Thomas Heide Clausen

Emmanuel Baccelli
email: emmanuel.baccelli@inria.fr

A Simple address Autoconfiguration Mechanism for OLSR

In this paper, we develop a simple autoconfiguration mechanism for OLSR networks. The mechanism aims at solving the simple, but common, probem of one or more new nodes emerging in an existing network. We propose a simple solution, which allows these new nodes to acquire an address and participate in the network. Our method is simple, both algorithmically and in the requirements to the network. While we recognize that this is a partial solution to the general autoconfiguration problem, we argue that the mechanism described in this paper will satisfy the requirements from a great number of real-world situations.

I. INTRODUCTION

A Mobile Ad-hoc NETwork (MANET) is a collection of nodes which are able to connect on a wireless medium forming an arbitrary and dynamic network, routing traffic through multi-hop-paths in order to ensure connectivity between any two nodes in the network. Implicitly herein is the ability for the network topology to change over time as links in the network appear and disappear.

In order to enable communication between any two nodes in such a MANET, a routing protocol is employed. The abstract task of the routing protocol is to discover the topology (and, as the the network is dynamic, continuing changes to the topology) to ensure that each node is able to acquire a recent image of the network topology for constructing routes.

One of the proposed routing protocols for MANETs is OLSR [START_REF] Clausen | The optimized link state routing protocol -evaluation through experiments and simulation[END_REF]. A proactive routing protocol, OLSR ensures that all nodes at all times have sufficient topological information to construct routes to all destinations in the network. This is achieved through periodic message exchange.

An issue, complimentary to that of routing, emerges with respect to bootstrapping of the network. OLSR do very well with the task of discovering paths in a MANET , however a prerequisite to the correct functioning of OLSR, and indeed of any MANET routing protocols, is, that all nodes are identifiable by an unique IP-address. Subsequently, a mechanism for assigning (unique) addresses to MANET nodes is required.

A particularity of MANETs is, that the roles of "terminal" and "network forming node" (router) are not clearly seperate. In principle, all nodes may act in both capacities simultaneously. An additional constraint is, that no assumptions with respect to a preexisting infrastructure can be made. Traditional mechanisms for host autoconfiguration, such as DHCP [START_REF] Droms | Dynamic host configuration protocol[END_REF] or ZeroConf [START_REF] Williams | Requirements for automatic configuration of ip hosts[END_REF] or similar mechanisms all assume the presense of a "server", which can coordinate and assign addresses. Further, these mechanisms work on the assumption that direct communication between the "server" and all hosts in the local network is possible. Due to the multi-hop nature of MANETs, direct communication between an arbitrary host in the network and (any) server cannot be assumed.

In order to ensure the true autonomy of MANETs, a specific mechanism -or adaptation of mechanism -for address autoconfiguration of MANETs is required.

A. Problem Statement

The issue of autoconfiguration in MANETs is complex since, for a complete solution, issues such as ensuring uniqueness of addresses in independant MANETs which later merge, must be addressed: independant MANET must somehow select nonoverlapping address-spaces, duplicate address detection, conflict resolution -and the issue of how to deal with ongoing data streams without loosing data or the requirement of specific application behavior.

In this paper, we aim for a simple solution to a simple problem: the connected case. A common situation occurs, in which an efficient and simple address autoconfiguration mechanism is desireable and sufficient. This situation is, where an OLSR network acts as an edge-extension to the Internet. I.e., nodes are interrested in maintaining connection to each other and to the Internet. The implication is also, that nodes join or leave the OLSR network, but do not migrate (alone or in groups) between OLSR-networks with the expectation of maintaining connectivity. The topic of nodes migrating between OLSR networks may better be addressed through mechanisms such as NEMO [START_REF] Devarapalli | Network mobility (nemo) basic support protocol[END_REF].

The mechanism, developed in this paper, is therefore targeted explicitly at the connected case described above. We recognize that this is a particular solution to a particular problem, and we aim at developing a simple and light-weight mechanism, efficient for these stated scenarios.

