Adour Estuary Research Articles

Keystone microbial taxa organize micropollutant-related modules shaping the microbial community structure in estuarine sediments

The fluctuation of environmental conditions drives the structure of microbial communities in estuaries, highly dynamic ecosystems. Microorganisms inhabiting estuarine sediments play a key role in ecosystem functioning. They are well adapted to the changing conditions, also threatened by the presence of pollutants. In order to determine the environmental characteristics driving the organization of the microbial assemblages, we conducted a seasonal survey along the Adour Estuary (Bay of Biscay, France) using 16S rRNA gene Illumina sequencing. Microbial diversity data were combined with a set of chemical analyses targeting metals and pharmaceuticals. Microbial communities were largely dominated by Proteobacteria (41 %) and Bacteroidota (32 %), showing a strong organization according to season, with an important shift in winter. The composition of microbial communities showed spatial distribution according to three main areas (upstream, middle, and downstream estuary) revealing the influence of the Adour River. Further analyses indicated that the microbial community was influenced by biogeochemical parameters (Corg/Norg and δ13C) and micropollutants, including metals (As, Cu, Mn, Sn, Ti, and Zn) and pharmaceuticals (norfloxacin, oxolinic acid and trimethoprim). Network analysis revealed specific modules, organized around keystone taxa, linked to a pollutant type, providing information of paramount importance to understand the microbial ecology in estuarine ecosystems.

Assessing and predicting the changes for inorganic mercury and methylmercury concentrations in surface waters of a tidal estuary (Adour Estuary, SW France)

Total and dissolved concentrations of inorganic mercury (IHg) and methylmercury (MeHg) in water (Adour Estuary) were determined during three sampling campaigns and related to biogeochemical variables (nutrients, organic matter). Factors (sampling time, sample type) were included in analysis of covariance with effect separation. The urban estuary suffered historically from anthropogenic sources, however, decreased emissions have reduced Hg concentrations. Total IHg (0.51–3.42 ng L−1) and MeHg (25–81 pg L−1) concentrations are additively described by suspended particulate matter and particulate organic carbon. Higher total concentrations, carried by organic-rich particles, were found near specific discharge points (0.79–8.02 ng L−1 and 34–235 pg L−1 for IHg and MeHg, respectively). The associated high dissolved MeHg concentrations could not be explained only by biogeochemical variables. Better efficiency of the models is found for total than for dissolved concentrations. Models should be checked with other contaminants or with estuaries, suffering from downstream contamination.

Prioritization based on risk assessment to study the bioconcentration and biotransformation of pharmaceuticals in glass eels (Anguilla anguilla) from the Adour estuary (Basque Country, France)

The presence of contaminants of emerging concern in the aquatic environment directly impacts water-living organisms and can alter their living functions. These compounds are often metabolized and excreted, but they can also be accumulated and spread through the food chain. The metabolized contaminants can also lead to the formation of new compounds with unknown toxicity and bioaccumulation potential. In this work, we have studied the occurrence, bioconcentration, and biotransformation of CECs in glass eels (Anguilla anguilla) using UHPLC-HRMS. To select the target CECs, we first carried out an environmental risk assessment of the WWTP effluent that releases directly into the Adour estuary (Bayonne, Basque Country, France). The risk quotients of every detected contaminant were calculated and three ecotoxicologically relevant contaminants were chosen to perform the exposure experiment: propranolol, diazepam, and irbesartan. An experiment of 14 days consisting of 7 days of exposure and 7 days of depuration was carried out to measure the bioconcentration of the chosen compounds. The quantitative results of the concentrations in glass eel showed that diazepam and irbesartan reached BCF ≈10 on day 7, but both compounds were eliminated after 7 days of depuration. On the other hand, propranolol's concentration remains constant all along with the experiment, and its presence can be detected even in the non-exposed control group, which might suggest environmental contamination. Two additional suspect screening strategies were used to identify metabolization products of the target compounds and other xenobiotics already present in wild glass eels. Only one metabolite was identified, nordiazepam, a well-known diazepam metabolite, probably due to the low metabolic rate of glass eels at this stage. The xenobiotic screening confirmed the presence of more xenobiotics in wild glass eels, prominent among them, the pharmaceuticals exemestane, primidone, iloprost, and norethandrolone.

