Maria Potop-Butucaru

Blockchains and the commons Invited Paper

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Introduction

Blockchain systems became today one of the most appealing area of research motivated mainly by the recent speculations on crypto-currencies such as Bitcoin [START_REF] Reischuk | A new solution for the byzantine generals problem[END_REF] or Ethereum [85]. A blockchain is a distributed ledger that mimics the functioning of a classical traditional ledger (i.e. transparency and falsificationproof of documentation) in an untrusted environment where the computation is distributed. The set of participants to the system are not known and it varies during the execution. Moreover, each participant follows his own rules to maximize its welfare.

Blockchain systems maintain a continuously-growing list of ordered blocks that include one or more transactions 1 that have been verified by the members of the system, called miners. Blocks are linked using cryptography and the order of blocks in the blockchain is the result of a form of agreement among the system participants. Participants strongly agree only on a prefix of the blockchain, the suffix of the blockchain may be different from one participant to another.

Blockchain systems, beyond their incontestable assets such as decentralization, simple design and relative easy use, are not free of incidents and limitations. The most popular incident reported for Ethereum for example was the 60 million dollars ether theft which was possible by simply exploiting an error in the code and the lack of system specification.

A recent scientific analysis, [START_REF] Garay | Sok: A consensus taxonomy in the blockchain era[END_REF], focus on several limitations of the most popular blockchain, Bitcoin, such as: weak security, low quality of services, storage limitations, low throughput and high cost and weak consistency.

Therefore, despite their large publicity blockchains are far from being a technology ready to be used in critical economical applications and scientists multiply their efforts in warning about the risks of using this technology before understanding and fully mastering it. Interestingly, many recent attempts to alarm on vulnerabilities of popular blockchains like Bitcoin are target of defenders brigading.

Nevertheless, once fully mastered, Blockchain systems will be the technical solution for fully decentralizing activities that were for centuries centralized such as for example administration or banking. The applications of tomorrow that potentially will be blockchainized are all different from each other. These applications may range from IoTs to notary passing by administration, banking or health. These applications have various consistency and quality of services requirements. Therefore, we advocate that there will not be only one blockchain but a family of modular blockchains that will have to offer various qualities of services and that will be eventually interconnected.

It should be noted that differently from classical distributed applications, some blockchains have a strong economical aspect since participants should be constantly incited to participate to the system welfare by rewarding their contribution. This contribution is materialized either in the energy spent in solving cryptographic puzzles in order to generate blocks or in the bandwidth spent to route transactions and blocks. If participants massively leave the system then the system collapses, phenomenon known in economy as the tragedy of commons. In order to avoid this phenomenon, blockchains have to cross-over new distributed formally verified and proven algorithms with game theory tools and also government rules issued from self-organized micro-economies.

State of the art

The birth of blockchains systems, as for the case of P2P systems in the early 2000, was in the non academic research. After the releasing of the most popular blockchains (e.g. Bitcoin [START_REF] Reischuk | A new solution for the byzantine generals problem[END_REF] or Ethereum [85]) with a specific focus on economical transactions their huge potential for various other applications became evident.

Their popularity, transformed blockchains in a huge social experiment that confirmed the fact that blockchains can be a viable alternative for distributed systems of tomorrow. Starting with this point, blockchain area started to became the focus of the academical research.

Interestingly, only recently distributed computing scientist started to investigate theoretical aspects of blockchains and several directions of research can be identified: blockchains based on proof-of-work and its alternatives such as proofof-stake, proof-of-space or proof-of-authority, blockchains using as underlying building block the achievements in classical practical Byzantine fault-tolerance and finally sortition based blockchains.

The theoretical study of proof-of-work based blockchains has been pioneered by Garay et al [START_REF] Kokoris-Kogias | Enhancing Bitcoin Security and Performance with Strong Consistency via Collective Signing[END_REF]. They decorticate the pseudo-code of Bitcoin and analyze its agreement aspects considering a synchronous round-based communication model. That is, messages sent in a round are assumed to arrive in the next round. This study has been extended by Pass et al [77] to round based systems where messages sent in a round can be received later. The major criticisms for the proof-of-work approach are as follows: it is assumed that the honest miners hold a majority of the computational power, the generation of a block is energetically costly which yielded to the creation of mining pools and finally, the multiple blockchains that coexist in the system. Interestingly, the two alternatives for proof-of-work such as proof-of-stake (the power of block building is proportional to the amount of money they own in the system) or proof-of-authority (the power of block building is proportional to the amount of authority they own in the system) have not yet been fully analyzed from a theoretical point of view. The line of research that addresses the consensus in proof-of-stake based blockchains is pioneered by Daian et al. [START_REF] Garay | The bitcoin backbone protocol: Analysis and applications[END_REF] that proposes a protocol for weakly synchronous networks. The execution of the protocol is organized in epochs. Similar to Bitcoin-NG [START_REF] Golan-Gueta | SBFT: a scalable decentralized trust infrastructure for blockchains[END_REF] described below in each epoch a different committee is elected and inside the elected committee a leader will be chosen. The leader is allowed to extend the new blockchain. The protocol is validated via simulations and only partial proofs of correctness are provided.

In order to overcome the drawbacks of Bicoin, [START_REF] Golan-Gueta | SBFT: a scalable decentralized trust infrastructure for blockchains[END_REF] proposes a mix between proof-of-work blockchains and proof-of-work free blockchains referred as Bitcoin-NG. The idea is that the execution of the system is organized in epochs. In each epoch a leader elected via a proof-of-work mechanism will decide the order transactions that will be committed in the blockchain till the next epoch. Bitcoin-NG inherits the drawbacks of Bitcoin: costly proof-of-work process, forks, no guarantee that a leader in an epoch is unique, no guarantee that the leader do not change the history at will if the leader is corrupted.

Later, [START_REF] Ostrom | Trust and reciprocity: Interdisciplinary lessons for experimental research[END_REF] initiates an alternative to the proof-of-work based blockchains, named Byzcoin. Their research build on top of practical Byzantine fault-tolerance [START_REF] Decker | Bitcoin Meets Strong Consistency[END_REF] enhanced with a scalable collective signing process. [START_REF] Ostrom | Trust and reciprocity: Interdisciplinary lessons for experimental research[END_REF] is based on a leaderbased consensus over a group of members chosen based on a proof-of-membership mechanism. As in Bitcoin, when a miner succeeds to mine a block it is included in the voting members set that excludes one member. This protocol also inherits some of the Bitcoin problems and vulnerabilities. Also Byzcoin voting core can be totally corrupted by a dynamic adversary. More recently, SBFT [START_REF] Lamport | The byzantine generals problem[END_REF] and Hyperledger Fabric [START_REF] Banu | An improved byzantine agreement algorithm for synchronous systems with mobile faults[END_REF] build also on top of [START_REF] Decker | Bitcoin Meets Strong Consistency[END_REF]. In the same spirit, [START_REF] Garay | Reaching (and maintaining) agreement in the presence of mobile faults[END_REF] proposes for the first time a leader-free algorithm to solve Consensus among participants in a consortium Blockchain where the specifications has been adapted to the Blockchain scenario. The same specification is then considered in DBFT [START_REF] Fischer | Impossibility of distributed consensus with one faulty process[END_REF], an evolution of the consensus algorithm in [START_REF] Garay | Reaching (and maintaining) agreement in the presence of mobile faults[END_REF], in Tendermint Consensus algorithm [START_REF] Croman | On scaling decentralized blockchains -(A position paper)[END_REF]. In the same line of research have been proposed recently SBFT [START_REF] Lamport | The byzantine generals problem[END_REF] and Hot-Stuff [START_REF] Anceaume | Blockchain abstract data type[END_REF].

In order to avoid some of the previously cited problems, Micali [START_REF] Poon | The bitcoin lightning network[END_REF] introduced (further extended in [START_REF] Bonnet | Tight bound on mobile byzantine agreement[END_REF][START_REF] Eyal | Bitcoin-ng: A scalable blockchain protocol[END_REF]) the sortition based blockchains that completely replace the proof-of-work mechanism by sortition. These works focus again the agreement aspects of blockchains using probabilistic ingredients. More specifically, the set of nodes that are allowed to produce and validate blocks are randomly chosen and they change over the time. Interestingly, the study focuses only on synchronous round-based communication models which do not reflect the reality of blockchain technologies.

In another line of research, Pass et al. address in [78] one of the vulnerabilities of Bitcoin studied formally in Eyal and Sirer [START_REF] Gotsman | cause i'm strong enough: reasoning about consistency choices in distributed systems[END_REF]. In [START_REF] Gotsman | cause i'm strong enough: reasoning about consistency choices in distributed systems[END_REF] the authors prove that if the adversary controls a coalition of miners holding even a minority fraction of the computational power, this coalition can gain twice its share. Fruitchain proposed in [78] overcomes this problem by ensuring that no coalition controlling less than a majority of the computing power can gain more than a factor 1 + 3δ by not respecting the protocol, where δ is a parameter of the protocol.

A full overview of the agreement protocols designed for blockchain systems can be found in [START_REF] Izumi | When expanders help self-healing distributed r-tree overlays[END_REF].