The address autoconfiguration mechanism in this paper is developed as an extension to OLSR [START_REF] Clausen | The optimized link state routing protocol -evaluation through experiments and simulation[END_REF], taking direct advantage of the mechanisms and features of OLSR.

B. Paper outline

The remainder of this paper is organized as follows: section II presents OLSR with sufficient details to define the address autoconfiguration mechanism as an extension to OLSR. Section III presents an overview of the address autoconfiguration mechanism. Section IV, sections V and section VI, describe the beaconing employed, the mechanism through which a locally unique address is acquired, and the mechanism through which a globally unique mechanism is assigned to an arriving node.

Section VII evaluates the overhead of the mechanism proposed. The paper is concluded by section VIII.

II. OLSR PROTOCOL OVERVIEW

In this section, relevant aspects of OLSR [START_REF] Clausen | RFC 3626: Optimized linkstate routing protocol[END_REF] are described, with the purpose of designing an address autoconfiguration mechanism which takes advantage of the possible optimizations in OLSR and integrates as a natural extension to the extension mechanism of OLSR. For further details on OLSR, as well as on performance characteristics of OLSR, please refer to [START_REF] Clausen | RFC 3626: Optimized linkstate routing protocol[END_REF] and [START_REF] Thomas Heide Clausen | Comparative study of routing protocols for mobile ad-hoc networks[END_REF].

OLSR is a proactive link-state routing protocol. I.e. it employs periodic control message exchange in order to accomplish topology discovery and topology maintenance. The result of this message exchange is a topology map in each node, from which a routing table can be constructed.

More specifically, OLSR employs two types of control messages: HELLO messages and TC messages.

HELLO-messages are exchanged periodically between negihbor nodes, to permit tracking the status links to neighbor nodes, including disappearing and emerging nodes.

TC-messages are likewise emitted periodically to diffuse link-state information to the entire network. TC-messages are flooded using an optimized flooding mechanism, specific to OLSR 1 .

Control-messages, intended for all nodes in the network, are flooded using a mechanism called MPR-flooding. The objective of MPR-flooding is to reduce the cost of performing a flooding operation. This is achieved through having each node select a minimal set of "relay nodes" (called MPRs), responsible for relaying flooded packets. Figure 1 illustrates classical flooding (left) and MPR flooding (right), with the packet originated by the central node and retransmitted as indicated by the arrows. [START_REF] Qayyum | Multipoint relaying: An efficient technique for flooding in mobile wireless networks[END_REF] has more details regarding MPR flooding.

As a platform for extensions, OLSR control traffic is transmitted in an unified packet format, allowing messages to be piggybagged together, as well as allowing extensions to take advantage of the MPR flooding mechanism. The OLSR packet format is given in figure 2.

As can be seen in figure 2, a packet is a collection of messages, each with individual headers, permitting individual treatment (including flooding behavior) of each message. Refer to [START_REF] Clausen | RFC 3626: Optimized linkstate routing protocol[END_REF] for further details. 1 For details regarding TC message generation, MPR flooding etc., please refer to [START_REF] Clausen | RFC 3626: Optimized linkstate routing protocol[END_REF] and [START_REF] Qayyum | Multipoint relaying: An efficient technique for flooding in mobile wireless networks[END_REF].

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Time To Live | Hop Count | Message Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

+-+-+ | | : MESSAGE : | | +

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message Type | Vtime | Message Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Originator Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Time To Live | Hop Count | Message Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | :

MESSAGE : | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : (etc) Fig. 2. Generic OLSR packet format. Each packet encapsulates several control messages into one transmission.

III. OVERVIEW AND PHILOSOPHY

With the description of OLSR in section II, as well as the problem statement in section I-A, this section will outline the functioning of our address autoconfiguration mechanism.

