economic, social, epistemic, cultural, symbolic, everything. And it is always only one of the co-productions of power among others in any context. Each of these relations of power is a co-component in the co-constitution of other relations of power.

Decolonial queerness/sexualities as a set of analytics and practices is only transformative if it includes a critique of all relations of power, concerns with all conditions of subalterneity, and an accounting for all positionalities and relationalities within a social context including dominant and subaltern subjects. For example, in contexts of misogynarchy wherein obligatory heterosexuality is organized into serial monogamous heterosexual familial units, we will want to understand the operability of both the subject positions therein and the stigmatized inside-outsider variants, such as mistresshood, the more or less organized institution of sex work, and the racialized-sexualized others who provide a backdrop to the construction of proper subjects. These positions (which have greater or lesser moral acceptability here or there) are not aberrations of the misogynarchy in question; rather, they are absolutely necessary and integral to keeping the system in place. Without the stigmatized inside-outsider sexual positions the system could collapse, as they perform the function of catharsis.

I also want to mention that sex work is work and that nearly everything, every other kind of job, is also organized in the context of relations of power to keep the prevailing relations of power intact. For example, I work in the Academy and thus inside the academic industrial complex whose function it is to reproduce dominant ideology that its subjectsstudents and facultyare supposed to internalize. Certainly, those of us with radically critical stances try to resist this ideological reproduction from within. Concurrently, however, insofar as we are within the system, we are objectively in a position of participating in bolstering the institution and thus enabling (even if unwillingly) its dominant ideological work. We are caught in it. We need to work together to get free together. To do so we need totally to transform the University, to start over anew, even as we work to transform all of life, everything about the current order, including ourselves as subjects and as subjects-in-relationality.

This work of collective liberation and the reinvention of life is particularly difficult given the intensity of relations of power in which we are co-constituted as subjects and live. Foucault helps us understand how we are "formed" as subjects within the relations of power that saturate our contexts, and how these relations of power are part of our very composition as subjects [START_REF] Foucault | Le pouvoir, une bête magnifique[END_REF](Foucault [ ] 2001[START_REF] Foucault | Le sujet et le pouvoir[END_REF] 2001). However, with decolonial queerness/ sexualities in mind, we can more accurately speak about subjects not as "formed" in ostensibly monolithic power in the abstract, but instead as coconstituted in multiplicities of relations of power in-context. Foucault, though he brings us much to think with, ignored colonialism and many kinds, dimensions and registers of racism, and thus the necessity for us of this theoretical re-envisioning. Anyhow, to contest and transform the relations of power that are co-components of our subjectivity, that saturate our contexts, we need to take apart, destroy, recompose, parts of ourselves.

These relations of power that extend across the social realm and into our most intimate lives are more complex than they initially might seem. Colonialism, coloniality, racism, capitalism, misygynarchies, specismall carry forms of genealogical accumulation that are fully operative in the present. This means that every contemporary context is an amalgamation of past and present manifestations of power that interact and enact in the present. For instance, we cannot think of queerphobia on the planet today without recalling immense historical forms of violence such as this: the British colonizers outlawed homosexuality in fifty-six countries that they colonized. They imposed their own binary gender modelsand accompanying gender ideologyinto the colonized spaces, and denigrated and displaced pre-existing models, thereby destroying or worsening the status of women and queers in-context. These actions affected different segments of the colonized population variably. The British constructed Brahmin men as effeminate, incapable children, and Dalits and Adivasi (original peoples whom the British called "tribals") as hypersexed and out of control [START_REF] Nandy | The Intimate Enemy[END_REF]. They mobilized these controlling images simultaneously to undermine Brahminical authority, block Dalit-led anti-caste movements, and instrumentalize deadly caste oppression, all in the service of ever-more murderous colonial divide-andrule strategies. They thereby severely delayed Indian societal transformation, with deep consequences for today. SJ, SB 2: Given the disparate genealogies of decolonial queer critique, do you see a coherent field that has emerged in the recent decade in the field of activism and academic scholarship? PB: Multiple sites of knowledge production and activism within and outside the University have emerged in recent times. I do not see coherency at this time, but rather convergences of what I call co-motion or multiple kinds of coming together, including solidarity, networks, unions, coalitions, etc. (Bacchetta forthcoming). In brief, co-motion is a large rubric to enable a discussion about many kinds and ways of coming together.

We live in a very exciting historical moment with much potentiality. Across the planet, autonomous feminist and queer movements are arising. We are creating convergent critiques, activisms and artivisms across some same and disparate relations to colonialism, coloniality, and all other relations of power. For example, there are common threads in the decolonial queer perspectives of Palestinian queers in Al Qaws, and Ugandan and African queers and allies who denounced the British proposition to boycott Uganda because of a law (financed largely by US evangelists) in Uganda calling for the death penalty for homosex. Al Qaws insists that the liberation of Palestinian queers cannot happen without the liberation of Palestine and thus all Palestinians from colonial occupation by the Israeli entity [START_REF] Maikey | Signposts from Al Qaws: A Decade of Building a Queer Palestinian Discourse[END_REF][START_REF] Maikey | Liberation in Palestine: A Queer Issue[END_REF]. Ugandan queers and allies maintain their oneness with the Ugandan people as a whole, and on that basis oppose British imperial intervention [START_REF] Ekine | Queer African Reader[END_REF]. Both positions are decolonial and queer at the same time. They bring into relief the complexities of subjects co-constituted in and caught in multiplicities of relations of power.

During this period, we should not miss the opportunity for polylogues and building solidarities across similar and disparate conditions. For example, in Paris in 2018during the Queer of Colour Day that we created as part of Queer Weekwe organized a first polylogue among queers under occupation: from Palestine, Kurdistan and Kashmir. Queers have always been leaders, intellectuals and activists in these broad decolonial struggles. But, today, they are increasingly openly so. Whether "out" or not (and "outness" is yet another discussion, one that I will not address here), today they often bring with them the resolve to engage every relation of power, for the liberation of all subjects, especially the most subaltern of subjects, including queers. Our existence in the world and in social movements can operate as a productive provocation, a useful incitement for total freedom. This bodes well for queer futurity and for the future of the world! An example of coming together in polylogue across some very different conditions is the work of the Transnational Decolonial QTPOC Facebook group. It has queer of colour and global southern queer membership from sites dispersed across the planet and its membership is constantly expanding. It can exist only because of the prior and ongoing work of groups such as the Decolonizing Sexualities Network in bringing many of us together. These are spaces for sharing knowledge, developing analyses, and coordinating transnational action.

Until now, our most radical decolonial queer/sexualities analyses, activisms and artivisms, and the subjects who produce them, have been kept out of the universities. The University and the entire academic industrial complex is a contradictory formation. It is both an apparatus of the state designed to reproduce the dominant order, and a site of the potential upheaval of that order. If we can bring radically critical subjects inside to constitute a critical mass, we could implode the university and recreate it differently. My thinking on this is deeply indebted to collective conversations [START_REF] Bacchetta | On the Need to Claim (Physical) QTBIPOC Spaces[END_REF][START_REF] Bacchetta | Queer of Color Space-Making in and Beyond the Academic Industrial Complex[END_REF]. I have with others considered how we might create spaces of freedom inside the University, as sites of convergence to resist co-optation, to de-alienate ourselves, to invent a sociality and politics of transformation. Unfortunately, today, the University offers some people of colour the option of becoming-functionallywhite, and some queers the option of becoming-functionally-straight (and some accept). Together, we can better resist the University's "diversity management" strategies that otherwise control us, deactivate us, render us ineffective. SJ, SB 3: You have actively contributed to queer-of-colour activism both in the United States and France, where you have enabled, among other contributions, productive dialogues between queer and trans mainstream movements and decolonial and people-of-colour political mobilization. We can, for instance, cite your role in Dyketactics (1970s in the United States) and more recent involvement with the various groups in France in the last decades, especially the Lesbiennes of Color (LOCs) and Queer et Trans Révolutionnaires (QTR). In what ways has this experience been notable in establishing a decolonial critique of power hierarchies for you? In other words, do you reckon that decolonial queerness steps beyond gender and sexuality frameworks to posit, perhaps daringly imagine even, a critique of all formations of power imbalances in a transnational bind? PB: I think that the critique of all relations of powerof what I call co-formations and co-productions of poweris absolutely essential for a truly transformative decolonial queerness/sexualities politics. Wherever we are located, we are part of the planet, not as some isolated unit somewhere. All relations of power across the planet are interconnected, as are all subjects and beings-becoming.