Another interesting line of research, has been opened by Decker et al.

[52] related to the blockchains consistency. They propose PeerCensus system that targets to provide the linearizability of transactions. PeerCensus combines, similar to Byzcoin, the proof-of-work blockchain and the classical results in practical byzantine agreement fault tolerance. This line of research has been continued in [START_REF] Avizienis | Basic concepts and taxonomy of dependable and secure computing[END_REF][START_REF] Biais | The blockchain folk theorem[END_REF][START_REF] Eyal | Majority is not enough: Bitcoin mining is vulnerable[END_REF][START_REF] Belotti | Game theoretical analysis of Atomic Cross-Chain Swaps[END_REF].

All the above mentioned studies leave a huge unexplored space in the theoretical distributed aspects of blockchains. Moreover, even though a strong effort has been recently dedicated to formalizing blockchain systems, it comes to evidence that blockchains still lack of formalization and theoretical understanding of their properties and their level of consistency face to system asynchrony, churn and partitions, rational and irrational behaviors and multiple types of faults and attacks. This important drawback limits drastically the integration of blockchains in industrial applications despite the huge interest of the main industrial actors in this technology. In the following we detail open research directions that may help in integrating blockchain solutions in practical applications.

Explore novel models of reliability for blockchains

Faults are studied in distributed systems for decades [START_REF] Blin | Fast self-stabilizing minimum spanning tree construction -using compact nearest common ancestor labeling scheme[END_REF] and most of the time in isolation. Interestingly, faults and behaviors are defined in the distributed systems literature in a verbose mode which, in most of the cases leaves the place to the interpretation.

In a very popular paper, Laprie et al. [START_REF] Blin | A super-stabilizing log(n)log(n)approximation algorithm for dynamic steiner trees[END_REF] the authors describe and classify the distributed system faults, errors and failures. Interestingly, Byzantine Altruistic and Rational model, a.k.a BAR [START_REF] Anta | Atomic appends: Selling cars and coordinating armies with multiple distributed ledgers[END_REF]. BAR model identifies three categories of processes: altruistic, those who follow a prescribed protocol; rationals, those who act in order to maximise their utility function; and Byzantines, those who may rationally deviate from a prescribed protocol. This later behavior can be seen as rational Byzantine behavior. In [START_REF] Anceaume | Incentives for P2P fair resource sharing[END_REF] the authors introduce the notion of robustness of a distributed system by introducing the notions of k-resiliency and t-immunity. In a k-resilient equilibrium there is no coalition of k players having an incentive to simultaneously change strategy to get a better outcome. On the other hand, the concept of t-immunity evaluates the risk of a set of t players to have a Byzantine behavior. It should be noted that the property of t-immunity is often impossible to be satisfied in practical systems [START_REF] Amoussou-Guenou | Rationals vs byzantines in consensus-based blockchains[END_REF].

In the context of blockchains, Micali et al [START_REF] Eyal | Bitcoin-ng: A scalable blockchain protocol[END_REF] advocate that blockchains should be tolerant to churn and a to very powerful dynamic adversary. Informally speaking, this adversary "can corrupt any user he wants, at any time; totally control and perfectly coordinate all corrupted users and schedule de delivery of messages". Moreover, Blockchains area brings a new direction of research by exposing rational behavior with effects similar to the irrational ones. This type of behavior is extensively studied in economics theories as for example the Elinor Ostrom work [START_REF] Wood | Ethereum: A secure decentralised generalised transaction ledger[END_REF][START_REF] Nolan | Re: Alt chains and atomic transfers[END_REF][START_REF] Sasaki | Mobile byzantine agreement on arbitrary network[END_REF].

The hierarchy of Laprie et al. [START_REF] Blin | A super-stabilizing log(n)log(n)approximation algorithm for dynamic steiner trees[END_REF] extended with the BAR model or the (k, t)robustness model totally covers complex faults experienced in blockchains such as dynamic adversaries, churn, transient faults, rational and irrational behaviors or combinations. Therefore, several research directions need to be explored in this context.

Blockchains robustness to dynamic adversaries

The dynamic adversary that affects blockchains described by Micali in [START_REF] Eyal | Bitcoin-ng: A scalable blockchain protocol[END_REF] has a Byzantine flavor and has similarities with Mobile Byzantine Adversaries studied in classical distributed systems. Intuitively, a mobile byzantine adversary can move agents from a process to another in order to deviate the process computation. When a process is infected by an adversarial agent, it behaves arbitrarily until the adversary decides to "move" the agent to another process. Most of the literature on Mobile Byzantine Adversaries [START_REF] Blin | A new self-stabilizing minimum spanning tree construction with loop-free property[END_REF][START_REF] Daian | Snow white: Provably secure proofs of stake[END_REF][START_REF] Hess | Understanding knowledge as a commons[END_REF][START_REF] Zakhary | Atomic commitment across blockchains[END_REF]81,83,[START_REF] Cholvi | Brief announcement: Implementing byzantine tolerant distributed ledger objects[END_REF] considered so far synchronous round-based models and only between two consecutive rounds, Byzantine agents are allowed to move from one process to another. Hence the set of faulty processes at any given time has a bounded size, yet its membership may evolve from one round to the next. It is obvious that adversaries described so far by the classical distributed literature do not match the Micali's description of dynamic adversary in blockchains. A challenge would be to explore Mobile Byzantine Adversaries decoupled from the synchronous communication of the system. However, this line of research still does not cover the dynamic adversary in blockchains and further research is needed in this direction.

Therefore, the main challenge will be the formal specification of the robustness of blockchains face to dynamic adversaries.

Robustness to rationality and irrationality

Common resources in blockchain systems can be seen at different levels. Participants gain a financial benefit from generating blocks. However, they bring to the system their energy. Moreover, the system itself uses participants as resources since functionalities of the system such as routing, overlay maintenance, mining or agreement, are totally dependent on the presence of the participants. The risk in these systems, as the one advertised recently for Bitcoin, is the fact that participants will leave the system and hence the system collapses. This phenomenon is known in economy as the tragedy of commons. Commons have similarities to the fair resource sharing in P2P networks where peers express rational behaviors. Each peer in a resource sharing system gains a certain benefit from using the system and pays a certain cost participating to it. The incentives solutions proposed so far in P2P networks (e.g. [START_REF] Attiya | Distributed Computing: Fundamentals, Simulations and Advanced Topics[END_REF]) are most of the time evaluated in an empirical model without no formalization. Also these solutions are not designed to cope with dynamic adversaries.

In order to avoid the tragedy of commons phenomenon in blockchains, new solutions have to be designed by combining self-organized micro-economies theories (in particular the work of Nobel Prize Elinor Ostrom) with on-the-shelf tools issued from mechanisms and game theories.

A first step would be to understand the effect of various behaviors on blockchain systems. From the game perspective point of view rationality in blockchains has been studies in [START_REF] Bonomi | Stabilizing server-based storage in byzantine asynchronous message-passing systems: Extended abstract[END_REF] (for the case of Bitcoin protocol) or [82] (for the case of proof-of-stake protocols). Recently, in [START_REF] Anta | Formalizing and implementing distributed ledger objects[END_REF] the authors explore the robustness of Tendermint consensus core to rational and Byzantine behaviors. They analyze equilibrium interactions between Byzantine and rational committee members and derive conditions under which consensus properties are satisfied or not in equilibrium. However, the proposed framework is not general enough to be applied to other blockchain building blocks.

The challenge here will be to define a unified framework for specifying rational and irrational behaviors all together with mobility of faults and attacks and propose incentive rules tolerant to these behaviors.

One possible solution is first to extend first model proposed in [START_REF] Anta | Atomic appends: Selling cars and coordinating armies with multiple distributed ledgers[END_REF] to the specificities of blockchain systems. In [START_REF] Anta | Atomic appends: Selling cars and coordinating armies with multiple distributed ledgers[END_REF] the authors define a Byzantine Altruistic Rational Tolerant (BART) the protocol that guarantees the specified set of safety and liveness properties in the presence of all rational deviations A protocol is said Incentive-Compatible Byzantine Fault Tolerant (IC-BFT) if any rational user is incentivized to follow the prescribed protocol, also in presence of byzantine users. Then, to make practical the model proposed in [START_REF] Anceaume | Incentives for P2P fair resource sharing[END_REF] by relaxing the requirements in terms of t-immunity. Then, propose combined rules resulted from various theories (games theory, mechanisms theory) that will be encoded in incentive rules.

Formal abstractions for blockchains consistency

A large number of political, economical and social organisms invoke the possibility of blockchainize their activity. Obviously, the data that will be stored on the blockchain in each of these applications may have various levels of coherency: starting with very strong coherency for the case of banking or notary applications and finishing with weak coherency for applications such as IoTs. Identifying the exact requirements of coherency for representative applications in each class will be core of the current workpackage.

Studying the level of coherency provided by existing blockchains is related to the distributed shared register area. However, the similarity is moderated. A distributed register is a shared variable accessed by a set of processes through two operations, namely write() and read(). Informally, the write() operation updates the value stored in the shared variable while the read() obtains the value contained in the shared variable. The classical registers definitions [START_REF] Ostrovsky | How to withstand mobile virus attacks (extended abstract)[END_REF] have been extended to the self-stabilizing area in [START_REF] Buchman | The latest gossip on bft consensus[END_REF]. This work considers that the system can be hit by arbitrary errors.