We will use the following two terms for the remainder of this paper: a new node is a node which is not yet assigned an address, and thus not is part of an OLSR network. An OLSR-node is a node which is assigned an address and which is part of the OLSR network. A configurating node is an OLSR-node, which is currently assisting a new node in acquireing an address.

• OLSR-nodes behave as specified by [START_REF] Clausen | RFC 3626: Optimized linkstate routing protocol[END_REF]. Additionally, they emit ADDR BEACON messages, to signal to new nodes that they may act as configurating nodes. This is detailed in section IV.

• New nodes do not emit HELLO and TC messages, however listen for ADDR BEACON messages. From among the OLSR-nodes emitting ADDR BEACON messages, one configurating node is selected, and a request-for-address-configuration is issued through an ADDR CONFIG message. The goal is for the configurating node to provide the new node with first a temporary address, then a permanent global address. This process of acquireing a local, temporary address is detailed in section V, and the task of acquireing a global address is detailed in section VI.

IV. LOCAL BEACONING Each OLSR-node, must ensure that it has the ability to provide temporary addresses from a private address space to new

The "Assigned Global Address" field is discussed in section VI.

VI. GLOBAL ADDRESS ASSIGNMENT

When the HELLO message exchange commences between the new and configurating node, local address assignment is completed, and acquireing a global address can commence. The configuration node is in charge of acting on behalf of the new node, wrt. acquireing a global address. Since the configuration node is already part of the OLSR network, a multitude of different mechanisms can be employed. One such mechanism for acquireing a global adddres would be for the configurating node to act as a modified DHCP proxy [START_REF] Patrick | Dhcp relay agent information option[END_REF] and transmit a request to an existing DHCP server in the network.

Another option would be to consult the nodes topology table. This table (in a stable state) contains all destinations (thus addresses) of the network. The configurating node can thus pick a non-used address and assign to the new node. To prevent duplicate address assignment, the configurating node includes the selected address in a few TCs. If a node receives a TC containing its own address (or an address, which the node has claimed for a new node) AND if the originator of the message is not the node itself nor an MPR of the node, a duplicate address assignment is detected. The detecting node can then communicate this to the originator of the "offending" TC, with the purpose of resolving the conflict.

Once the configurating node has acquired a globally unique address, this is assigned to the new node through an ADDR CONFIG message, containing the "Assigned Local Address" and "Originator Address" as before, but with a non-zero address in the "Assigned Global Address" field. This is then the ticket for the new node to participate fully in the OLSRnetwork.

The configurating node will continue to transmit this ADDR CONFIG message periodically until either the HELLO messages from the new node's assigned local address cease, or until an ADDR CONFIG message from the new node is received, listing the new nodes global address in both the originator field and the "Assigned Global Address" field, and with the "Assigned Local Address" and "MAC address" as before.

VII. OVERHEAD EVALUATION

The overhead, incuring from the mechanism specified in this paper, comes from primarilly three sources: periodic beaconing of ADDR BEACON messages address request/replies through ADDR CONFIG messages and discovery of a globally unique address.

ADDR BEACON messages and ADDR CONFIG messages are local, i.e. no flooding operations incur. ADDR CONFIG messages are furthermore only transmitted while nodes are being configured, and are of limited size (24 bytes + size of MAC address). Each configuration cycle incurs 4 messages. The overall overhead, incured through this procedure, is negligible.

ADDR BEACON messages are transmitted in the same OLSR packets as OLSR HELLO messages (MTU permitting), thus the number of transmissions required remains constant as compared to OLSR. Except when an node configuration is ongoing, the additional overhead incured from ADDR BEACON amounts to 20 bytes.

Discovery of a globally unique message depends on the mechanism employed. Assuming the decentralized mechanism, where an unused address is picked from the topology table and is probed through including this address in a TC emission, the additional overhead per TC message for that node is 4 bytes. This is offset by the fact that if address is assigned to the new node, topological information is already present in the network, allowing the node immediate participation.