It is important to note that such critique, as some of us propose today, is not new. It has many genealogical strands in our own lives and far beyond. In my own little life, my decolonial queer critique began when I was quite young, in the collective Dyketactics! (Bacchetta 2019a). I was a co-founder of Dyketactics! in the mid-1970s in Philadelphia in Turtle Island or the territory that the colonizers named the United States. Philadelphia is located on the land of the Lenape people. Dyketactics! included lesbians of colour of many heritages and positionalities, and a few white women. Today, Dyketactics! is known as the first queer collective in the United Statesand possibly the worldto take the police to court for their brutality targeting queers. I am one of the Dyketactics! Six in that lawsuit. However, we did much more. To my mind, some of our most significant work was epistemological. We began nearly every statement by denouncing US (settler) colonialism, genocide, the colonial theft of land, the system of enslavement, racism, capitalism, exploitation of workers, and all relations of power. We published the statements in many feminist and dyke media of the day.

Importantly, different Dyketactics! members were simultaneously part of many movements: for Native American land and sovereignty, for Puerto Rican independence, for Black liberation, etc. (Bacchetta 2019a). And as a collective we supported all these movements and worked in solidarity with them. We were an autonomous group of dykes, but we were implicated in many nondyke movements at once. I want to put into relief the fact that Dyketactics! is an example, and not an exception, of a radically critical and constructive queer collective that produced a revolution-oriented analytic and practice pertaining to all relations of power. This analytic, these practices, were formulated by dykes of colour and some white allies together. Today, the early existence of such subjects and politics has been erased, such that current generations imagine that they are alone, or the first, or the only ever. We need to archive better our most radically critical queer-of-colour and global southern queer historiography so that present and future generations do not need to waste time reinventing the wheel and can instead take everything to the next level. Shortly thereafter, in France, where I ended up in political exile, I was of course involved in activism on multiple fronts, too. From the 1980s I was part of the feminist, lesbian and anti-racism movements at once. In the early 1980s I was a member of the advisory board of the Maison des femmes de Paris (Paris Women's Centre), which at the time housed about fifteen groups, more than half of which were groups of women of colour. They included the MODEFEN (Movement of Black women), the Nanas Beurs (second generation Arab-French women), the Groupe femmes Tunisiennes (Tunisian Women's Group), Groupe femmes Latino-américaines (Latin American Women's Group) and many other groups. In 1984 I cofounded the coalitional group called Collectif féministe contre le racisme et l'anti-semitisme (Feminist Collective against Racism and Anti-semitism) and in 1985 the Collectif lesbian contre le fascisme et le racisme (Lesbian Collective against Fascism and Racism). In France, we have had autonomous lesbian-of-colour groups only since 1999 with the founding of the Groupe du 6 Novembre: Lesbiennes Issues du Colonialisme, de l'Esclavage et de l'Immigration (6 November Group: Lesbians Born in/Out of Colonialism, Slavery and Immigration) (Bacchetta 2019b). Today in France I am a part of a broad decolonial queer-of-colour movement that includes many groups.

My experiences in the United States, France, India, Italy and, more recently, Brazil, are all formational to my thinking on decolonial queerness/sexualities and all co-formations and co-productions of power. Despite the varied contexts, each with its own relation to colonialism, coloniality, slavery, racism, capitalism, misogynarchy and other relations of power, my experiences are formed in some same and different relations of power. To think with them together, and to recognize the limits of thinking with them together (for they are only a few of the many sites across the globe) has been formational. To give just one example, considering these contexts together led me to what I call situated planetarity, or the necessity of analysing any specific context not as a bounded entity but rather as a temporally-spatially scaled site of many simultaneous relations of power, located within a larger context of planetary co-productions and co-formations of power (Bacchetta forthcoming). This is to take account of both the particularities of any site and its interconnections with other sites and the entire planet.

Radically critical queers of colour and queers in the global South(s), with our different kinds of subalterneities, with our experiences and perspectives, are particularly well positioned to develop analytics of all relations of power at once. But because our very composition induces in us (as in all people) forms of alienation, there is no guarantee that we will do that. We are co-constituted in subalterneity at the nexus of so many relations of power. If we can de-alienate, understand how power composes us and operates within us and in our contexts, then we can open up a space for configuring interesting analytics and practices that are oriented towards total liberation, transformation and the invention of new ways of life.

Our liberation and more broadly transformation are entwined with the liberation of all other kinds of subalterns, of all people, across what I call the subaltern-to-dominant-continuum in the sense of Antonio Gramsci (Bacchetta forthcoming). Thus, while we create for ourselves as queer people of colour, and global southern queers, autonomous spaces for de-alienation processes, reflection about relations of power, and oppositional and non-oppositional resistance, we can also participate in spaces of polylogue with allies and people as-yet-to-become-allies who may be differently positioned from ourselves. As you mention, I have contributed to such spaces. I work in Paris with the queer and broader non-queer decolonial movement, in India with the queer and non-queer feminist movement, in the United States with queer and non-queer movements against racism, Islamophobia and fascism, and in Brazil with queer and non-queer movements against fascism. For freedom we need all of us, everyone.

Though I am deeply engaged in broad movements, I want to issue a word of caution: as queers of colour and queers in the global South(s), in many places building our ongoing autonomy is a real necessity. We need to pay attention to where our primary emotional, sexual, political and intellectual energy is going. Communities of colour, communities of queers in the global South (s), are generally positioned in extremely deadly subalterneity. If we can think, feel, re-energize, activate in autonomy, we will work more effectively in broader movements without getting engulfed or co-opted by them. SJ, SB 4: And last, how can decolonial queerness, in all its contradictions and molten formations, partake in the processes of healing in both personal and collective spheres? PB: Decolonial queerness/sexualities has a central role to play in healing in both personal and collective dimensions. As radically critical queers of colour and queers in the global South(s), our subjectivities are particularly multiply subalternized, destroyed and distorted in colonialism, racism, Islamophobia, capitalism and misogynarchies. However, we are particularly resistant and somehow also other-than-completely-alienated. We have to be so in order to surpass the conditions in which we are co-constituted and live. We are out-of-sync in relation to every register, every temporality, and every human and spatial scale. We are other to the heteronormative familial, community, obligatory institutions such as schools and universities, broader society, and ideologies of docile patriotism upon which national normativity depends. The compelling question is: How do we sustain queer-of-colour life, and global southern queer life, under such conditions? There is something beautiful and miraculous in our resilience. It is my hope that together we can do something to create better conditions for future generations of queers of colour, queers in the global South(s), and our allies, who live with broken and erased genealogies, who often have no idea that others have preexisted them, are still here.