Variability of subtidal flow in a narrow meandering stratified estuary

A 3D numerical model was set up to investigate the spatial and temporal variations of subtidal circulation in a narrow meandering stratified estuary: the Adour Estuary (France). The hydrodynamic model was validated with field data and reproduced satisfactorily tide, current and salinity observations. The results highlighted a fortnightly switch in the residual flow structure and a variation in residual circulation intensity between low and high river discharge conditions. During neap tides, an exchange flow was observed as the baroclinicity dominated the residual circulation. During spring tides, the advection and barotropic pressure gradient drove the subtidal flow and an outflow over the whole water column took place. Residual flows were twice as high during high river discharge as during low river discharge. Secondary cross-channel residual circulation was driven by the estuarine morphology and the along-channel residual circulation. A secondary circulation cell was observed when a density-driven exchange flow occurred (neap tide). The meanders induced a variation of the sense of rotation of the circulation cell. A constriction of the channel has been shown to strengthen the residual circulation by flow conservation. Finally, we observed that the freshwater Froude number and the Mixing number Geyer and MacCready (2014) were able to distinguish residual flow pattern between idealised simulation cases.

The contribution of short-wave breaking to storm surges: The case Klaus in the Southern Bay of Biscay

This study investigates the contribution of short-wave breaking to storm surges through a high-resolution hindcast of the sea state and storm surge associated with the extra-tropical storm Klaus. This storm made landfall in January 2009 in the Southern Bay of Biscay and produced the largest storm surges observed in this region over the last 20 years, with 1.70m in the Arcachon Lagoon and 1.10m in the Adour Estuary. A fully-coupled 3D modelling system, which uses a vortex force formalism to represent wave-current interactions, is applied with a spatial resolution down to 35m in the surf zones in order to properly compute the wave-induced setup. Modelling results reveal that the wave setup contributes by up to 40% and 23% to the storm surge peak in the Adour Estuary and the Arcachon Lagoon respectively. Accounting for wave forces in the circulation model improves storm surge predictions by 50 to 60%. This is explained by the dominant role played by wave forces in the momentum balance at the inlets under storm waves. Numerical experiments further reveal that the wave-induced setup can be tidally-modulated, although this phenomenon seems to be site-specific. Finally, a sensitivity analysis highlights the importance of the model grid resolution in the surf zones to correctly resolve the wave setup along open-ocean coasts. Inside the lagoon, the storm surge and wave setup are less sensitive to the grid resolution while tidal propagation cannot be accurately represented with a resolution of 1000m, which is typically used in operational storm surge forecast.

Microplastics in a salt-wedge estuary: Vertical structure and tidal dynamics

The abundance and distribution of microplastics in estuaries have been barely documented, and generally without accounting for the vertical structure in the water column. This study presents the very first data on the occurrence and distribution of microplastics in the Adour Estuary, SW France. The experimental data set was complemented by numerical simulations to gain understanding of the behaviour of suspended microplastics. Microplastics were found throughout the water column with a mean abundance of 1.13 part/m3. Films and fragments were the most abundant types of particles collected. Numerical simulations demonstrated that vertical distribution of microplastics in the water column is highly dependent on particle characteristics and on the local hydrodynamics. The main trend is that neutrally-buoyant microplastics are easily flushed out while heavier microplastics are prone to entrapment in the estuary, in particular under low discharge conditions. The present study suggest that estuaries could be a sink of microplastics.

Hydrodynamics and SPM transport in an engineered tidal estuary: The Adour river (France)

The present paper reports on a series of field experiments aiming to characterise the functioning of a man-engineered strongly forced salt-wedge estuary: the lower estuary of the Adour river, France. Bottom-moored velocity measurements and surface boat surveys have been performed under low river discharge conditions, for both neap and spring tides, in order to provide a well-documented reference framework to understand the dynamics of water masses, turbulence and suspended particulate matter (SPM) transport in the lower estuary. An additional campaign has been carried out in high river discharge conditions. This first documented in-situ study of the Adour lower estuary demonstrates its variability in terms of hydrological regimes, from salt-wedge to partially mixed regimes depending on tidal and discharge conditions. Turbulent properties showed a significant response to the variations of salinity structure, with higher values when stratification is minimal. At spring tide, a tidal variation between mixing conditions on the ebb and the flood is revealed by ADCP measurements, with higher values extended up to the surface during the ebb. The link between turbulent mixing and suspended sediment concentration is straightforward during the ebb. During the flood, the suspended sediment concentration (SSC) seems related to the salt-wedge entrance re-suspension and stratification-induced turbulence damping. No stable Estuarine Turbidity Maximum (ETM) has been observed during the field experiment in the lower Adour estuary.