VIII. CONCLUSION

In this paper, we have presented a simple solution to a common case of autoconfiguration of a new node in an existing OLSR network. Our approach is based on a "new" node associating with an existing node, and thereby acquireing first a local, then global, address by way of this existing node. We notice, that our approach permits both the use of "serverless" autoconfiguration, taking advantage of the fact that OLSR is a proactive table-driven protocol in order to select global addresses and verify their uniqueness, as well as the use of existing configuration servers, such as DHCP-servers.

Our proposed solution is simple, both conceptually and in terms of the overhead incuring, however solves the problem of autoconfiguration in an OLSR network, where the situation is that a node emerges in an existing network.

We have described the situation addressed by this protocol in terms of "the connected case": a requirement for the mechanism to work is, that a new node can associate itself with a node which is already part of the network. The implication of this is, that we avoid the "disconnected case", where two MANETs configure independantly, then merge at a later point with potential addressing conflicts which must be resolved. We believe that the "connected case" is a quite common situation (e.g. when OLSR networks are applied as extensions to an existing infrastructure -in which case the network formation naturally happens from the existing infrastructure), and the a simple solution, such as the one presented in this paper, is required.

Fig. 1 .

 1 Fig. 1. Two hop neighbors and "multipoint relays" (the solid circles) of a node. (a) illustrates the situation where all neighbors retransmit a broadcast, (b) illustrates where only the MPRs of a node retransmit the broadcast.

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ADDR_BEACON | Vtime | Message Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Originator Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | 0 | Message Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address Sequence Start | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address Sequence Stop | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | :

Currently Used Addresses : | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Fig. 3. ADDR BEACON message format. nodes. It is important that, within a region, these temporary addresses are unique, i.e. that no two new nodes within the same neighborhood are assigned the same temporary address. In order to ensure this, a predefinined address space is allocated for use for "temporary addresses". The task is to ensure that this address space is divided, without overlap, between nodes in a region of the network:

• Each OLSR-node will, independently, select a continous address-sequence from the address-space allocated for "temporary addresses";

• Each OLSR-node will signal, in periodic ADDR BEACON messages, this selected sequence. ADDR BEACON's are transmitted to neighbor nodes only, i.e. are not forwarded;

• Each node will record the address-sequences, selected by all neighbor nodes.

• If, upon receiving an ADDR BEACON message, a node detects that a conflicting address-sequence is selected, arbitration must happen.

-If no nodes in the conflict are acting as configurating nodes, arbitration is carried out simply by having the conflicting node with the lowest ID (IP-address) select a new, unused address-sequence. -If one or more conflicting nodes are acting as configurating node(s), arbitration must aim at allowing ongoing configuration sessions to complete. To accomodate this, all configuration nodes "narrow" their selected address-sequence to contain only the address(es) which are currently assigned to new nodes. This is included in the next ADDR BEACON. Nodes which are not currently acting as configuration nodes, select non-conflicting address sequences. If a conflict between two configurating nodes remains, the node which has the lowest ID (IP address) must yield. The ADDR BEACON message has the format specified in figure 3. [START_REF] Clausen | RFC 3626: Optimized linkstate routing protocol[END_REF] specifies the values of Message Size, Originator Address, Message Sequence Number and Vtime.

In case of "narrowing down" the address-sequence to only currently used addresses, the "Address Sequence Start" and "Address Sequence Stop" are both set to zero.

Each node will send ADDR BEACON messages, listing both its address-sequence and the addresses which currently in use'. In case of a conflict, a recipient node can detect if the node with which it is conflicting is active as configurating node and if both nodes are active as configurating nodes, detect a conflict in the addresses actually selected.

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Originator Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Configurating Node Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | : MAC Addresses : | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |

Assigned Local Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Assigned Global Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Fig. 4. ADDR CONFIG message format.

ADDR BEACON messages are transmitted piggybagged in the same OLSR packet as OLSR HELLO messages.

V. ACQUIREING A LOCAL ADDRESS

The first task of a new node is to associate itself with an OLSR-node.

