Trends in concentrations of selected metalloid and metals in two bivalves from the coral reefs in the SW lagoon of New Caledonia.

The concentrations of nine elements (Ag, As, Cd, Co, Cr, Cu, Mn, Ni and Zn) were measured in the oyster Isognomon isognomon and the edible clam Gafrarium tumidum from different sites along the SW New Caledonian coast which is subjected to important chemical inputs due to intense land-based mining activities (New Caledonia is the third world producer of nickel). Results indicate that concentrations in the two organisms mirrored the geographical differences in contamination levels as established through element analyses in sediment. On the basis of organism analyses, two out of the seven investigated stations can be considered as relative "reference" sites, except for As, for which very high levels were detected in clam and oyster tissues (up to 441 microg g(-1) dry wt for clams). Overall, our results indicate that both tropical organisms investigated could be used as valuable bioindicator species for surveying metal contamination in the coastal waters of New Caledonia with reasonable perspectives of wider application to other coral reef environments.

Nickel bioaccumulation in bivalves from the New Caledonia lagoon: seawater and food exposure.

The New Caledonian lagoon is submitted to intense heavy metal input from land-based Ni mining. Therefore, the use of sentinel species is strongly recommended in order to develop and implement coastal zone management programmes of the metal contamination. The tropical oysters Isognomon isognomon and Malleus regula and the clam Gafrarium tumidum were previously proposed as such possible sentinel organisms and were thus investigated in this context. The three species were exposed to Ni via seawater or food using radiotracer techniques. Results indicate that uptake and retention efficiencies of Ni are independent of the dissolved Ni concentrations in the surrounding seawater. Hence, for the three species, body concentrations of Ni taken up from the dissolved phase are directly proportional to the ambient dissolved concentrations. Biokinetic patterns indicated that the major part of Ni was rapidly lost from bivalves during the first days of depuration, whereas 7 to 47% of 63Ni were retained in tissues with a biological half-life not significantly different from infinity. Finally, feeding experiments showed that Ni ingested with food (phytoplankton) was assimilated more efficiently in clams (assimilation efficiency, AE = 61%) than in oysters (AE = 17%), and was strongly retained (T(b1/2) > or = 35 d) in the tissues of both bivalves. It is concluded that the investigated species examined are efficient bioaccumulators of Ni from both the surrounding seawater and the food, and that they would be useful bioindicators for monitoring the status of Ni contamination in tropical coastal waters.

Impact of zinc and nickel on oxygen consumption of benthic microbial communities assessed with microsensors.

In this study, the effect of zinc and nickel on oxygen consumption in sediments was determined using oxygen microsensors. Sediments from the southwest lagoon of New Caledonia, in the vicinity of the city of Nouméa, were incubated nearby in situ conditions and exposed to Zn and Ni concentrations of 20 and 60 mg l(-1). The depth distribution of oxygen consumption was estimated from the steady-state oxygen microprofiles, and the effects of metal were compared on the distributions before and after spiking. In most cases, metal had a strong effect on oxygen consumption at the surface. After 6 h exposure, oxygen consumption was only 10-40% of the initial value. However, the strong decrease in oxygen consumption observed at the sediment surface was counterbalanced by an increase of oxygen consumption deeper in the sediment. This is probably due to (i) a downward migration of aerobic microbial microorganisms living at the surface in order to escape the toxic effect of metal or/and (ii) a switch of the facultative aerobes from the low efficiency fermentation mode to the high-energy aerobic respiration mode.

A Microsensor Study of the Interaction between Purple Sulfur and Green Sulfur Bacteria in Experimental Benthic Gradients.

> Abstract The interaction between the purple sulfur bacterium Thiocapsa roseopersicina and the green sulfur bacterium Prosthecochloris aestuarii was studied in a gradient chamber under a 16-hours light-8-hours dark regime. The effects of interaction were inferred by comparing the final outcome of a mixed culture experiment with those of the respective axenic cultures using the same inoculation densities and experimental conditions. Densities of bacteria were deduced from radiance microprofiles, and the chemical microenvironment was investigated with O2, H2S, and pH microelectrodes. P. aestuarii always formed a biofilm below the maximal oxygen penetration depth and its metabolism was strictly phototrophic. In contrast, T. roseopersicina formed a bilayer in both the mixed and the axenic culture. The top layer formed by the latter organism was exposed to oxygen, and chemotrophic sulfide oxidation took place during the dark periods, while the bottom layer grew phototrophically during the light periods only. In the mixed culture, the relative density of P. aestuarii was lower than in the axenic culture, which reflects the effects of the competition for sulfide. However, the relative density of T. roseopersicina was actually higher in the mixed culture than in the corresponding axenic culture, indicating a higher growth yield on sulfide in the mixed culture experiment. Several hypotheses are proposed to explain the effects of the interaction.

Phytoplankton distribution and productivity in a highly turbid, tropical coastal system (Bach Dang Estuary, Vietnam).

Phytoplankton diversity, primary and bacterial production, nutrients and metallic contaminants were measured during the wet season (July) and dry season (March) in the Bach Dang Estuary, a sub-estuary of the Red River system, Northern Vietnam. Using canonical correspondence analysis we show that phytoplankton community structure is potentially influenced by both organometallic species (Hg and Sn) and inorganic metal (Hg) concentrations. During March, dissolved methylmercury and inorganic mercury were important factors for determining phytoplankton community composition at most of the stations. In contrast, during July, low salinity phytoplankton community composition was associated with particulate methylmercury concentrations, whereas phytoplankton community composition in the higher salinity stations was more related to dissolved inorganic mercury and dissolved mono and tributyltin concentrations. These results highlight the importance of taking into account factors other than light and nutrients, such as eco-toxic heavy metals, in understanding phytoplankton diversity and activity in estuarine ecosystems.