The intensity of our co-constitutions in power and of multidimensional alienation, and the many other kinds of murderous harm, mean that our healing also must be multifaceted, multidimensional. We might begin to conceptualize healing modalities by reimagining ourselves outside the confines of the concept of the subject that is grounded in neoliberalism and coloniality, that we are induced into exclusively thinking with and into embodying. A position of not-neoliberalism means that we walk away from the notion of the individualized, bounded, internally coherent subject that capitalism needs. A stance for not-coloniality means that we refuse the idea of the submissive, compliant subject who upholds the order of the dominant. Instead, we can begin with a concept of the subject, of ourselves, in collectivity and in fusionality with ancestors, progenitor, peers, all beings-becoming, perceptible and imperceptible entities and energies.

With this, our healing could take the form of de-alienation and the creation of ourselves anew as subjects with the world. This means recentring ourselves in our (singular and collective) hearts, our affect, our corporeality, our politics. We will need to induce the reordering of every cell in our bodies. This is only possible to do together, not alone. As Audre Lorde helped us to understand, the work of healing is not a luxury and it is political. Healing is also tied up with our invention of spaces of freedom where we create the conditions for expression, loving interrelationality, and polylogues for analysis and political solidarity. One such space is the Decolonial Café that you (Suhraiya and Sandeep) both organized in Paris in May 2019.

In sum, to the extent that we actively work to bring into relief multiple relations of power and strive towards dismantling them, we will be in an excellent position to heal ourselves, to work well with allies, to transform the world, and to invent other modes of life, together.


Dahédrey Payandi-Rolland
email: dahedrey.payandirolland@hotmail.com

Liudmila S Shirokova

Fabian Labonne

Pascale Bénézeth

Oleg S Pokrovsky

Impact of freeze-thaw cycles on organic carbon and metals in waters of permafrost peatlands

Keywords: ice, moss, lichen, peat, thermokarst lake, river

Despite the importance of soil and surface waters freezing in permafrost landscapes, the behaviour of dissolved organic carbon (DOC), nutrients and metals during periodic freezethaw cycles (FTC) remains poorly known. The on-going climate warming is likely to increase the frequency of FTC in continental aquatic settings, which could modify the chemical composition of waters. In this study, we conducted 9 repetitive cycles of overnight freezing (-20 °C) and 5 hours thawing (4°C) in the laboratory using representative 0.22 µm-filtered waters from NE European permafrost peatland: leachates of vegetation and soil, and natural surface waters (depression, thermokarst lake and river). Only minor (< 5% -15%) changes of DOC concentrations, SUVA254 and molecular weight were observed in all leachates and the depression water. In contrast, several trace elements (Fe, Al, P, Mn, As, and REE) exhibited sizable variations during FTC (> 10%). The leachates and the depression water were enriched in trace elements, whereas the thermokarst lake and the river demonstrated a decrease in concentration of Fe (-39 and -94 %, respectively), Al (-9 and -85 %), and Mn (-10 and -79 %) during FTC. Overall, the observations demonstrated an increase in aliphatic low molecular weight organic matter (OM), and the precipitation of Fe, Al hydroxides and organo-mineral particles. Therefore, enhanced of frequency of FTC can favour the release of metals and toxicants from acidic OM-rich surface waters and maintain stable OM-metals-colloids in large lakes and rivers, thus regulating aquatic transport of DOC and metals from soils to the Arctic Ocean.

Introduction

In Arctic regions, strong permafrost thaw occurs mostly where soil temperature fluctuates around 0 °C [START_REF] Romanovsky | Permafrost thermal state in the polar Northern Hemisphere during the international polar year 2007-2009: a synthesis[END_REF] which corresponds to discontinuous and sporadic permafrost zones. This is especially important in Arctic wetlands such as permafrost peatlands which contain a huge amount of organic carbon (OC) [START_REF] Hugelius | Estimated stocks of circumpolar permafrost carbon with quantified uncertainty ranges and identified data gaps[END_REF][START_REF] Schuur | Climate change and the permafrost carbon feedback[END_REF]. The dissolved OC (DOC) is the main form of terrestrial C exported from soil solutions to rivers and lakes in the wetlands [START_REF] Chapin | reconciling carbon-cycle concepts, terminology, and methods[END_REF][START_REF] Cole | Plumbing the global carbon cycle: integrating inland waters into the terrestrial carbon budget[END_REF]. The transformation of DOC in surface waters involves photo-and bio-degradation which is controlled by light intensity [START_REF] Selvam | Photoreactivity of dissolved organic carbon in the freshwater continuum[END_REF], temperature [START_REF] Boddy | Turnover of low molecular weight dissolved organic C (DOC) and microbial C exhibit different temperature sensitivities in Arctic tundra soils[END_REF], origin of OC and microbial diversity [START_REF] Young | Degradation of surface-water dissolved organic matter: influences of DOM chemical characteristics and microbial populations[END_REF]. Transition periods (i.e. early spring and late autumn) appeared to be essential times for the export [START_REF] Manasypov | Seasonal dynamics of organic carbon and metals in thermokarst lakes from the discontinuous permafrost zone of western Siberia[END_REF]Pokrovsky et al., 2020) and biodegradation (Payandi-Rolland et al., 2020b) of DOC in rivers and lakes of Arctic wetlands when the majority of DOC originates from leaching of supra-permafrost-soil and vegetation [START_REF] Ma | Dissolved organic carbon in permafrost regions: A review[END_REF][START_REF] Vonk | Reviews and syntheses: Effects of permafrost thaw on Arctic aquatic ecosystems[END_REF]. These periods are dominated by recurrent freeze-thaw cycles (FTC, [START_REF] Henry | Climate change and soil freezing dynamics: historical trends and projected changes[END_REF]. Furthermore, full freezing of thermokarst lake basins in western Siberia was reported to lead to massive coagulation of organo-ferric colloids producing macroscopic, organic-and Fe-rich amorphous particles capable of precipitating to the lake bottom [START_REF] Manasypov | Seasonal dynamics of organic carbon and metals in thermokarst lakes from the discontinuous permafrost zone of western Siberia[END_REF].

Since the 1970s, a lot of attention is given to the impact of freezing and thawing on soil properties [START_REF] Deluca | Effect of freeze-thaw events on mineralization of soil nitrogen[END_REF][START_REF] Kim | Effects of a controlled freeze-thaw event on dissolved and colloidal soil organic matter[END_REF][START_REF] Larsen | Repeated freeze-thaw cycles and their effects on biological processes in two arctic ecosystem types[END_REF][START_REF] Ren | Effect of freeze-thaw cycling on the soil-freezing characteristic curve of five Canadian soils[END_REF][START_REF] Schimel | Microbial response to freeze-thaw cycles in tundra and taiga soils[END_REF][START_REF] Xiao | Effects of freeze-thaw cycles on aggregateassociated organic carbon and glomalin-related soil protein in natural-succession grassland and Chinese pine forest on the Loess Plateau[END_REF]. In contrast, studies on the influence of freezing and thawing on dissolved solutes of continental waters are only at their beginning [START_REF] Chen | Effect of freezing-thawing on dissolved organic matter in water[END_REF][START_REF] Hentschel | Repeated freeze-thaw events affect leaching losses of nitrogen and dissolved organic matter in a forest soil[END_REF][START_REF] Pokrovsky | Freeze-thaw cycles of Arctic thaw ponds remove colloidal metals and generate low-molecular-weight organic matter[END_REF][START_REF] Savenko | Phase fractionation of chemical elements during the formation of ice in fresh surface waters[END_REF][START_REF] Spencer | Freeze/thaw and pH effects on freshwater dissolved organic matter fluorescence and absorbance properties from a number of UK locations[END_REF].