Thus, the new node listens for ADDR BEACON messages and selects one "configurating node". An ADDR CONFIG message is then created and transmitted, in order to request address configuration from the selected configurating node. Abcent an IP-address, the MAC address of the new node must be included, to uniquely identify the new node.

Upon receiving an ADDR CONFIG message, the configurating node assigns a local address to the new node, and signals this assignment through an ADDR CONFIG message. Additionally, the configurating node marks the assigned address as "used" in its ADDR BEACON messages.

Upon receiving a local address through an ADDR CONFIG message, the new node will start sending HELLO messages, including only the configurating node as neighbor. This allows the new and configurating node to track each other (i.e. allows both nodes to "reset", should the link disappear before a global address was assigned to the new node), while does not cause the new node to be advertised to the network: adverticing a node with a non-unique address might lead to data loss, loops etc.

If a new node does not receive an ADDR CONFIG reply, it may either retransmit the ADDR CONFIG to the same configurating node -or give up and select an alternative configuration node. Absent the HELLO message exchange described above the configuration node may retransmit its ADDR CONFIG reply -or give up, in which case any temporarilly assigned addresses will be reclaimed.

An ADDR CONFIG message has the format specified in figure 4. [START_REF] Clausen | RFC 3626: Optimized linkstate routing protocol[END_REF] specifies the values of Message Size, Originator Address, Message Sequence Number and Vtime.

If the "Assigned Local Address", "Assigned Global Address" and "Originator Address" fields are both set to zero, the ADDR CONFIG message is a request to the "Configurating Node" to perform local address assignment.

If the "Assigned Local Address" is non-zero (i.e. contains an actual address) and "Originator Address" is non-zero, but the "Assigned Global Address" field is set to zero, the ADDR CONFIG message is an assignment of a temporary local address. I.e. this is the reply, generated by a configurating node.


Marc Metian

Laëtitia Hédouin

C Barbot

Jean-Louis Teyssié

Scott W Fowler

Françoise Goudard

Paco Bustamante

Jean-Pierre Durand

Jacques Piéri

Michel Warnau
email: m.warnau@iaea.org

J.-L Teyssié

J.-P Durand

Use of Radiotracer Techniques to Study Subcellular Distribution of Metals and Radionuclides in Bivalves from the Noumea Lagoon, New Caledonia

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

New Caledonia is the third largest producer of nickel in the world, and this small South Pacific island is estimated to contain no less than 20% of the total stock of Ni on the planet [START_REF] Connell | New Caledonia: an infinite pause in decolonization? The Round Table[END_REF]. Metal contamination resulting from the nickel mining industry and related activities constitutes a long lasting threat to the marine ecosystems sheltered by the second largest reef system in the world [START_REF] Labrosse | New Caledonia[END_REF]. However, as almost a rule when it concerns tropical ecotoxicology, available information on metal contamination in New Caledonia waters is extremely scarce and very little is known about the extent of local contamination and possible environmental impacts [START_REF] Labrosse | New Caledonia[END_REF]. Moreover, a new extraction process for Ni ("lixiviation", viz. acidic extraction) has recently been tested at the industrial level and should be implemented in the near future (2006)(2007). This process will result inevitably in increased discharges of co-occurring metals in Ni ores (e.g. Co and Cr). Thus, basic information on metal metabolism and behaviour is needed in order to assess the possible impact of these additional metal inputs on local ecosystems.

The objective of the present study was to determine the potential toxicity of metals in two species commonly found in the lagoon: the edible clam Gafrarium tumidum and the oyster Isognomon isognomon. Contaminant partitioning within the cells (soluble vs. insoluble fractions) determines the likelihood of inducing deleterious effects (reaction with cellular components) [START_REF] Viarengo | Biochemical effects of trace metals[END_REF] as well as to being transferred to higher trophic levels [START_REF] Reinfelder | The accumulation of element ingested by marine copepods[END_REF]. Therefore, the subcellular distribution of five metals (Cd, Co, Cr, Zn, Ag) and two anthropogenic radionuclides ( 134 Cs, 241 Am) was examined in the gills and visceral mass of both species following direct seawater exposure to these elements using highly sensitive radiotracer techniques.