It should be noted that none of the above mentioned classical definitions captures the behavior of the popular blockchains such as Ethereum and Bitcoin. That is, values written in a classical register are potentially independent, and during the execution, the size of the register remains the same. In contrast, a new block cannot be written in the blockchain if it does not depend on the previous one, and successive writings in the blockchain increase its size. Also, differently from stabilizing registers, the prefix of the blockchain eventually converges, while no guarantees hold for the last created blocks.

The challenge here is to define new consistency abstractions that will capture the semantics of blockchains.

Defining new consistency abstractions for blockchains

The first effort in specifying the properties of permissionless blockchain systems is due to Garay and Kiayias [START_REF] Kokoris-Kogias | Enhancing Bitcoin Security and Performance with Strong Consistency via Collective Signing[END_REF]. They characterized Bitcoin blockchain via its quality and its common prefix properties, i.e., they define an invariant that this protocol has to satisfy in order to verify with high probability an eventually consistent prefix. This line of work has been continued by [78]. In order to model the behavior of distributed ledgers at runtime, Girault et al. [START_REF] Lamport | On inter-process communications, part I: basic formalism and part II: algorithms[END_REF] present an implementation of the Monotonic Prefix Consistency (MPC) criterion and showed that no criterion stronger than MPC can be implemented in a partition-prone message-passing system. On the other hand, the proposed formalization does not propose weaker consistency semantics more suitable for proof-of-work blockchains as BitCoin. In the same line of research, in [START_REF] Avizienis | Basic concepts and taxonomy of dependable and secure computing[END_REF], Anceaume et al. propose a new data type to formally model distributed ledgers and their behavior at runtime. They provide consistency criteria to capture the correct behavior of current blockchain proposals in a unified framework. In parallel and independently of [START_REF] Avizienis | Basic concepts and taxonomy of dependable and secure computing[END_REF], Anta et al [START_REF] Bentov | The sleepy model of consensus[END_REF] propose a formalization of distributed ledgers modeled as an ordered list of records. The authors propose three consistency criteria: eventual consistency, sequential consistency and linearizability.

Providing an unified framework able to capture the specificity of blockchain systems is still an open problem.

Moreover, formalizing the definition of this class of blockchain consistency will help in further proving the correctness and formally verifying algorithms that implement them. The semantic of the consistency can be express in terms of events and partial orders to these events. Note that for the classical consistency criteria the recent work of Gotsman et al. [START_REF] Lewis | Databases and transaction processing: an application-oriented approach[END_REF] provided a rich formalism based on token systems. However, this formalism should be extended to the blockchain context.

Design and formally prove new consistency algorithms tolerant to complex behaviors

It should be noted that existing effort for implementing coherency in blockchains (e.g. [START_REF] Kokoris-Kogias | Enhancing Bitcoin Security and Performance with Strong Consistency via Collective Signing[END_REF][START_REF] Poon | The bitcoin lightning network[END_REF][START_REF] Bonnet | Tight bound on mobile byzantine agreement[END_REF]) concentrate on solving the agreement (consensus) problem. However, it is already folklore that consensus is impossible to solve deterministically in asynchronous environments [START_REF] Herlihy | Atomic cross-chain swaps[END_REF]. As pointed out in the state of the art section implementing blockchain probabilistic consensus in asynchronous environments subject to dynamic faults is still an open problem. The deterministic implementation of registers (even with strong consistency guarantees) in various models characterized by the presence of multiple types of faults (crashes, byzantine, dynamic byzantine or transient) have been investigated in the past [START_REF] Buhrman | Optimal resiliency against mobile faults[END_REF][START_REF] Buchman | The latest gossip on bft consensus[END_REF][START_REF] Castro | Practical byzantine fault tolerance and proactive recovery[END_REF][START_REF] Crain | Dbft: Efficient byzantine consensus with a weak coordinator and its application to consortium blockchains[END_REF]. In blockchain systems, recent effort has been directed to both formalizing and implementing consistency criteria in systems prone to faults or Byzantine behaviors [START_REF] Avizienis | Basic concepts and taxonomy of dependable and secure computing[END_REF][START_REF] Biais | The blockchain folk theorem[END_REF][START_REF] Eyal | Majority is not enough: Bitcoin mining is vulnerable[END_REF][START_REF] Belotti | Game theoretical analysis of Atomic Cross-Chain Swaps[END_REF]. None of the above proposed solutions work with the severe model of blockchain adversary including rationality , irrationality, churns or partitions. Therefore, the implementation of blockchain objects with various consistency guarantees in a asynchronous environment with dynamic models of adversary when the size of the network is unknown is a real challenge that might be mitigated by combining the framework in [START_REF] Crain | Dbft: Efficient byzantine consensus with a weak coordinator and its application to consortium blockchains[END_REF] with abstractions such as k-quorums defined in [START_REF] Androulaki | Hyperledger fabric: a distributed operating system for permissioned blockchains[END_REF] and sortition techniques or intersecting sets (i.e. the secure version of the classical distributed quorum systems).

Develop correct-by-construction agreement algorithms for blockchains

The core of blockchains technologies is the agreement problem, studied in an environment where participants to the agreement may be controlled by a dynamic adversary. This form of agreement is known in distributed computing as Byzantine Agreement. Briefly stated, it requires that processors, some of which being potentially malicious, start the computation with an initial value and decide on the same value.

Byzantine Agreement, introduced by Lamport et al. [START_REF] Pass | Analysis of the blockchain protocol in asynchronous networks[END_REF], has been studied for decades in static distributed systems under different aspects (e.g., possibility, complexity, cost) in various models (from synchronous to asynchronous, from authenticated to anonymous) with different methodologies (deterministic or probabilistic).

Feasibility of blockchain agreement face to complex faults and behaviors

Garay et al [START_REF] Kokoris-Kogias | Enhancing Bitcoin Security and Performance with Strong Consistency via Collective Signing[END_REF] and [START_REF] Poon | The bitcoin lightning network[END_REF] pioneered the study of Byzantine Agreement in blockchains. However, their studies are restricted to only round-based synchronous systems.

In [START_REF] Bonomi | Self-stabilizing mobile byzantine-tolerant regular register with bounded timestamp[END_REF] the authors study deterministic Byzantine Agreement in environments where the set of nodes controlled by the adversary may change over time. Contrary to other approaches, the model considers that a process previously affected by the adversary may send messages (based on a corrupted state), it will behave correctly in the way it sends those messages: i.e., send messages according to the algorithm. This behavior is very similar to the way the adversary acts in blockchains systems. Interestingly, in order to implement Byzantine Agreement under the assumption of dynamic Byzantine adversary a system needs at least 5f + 1 nodes while in the case of static Byzantine adversary only 3f + 1 are sufficient, where f is the number of nodes controlled by the Byzantine. These studies leaves a huge avenue to be explored. First, there is no extension of [START_REF] Bonomi | Self-stabilizing mobile byzantine-tolerant regular register with bounded timestamp[END_REF] to round-free environments. Second, in the same model of adversary there is no study related to feasibility of the agreement problem when the adversary movement is decoupled from the synchronous round of computation.

The above works do not implement agreement in asynchronous systems prone to dynamic adversary, rationality or churn.

An interesting challenge would be to explore the asynchronous probabilistic Byzantine agreement in systems prone to dynamic adversary and churn and where processes may have rational behaviors. One of possible solutions would be to considered the methodology proposed in [START_REF] Bonomi | Self-stabilizing mobile byzantine-tolerant regular register with bounded timestamp[END_REF] to round free churn exposed systems combined with sortition techniques and incentives rules issues from games and mechanisms theories.

New abstractions for blockchain agreement

Agreement in blockchains has an Approximate agreement flavor since the agreement on blockchains should be guaranteed not on an exact value. In systems hit by Mobile Byzantine Adversaries (the closes to the blockchain dynamic adversary) [39] formalized the approximate agreement and prove lower bounds on problem solvability in various dynamic adversary models and further propose an optimal algorithm for approximate agreement in round based systems. The lower bounds range from n > 3f + 1 to n > 6f + 1 depending on the type of adversary.

The previous results do not cover the blockchain agreement for several reasons: blockchains are not round-based, the adversary is not bounded to the rounds change, the agreement value is not a real value but a prefix of an ever changing blockchain.

Formalizing the bockchain approximate agreement and then proposing solutions in asynchronous environments hit by a dynamic adversary and rationality is the scientific lock here.

5 Develop correct-by-construction overlays and routing algorithms for blockchains

Blockchain underlying overlays and the associate routing are totally unexplored from theoretical point of view. However, the performances of blockchains technologies heavily depend on the performances of the underlying routing process.

Recently, Lightning technologies imposed themselves as a viable direction for improving the blockchains throughput. This technology builds on top of blockchains (e.g. Bitcoin) an overlay of secured channels opened by two parties involved in long term multi-transactions. This overlay is further used to route transactions. Although blockchain technologies make strong assumptions on their underlying overlays there is no academic research that focus these overlays. The only prior research on the overlays topic has been developed in the context of dynamic networks such as P2P or wireless networks.