Most of them focused on surface water with near circum-neutral pH such as rivers [START_REF] Chen | Effect of freezing-thawing on dissolved organic matter in water[END_REF][START_REF] Savenko | Phase fractionation of chemical elements during the formation of ice in fresh surface waters[END_REF] and ponds [START_REF] Pokrovsky | Freeze-thaw cycles of Arctic thaw ponds remove colloidal metals and generate low-molecular-weight organic matter[END_REF]. The main impact noticed by those studies is the decrease of aromaticity in waters due to FTC [START_REF] Chen | Effect of freezing-thawing on dissolved organic matter in water[END_REF] and the generation of low molecular weight (LMW) organic components [START_REF] Pokrovsky | Freeze-thaw cycles of Arctic thaw ponds remove colloidal metals and generate low-molecular-weight organic matter[END_REF].

The impact of freezing on dissolved organic matter (DOM) in high latitudes is still poorly known. In this study, we conducted freeze-thaw experiments aimed to characterize the impact of FTC on DOC, major and trace elements in waters of one of the largest permafrost peatlands of Europe, the Bolshezemelskaya Tundra (BZT). For this, we used DOM originated from vegetation, soil (peat) and natural water bodies of this peatland. Based on a few available studies dealing with FTC of surface waters [START_REF] Fellman | An evaluation of freezing as a preservation technique for analyzing dissolved organic C, N and P in surface water samples[END_REF][START_REF] Pokrovsky | Freeze-thaw cycles of Arctic thaw ponds remove colloidal metals and generate low-molecular-weight organic matter[END_REF][START_REF] Xue | The migration and transformation of dissolved organic matter during the freezing processes of water[END_REF], we hypothesized that: 1) DOC and trace metals (TM) present in the form of organic and organo-mineral colloids could coagulate under freezing which could favour the transport of mineral under more stable forms (colloids) through lakes and rivers to the Arctic Ocean; 2) OC originated from fresh plant biomass (i.e. leachates not subjected to previous freezing) would be more sensitive to FTC compared to natural waters, as fresh DOC is known to be composed of more labile components, and 3) the FTC could have a more pronounced effect on the degradation of OC from the permafrost compared with active layer OC since the latter has undergone previous seasonal freezing cycles. Overall, we aimed at providing an empirical assessment on the degree of DOC and inorganic solutes concentrations change after repeating freezing and thawing of representative surface waters from permafrost peatlands.

Materials and methods

Substrates origins and use

The DOM used in this study originated from: i) natural surface waters of a permafrost peatland, ii) aqueous leachates of ground vegetation and iii) aqueous leachates of peat soil.

The sampling sites are located in the NE European permafrost peatlands (BZT) within the discontinuous permafrost zone. Natural waters were collected from a small depression, a thermokarst lake and the Pechora River (Supplementary Table S1). Waters were filtered onsite through a 0.45 µm sterile filtration unit and re-filtered in the laboratory through a 0.22 µm Nalgene sterile filter before freezing.

The soil and ground vegetation for leachates were collected at the shore of the thermokarst lake (Table S1 and Figure S1). The peat core was sampled using a Russian peat corer (see details of sampling and handling in [START_REF] Morgalev | Bacteria primarily metabolize at the active layer/permafrost border in the peat core from a permafrost region in western Siberia[END_REF]. The soils included two subsamples of peat, one collected in the active (unfrozen) layer (20-30 cm below the surface)

and the second one in the permafrost zone (> 30-40 cm). Both subsamples represent oligotrophic peat composed of Sphagnum angustifolium, cotton-grass Eriophorum, Scheuchzeria palustris, bog-sedge Carex limosa, lichens, dwarf shrubs and some amount of wood debris (pine) and green mosses (Payandi-Rolland et al., 2020b). Ground vegetation contained mosses (Sphagnum fuscum and Sphagnum augustifolium) and lichens (Cladonia genus), which colonize hollows and mounds of the palsa and are reported to be widely spread species in the tundra [START_REF] Malmer | Vegetation, climatic changes and net carbon sequestration in a North-Scandinavian subarctic mire over 30 years[END_REF]. Aqueous leachates were prepared by reacting 10 g of organic substrate (10 gwet for frozen peat and 10 gdry for vegetation) with 1 L of sterile MilliQ water under constant agitation (70 rpm) at 25 ± 2 °C in the dark and aerobic conditions (Payandi-Rolland et al., 2020b;Shirokova et al., 2017aShirokova et al., , 2017b)). After 3 days of reactions, the suspension was centrifuged at 4500 g for 5 minutes and then sterile filtered (< 0.22 µm) using 500 mL Nalgene units and a manual vacuum pump.

Incubation, freeze-thaw cycles and sampling

A 150 mL aliquot of each leachate and natural waters were placed in pre-cleaned (rinsed 3 times with HCl (1 M) and MQ water and then autoclaved at 121 °C for 20 min) 200 mL polypropylene Nalgene bottles. Experiments with each substrate were performed in duplicate. A cycle of freeze-thaw (adapted from [START_REF] Pokrovsky | Freeze-thaw cycles of Arctic thaw ponds remove colloidal metals and generate low-molecular-weight organic matter[END_REF] started from placing water-filled Nalgene bottles in the freezer (-20 °C) overnight. After the total freeze of the liquid, bottles were transferred into the refrigerator compartment (at 4 °C) and kept there for 5 hours until the ice melted completely. These temperatures can be observed at the permafrost peatlands during the year in winter [START_REF] Lim | Dispersed ground ice of permafrost peatlands: Potential unaccounted carbon, nutrient and metal sources[END_REF], early spring and late autumn ("Climate & Weather Averages in Naryan-Mar, Russia," 2015; [START_REF] Serikova | High carbon emissions from thermokarst lakes of Western Siberia[END_REF] and are consistent with the range used in previous works [START_REF] Chen | Effect of freezing-thawing on dissolved organic matter in water[END_REF][START_REF] Pokrovsky | Freeze-thaw cycles of Arctic thaw ponds remove colloidal metals and generate low-molecular-weight organic matter[END_REF][START_REF] Spencer | Freeze/thaw and pH effects on freshwater dissolved organic matter fluorescence and absorbance properties from a number of UK locations[END_REF][START_REF] Xue | The migration and transformation of dissolved organic matter during the freezing processes of water[END_REF]. All filtrates have undergone 9 FTC, which corresponds to the average number of annual freeze-thaw cycles observed in high-latitude boreal and Arctic regions [START_REF] Henry | Climate change and soil freezing dynamics: historical trends and projected changes[END_REF][START_REF] Pokrovsky | Freeze-thaw cycles of Arctic thaw ponds remove colloidal metals and generate low-molecular-weight organic matter[END_REF].

After total ice melt in each cycle, we sampled 15 mL of a homogeneous aliquot from the bottle. Before and during the sampling, bottles were vigorously stirred with a magnetic stir bar to provide homogeneous suspension and avoid selective removal of either dissolved or particulate phase. A non-filtered subsample was used to measure pH (uncertainty of ± 0.01 pH units) and specific conductivity (± 0.1 µS.cm -1 ). The rest of the 15 mL was filtered through a 0.22 µm filter (sterilizing filtration, Minisart® syringe filter). In the filtrates, the DOC was measured with Shimadzu TOC-VSCN with an uncertainty of 2%. The UV-absorbance of water samples was measured using a 10 mm quartz cuvette on a CARY-50 UV-vis spectrophotometer, between 200 and 700 nm. The UV and visible spectra allowed assessment of (i) the molecular weight (MW) of DOC via the Weight-Average Molecular Weight index (WAMW, using the absorbance at 280 nm, [START_REF] Chin | Molecular weight, polydispersity, and spectroscopic properties of aquatic humic substances[END_REF], (ii) the humification index using the ratio of absorbance at 250 and 400 nm (the E2:E4 ratio, [START_REF] Park | characteristics of dissolved organic carbon in the leachate from Moonam Sanitary Landfill[END_REF] and (iii) the aromaticity of incubated water via specific UV absorbance (SUVA254) [START_REF] Abbt-Braun | Basic characterization of Norwegian NOM samples -Similarities and differences[END_REF]. Major cations (Na, Ca, K and Mg), Si, P and trace elements (TE) were measured with a quadrupole ICP-MS (Agilent 7500 ce) using In+Re as an internal standard. The international geo-standard SLRS-5 (Riverine Water Reference Material for Trace Metals) was used to check the validity and reproducibility of analyses.