Dynamics of microbial communities across the three domains of life over an annual cycle with emphasis on marine mucilage in the Southern Bay of Biscay resolved by microbial fingerprinting

Marine mucilage has been described as worldwide phenomena occurring sporadically, or frequently in certain coastal areas. They are transitory phenomena that can remain in the photic zone for several days or weeks. Their occurrence has been more frequent and persistent, and their magnitude has increased during the last decades. Their formation typically reflects an imbalance in the marine microbial communities, and has frequently be linked to global changes. Recurrent marine mucilage events have been observed in recent years in the Southern Bay of Biscay near the French coast, mainly south of the Adour estuary. In this study, we used a fingerprinting method (T-RFLP) targeting 16S and 18S rRNA genes to investigate marine microbial community composition and dynamics over an annual cycle across the three domains of life in coastal water at three depths, in the Adour River upstream of the mouth and in the marine mucilage. In line with studies conducted in marine environments we highlighted that the dynamics of marine microbial communities in the Bay of Biscay coincided with two environmental contexts: winter and spring on one hand and summer and autumn on the other hand. More interestingly, this dynamics affects also the marine pelagic mucilage that was observed during the sampling campaigns. Thus, the composition of the marine mucilage that appears in winter/spring context diverges from the composition of the marine mucilage that occurred during the summer/autumn context. We highlighted also that marine microbial communities were partly different from marine mucilage communities with an enrichment of some microorganisms suggesting that marine mucilage behaves as a microhabitat in seawater and possesses distinct microbial assemblages from the surrounding communities.

Occurrence and Fate of Organic and Organometallic Pollutants in Municipal Wastewater Treatment Plants and Their Impact on Receiving Waters (Adour Estuary, France).

To achieve a "Good Environmental Status by 2015," as demanded by the water framework directive, monitoring programs are needed to furnish data on target compounds. In this study, a first evaluation of influents and main emissions of 3 local wastewater treatment plants (WWTP) in the Adour estuary (southwest of France) was performed for 23 pollutants (10 musk fragrances, 5 alkylphenols, and 8 organometallics), as well as receiving estuarine water from the same area. High frequency of occurrence of these compounds was found in influents samples (musks: 22-100%; alkylphenols 11-100%; organometallics 0-100%) and effluents (musks: 0-100%; alkylphenols 0-100%; organometallics 0-100%). The removal efficiencies were calculated and varied from negative values up to 98% with the lowest values for synthetic musk compounds. Temporal variability of the target compounds also was studied, and a few tendencies were observed. Estimation of the daily output of each WWTP into the estuary also showed that galaxolide, nonylphenol, monobuthyltin, and inorganic mercury were the compounds discharged into the environment at the highest concentrations. Finally, the occurrence of these compounds in estuarine waters was evaluated; most of them were present at concentrations below the limits of quantification (musks: 0.53-41.5ng/L; alkylphenols 3.4-410ng/L; organometallics 0.02-0.70ng/L) suggesting a low impact in the resulting receiving waters.

Migratory behavior, metabolism, oxidative stress and mercury concentrations in marine and estuarine European glass eels (Anguilla anguilla)

The relationships between the migratory behavior, methylmercury (MeHg) concentrations, oxidative stress response and detoxification processes were investigated in glass eels collected in marine (Molliets) and estuarine (Urt) waters (Adour estuary, South West France) at the end of the fishing season (April). Glass eel migratory behavior was investigated in an experimental flume according to their response to dusk. Fish responding to the decrease in light intensity by ascending in the water column and moving with or against the flow were considered as having a high propensity to migrate (migrant). Glass eels still sheltering at the end of the 24h catching period were considered as having a low propensity to migrate and were called non-migrant. Our results provide some evidence that estuarine glass eels were bigger, presented a higher propensity to migrate and a lower oxidative stress response than marine glass eels. This might reflect a selection process, some marine glass eels progressively settling or dying before reaching Urt and/or a change in feeding behavior. In April, glass eels restart feeding in the Adour estuary which might decrease the oxidative stress possibly related to starvation, and enhance migration. MeHg concentrations was significantly higher in non-migrant than in migrant glass eels and it is suggested that non-migrant glass eels might present a higher vulnerability to stress (at least contamination and/or starvation), although the underlying mechanisms remain to be elucidated.