Another interesting point to be explored is the liveness of the overlay and more generally of the system. In blockchains the welfare of participants is a crucial factor. When participants desert the system in proof-of-work based blockchains the security of the system sinks which yields to the global sink of the system. This phenomenon is known in economy as the tragedy of commons.

New abstractions for blockchain overlays

Expenders theory proved recently its effectiveness for constructing overlays resilient to churn and partitions. The (node) expansion of an undirected graph is a characterization of the graph robustness. That is, graphs with good expansion are hard to be partitioned into a number of large connected components. In this sense, the expansion of a graph can be seen as a good evaluation of its resilience to faults and churn. However, the expansion of tree overlays is trivially O(1/n). This weakness to faults explains why tree overlays are not pervasive in real applications.

In [START_REF] Ostrom | Governing the commons[END_REF] the authors measured the robustness of tree overlay networks by evaluating their graph expansion and proposed a logarithmic algorithm for the construction of a constant degree self* expander that improve the resilience to churn of P2P tree-overlays.

The existing works are not tolerant to dynamic adversaries which can disconnect the overlay before its stabilization. The unexplored yet research direction concerns the construction of constant degree expenders tolerant to dynamic Byzantine behavior and multiple types of faults. A possible solution would be to extend the methodology in [START_REF] Ostrom | Governing the commons[END_REF] with sortition techniques.

New formally verified routing protocols for blockchains

In order to increase the throughput in Bitcoin, the non academical research in blockchains proposed recently lightning routing networks [79]. Secured channels between two or more participants are opened on top of Bitcoin and transactions are routed on top of the virtual network formed by these channels. Routing in lightning networks has some similarities with routing in P2P or mobile wireless networks or delay tolerant networks. Flare, [?], for example the most prominent lightning routing was inspired by the wireless ZRP routing.

Interestingly, there is no formal academic research on this topic so far and our preliminary studies show that Flare (and its derivates) present severe limitations such as weak resilience to churn or deadlocks. Moreover, none of these lightning routing protocols has been exposed to multiple types of faults, attacks or dynamic adversaries.

The most studied overlay for routing in classical distributed systems and networking theory is the minimum spanning tree (MST). Research on spanning trees tolerant to multiple faults has been conducted in [START_REF] Bonomi | Stabilizing byzantine-fault tolerant storage[END_REF][START_REF] Bonomi | Optimal mobile byzantine fault tolerant distributed storage: Extended abstract[END_REF][START_REF] Bonomi | Tight self-stabilizing mobile byzantine-tolerant atomic register[END_REF][START_REF] Bonomi | Approximate agreement under mobile byzantine faults[END_REF]. None of the above cited algorithms is resilient to dynamic adversaries in conjunction with churn and attacks. The challenge here will be the design of new routing algorithms optimized for the context of lightning networks subject to multiple types of faults, attacks, rationality and dynamic adversaries.

Blockchains Interoperability

There are currently several operational systems for achieving interoperability between different blockchains such as Cosmos [START_REF]Decred cross-chain atomic swapping[END_REF] or Polkadot [START_REF] Aiyer | Byzantine and multi-writer k-quorums[END_REF]. These systems can be classified into two categories according to their decentralization level: systems that use a trusted third-party to validate transactions or systems that realize it directly between blockchains without the need of a trusted third-party.

In order to execute an exchange or a swap (i.e., a set of transactions between parties), transacting agents (i.e., blockchain users) are provided with a protocol to stick to. A protocol in this case consists of a specific sequence of instructions agents should perform to preserve the ACID properties [START_REF] Pass | Fruitchains: A fair blockchain[END_REF] of the individual transactions or exchanges.

The first atomic swap solution has been proposed for Bitcoin by Nolan [73] making use of hash-time locked contracts enabling conditional assets transfers. Decred [4] implements Nolan's algorithm on UTXO-based premissionless. In [START_REF] Nakamoto | Bitcoin: A Peer-to-Peer Electronic Cash System[END_REF] the authors generalize and prove correct Nolan's scheme. Other projects such as BartherDEX [START_REF] Abraham | Distributed computing meets game theory: Combining insights from two fields[END_REF], part of the Komodo project [START_REF]Polkadot: Vision for a Heterogeneous Multi-Chain Framework[END_REF], represents a cross-chain solution that matches orders and defines the swap protocol or Blockchain.io [START_REF]Cosmos: A Network of Distributed Ledgers[END_REF] implements atomic cross-chain swaps by combining centralized components (order matching) with decentralized ones (trade settlement and execution). These projects are not yet formally proved correct.

The academic research focuses on hybrid swap protocols, replacing decentralized commitment/locking schemes (hash-locks) with centralized ones, resulting more attractive and efficient. AC3TW and AC3WN [86] protocols propose atomic cross-chain swaps respectively with centralized and distributed trusted authorities (i.e. witnesses) It should be noted that different swap protocols differ essentially in the involved parties. The set of swap participants can be composed only of the asset owners (e.g., as in [START_REF] Nakamoto | Bitcoin: A Peer-to-Peer Electronic Cash System[END_REF]) or by owners accompanied by a trusted third party (e.g., as in the AC3TW protocol [86]).

In [START_REF] Blin | Self-stabilizing minimum degree spanning tree within one from the optimal degree[END_REF] the authors propose a generic game theoretical framework that formalizes the swap problem and characterize equilibria of two representative recent protocols presented in [START_REF] Saleh | Blockchain Without Waste: Proof-of-Stake[END_REF] and [86] respectively. In the case of the protocol proposed in [START_REF] Saleh | Blockchain Without Waste: Proof-of-Stake[END_REF] and generalised in [START_REF] Nakamoto | Bitcoin: A Peer-to-Peer Electronic Cash System[END_REF], following the protocol is the unique subgame perfect equilibrium (in dominant strategies), while in the case of the protocol proposed in [86], following the protocol is a Nash equilibrium.

This work opens several research directions. Swap protocols and more generally blockchain intercommunication are not yet properly formalized and analyzed.

An important challenge in this area is to fully formalize the problem and analyze the robustness of protocols that implement it to both rational and irrational behaviors, dynamic adversaries and attacks and coalitions.

Conclusions

Blockchains evolve in a very complex environment. Differently, from the classical distributed systems where faults are considered to appear in isolation and to affect the same node of the system during the whole computation, in blockchains environments faults do not follow the same pattern. Blockchains have to face in the same time classical pattens of faults such as crash faults, transient faults, Byzantine faults but also attacks, dynamic faults, churn and selfish or rational/irrational behaviors. Therefore, before addressing the algorithmic core of blockchains a fully characterization of the adversarial environment is necessary. Interestingly, faults and errors in most of the cases (even in classical distributed system) have only a verbose definition. When systems have to be resealed for an industrial or critical economical use automatic verifications and mathematical proofs are necessary. Therefore, verbose definitions are not precise enough. In this paper we discuss five important challenges in this area. The first challenge will be to explore and formalize blockchains robustness. The second challenge will be to formally define universal abstractions for characterizing blockchains consistency. The third challenge will be to provide new correct-by-construction abstractions for agreement in blockchains. The effectiveness of these building blocks will be insured by a formal verification and proof using formal methods tools. The fourth challenge is to develop optimized overlays and communication primitives for blockchains resilient to nodes churn, various attacks and adversary dynamic behaviors and target to avoid the partition or the sink of the system in a tragedy of commons. Finally, the formalization of blockchains interoperability is the fifth challenge.


H Alout
email: alout@isem.univ-montp2.fr

M Weill

Amino-acid substitutions in acetylcholinesterase 1 involved in insecticide resistance in mosquitoes

Keywords:

In natural populations of mosquitoes, high level of resistance to carbamates (CX) and organophosphates (OP) is provided by insensitive acetylcholinesterase (AChE1). Different alleles conferring resistance have been identified at the ace1 locus. They differ from the wildtype by only one amino-acid substitution. The comparison of the biochemical characteristics of mutated recombinant proteins and AChE1 in resistant mosquito extracts confirmed the role of each substitution in insensitivity. Selection of these different resistant alleles in field populations depends likely on the insecticides used locally. Theoretical modelling studies are initiated to develop novel strategies of mosquito control.