Treatment of experimental results

Statistical treatment involved the least square method for regression and the Pearson correlation. All statistical analysis has been accomplished under R-software (version 3.6.2).

Boxplot analyses were used to detect outliers. Boxplot values combine the median (Q2), the first quartile (Q1), the third quartile (Q3), and the interquartile range (IQR) which is defined as the difference between Q3 and Q1. Outliers are defined as values located outside the first (D1)

and the last (D9) decile, as "D1 = Q1 -1.5*IQR" and "D9 = Q3 + 1.5*IQR" [START_REF] Tukey | Exploratory data analysis[END_REF]. Linear regression analyses have been performed using the function "lm" of the package "stats". The percentage of component gain or loss during FTC (FTC0-9, %) was calculated as a difference in concentration between the final and initial concentration of a component, normalized to its initial concentration. Trace element complexation with OM and saturation indices of the solution with respect to common solid phases were calculated using the Visual MINTEQ (vMINTEQ) software, following the model specifications for organic-rich waters of permafrost peatlands describe elsewhere [START_REF] Pokrovsky | Organic and organo-mineral colloids in discontinuous permafrost zone[END_REF][START_REF] Pokrovsky | Freeze-thaw cycles of Arctic thaw ponds remove colloidal metals and generate low-molecular-weight organic matter[END_REF]. A dataset combining the measured parameters can be found in Payandi-Rolland et al. (2020a).

Results and Discussion

Chemical composition of aquatic substrates

The dissolved (<0.22 µm) concentrations of DOC, major and trace elements of substrates used in this study are reported in Table 1. The water samples ranged from acidic (peat and vegetation leachates) to circum-neutral (depression, thermokarst lake, and Pechora River). The DOC of the thermokarst lake and the river was < 10 mg L -1 , which is 2 times lower than the leachates of peat and moss, 5 times lower than the lichen leachate, and 10 times lower than the water of depression. Such a wide range is comparable to the one reported for the hydrological continuum (depression  lake  stream  river) of this region (Shirokova et al., 2019). In all substrates, the DIC was below 1.5 mg L -1 except for the Pechora River (10.4 mg L -1 ). The SUVA254 ranged from low (< 2 L mgC -1 m -1 ) for the lichen leachate and the depression to near 4 L mgC -1 m -1 in other natural waters, and finally to values > 5 L mgC -1 m -1

for leachates of soils and moss. The WAMW of all substrates was lower than 3 kDa and ranged from ≤ 1 kDa in lichen leachate and the depression to near 2-2.5 kDa in other natural waters and in soil and moss leachates. Considering that the MW of fulvic acids (Fa) is approximately 600 to 1000 Da and that the humic acid (Ha) MW is between 1500 and 5000 Da, [START_REF] Garcia-Mina | Stability, solubility and maximum metal binding capacity in metal-humic complexes involving humic substances extracted from peat and organic compost[END_REF][START_REF] Malcolm | The uniqueness of humic substances in each of soil, stream and marine environments[END_REF], 3 groups of samples could be distinguished in terms of DOM character:

i) the humic OM group with a strong influence of Ha (i.e. leachates of soils and moss), ii) the "mixed" OM group with both influence of humic and fulvic acids (HFa) (i.e. the thermokarst lake and the Pechora River) and iii) the Fa OM group (i.e. lichen leachate and the water from the depression).

The leachate of lichen and the water from depression exhibited the highest DOC concentration (respectively 53 and 107 mg L -1 ), and lowest WAMW (758 ±1 and 1121 ±7 Da) and SUVA254 (0.7 and 1.6 L mgC -1 m -1 ). This indicates that the DOC of these samples is mostly composed of simple OC molecules with the dominance of aliphatic components [START_REF] Ågren | Dissolved organic carbon characteristics in boreal streams in a forest-wetland gradient during the transition between winter and summer[END_REF][START_REF] Tranvik | Colloidal and dissolved organic matter in lake water: carbohydrate and amino acid composition, and ability to support bacterial growth[END_REF][START_REF] Weishaar | evaluation of specific ultraviolet absorbance as an indicator of the chemical composition and reactivity of dissolved organic carbon[END_REF]. Conversely, other waters exhibited lower DOC concentrations with a high proportion of aromatic compounds (Table 1).

Considering the observed large difference in concentration of DOC, aromatic/aliphatic composition and the molecular weight of waters, it is anticipated that the effect of FTC could be primarily linked to these parameters. The concentrations of major and trace elements strongly varied among samples with general dominance of alkali and alkaline earth metals such as Ca, Na, Mg and K (Table 1). The highest concentrations of metals (Fe, Al, Mn) were observed in the moss leachate and depression waters (Table 1). Other trace elements exhibited concentrations lower than 2 µg L -1 .

Visual impact of the freezing process on DOM and removal of outliers

After full freezing of experimental reactors, ice was transparent at the borders and brown in the centre of bottles, similar to other FTC experiments (e.g., [START_REF] Pokrovsky | Freeze-thaw cycles of Arctic thaw ponds remove colloidal metals and generate low-molecular-weight organic matter[END_REF].

The concentration of brownish components (chromophoric components) in the centre of the bottle reflects the progressive freezing of water from bottle edges to the centre with a relative accumulation of DOM in the remaining liquid [START_REF] Petzold | Vacuum-assisted freeze concentration of sucrose solutions[END_REF][START_REF] Shafique | Forced migration of soluble and suspended materials by freezing front in aqueous systems[END_REF][START_REF] Xue | The migration and transformation of dissolved organic matter during the freezing processes of water[END_REF][START_REF] Zaritzky | Physical-chemical principles in freezing[END_REF]. Further, it was shown that a high concentration of DOM and low freezing temperatures diminish the proportion of DOM incorporated in the ice phase [START_REF] Elliott | Freeze-thaw cycle amplitude and freezing rate effects on extractable nitrogen in a temperate old field soil[END_REF][START_REF] Hentschel | Repeated freeze-thaw events affect leaching losses of nitrogen and dissolved organic matter in a forest soil[END_REF][START_REF] Shafique | Forced migration of soluble and suspended materials by freezing front in aqueous systems[END_REF][START_REF] Xue | The migration and transformation of dissolved organic matter during the freezing processes of water[END_REF]. The high concentration of DOC in leachates of soils and moss (around 20 mg L -1 ), in the leachate of lichen (53 mg L -1 ) and in the water from the depression (106 mg L -1 ) associated to the -20 °C temperature used in this study promotes the segregation of DOM in the remaining fluid located at the centre of the bottle during the freezing process. Such a formation of brown coagulates in the ice was interpreted as a change in particles density and diameter that occurs during the freezing [START_REF] Fellman | An evaluation of freezing as a preservation technique for analyzing dissolved organic C, N and P in surface water samples[END_REF][START_REF] Giesy | Particulate formation due to freezing humic waters[END_REF], and presumably, leads to the formation of organo-metal aggregates rich in Fe and Al hydroxides (see section 3.5). It is worth noting that after the total thaw of waters, no visual aggregations of OM (brownish area) can be distinguished in liquid. Furthermore, conversely to the FTC experiments on an organicrich bog and fen waters [START_REF] Pokrovsky | Freeze-thaw cycles of Arctic thaw ponds remove colloidal metals and generate low-molecular-weight organic matter[END_REF], no brown/yellow precipitates have been observed at the bottom of the containers. From this observation, we hypothesized apparent reversibility of organic colloids and particles aggregation-dissolution during FTC of studied waters.