Spatial and seasonal variability of sediment oxygen consumption and nutrient fluxes at the sediment water interface in a sub-tropical lagoon (New Caledonia).

In order to quantify the spatial and seasonal variations of sediment oxygen consumption and nutrient fluxes, we performed a spatial survey in the south west lagoon of New Caledonia during the two major seasons (dry and wet) based on a network of 11 sampling stations. Stations were selected along two barrier reef to land transects representing most types of sediments encountered in the lagoon. Fluxes were measured using ex-situ sediment incubations and compared to sediment characteristics. Sediment oxygen consumption (SOC) varied between 500 and 2000 micromol m(-2)h(-1), depending on season and stations. Nutrient effluxes from sediment were highly variable with highest fluxes measured in muddy sediments near the coast. Inter-sample variability was as high as seasonal differences so that no seasonally driven temperature effect could be observed on benthic nutrient fluxes in our temperature range. Nutrient fluxes, generally directed from the sediment to the water column, varied between -5.0 and 70.0 micromol m(-2)h(-1) for ammonia and between -2.5 and+12.5 micromol m(-2)h(-1) for PO(4) and NO(2+3). SOC and nutrient fluxes were compared to pelagic primary production rates in order to highlight the tight coupling existing between the benthic and pelagic compartments in this shallow tropical lagoon. Under specific occasions of low pelagic productivity, oxygen sediment consumption and related carbon and nutrient fluxes could balance nearly all net primary production in the lagoon. These biogeochemical estimates point to the functional importance of sediment biogeochemistry in the lagoon of New Caledonia.

Hydrotaxis of cyanobacteria in desert crusts.

We studied the migration of cyanobacteria in desert crusts from Las Bárdenas Reales (Spain). The crusts were almost exclusively colonized by the filamentous cyanobacterium Oscillatoria, which formed a dense layer approximately 600 microm thick located between 1.5 and 2.1 mm deep. Laboratory and field experiments showed that saturation of the crust with liquid water induced a migration of the cyanobacteria leading to a significant greening of the surface within a few minutes. Under light and rapid evaporation, the green color rapidly disappeared and the crust surface was completely devoid of filaments within 60 min. In contrast, 260 min was required to recover the original white color of the crust when slow evaporation was experimentally imposed. The up and down migration following wetting and drying occurred also in the dark. This demonstrates that light was not a required stimulus. Addition of ATP synthesis inhibitors prevented the cyanobacterium from migrating down into the crust, with filaments remaining on the surface. Therefore, the disappearance of the green color observed during desiccation can only be attributed to an active cyanobacterial motility response to the decrease in the water content. The simplest explanation that can account for the evidence gathered is the presence of a mechanism that links, directly or indirectly, these motility responses to gradients in water content, namely a form of hydrotaxis.

Monitoring of oxygenic and anoxygenic photosynthesis in a unicyanobacterial biofilm, grown in benthic gradient chamber.

In order to assess the role of cyanobacteria in the formation and dynamics of microenvironments in microbial mats, we studied an experimental biofilm of a benthic, halotolerant strain, belonging to the Halothece cluster of cyanobacteria. The 12-week-old biofilm developed in a sand core incubated in a benthic gradient chamber under opposing oxygen and sulfide vertical concentration gradients. At the biofilm surface, and as a response to high light irradiances, specific accumulation of myxoxanthophyll was detected in the cells, consistent with the typical vertical distribution of sun versus shade species in nature. The oxygen turn-over in terms of gross photosynthesis and net productivity rates was comparable to oxygen dynamics in natural microbial mats. Sulfide blocked O(2) production at low irradiances in deep biofilm layers but the dynamics of H(2)S and pH demonstrated that sulfide removal by anoxygenic photosynthesis was taking place. At higher irradiances, as soon as H(2)S was depleted, the cells switched to oxygenic photosynthesis as has been postulated for natural communities. The similarities between this experimental biofilm and natural benthic microbial mats demonstrate the central role of cyanobacteria in shaping microenvironmental gradients and processes in other complex microbial communities.

Dynamics of anoxygenic photosynthesis in an experimental green sulphur bacteria biofilm.

The dynamics of sulphide oxidation in an experimental biofilm of the green sulphur bacterium, Prosthecochloris aestuarii, were studied using a newly developed light-dark cycling procedure. The biofilm was grown for 6 weeks in a benthic gradient chamber, in which gradients of light, sulphide and oxygen were imposed experimentally. The H2S concentrations and pH were measured with microsensors as a function of depth in the biofilm and of time after a change in illumination status. The sulphide oxidation rates were calculated as a function of time and depth in the biofilm using a numerical procedure to solve the non-stationary general diffusion equation. A close agreement was found between the areal rates of anoxygenic photosynthesis during the cycling procedure and the steady state before the cycling experiment. For the different layers of the biofilm, the maximum activity was observed after 10-12min of light exposure. After this maximum, sulphide oxidation decreased concomitantly with sulphide concentration, indicating sulphide limitation of anoxygenic photosynthesis. This lag time limits the application of the standard dark-light shift method with a brief light exposure of a few seconds and, therefore, the numerical procedure described in this study enables the depth distribution of anoxygenic photosynthesis rates in microbial mats to be determined more accurately.