Introduction

Mosquitoes are among the most important vectors of human disease. The use of insecticide to control them has led to selection of resistant populations and hence to control program failures. In many arthropods, selection of insensitive insecticide target is a common resistance mechanism. This is the case of acetylcholinesterase (AChE, EC 3.1.1.7), the target of OP and CX insecticides. In Diptera, two AChE genes are usually present (ace-1 and ace-2), and ace1 codes the synaptic enzyme [START_REF] Weill | A novel acetylcholinesterase gene in mosquitoes codes for the insecticide target and is non-homologous to the ace gene in Drosophila[END_REF]. In a few Diptera, such as true flies, only the ace-2 gene is present and codes the synaptic AChE [START_REF] Huchard | Acetylcholinesterase genes within the Diptera: takeover and loss in true flies[END_REF]. In mosquitoes, a single aminoacid change, located near the active site of the synaptic AChE1, is sufficient to provide a high resistance level. To date, only three single amino-acid substitutions have been described: G119S in Culex pipiens, C. vishnui, Anopheles gambiae and A. albimanus; F290V in C. pipiens and F331W in C. tritaeniorhynchus (according to Torpedo californica AChE nomenclature), suggesting a very high structural constraint of the AChE1 enzyme [START_REF] Weill | Insecticide resistance in mosquito vector[END_REF][START_REF] Alout | A new amino-acid substitution in acetylcholinesterase 1 confers insecticide resistance to Culex pipiens mosquitoes from Cyprus Island[END_REF][START_REF] Nabeshima | An amino acid substitution attributable to insecticide-resistance in a Japanese encephalitis vector mosquito, Culex tritaeniorhynchus[END_REF]. These substitutions have been shown to provide insensitivity by means of in vitro experiments with mutated recombinant enzymes [START_REF] Weill | Insecticide resistance in mosquito vector[END_REF][START_REF] Alout | A new amino-acid substitution in acetylcholinesterase 1 confers insecticide resistance to Culex pipiens mosquitoes from Cyprus Island[END_REF][START_REF] Alout | Different amino-acid substitutions confer insecticide resistance through acetylcholinesterase 1 insensitivity in Culex vishnui and Culex tritaeniorhynchus (Diptera: Culicidae) mosquitoes from China[END_REF][START_REF] Benting | Biochemical evidence that an S431F mutation in acetylcholinesterase-1 of Aphis gossypii mediates resistance to primicarb and omeothoate[END_REF]. Our aim is to understand how resistance-associated mutations are selected in natural populations in order to delay its occurrence and its spread. The present study focuses on characterization of mutated recombinant AChE1 proteins and their biochemical properties. Preliminary data are presented.

Materials and methods

Mosquito samples

Two C. pipiens reference strains were used: the susceptible one SLAB [START_REF] Georghiou | Carbamates-resistance in mosquitoes; selection of Culex pipiens fatiguans Wied (=Culex quinquefasciatus) for resistance to Baygon[END_REF] and the resistant homozygous G119S SR [START_REF] Berticat | Insecticide resistance genes induce a mating competition cost in Culex pipiens mosquitoes[END_REF]. ACE-R strain homozygous for the F290V mutation was established from the Mitsero population collected in Cyprus in 1987 [START_REF] Wirth | Organophosphate resistance in Culex pipiens from Cyprus[END_REF]. C. tritaeniorhynchus larvae harboring the F331W AChE1 were sampled in China in 2003 [START_REF] Alout | Different amino-acid substitutions confer insecticide resistance through acetylcholinesterase 1 insensitivity in Culex vishnui and Culex tritaeniorhynchus (Diptera: Culicidae) mosquitoes from China[END_REF].

Site-directed mutagenesis

Expression vectors pAc5.1/V5-His (Invitrogen) containing C. pipiens WT and G119S AChE1 complete coding sequences were already described [START_REF] Weill | Insecticide resistance in mosquito vector[END_REF]. Mutations were introduced into the WT vector by means of a PCR-based strategy [START_REF] Alout | A new amino-acid substitution in acetylcholinesterase 1 confers insecticide resistance to Culex pipiens mosquitoes from Cyprus Island[END_REF][START_REF] Alout | Different amino-acid substitutions confer insecticide resistance through acetylcholinesterase 1 insensitivity in Culex vishnui and Culex tritaeniorhynchus (Diptera: Culicidae) mosquitoes from China[END_REF].

Production of wild-type and mutated AChE1 in Drosophila S2 cells

S2 cells (20 × 10 6 ) were transfected using Fugene6 (Roche), in OptiMEM medium, according to the manufacturer protocol. Cells were maintained in serum-free Schneider's medium to prevent AChE activity from foetal bovine serum. Four days after transfection, cells were collected by centrifugation at 250 × g for 3 min and homogenised in 500 l phosphate buffer (0.25 M, pH 7.0) containing 1% Triton X-100. The homogenate was centrifuged for 10 min at 9000 × g and the supernatant was used as source of enzyme, after dilution in phosphate buffer.

AChE1 inhibition characteristics

The relative AChE1 activity was determined spectrophotometrically at room temperature, as described by Ellman et al. [START_REF] Ellman | A new and rapid colorimetric determination of acetylcholinesterase activity[END_REF], using 100 l of supernatant containing the recombinant AChE1 WT, G119S, F331W or F290V. Inhibition curves were performed by incubating samples (100 l) for 15 min with 10 l of inhibitor solutions at various concentrations. We then added 100 l of 1.6 mM substrate and residual AChE1 activity was estimated by measuring changes in optical density at 412 nm for 15 min. We analysed three to five replicates for each assay. Replicates were performed with distinct batches of production.

The irreversible inhibition reaction is pseudo-first order and the residual activity follows the equation [E]/[E 0 ] = e -k i t[I 0 ] , when inhibitors are in excess compared to enzyme. k i is the bimolecular rate constant, t represents time of incubation and [I 0 ] is the initial inhibitor concentration. Insensitivity ratios were calculated by dividing the k i of wild-type (WT) recombinant AChE1 by the k i of mutated recombinant AChE1.

Three-dimensional modelling

Three-dimensional structures of AChE1 were created by automated homology modelling using T. californica and D. melanogaster AChE as previously described [START_REF] Weill | The unique mutation in ace-1 giving high insecticide resistance is easily detectable in mosquito vectors[END_REF]. RMS deviation is 1.1 Å on 528 carbon atoms.

Results and discussion

Different insensitive AChE1 were characterized on individual mosquitoes from different field samples using the biochemical TPP test [START_REF] Bourguet | Determination of Ace.1 genotypes in single mosquitoes: toward an ecumenical biochemical test[END_REF]. The cDNA of ace1 were then sequenced from homozygous resistant individuals and compared with susceptible ones. Once identified, the mutations were introduced into an expression vector containing the wild-type ace1 cDNA to produce the mutated recombinant enzymes in Drosophila S2 cells. Then, sensitivity to 10 insecticides was investigated to compare the recombinant AChE1s with the enzyme extracted from resistant mosquito heads (Fig. 1). The superimposition of inhibition patterns of recombinant and mosquito extracted enzymes for all insecticides (data not shown) confirmed the role of each AChE1 mutation in resistance. The different mutated AChE1 displayed a strong insensitivity to specific insecticides (Table 1), and there was a good relationship between the highest insensitivities (resistance ratios) and the insecticide(s) used locally in control programs. Thus, F290V allele was observed in resistant C. pipiens sampled in Cyprus Island where the main insecticide used is dichlorvos. F331W is the resistant allele found in C. tritae- niorhynchus from China where the species is controlled by the use of fenitrothion and dichlorvos. G119S resistant allele was found in C. pipiens from France, where chlorpyrifos has been the only insecticide used to control this species until the late 1990s. Using a structural model of C. pipiens AChE1 created by automated homology, the mutated positions were located. The 119 position is close to the catalytic Serine (S200) and the G-to-S substitution would reduce accessibility to inhibitors and substrate by steric hindrance (Fig. 2b). F331 is implicated in substrate guidance, as a component of the choline binding site (Fig. 2c). Substitution affecting this position would alter stabilization of intermediates. Also due to its bigger size, tryptophane residue might reduce accessibility for other ligands [START_REF] Ordentlich | Dissection of the human acetylcholinesterase active center determinants of substrate specificity. Identification of residues constituting the anionic site, the hydrophobic site and the acyl pocket[END_REF]. The F290 (Fig. 2d) belongs to the acyl binding pocket and is involved in substrate specificity. Although this substitution involved a residue smaller than the WT, it might hamper the gorge entrance because of a loss of stabilization process [START_REF] Harel | Conversion of acetylcholinesterase to butyrylcholinesterase -modeling and mutagenesis[END_REF]. These residues are important features of cholinesterases, particularly of acetylcholinesterase which has been naturally selected for its high catalytic activity. Therefore, these mutations in AChE may affect normal cholinergic function leading to a selective disadvantage as demonstrated for the G119S AChE1 in C. pipiens [START_REF] Raymond | Insecticide resistance in the mosquito Culex pipiens: what have we learned about adaptation?[END_REF]. Further work includes the determination of specific activity and enzymatic stability to estimate relative selective value, and crystallization for structural studies.

Fig. 1 .

 1 Fig. 1. Comparison of residual AChE1 activities of recombinant enzymes (black symbols) and mosquito head extracts (empty symbols) in presence of increasing doses of insecticides. For each mutated AChE1, only one insecticide is represented. Residual activity of susceptible head extract and WT recombinant AChE1 are included as control (squares).

Fig. 2 .

 2 Fig. 2. 3D view point of the entrance of the catalytic gorge of AChE1. The backbone of the enzyme structure is rendered as ribbon with secondary structure. (a) The catalytic triad is represented as black sticks. G119, F290, F331 are represented as light grey van der Waals spheres. Amino-acid substitutions are represented in dark grey: (b) the G119S, (c) the F290V and (d) the F331W.