MATERIALS AND METHODS

Both bivalve species were collected in August 2002 by SCUBA diving in Dumbéa bay (G. tumidum) and Maa bay (I. isognomon) (Nouméa, New Caledonia) and were immediately shipped to the IAEA-MEL premises in Monaco where they were acclimated to laboratory conditions (open circuit aquaria; water renewal 10% hr -1 ; S: 36 p.s.u.; T: 25±0.5°C) for 6 wk prior to experimentation. The organisms were then experimentally exposed for 28 days to radiotracers of five heavy metals ( 109 Cd, 57 Co, 51 Cr, 65 Zn, 110m Ag) and two radionuclides ( 134 Cs, 241 Am) directly via sea water. Periodically during the exposure phase, the bivalves were transferred to unlabelled sea water for a short time (1-2 hrs) where they fed on mixed phytoplankton cultures before being returned to the labelled sea water for further uptake. At the end of the experiment, 6 individuals of each species were collected and dissected. The gills and visceral mass were separated, pooled, and processed for subcellular fractioning, using differential centrifugation [START_REF] Boisson | Comparative radiotracer study of cadmium uptake, storage, detoxification and depuration in the oyster Crassostrea gigas : potential adaptive mechanisms[END_REF]. Briefly, homogeneized tissues were centrifuged successively: -at 900 × g for 10 min (to sediment nuclei and heavy lysosomes), -at 12,000 × g for 15 min (to sediment lysosomes and mitochondria), -at 45,000 × g for 30 min (to sediment light mitochondria and plasma membranes), -and finally at 115,000 × g for 70 min (to separate microsomes from the cytosolic fraction, the latter constituting the supernatant).

Distribution of the radiotracers among the different subcellular fractions was determined using a high-resolution γ-spectrometry system consisting of 4 coaxial Ge (N-or P-type) detectors (EGNC 33-195-R, Intertechnique) connected to a multichannel analyzer and a computer equipped with a spectra analysis software (Interwinner, Intertechnique). The detectors were calibrated with appropriate standards for the counting geometry used, and all measurements were corrected for background and physical decay of the radiotracers. Counting times were adapted to obtain count rates with relative propagated errors less than 5%.

RESULTS AND DISCUSSION

Measurements of specific enzymatic markers (acid phosphatase for lysosomes, glucose-6-phosphatase for microsomes, 5'-nucléotidase for plasma membranes; [START_REF] Boisson | Comparative radiotracer study of cadmium uptake, storage, detoxification and depuration in the oyster Crassostrea gigas : potential adaptive mechanisms[END_REF])) indicated that the purity of the different subcellular fractions was good. Results of the subcellular distribution of the different metal radiotracers and radionuclides in gills and visceral mass are given in Table 1.

Globally, the distributions in both tissues were similar for each bivalve species.

The only main departure from this was observed for 57 Co in the clam where the cytosolic fraction was much lower in the gills (25%) than in the visceral mass (79%). Cr, Co, Zn, Cd and 134 Cs were mainly found in the cytosolic fraction (30-87%) whereas 110m Ag and 241 Am were mainly associated with membranes and organelles (65-96%). These results are in agreement with those reported for other bivalves from temperate waters, e.g., the scallop Chlamys varia (Bustamante and Miramand in press) and the oyster Crassostrea gigas [START_REF] Milcent | Identification of Cs-137 and Am-241 binding sites in the oyster Crassostrea gigas[END_REF].