Preliminary analysis of the DOC concentration evolution during the FTC demonstrated some clear outliers. A boxplot repartition on data revealed that some of the data points can be statistically considered as outliers (see description of outliers given in section 2.3; Figure 1 and Table S2). These outliers are believed to be sampling artefacts. Some samples demonstrated an increase in certain elements concentration during FTC, which is counterintuitive given that all samples were 0.22 µm filtered prior to the incubation and also 0.22 µm filtered at each sampling. Furthermore, these increases were non-systematic and non-reproducible among replicates. We hypothesize that FTC can generate aggregates of DOC and metal hydroxides (see section 3.5) thus producing some "islands" of solute-rich and solute-poor fluids. During the sampling procedure, even under continuous stirring, one can randomly collect some aggregate-enriched aliquots. Because i) each sampling are independent and ii) the presence of such aggregates, revealed by abnormal DOC concentrations, can biased other measured parameters (absorbance and trace elements concentrations), we have removed the outliers (about 7% of all collected data, see Table S2).

In the rest of this study, results and discussions will only deal with the new data set (i.e. without outliers).

Quantification of freeze-thaw cycles impact on DOC concentration and quality

The evolution of DOC concentration during FTC (Figure 1) exhibited variable trends among samples. The DOC concentration slightly increased between the initial stage (prior to the first freezing, i.e. FTC0) and after the last freezing (FTC9) in the active layer leachate (5 ± 2%), the thermokarst lake (8%) and the Pechora River (14 ± 4%) (Figure 2 and Table S3). The variations of DOC in other samples were lower than 5%. However, only the increase of thermokarst lake DOC with FTC was superior to 5% and significant (p_value < 0.05; Figure 2 and Table S4). Former studies demonstrated generally decreasing DOC concentration in the course of FTC with a range of DOC loss from 7 to 25% [START_REF] Fellman | An evaluation of freezing as a preservation technique for analyzing dissolved organic C, N and P in surface water samples[END_REF][START_REF] Pokrovsky | Freeze-thaw cycles of Arctic thaw ponds remove colloidal metals and generate low-molecular-weight organic matter[END_REF]. [START_REF] Fellman | An evaluation of freezing as a preservation technique for analyzing dissolved organic C, N and P in surface water samples[END_REF] demonstrated that the initial concentration of DOC strongly influenced the removal efficiency, with a loss of 7% when the concentration was < 5 mg L -1 and a 20% loss at higher initial DOC concentrations. In the present study, all initial DOC concentrations were superior to 7 mg L -1 and waters with the lowest initial concentration (from 7 to 10 mg L -1 , Table 1) exhibited an increase in DOC (i.e. the Pechora River and the thermokarst lake, Figure 2 and Table S3Erreur ! Source du renvoi introuvable.). Therefore, the initial concentration of DOC does not appear to control the DOC removal efficiency in our study. Overall, low variations of DOC (< 5%, at the exception of the thermokarst lake and the Pechora River water) highlight the high stability of DOC when subjected to FTC. This observation is in agreement with the study by [START_REF] Xue | The migration and transformation of dissolved organic matter during the freezing processes of water[END_REF] showing that DOC concentrations of lotic waters (which froze on 1 m thick during 3 to 5 months per year in the northern China) were highly stable (from 0.68% increase to 4.3% decrease) through FTC.

The SUVA254 values presented in this work do not take into account the iron corrections suggested by [START_REF] Poulin | Effects of iron on optical properties of dissolved organic matter[END_REF] because the differences between the uncorrected and corrected SUVA254 values were below 5% for all incubated substrates at the exception of the depression (maximum 5.9%), see Table S5. The SUVA254 and WAMW values decreased (from -6.5% to -(17.0 ± 1)%, and from -5.6 % to -(14.1 ± 1)%, respectively) between FTC0 to FTC9 in the thermokarst lake and the Pechora River, whereas in other samples, the variations were below 5% (Figure 2, Figure S2 and Table S3). Given that the waters were sterile filtered (< 0.22 µm) before the experiments, the observed decrease of SUVA254 and WAMW can be only abiotically driven. This suggests that the FTC can provide an increase in non-aromatic low molecular weight components in waters from moss leachates, thermokarst lake and Pechora River. At the same time, both variations of SUVA254 and WAMW in waters of thermokarst lakes and Pechora River were not significantly correlated to the number of FTC (Table S4).

The evolution of E2:E4 ratio between the 0 and 9 th cycle of freezing was null to positive in all leachates (< 5% increase), clearly positive in the depression (an increase of 8.5%), and negative in the thermokarst lake and the Pechora River (-5.2% and -(5.6 ± 2)%, respectively,

Figure 2 and Table S3). In the present study, the relative removal of humic-like aromatic components in waters from the HFa group (see section 3.1, i.e. the thermokarst lake and the Pechora River) is corroborated by a decrease of SUVA254 and WAMW values through FTC in these samples (Figure 2 and Figure S3). Unlike well-known biotically-driven degradation of humic-like aromatic components in streams and rivers waters demonstrated by the decrease of E2:E4 ratio [START_REF] Hutchins | The optical, chemical, and molecular dissolved organic matter succession along a boreal soil-stream-river continuum[END_REF], in this study the decrease of this ratio is imputed to FTC (abiotic phenomenon) which appear to be also capable of selective removal of humic-like aromatic compounds in the lake and river water. In contrast, in leachates and depression waters, while the E2:E4 ratio slightly (< 10 %) increased with FTC, no evidence of selective removal of LMW components have been detected (the WAMW variations were < 5%).

Trace element behaviour during FTC

A clear decrease of Fe concentration during FTC in thermokarst lake and the Pechora River (-39.0% and -94.4%; respectively; Figure 3, Figure S4 and Table S3) suggests the removal of this element in the form of secondary phases. The saturation index (SI) calculated via the vMINTEQ software indicates that Fe can precipitate in the form of akaganeite (FeO0.83(OH)1.17Cl0.17, SI: 1.8 to 2.1), goethite (Fe(OH)3, SI: 0.7 to 1.0) and lepidocrocite (Fe 3+ O(OH), SI: 0.6 to 0.9) in all surface waters. In the case of thermokarst lake and the Pechora River, a significant (p < 0.05) increase in (Corg/Fe) molar ratio in the course of FTC was observed, which suggested a progressive depletion of Fe relative to Corg in the colloidal fraction (Figure 4A and C, and Table S4).

Aluminium concentration decreased during FTC of moss leachate (-33.4%), thermokarst lake (-9.4%) and Pechora River (-85.1 ±7%) samples (Figure 4, Figure S5 and Table S3). The SI calculation did not indicate any supersaturation concerning Al oxyhydroxides.