Table 1

 1 Bimolecular velocity constant (k i ) and resistance ratio to organophosphates and carbamate insecticides observed in wild-type (WT) and mutated AChE1

	Pesticide class	Insecticide	k i (1/mol/s)				Resistance ratio (WT/mutant)
			WT	G119S	F290V	F331W	G119S	F290V	F331W
	Carbamates	Aldicarb	167	50	7	N/A	3	25	-
		Propoxur	2895.4	0.029	58	49.4	99624	50	59
		Eserine	48135.2	948.5	8852	1006.7	51	5	48
	Organophosphorous	Dichlorvos	1711.5	373.9	25	11.2	5	68	153
		Malaoxon	3209.01	43.03	372	60.6	75	9	53
		Paraoxon-ethyl	4557.2	37.9	917	381.3	120	5	12
		Paraoxon-methyl	6261.5	26.5	N/A	217.6	237	-	29
		Fenitroxon	17114.7	252.5	181	16.2	68	95	1053
		Chlorpyrifos-oxon	54236.9	89.9	14531	3500.7	604	4	15

Acknowledgements

This work was financed in part by the ANR Programme SEST "Morevol" (Ministère délégué à la Recherche). Contribution 2007-032 of the Institut des Sciences de l'Evolution de Montpellier (UMR CNRS 5554).


V Blanfort

S Fontaine

C Picon Cochard

K Klumpp

J Huguenin

J F Soussana

Evaluation of carbon sequestration in tropical grassland, ecological processes and farm practices in Amazonian cattle systems

Introduction

The humid tropics have seen a considerable expansion in ruminant farming since the 1970s (at present 25% of the world's ruminant stock). This situation is often criticized for its negative environmental impacts: deforestation, loss of biodiversity, greenhouse gas (GHG) production (carbon dioxide, methane, nitrous oxide). The increase of direct emissions from the agriculture sector (17%) mostly occur in emerging countries like Brazil (IPCC, 2007). Although ruminant production systems are viewed as a major source of GHG emissions (18% of global emissions according to FAO soil carbon (C) sequestration (SOC) the world's permanent pastures could potentially offset up to 4% of the global GHG emissions [START_REF] Soussana | Mitigating the greenhouse gas balance of ruminant production systems through carbon sequestration in grasslands[END_REF]Steinfeld et al., 2010). This research, started in 2010, aims at identifying the carbon dynamics in pastures and define the management practices likely to compensate a part of the C losses resulting from their establishment (which was detrimental to the forest, Figure 1).

Context and Hypothesis

Livestock production produces 9% of global anthropogenic CO 2 emissions, the largest share (i.e. 7%) of this derives from land-use changesespecially deforestation (25% of CO 2 global emissions) -caused mainly by expansion of pastures and forage crops. In Amazonia, more than 50% of these pastures are abandoned and replaced by other grasslands established on newly deforested areas. Therefore it is important to know the fate of carbon in the soil after the forest has been transformed into grassland and to understand how these pastures can continue storing carbon in the soil (Figure 1) as well as keeping their fertility level.

Methods and Results

Carbon sequestration can be determined directly by measuring changes in C stocks (most of the Amazonian studies) but SOC stocks displays a high spatial variability which limits the accuracy of direct determinations of C stock changes. An alternative is to measure the net balance of C fluxes exchanged at the system boundaries (eddy flux covariance). There has not yet been any direct comparison at the same site between C flux and C stock change measurements. The originality of this research consists in associating the two approaches and to compare C balance -E-mail: blanfort@cirad.fr Advances in Animal Biosciences in deforested grassland to the original forest where similar measurements are in progress. While the Amazonian forest has considerable potential for trapping carbon-up to 300 t per hectare in French Guiana-the estimated quantity of carbon stored in the soil of grassland is very high. Carbon accumulation in grassland ecosystems (SOC) occurs mostly below ground due to active rhizodeposition, and the residence time in organic matter depends on management practices [START_REF] Fontaine | Stability of organic carbon in deep soil layers controlled by fresh carbon supply[END_REF]. In Amazonia, the reconstitution of C in the soil after deforestation could reach more than 100 t/ha after decades according to studies based on C stock changes under pasture (Figure 1). The sink activity of temperate grassland can reach 1 t of carbon per hectare per year [START_REF] Soussana | Mitigating the greenhouse gas balance of ruminant production systems through carbon sequestration in grasslands[END_REF] and need to be validated in tropical conditions.

Conclusions

Current references on GHG emissions and mitigation by soil carbon (C) sequestration need to be completed to identify indicators and management practices able to strengthen the stability of deforested areas in humid tropics while attenuating their negative environmental impacts by more effectively providing certain ecosystemic services. These results could be adapted to tropical conditions at farm level to estimate the farm GHG balance.

Introduction

Every year Haiti imports more than 40 million Euros worth of milk products. Milk is no less than the second category of food imports into the country. To deal with this situation, Haitian producers organize and develop sustainable alternate strategies with support from young professionals. ''Le `t Agogo'' has been developed by the NGO Veterimed (Veterimed, 2007(Veterimed, , 2010)).

Veterimed is a Haitian NGO that works to support development in Haiti. It was created in 1991 by a group of Haitian professionals who wanted to contribute to national development in the rural sector. Some 700000 families of small farmers ensure around 90% of the livestock production in Haiti (Veterimed, 2010). For the most part, these producers did not receive any technical support. Veterimed's mission is to help small Haitian farmers increase their income and improve their quality of life through training, research and technical support in the areas of animal health and production. The focus consists in step-by-step improvements to traditional production issues in order to increase profitability and sustainability, and thus contribute to significant improvements in farmers' incomes. Their objective is to work with farmers to ensure sustainable alternatives in livestock development. Veterimed supports small, family farming because it creates and maintains jobs in the countryside, is more environmentally-sound and socially more equitable. Local vegetal and animal resources are used.

For about 10 years, ''Le `t agogo'' (milk in abundance) project has been contributing to decrease milk importation, developing local milk production based on network of small farms. The ''Le `t agogo'' project works to increase milk production in order to increase farmer's family income, as well as commercialization opportunities. Today ''Le `t agogo'' is also the brand name for the milk products (yogurt and sterilized milk) produced by a network of micro transformation units and distributed across the country with support from many youth and rural organizations.

-E-mail: mushe78@gmail.com Advances in Animal Biosciences

Fig. 1

 1 Fig. 1 Dynamics of SOC after deforestation and under grassland in Amazonia (hypothesis 1 or 2 depending on pasture management practices)

Acknowledgements

These researches are co-funded by Europe FEDER (Fonds europe ´en de de ´veloppement regional).


Hélène Hayes
email: helene.hayes@jouy.inra.fr

Didier Boichard

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

E D I T O R I A L Open Access

Genetics Selection Evolution: news for the period January 2009 -May 2010 and editorial announcements Helene Hayes * , Didier Boichard

It is now nearly a year and a half since the journal Genetics Selection Evolution (GSE) is published as an open access journal and it is time for an update on its activity and to share some changes on the editorial board.

Comparing statistics between 2008 and 2009 provides some initial indication on the consequences of the "Open Access" conversion.

The total number of manuscripts submitted in 2008/ 2009 was similar i.e. 98/93. Except for manuscripts originating from China (22 in 2008China (22 in /12 in 2009)), there was little variation between 2008 and 2009: origin and number of manuscripts were quite stable with a large range of origins concentrating over Norway (8 in 2008Norway (8 in /10 in 2009)), USA (8/9), the Netherlands(7/10), France (7/8), Spain (6/5), Australia (3/6), UK (3/4), Germany (2/4) and Italy (1/4). Thus, in 2009 introduction of an author pay charge (APC) had little effect.

The total number of manuscripts published was higher in 2009 (54) than in 2008 (40), which is partly due to the success of the call for sponsored papers in spring 2008 to promote new Open Access GSE i.e. eight sponsored papers were published. The increase in number of published papers is made possible by the fact that with the Open Access online model, there is no restriction in page number as before with the paper version.

Over 2008 and 2009, the number of submissions per field was as follows: methodological and quantitative genetics (25 in 2008/37 in 2009), applied and experimental genetics (25/29), population genetics and genetic diversity (24/12), molecular genetics (16/7) and others (8/8). Thus, about three quarters of the submitted manuscripts concerned in equal proportions methodological, applied and population genetics in 2008 while in 2009, we observed a clear increase in manuscripts dealing with methodological and quantitative genetics and a clear decrease in manuscripts on genetic diversity.

Clearly, GSE is established as a journal specialized on the quantitative and applied aspects of genetics and selection in farm animals.

GSE works with many peer reviewers all experts in their own field. Thorough peer reviewing is essential to guaranty the quality and the relevance of the papers published in GSE. We are well aware that reviewing manuscripts is a heavy and time-consuming task and we are grateful to all the referees for supporting GSE in this way.

We have decided to go further and to offer referees, sending back their review on time, a 20% discount on author pay charge if they publish an article in Genetics Selection Evolution. This offer will hold for a year after submission of the review.

Another change is the possibility for referees to upload an attached file with their evaluation report, which is useful in cases where special symbols and mathematical equations are necessary.