The predominant distribution of Ag in the insoluble fraction (viz. the noncytosolic fractions) could be due to specific Ag storage/detoxification in these two bivalve species. Indeed, it is well documented that various bivalves are able to trap Ag as non-toxic Ag 2 S precipitates within their tissues [START_REF] Berthet | Bioaccumulation, toxicity and physico-chemical speciation of silver in bivalve mollusks: ecotoxicological and health consequences[END_REF][START_REF] Berthet | Effets chimiques et histologiques de la décontamination de l'huître Crassostrea gigas Thunberg préalablement exposée à l'argent[END_REF]. This kind of sequestration can inhibit the deleterious effects that could be caused by this highly toxic element, even if present in high concentrations. In addition, preferential distribution in the insoluble subcellular fraction indicates that Ag is not likely to be bioavailable to higher trophic levels (Bustamante and Miramand in press;[START_REF] Reinfelder | The accumulation of element ingested by marine copepods[END_REF].

Preferential distribution of most radioelements in the cytosol suggests that, once incorporated into the cells, a large part of these metals could be toxic, since they are likely to bind with key soluble components of the cells (e.g. proteins, enzymes, DNA). However, in the case of Cd and Zn, a substantial fraction of the cytosolic metal is most probably detoxified as "metal-metalloprotein" complexes, e.g. approximately 40% in the case of Cd in oysters [START_REF] Boisson | Comparative radiotracer study of cadmium uptake, storage, detoxification and depuration in the oyster Crassostrea gigas : potential adaptive mechanisms[END_REF]. Furthermore, the metals preferentially associated with the cytosolic fraction are likely to be readily bioavailable to higher trophic levels preying on these organisms [START_REF] Reinfelder | The accumulation of element ingested by marine copepods[END_REF]. This fact is of particular concern here since the clam G. tumidum is consumed by local populations, and could therefore be a non-negligible source of human exposure to metals through seafood consumption.

Progress in Ni ore exploitation planned in New Caledonia will result in an increased input of dissolved metals to New Caledonian lagoon waters [START_REF] Morreton | Caractéristiques physicochimiques des eaux. Goro-Nickel/IRD Convention[END_REF]). Such a situation could result in an increased contamination of the local bivalves. Our findings indicate that subcellular partitioning of metals cooccurring in Ni ores will be preferentially cytosolic. Therefore, metal exposure of organisms (including man) preying on these two bivalves could be enhanced as well. Monitoring of metal contamination levels in edible species is therefore recommended following the industrial implementation of the acidic lixiviation process in New Caledonia.

Table 1 .

 1 Subcellular partitioning (mean %) of radioisotopes in gills and visceral mass of two bivalves. Co 65 Zn 109 Cd 110m Ag 134 Cs 241 Am 51 Cr 57 Co 65 Zn 109 Cd 110m Ag 134 Cs 241 Am

				Gafrarium tumidum					Isognomon isognomon
	Gills Cr 57 Nuclei 51 18 28	28	30	73	20	25	17	16	22	14	23	17	27
	Lysosomes + mitochondria	6	7	6	2	6	7	6	10	19	30	15	34	12	36
	Membranes	10	17	16	1	6	13	25	19	8	15	10	23	16	10
	Microsomes	10	22	19	1	5	13	27	10	5	0	6	7	11	4
	Cytosol	57	25	31	67	10	48	17	44	52	33	54	13	45	22
	Visceral mass
	Nuclei	28	9	24	10	49	27	42	25	20	28	24	43	26	35
	Lysosomes + mitochondria	13	6	10	2	12	11	22	22	10	20	15	27	19	47
	Membranes	7	3	15	1	3	6	19	6	2	5	3	19	6	6
	Microsomes	6	3	12	1	2	5	13	7	3	6	3	4	7	3
	Cytosol	45	79	40	87	35	51	4	39	65	41	54	7	42

Acknowledgements. The IAEA-Marine Environment Laboratory operates under a bipartite agreement between the International Atomic Energy Agency and the Government of the Principality of Monaco. We thank Dr. R. Fichez and L. Breau (IRD-Nouméa) for providing the organisms and taking care of their shipment and Mr. O. Cotret (IAEA-MEL) for skillful technical assistance.