However, the formation of organo-mineral (Fe, Al) coagulates via coprecipitation of Fe and Al oxyhydroxides is likely given the sizable correlation (p < 0.05) between aqueous concentrations of Fe and Al in the course of the FTC sequence (Figure S6) in all waters (with an exception of moss leachate). In the case of the Pechora River, a significant (p < 0.05) evolution of the molar ratio (Corg/Al)molar with the number of FTC (Figure 4B and Table S4)

suggests the formation of mixed Fe-Al-OM sub-colloidal particles as it is reported in surface waters of permafrost peatlands [START_REF] Pokrovsky | Organic and organo-mineral colloids in discontinuous permafrost zone[END_REF]. Other trivalent hydrolysates such as Y and Ga punctually correlated with DOC and Fe during FTC (Figure S7 and Table S6). Light REE (LREE: La, Ce and Pr) correlated with Fe in the leachate of the active layer and the permafrost, and in waters of the depression and the Pechora River, whereas heavy REE (HREE: Nd, Dy, Yb) significantly correlated with Fe in the Pechora River (Table S6).

Phosphate is known to be controlled by dissolved humic substances [START_REF] Fellman | An evaluation of freezing as a preservation technique for analyzing dissolved organic C, N and P in surface water samples[END_REF][START_REF] Francko | Epilimnetic phosphorus cycling: influence of humic materials and iron on coexisting major mechanisms[END_REF][START_REF] Jones | Phosphorus transformations in the epilimnion of humic lakes: abiotic interactions between dissolved humic materials and phosphate[END_REF] especially in wetlands where the high concentration of DOM and iron facilitates the complexation of DOC and PO4 [START_REF] Dillon | Effect of landscape form on export of dissolved organic carbon, iron, and phosphorus from forested stream catchments[END_REF].

However, this was not observed in this study, because there was no correlation between the concentration of PO4 and DOC, and the molar ratio between organic carbon and phosphate did not systematically vary during the FTC in all samples (Table S4). Chromium, vanadium and silicon concentrations decreased in waters of thermokarst lakes and Pechora River (Figure S8 and Table S3), whereas most of the divalent macro-and micro-nutrients (Co and Ni) exhibited a slight increase in concentration through FTC in all waters (Figure S8 and Table S3).

Manganese concentration clearly decreased in thermokarst lake and the Pechora River (respectively -10.4% and -(79.4 ±1) %) but remained stable or increased in other waters (Figure 3, Figure S9 and Table S3). The decrease of Mn concentration and the correlation between the molar ratio (Corg/Mn)molar and FTC number could indicate a formation of Mn (hydr)oxide particles or Mn coprecipitation with organo-ferric compounds in the water of the Pechora River.

Among tetravalent hydrolysates, Ti and Zr demonstrated an increase in the Pechora River and a decrease in other waters (Figure S7). Titanium concentration significantly correlated with Fe in all waters (except the moss leachate). Zirconium correlated with Fe in the leachate of the active layer, permafrost and in the depression. Hafnium and Th correlated with Fe only in water of the depression (Table S6). These types of correlation between Fe and insoluble tetravalent hydrolysates have been observed previously in ultrafiltration experiments on boreal forest [START_REF] Ilina | Speciation, Size Fractionation and Transport of Trace Elements in the Continuum Soil Water-Mire-Humic Lake-River-Large Oligotrophic Lake of a Subarctic Watershed[END_REF] and permafrost surface waters (Pokrovsky et al., 2016) and indicate the dominance of organo-ferric high molecular weight colloids in TE 4+ speciation.

Lead concentration increased in the course of FTC in all leachates and decreased in natural waters (Figure S7 and Table S3). Arsenic (As) decreased only in the Pechora River water (Figure 3, Figure S10 and Table S3). In riparian wetlands, the presence of As hotspots due to As association with Fe or OM was reported [START_REF] Guénet | Evidence of organic matter control on As oxidation by iron oxides in riparian wetlands[END_REF]. In the present study, linear regressions on As and DOC concentrations were non-significant (p > 0.05) in all samples (Table S6). In contrast, there was a strong linear regression (p < 0.03) between As and Fe concentrations for all studied samples, except the thermokarst lake (Figure S11 and Table S6).

It appears then, that the As is bound to the Fe and not to the OM of either vegetations and soils leachates or natural waters. In addition to its control by DOM, As in natural waters can be adsorbed to Fe-oxyhydroxides, such as nano-lepidocrocite [START_REF] Guénet | Evidence of organic matter control on As oxidation by iron oxides in riparian wetlands[END_REF]. Both the correlation between As and Fe concentrations and the supersaturation of solutions with respect to Fe-oxyhydroxides and lepidocrocite secondary phases (section 3.4) observed in the present study support this hypothesis.

Possible mechanisms of DOC and metal transformation during FTC.

Two colloidal pools are responsible for TE speciation in boreal aquatic environments:

i) small organic-rich colloids and LMW DOM, and ii) high molecular weight Fe-rich colloids [START_REF] Baalousha | Size-based speciation of natural colloidal particles by flow field flow fractionation, inductively coupled plasma-mass spectroscopy, and transmission electron microscopy/x-ray energy dispersive spectroscopy: colloids-trace element interac on[END_REF][START_REF] Krachler | Nanoscale lignin particles as sources of dissolved iron to the ocean[END_REF][START_REF] Stolpe | Abundance, size distributions and trace-element binding of organic and iron-rich nanocolloids in Alaskan rivers, as revealed by field-flow fractionation and ICP-MS[END_REF]. The LMW colloids dominate metal speciation in acidic thaw pond water and are highly stable during FTC. In contrast, high molecular weights colloids dominated mainly in neutral waters of the permafrost area and are subjected to strong removal during freezing [START_REF] Pokrovsky | Freeze-thaw cycles of Arctic thaw ponds remove colloidal metals and generate low-molecular-weight organic matter[END_REF]. In our study, the MW of Ha and HFa group can be indicative of LMW colloids (generally ranging between 1 to 50 kDa), while the Fa group (lichen leachate and depression waters) exhibited essentially dissolved fraction (LMW<1 kDa). The LMW1-50 kDa colloids are likely to dominate the metals speciation in the thermokarst lake and the Pechora River water. The stronger impact of FTC on the Pechora River and the thermokarst lake water suggests that mixed HFa OM of large terminal water bodies is most vulnerable to the freeze-thaw process. This observation can be attributed to the two binding patterns of metals and OM. These patterns depend on the maximum metal binding capacity of i) humic-like component which is reached at neutral to alkaline pH (7 to 9) due to maximal ionization of carboxylate and phenolic acids; and ii) fulvic-like component reached at acidic pH (5 to 6) when the majority of carboxylate moieties are dissociated [START_REF] Garcia-Mina | Stability, solubility and maximum metal binding capacity in metal-humic complexes involving humic substances extracted from peat and organic compost[END_REF]. Therefore, in contrast to more acidic plant and peat leachates, in the Pechora River, both the presence of humic acids (in HFa) and the neutral pH ( 7) and in thermokarst lake, the presence of fulvic acids (in HFa) and slightly acidic conditions (pH = 6.3), promote the aggregation of colloidal Fe via DOM -metal binding. Considering, based on our experimental observations, a relatively stable concentration of DOC, a decrease of SUVA254, WAMW, Fe concentrations, and an increase in Corg/Fe ratio in the course of FTC, we suggest preferential degradation of humic-like components in waters from thermokarst lake and Pechora River and formation of aliphatic LMW1-50kDa colloids. On the contrary, no evidence on the formation of organic and organo-ferric colloids in other waters during FTC has been detected in our study.