Over 2009 and 2010, several changes have taken place on the editorial board:

-(1) three associate editors André Eggen (Jouy-en-Josas, France), Catherine Montchamp-Moreau (Gif-sur-Yvette, France) and Miguel Perez-Enciso (Bellatera, Spain) have retired from the editorial board and we thank them warmly for their active collaboration with the journal during many years -(2) seven new associate editors have joined us bringing their expertise in their respective fields of interest:

▪ Paolo Ajmone-Marsan (Piacenza, Italy): animal molecular genetics applied to the investigation of genetic diversity and to the identification of genes and QTL controlling economic traits Last but not least, a new executive co-editor for GSE! After four and a half years as scientific and now executive editor of GSE, Philippe Baret will step down from this position to devote more time to his research and academic activities. Over the period, he has been actively involved in all the editorial decisions of GSE and especially in the successful adventure of moving GSE to Open Access. It has been a great pleasure to work with Philippe and we thank him for contributing his time and effort to the good management of GSE and for bringing GSE to where it is today.

We are pleased to announce that Jack Dekkers from Iowa State University (USA) has agreed to join the editorial board of GSE as our new executive editor. Jack Dekkers is professor of animal breeding and genetics and is internationally recognized for his work on quantitative genetics and animal breeding applied to dairy cattle, swine and poultry.

In his own words: "I am excited to join the GSE team because I agree with the philosophy and focus of GSE and hope I can do my part to ensure its continued success". Jack Dekkers will take up his position with GSE from mid-May 2010. We welcome him to the journal and look forward to working with him.


Dr Vinsensia Ade Sugiawati

Prof. P Florence Vacandio

Philippe Knauth

Thierry Djenizian
email: thierry.djenizian@mines-stetienne.fr

Knauth

Prof T Djenizian

Sputter-Deposited Amorphous LiCuPO 4 Thin Film as Cathode Material for Li-ion Microbatteries

Keywords: amorphous materials, LiCuPO 4, Li-ion microbatteries, radio frequency sputtering, thin films

We report the electrochemical performance of amorphous LiCuPO 4 thin films obtained by radio frequency sputtering as a cathode material for Li-ion microbatteries. The thin films were characterized by X-ray diffraction, scanning electron microscopy, profilometry and electrochemical techniques. Charge/ discharge profiles and cycling performance were evaluated in lithium electrochemical test cells. Cyclic voltammogram of the LiCuPO 4 film shows the typical redox reaction peak at ~1.9 V vs. Li/Li + . A discharge capacity of 160 mAh g À1 (50 mAh cm À2 ) is attained for the first cycle at C/10 to reach a stable capacity of 70 mAh g À1 (22 mAh cm À2 ) with good stability over 160 cycles. For comparison, the electrochemical performance of a crystalline LiCuPO 4 film was investigated. The first discharge could deliver a high capacity of around 375 mAh g À1 at C/10, but the capacity decayed quickly to a low capacity of 11 mAh g À1 over 50 cycles. The results show that the LiCuPO 4 amorphous materials can be considered as the exciting cathode candidate for Li-ion microbatteries.

Introduction

In recent years, Li-ion batteries have received great attention for a variety of energy storage devices due to their attractive performance including high specific energy, power density and thermal stability. [1] Much effort has been devoted to improving the electrochemical performance of Li-ion batteries, leading to the discovery of novel cathode materials with many improvements in terms of safety, cycle life, power and energy densities to fulfill the demands for different energy storage applications. In particular, cathode materials based on lithium transition metal phosphates LiMPO 4 (M = Fe, Co, Ni, Mn) have attracted much attention due to their excellent cycling behavior and rate capability. [2] Researchers have reported several strategies to enhance the performance of cathode materials. For instance, Zhang et al. [3] synthesized a mesoporous biocarbon nanowire coated LiFePO 4 with high-energy quantum dots by using a highenergy biomolecule ATP in order to improving the electrochemical performance of Li-ion batteries ; this nanostructured LiFePO 4 displayed a high discharge capacity of 197 mAh g À 1 at the C/10 rate, which is higher than the theoretical specific capacity of LiFePO 4 (170 mAh g À1 ). Also, Nagaraju et al. [4] fabricated thin sheets of reduced graphene oxide (rGO)wrapped LiFePO 4 composite using a co-precipitation method which shows an enhanced performance compared with the bulk LiFePO 4 . More recently, electrodes composed of Cu have been investigated as cathode materials for Li-ion batteries such as Cu 3 (PO 4 ) 2 , [5] Li 3-x Cu x PO 4 , [6] LiCuPO 4 , [START_REF] Amine | Active Material for Lithium Batteries[END_REF] etc. Besides good theoretical gravimetric capacity, low cost, and being environmentally benign, the potentials of Cu 0 /Cu 1 + /Cu 2 + redox couples are compatible with the stability domain of classical electrolytes. [8] However, the performance of thin film cathodes still needs to be improved to realize high performance Li-ion microbatteries. So far, LiCuPO 4 (LCP) material prepared by a solid state reaction method involving a stochiometric mixture of Li 2 CO 3 , CuO, and (NH 4 ) 2 HPO 4 has been reported as a cathode for Li-ion batteries. The LCP cathode has been considered as a promising cathode candidate due to its ability to intercalate more than one Li atom per active transition metal, leading to a high theoretical capacity (> 300 mAh g À1 ). [START_REF] Amine | Active Material for Lithium Batteries[END_REF] Since this has been the only study on LiCuPO 4 composite electrodes reported to date, further investigation on thin film cathodes is needed to verify their performance. In order to obtain high performance thin film cathodes, numerous studies on the fabrication of thin films using a variety of processing methods such as radiofrequency (r.f.) sputtering, [9] chemical vapor deposition, [10] pulsed laser deposition (PLD), [11] and sol-gel [12] have been reported. Among them, r.f. sputtering shows many advantages such as better surface adhesion, smaller grain size, good control of film thickness, film uniformity and low-temperature plasma production. [13] Moreover, it has been widely utilized to fabricate electrode components for all-solid-state Li-ion microbatteries owing to its excellent cyclability with almost no capacity loss. [14] In this work, we report for the first time the remarkable electrochemical properties of an amorphous LCP thin film deposited by radio frequency (r.f.) sputtering. We show that amorphous LCP layers reveal a good capacity over 160 cycles. Interestingly, the performance of the amorphous LCP film is higher than that of crystalline LCP slurries carrying binder and additives.

Results and Discussion

LCP thin films were prepared using r.f. sputtering technique. As can be seen in the Figure S1 (see Supporting Information), a gaseous plasma is generated by applying an electric field inside a vacuum chamber filled with an inert gas such as Argon at a specific pressure. The positively charged ions (Ar + ) are accelerated into the negatively charged LCP target material, leading to the the atoms ejection. Then, the atoms are diffused and deposited as a LCP thin film on the surface of the Ti foil. [15] Furthermore, Thornton developed a structure-zone model to prove the film morphology evolution upon sputtering with different argon pressure and homologous substrate temperature (T/Tm, where T is the substrate temperature, Tm is the melting temperature of the target). In the present experiment, we carried out the deposition process at room temperature with an argon pressure of 10 mTorr, meaning that LCP deposition is in zone 1 structures (T/Tm = ~0.1-0.5) which is amorphous and it is difficult to identify the columnar growth for certain materials, as reported previously. [16] Prior to battery assembly, the structural properties of the different LCP electrodes were studied by XRD (Figure 1a). The initial target and the slurry obtained from a crystalline powder show the same peaks corresponding to well-crystallized LCP phases. According to work conducted by Amine, [START_REF] Amine | Active Material for Lithium Batteries[END_REF] LCP material has orthorhombic symmetry with the following unit cell parameters : a = 5.31 A ˚, b= 13.43 A ˚, and c = 4.91 A ˚. It has been reported that the structure of LCP is different from other olivine materials due to a much larger unit cell compared to the olivine LiMPO 4 (M = Co, Fe, Ni, and Mn). For comparison, after deposition at room temperature, the XRD pattern of the asdeposited LCP thin film has no visible diffraction peak indicating that the film is amorphous. This phenomenon may be due to the deposition being performed at room temperature, which cannot provide enough energy to cause the recrystallization of the sputtered particles. This result is in agreement with previous work, reporting that crystalline phases with a regular atomic arrangement can be obtained by either substrate heating during the deposition process or a post-annealing treatment at high temperature. [9,17] However, such a heating treatment step of the thin films is not compatible with the polymer electrolytes and low-temperature substrate materials. [18] In addition, it will add complexity to the battery preparation process and also cause unwanted side reactions between the deposited film and the electrolyte, limiting the application of Li-ion microbatteries. Hence, lowtemperature processing of the thin film cathodes are favored to fabricate the microbatteries. [18,19] SEM images of the as-deposited LCP thin film obtained after 3 hours on a Si substrate are given in Figure 1b. The morphology observed from the top and cross sectional views (inset in Figure 1b) reveal that the layer is uniform and dense with a thickness of 850 nm measured by profilometry.

The cyclic voltammogram of the amorphous LCP film has been recorded to elucidate the electrochemical reactions undergone during cycling (Figure 2a). The cyclic voltammogram obtained at a scan rate of 0.5 mV/s exhibits a broad reduction peak in the voltage range between 1.7 V and 2.3 V vs. Li/Li + . It can be assigned to the insertion of Li + ions into the LCP structure and the typical peak for amorphous compounds. In addition, a broad oxidation peak marked by an increment of the current density is observed in the voltage range between 1.4 and 3.4 V vs. Li/Li + suggesting that Li + ions also reacted with other phases meaning a different conversion reaction may occur.