Spatial and seasonal variations of methylmercury in European glass eels (Anguilla anguilla) in the Adour estuary (France) and relation to their migratory behaviour.

Overall recruitment of European glass eels (Anguilla anguilla) has decreased significantly since the early 1980s. Due to their long life cycle, benthic/demersal habits and high lipid content, eels might accumulate high concentrations of contaminants, but data concerning glass eels are still scarce. This study provides original data on methylmercury (MeHg) concentrations in glass eels at spatial (marine and estuarine), annual and seasonal scales. The relationship between MeHg concentrations in glass eels and their propensity to migrate up estuaries was also investigated. MeHg data were individually related to the eels' energetic condition which was estimated by dry weight. Glass eel migratory behaviour was investigated in an experimental flume and related to the MeHg concentration and dry weight at the individual scale. Marine and estuarine glass eels were caught from 2004 to 2011. There was a strong inverse correlation between MeHg concentrations and dry weight. MeHg concentrations increased in marine and estuarine glass eels from 2004 to 2009 and from 2004 to 2010, respectively, and then, both groups decreased in 2011. On a seasonal time scale, MeHg concentrations were higher at the end of the fishing season (April). MeHg bioaccumulation is likely to result from different sources, but the lack of significant differences between marine and estuarine glass eels suggests that direct contamination during estuarine migration is low. Other sources such as maternal transfer or oceanic contamination are discussed.

Longshore sediment flux hindcast and potential impact of future wave climate change along the Gironde/Landes Coast, SW France

The West-facing Gironde/Landes Coast, SW France, comprises about 230 km of high-energy, nonengineered, sandy beaches. It extends from the Gironde estuary to the North, and to the Adour estuary to the South. Overall, the coast erodes at a rate of about 1 to 2 m/yr, with localized areas with stronger erosion or with a stable or slow accretive trend. The gradients in longshore sediment transport and the sediment budgets are presumably the most important factor controlling decadal scale evolution of the shoreline along the Gironde/Landes Coast. It is established that the net longshore drift, which is only disturbed at the tidal inlet system of the Arcachon Lagoon and shoreward of the Capbreton Canyon, is oriented southward (Michel, 1997). However, its intensity and longshore variability, as well as how it will evolve in the framework of climate change, are still a matter of debate. This important issue is addressed in this contribution. First, longshore sediment fluxes are hindcasted over the period 1958-2002, using an ERA-40 wind forced wave modelling and the most commonly used empirical longshore drift formulae (Charles et al, 2012). Results are (1) discussed with respect to the existing estimations based on smaller periods and using different datasets; (2) compared the shoreline evolution measured during this period and (3) further analyzed to address the respective contributions of the different representative wave classes (e.g. swell, storm waves, wind waves) to the total longshore drift. Wave projections, based on wind fields from one climate model (ARPEGE-Climat) for several emission scenarios (A2, A1B and B1), are carried out to estimate possible wave climate change in 2061-2100 (compared to a so-called REF scenario covering the 1961-2000 period). It shows that, for instance for the A2 scenario and the summer season, the modal significant wave height is likely to decrease together with a 5° northward rotation of incoming wave incidence. Based on the wave model outputs, longshore sediment fluxes along the Aquitanian coast are computed for the four scenarios. The analysis of the relative sediment flux evolution (A2, A1B and B1 scenarios with respect to present day REF scenario) shows an overall decrease of the southerly longshore drift magnitude. A thorough analysis of the wave components evolution responsible for this decreasing trend is done, as well as an analysis of the longshore drift gradients along the Aquitanian coast. The potential impact of longshore drift evolution on the shoreline evolution at the 2061-20100 is discussed.