Possible role of FTC in natural environments

Progressive accumulation of DOM and related elements in the remaining fluid due to full freezing of the water column is observed in shallow confined natural water bodies such as thermokarst lakes and ponds of permafrost peatlands [START_REF] Manasypov | Seasonal dynamics of organic carbon and metals in thermokarst lakes from the discontinuous permafrost zone of western Siberia[END_REF]. In the present study, the effect of freezing on concentrations of most dissolved components did not generally exceed 10% and was limited to simple physicochemical reactions given that all samples were sterile-filtered before the first freezing. In contrast, in a non-sterile FTC experiment that used various soils from marsh and wet grasslands, a strong DOC production (100 to 300 mg L -1 ) compared to samples not subjected to FTC was reported [START_REF] Yu | Response of soil constituents to freeze-thaw cycles in wetland soil solution[END_REF]. This increase was believed to be the consequence of microbial cells lysis, physical disruption of soils aggregates, and roots turnover [START_REF] Bochove | Effects of freeze-thaw and soil structure on nitrous oxide produced in a clay soil[END_REF][START_REF] Deluca | Effect of freeze-thaw events on mineralization of soil nitrogen[END_REF][START_REF] Ivarson | Effect of frost action and storage of soil at freezing temperatures on the free amino acids, free sugars and respiratory activity of soil[END_REF][START_REF] Tierney | Soil freezing alters fine root dynamics in a northern hardwood forest[END_REF]. In natural permafrost aquatic and soil environments, microbial communities are highly resistant to FTC [START_REF] Lipson | Plant-microbe competition for soil amino acids in the alpine tundra: effects of freeze-thaw and dry-rewet events[END_REF][START_REF] Stres | Frequent freeze-thaw cycles yield diminished yet resistant and responsive microbial communities in two temperate soils: a laboratory experiment[END_REF]. This suggests that both the capacity of micro-organisms to quickly recover and efficiently biodegrade OM, together with the abiotic transformation of DOC during and after repetitive freezing can become a sizable source of LMW DOC, as also reported in previous studies [START_REF] Pokrovsky | Freeze-thaw cycles of Arctic thaw ponds remove colloidal metals and generate low-molecular-weight organic matter[END_REF]. Further, whereas the freezing process can impact the DOM concentration, the thawing duration (hours or days) has little effect on the DOC aromaticity of leachates [START_REF] Hentschel | Repeated freeze-thaw events affect leaching losses of nitrogen and dissolved organic matter in a forest soil[END_REF][START_REF] Vestgarden | Effects of freeze-thaw on C and N release from soils below different vegetation in a montane system: a laboratory experiment[END_REF]. This implies that the rapid thawing of ice in nature could lead to the production of more easily biodegradable OM components.

The results of the present study demonstrated rather minor (i.e., ≤ 10%) effects of freeze-thaw cycles on DOC quantity and quality in various leachates of soils and vegetation which indicates strong stability of DOM in typical surface waters of boreal peatlands (except large river). As such, the FTC may be of sub-ordinary importance on DOM transformation in surface waters of permafrost peatlands relative to other biotic and abiotic factors such as heterotrophic degradation and photolysis (Payandi-Rolland et al., 2020b;[START_REF] Shirokova | The evolution of the ecosystems of thermokarst lakes of the Bolshezemelskaya tundra in the context of climate change[END_REF]. In contrast, the "mineral" part of organo-mineral colloids present in natural waters was subjected to removal from solution due to freezing as indicated by an increase in Corg/Fe and Corg/Al molar ratios in the course of FTC of river and lake water. In this regard, the impact of FTC on trace metal can be competitive to bio-and photo-transformation of trace metal colloids [START_REF] Oleinikova | Low biodegradability of dissolved organic matter and trace metals from subarctic waters[END_REF][START_REF] Oleinikova | Transformation of organo-ferric peat colloids by a heterotrophic bacterium[END_REF]. Therefore, the first hypothesis of this study was confirmed only for the large river, where both Fe and Al were removed preferentially to DOC.

For a thermokarst lake, this hypothesis was only partially confirmed (solely Fe removal). Such a removal of Fe, Al and other metal were not observed in vegetation and soil leachates and in the depression water, thus confirming a subordinary role of freezing on DOM and metal fate in surface waters of permafrost peatlands. This dismisses the second hypothesis of this study on the highest sensitivity of fresh leachates to freezing: regardless of the origin of vegetation leachates (moss and lichen), the change in concentration of DOC and metals during FTC was inferior to that of river and lake waters. In contrast, large water bodies such as the Pechora River or a thermokarst lake meet the ideal conditions for stabilization of OM-mineral colloids.

In these settings, the FTC can facilitate aquatic transport of C, nutrients and metals from peatlands surface waters to the Arctic Ocean. However, because the full freezing of large surface water bodies is unlikely compared to small depressions and soil porewaters, the overall impact of FTC on DOC and metal transformations in permafrost peatlands is expected to be low. As such, unlike the bio-and photo-degradation of permafrost DOM in Arctic waters [START_REF] Cory | Sunlight controls water column processing of carbon in arctic fresh waters[END_REF][START_REF] Vonk | Reviews and syntheses: Effects of permafrost thaw on Arctic aquatic ecosystems[END_REF], which has to be considered in models of the future arctic C balance [START_REF] Mcguire | Dependence of the evolution of carbon dynamics in the northern permafrost region on the trajectory of climate change[END_REF], the freeze-thaw cycles are likely to be of secondary importance for C transformation in Arctic continental waters.

Conclusions

This study demonstrated that the pH and OM composition (aromaticity, molecular weight, proportion of humic and fulvic components) are the main factor controlling the DOC (< 0.22 µm) removal from surface waters of permafrost peatlands under freeze-thaw cycles.

No quantitative relationship was established between both DOC quantity and quality and the number of FTC in the acidic, DOC-rich aqueous leachates of terrestrial vegetation and various peat horizons. We also observed sizable stability of DOC and metal complexes present in typical surface and soil waters of permafrost peatlands with respect to freeze-thaw cycles. The exceptions are the circumneutral, humic-and fulvic-rich waters of the thermokarst lake and the Pechora River, which likely exhibited formation of Fe-and Al-rich organo-mineral particles.

In contrast to DOC concentration which remained rather stable with FTC, trace element variations were pronounced and often exceeded 10%. Thus, in the Pechora River water, concentrations of metal micronutrients and trace elements sizably decreased during FTC. This was also partially pronounced in the thermokarst lake water. As such, large water bodies are most subjected by the FTC impact on OM-mineral colloids transport from lakes and rivers to the Arctic Ocean. This impact can be further accentuated due to enhanced FTC frequency under on-going climate warming in high latitudes.

Figures

Figure 1 :

 1 Figure 1: DOC concentration during FTC for each type of waters incubations. "*" indicates the

Figure 2 :

 2 Figure 2: Percent variation (Δ, %) of DOC quantity and quality parameters (DOC concentration,

Figure 3 :

 3 Figure 3: Percent variation of trivalent hydrolysates (Al, Light and Heavy Rare Earth Elements

Figure 4 :

 4 Figure 4: The molar ratio of Corg (A) to Al and (B) to Fe for the Pechora River substrate, and the

Acknowledgements

Supports of RFBR N o 19-55-15002, 20-05-00729 and of State Task AAAA-A18-118012390200-5. D.P-R was supported by a PhD. fellowship from the French « Ministère de l'Enseignement Supérieur, de la Recherche et de l'Innovation ». We are grateful to Carole Causserand (GET) for help with DIC and DOC concentrations analyses and to the ICP-MS team of the GET laboratory for the help with the trace elements analyses.

Conflicts of interest

The authors declare no conflict of interest.

Tables and Figures

Tables Table 1: General physical and chemical dissolved (<0.22 µm) parameters of different aquatic substrates before the first freezing (replicate mean).