Other reduction peaks with high current densities can be also observed at a potential of around 1.2 V vs. Li/Li + , corresponding to Li 3 PO 4 phases formation during the discharge process. [20] The electrochemical conversion reactions of the LCP cathode may be written as the following Equations (1-4): [5,[START_REF] Amine | Active Material for Lithium Batteries[END_REF]21] LiCuPO

4 þ 2Li þ þ 2e À ! Li 3 PO 4 þ Cu Ð LiCuPO 4 þ 2Li þ þ 2e À ð1Þ
LiCuPO 4 þ 2Li þ þ 2e À Ð Li 3 PO 4 þ Cu ð2Þ
Li x CuðIÞ y PO 4 þ Li þ þ e À Ð Li 3 PO 4 þ Cu ð3Þ
Li x CuðIIÞ y PO 4 þ Li þ þ e À Ð Li 3 PO 4 þ Cu ð4Þ
According to the Equation (1), the LiCuPO 4 phase is electrochemically reduced to form a nanocomposite consisting of lithium rich (i. e., Li 3 PO 4 ) and Cu during the discharge process. An irreversible conversion reaction at the first cycle probably takes place as previously reported. [22] However, the reversible reaction occurs after the first cycle which results in smaller particle sizes compared to the initial LCP particles. Due to the fact that the oxidation state of copper in LCP is + 2, it can be deduced that the Cu + may be formed during cycling and the conversion reaction of LCP, Li x Cu(I) y PO 4, and Li x Cu(II) y PO 4 into Cu and Li 3 PO 4 could have occurred as given in Equation (2-4). Simply, the electrochemical conversion reactions of the LCP cathode could be described through the reduction of Cu 2 + to intermediate of Cu + with one Li + insertion and then further reduction of Cu + to metallic Cu with another Li + insertion, and vice versa, totally delivering 2-electrons redox capacity. [23] Figure 1. XRD patterns of the initial target, crystalline LCP film and asdeposited LCP film (a). Corresponding SEM images (b) of as-deposited LCP film from the top view and cross-sectional view (inset in Figure 1b).

The electrochemical behavior was then studied by galvanostatic cycling experiments. The charge/discharge profiles for the amorphous LCP thin film at C/10 can be seen in Figure 2b. The film exhibits one pseudoplateau at around 2.3 V vs. Li/Li + in the 1 st cycle and two pseudoplateaus at around 1.8 V vs. Li/ Li + in the 2 nd and 3 rd cycles, which is in good correlation with the cyclic voltammogram of the amorphous LCP film.

The galvanostatic charge/discharge profiles obtained at C/ 10 of the half-cell battery show a first charge capacity of the amorphous LCP film of 118 mAh g À1 (37 mAh cm À2 ) and a discharge capacity of 160 mAh g À1 (50 mAh cm À2 ) leading to a coulombic efficiency of 74% (the corresponding Li + ions insertion for the charge and discharge capacities are 0.7 mol and 1 mol Li + ion per formula unit, respectively). The discharge capacities for the 2 nd , 3 rd , 30 th , and 50 th cycles are 128 mAh g À1 (50 mAh cm À2 ), 104 mAh g À1 (33 mAh cm À2 ), 70 mAh g À1 (22 mAh cm À2 ) and 70 mAh g À1 (22 mAh cm À2 ), respectively. The coulombic efficiency continuously increases from 76% to almost 100%. Moreover, it can be pointed out that the discharge curves overlapped together very well for the 30 th and 50 th cycles with a good capacity of 70 mAh g À1 (22 mAh cm À2 ), implying a good reversibility and stability of Li + ion insertion and extraction after the first few cycles.

For comparison, we also observed a crystalline LCP film prepared by mixing LCP powder with a binder and conductive agent. A SEM image of the crystalline LCP film is shown in Figure S2 (see Supporting Information). The charge/discharge profiles of amorphous and crystalline films can be seen in Figure 2c. A large discharge plateau observed for the crytalline LCP film at 1.9 V vs. Li/Li + at the first cycle and a charge pseudoplateau at around 2.7 V vs. Li/Li + correspond to the insertion/extraction of Li + ions and also conversion/deconversion reactions of Cu 0 /Cu 1 + /Cu 2 + redox couples. Figure 2d shows the capacity of the crystalline LCP film decreases rapidly up to a very low capacity, delivering a capacity of 11 mAh g À1 . This result is in good agreement with previous work reported by Zhong et al. [5] showing that almost no cycle performance of the as-prepared crystalline cathode could be observed due to the fact that the reversibility of the electrode is very sensitive to the particle size. Interestingly, herein, the amorphous LCP film shows better performance than the crystalline LCP film, preserving a good capacity over 50 cycles.

In light of these results, the electrochemical performance of amorphous LCP film compared to crystalline LCP film is shown in Table 1. When both amorphous LCP film and crystalline LCP film are cycled between 1 and 4.5 V vs. Li/Li + , amorphous LCP film shows improved performance compared to crystalline LCP film. For amorphous film, a discharge capacity of 70 mAh g À1 was obtained after 50 cycles, corresponding to 55% cycling efficiency considering the 1 st cycle reversible capacity, while for the crystalline film, the first discharge delivers a high capacity around 375 mAh g À1 at C/10 with a low cycling efficiency of about 7% and a very high irreversible capacity of 224 mAh g À1 . A high irreversible capacity loss observed at the first cycle may be associated with an inhomogeneous carbon distribution in crystalline LCP film and its large particles, leading to the difficulty in Li + diffusion. The better cycling performance of the amorphous sample may be due to the smaller sputtered particle of discharge products (i. e., Cu and Li 3 PO 4 ) in the amorphous sample, providing better physical contact among them which may facilitate the recharge reaction and improve the electrochemical cycling performance of the LCP cathode. [5] The cycling performance of amorphous LCP film at different kinetics is illustrated in Figure 3. The battery delivers a stable capacity of 70 mAh g À1 (22 mAh cm À2 ) at C/10, 55 mAh g À1 (17 mAh cm À2 ) at C/5, 40 mAh g À1 (13 mAh cm À2 ) at C/2, and 30 mA h g À1 (10 mAh cm À2 ) at 1C beyond 110 cycles. The capacity can be recovered after cycling at fast C-rate with a discharge capacity of 70 mA h g À1 (22 mAh cm À2 ) over 160 cycles, revealing that the reversibility of the conversion reaction of thin films could be improved by sputtering. It is important to notice that reasons for the capacity fading for the initial cycles can be attributed to the loss of the active material. Indeed, dissolution of Cu into an electrolyte is supposed to form an intermediate Cu + compound leading to the increasingly ineffective reconversion process during recharging. [23,24] Moreover, it is likely caused by the initial particle size of the LCP cathode and the insulating character of Li 3 PO 4 which formed after the discharge process. [25] The amorphous LCP film provides a good capacity suggesting that the amorphous component seems to be a potential cathode material for Li-ion microbatteries.

Conclusion

Amorphous LiCuPO 4 thin films were deposited onto Ti foils by r.f. sputtering at room temperature. Charge/discharge cycling results show that the LiCuPO 4 thin film has good electrochemical performance. The cyclic voltammogram of amorphous LCP film exhibits a broad reduction peak between 1.7 and 2.3 V vs. Li/Li + . A 850 nm thin film delivered a stable capacity of 70 mAh g À1 (22 mAh cm À2 ) at C/10 over 160 cycles with a high coulombic efficiency close to 100%. For comparison, the electrochemical performance of a crystalline LiCuPO 4 film has been studied. A first discharge could deliver a high capacity around 375 mAh g À1 at C/10, but the capacity decayed quickly up to a very low capacity of 11 mAh g À1 after 50 cycles. These results demonstrate that amorphous LiCuPO 4 thin film deposited by r.f. sputtering could be considered as a cathode material for Li-ion microbatteries.

Supporting Information Summary

The experimental section can be found in the supporting information. The supporting Figures have been also provided.

Figure 2 .

 2 Figure 2. (a) Cyclic voltammograms of the as-deposited LCP film at a scan rate of 0.5 mV/s and (b) Typical galvanostatic charge/discharge profiles. (c) Charge/discharge profiles of (&&&) crystalline LCP film and (-) amorphous LCP film and (d) discharge capacity of the amorphous and crystalline LCP films at C/10.

Table 1 .

 1 Discharge capacities and cycling efficiencies of both amorphous and crystalline LCP film in the potential window between 1 and 4.5 V vs. Li/ Li + .

	LCP	1 st cycle rever-	Irreversible 50 th cycle rever-
		sible		sible	Cycling
		capacity	capacity	capacity	efficiency
		(mAh g À1 )	(mAh g À1 )	(mAh g À1 )	(%)
	Amorphous 128	31	70	55
	Crystalline	151	224	11

Acknowledgements

We thank the re ´gion Provence Alpes Co ˆte d'Azur for the financial support.

Conflict of Interest

The authors declare no conflict of interest.