In situ experiments for element species-specific environmental reactivity of tin and mercury compounds using isotopic tracers and multiple linear regression

The fate of mercury (Hg) and tin (Sn) compounds in ecosystems is strongly determined by their alkylation/dealkylation pathways. However, the experimental determination of those transformations is still not straightforward and methodologies need to be refined. The purpose of this work is the development of a comprehensive and adaptable tool for an accurate experimental assessment of specific formation/degradation yields and half-lives of elemental species in different aquatic environments. The methodology combines field incubations of coastal waters and surface sediments with the addition of species-specific isotopically enriched tracers and a mathematical approach based on the deconvolution of isotopic patterns. The method has been applied to the study of the environmental reactivity of Hg and Sn compounds in coastal water and surface sediment samples collected in two different coastal ecosystems of the South French Atlantic Coast (Arcachon Bay and Adour Estuary). Both the level of isotopically enriched species and the spiking solution composition were found to alter dibutyltin and monomethylmercury degradation yields, while no significant changes were measurable for tributyltin and Hg(II). For butyltin species, the presence of light was found to be the main source of degradation and removal of these contaminants from surface coastal environments. In contrast, photomediated processes do not significantly influence either the methylation of mercury or the demethylation of methylmercury. The proposed method constitutes an advancement from the previous element-specific isotopic tracers' approaches, which allows for instance to discriminate the extent of net and oxidative Hg demethylation and to identify which debutylation step is controlling the environmental persistence of butyltin compounds.

Simultaneous determination of mercury and butyltin species using a multiple species-specific isotope dilution methodology on the European, Anguilla anguilla glass eel and yellow eel

A methodology to simultaneously determine mercury (MeHg, IHg) and butyltin (TBT, DBT, MBT) compounds in eel samples was assessed and validated using multiple isotopically enriched species. The developed methodology was able to analyse simultaneously the organometal species accurately and precisely and to correct for the potential transformations/degradations of the different species during the various steps of the analytical procedure. Low detection limits were achieved (0.007–0.17 µg Hg kg−1 for mercury (Hg) species and 0.42–0.71 µg Sn kg−1 for tin (Sn) species) allowing analysis of low-mass samples and thus the analysis at the individual organism scale, including glass eels for which samples dry weight ranged from 60 to 100 mg. The methodology was validated with certified reference materials (BCR-464, BCR-477, BCR-710, DOLT-4 and NIST-2977) and applied to the analysis of these pollutants in two developmental stages of the European eel, Anguilla anguilla: individual whole glass eels and muscle tissue from yellow eels. The Adour estuary (South-west France) was selected to monitor the bioaccumulation of organometal species in these organisms, according their developmental stage, their morphological parameters and the sampling site. The results suggest that the accumulation of methylmercury in glass eel tissue is related to weight, with higher concentrations in smaller individuals. Butyltin concentrations were very close to the limit of detection, and no significant differences were detected between glass and yellow eels.

Temporal genetic variation in European eel Anguilla anguilla (Linnaeus, 1748): A fine scale investigation in the Adour estuary

The genetic structure of arrival cohorts of European eels in the continental basin of Adour, France, was analysed. A regular sampling was carried out during three successive seasons of migration to ensure a representative sampling of complete recruitment in the estuary. Genetic variability within this sample was evaluated by analysis of highly polymorphic microsatellite markers. A high level of polymorphism was highlighted but a lack of genetic differentiation was observed between cohorts collected during the three successive seasonal migrations.

Heavy metals and organochlorinated compounds in the European eel (Anguilla anguilla) from the Adour estuary and associated wetlands (France)

Heavy metals and organic pollutants were investigated in the Adour estuary (South West France) and associated wetlands using the European eel (Anguilla anguilla) as a bioindicator. Heavy metals (Cu, Cd, Zn, Pb, and Ag) were measured in soft tissue of yellow eels. Mercury (total Hg and MeHg) and organochlorinated compounds (7 PCBs, 11 OCPs) were analysed in muscle. Concentrations in muscle were in agreement with moderately contaminated environments in Europe and were below the norms fixed for eel consumption for heavy metals and OCPs. Analyses of liver showed a higher pressure of Ag and Zn in the downstream estuary than in the freshwater sites whereas Cd was lower in the estuary probably because of the salinity influence. According to quality classes 100% of eels from freshwater sites indicated clean or slightly polluted environments. However, total mercury concentrations were close to the thresholds fixed by the European Community in the downstream estuary, whereas the sum of PCBs was found to be greatly above the fixed value. 100% of the individuals from the estuary were classified in quality classes corresponding to polluted or highly polluted sites. These first results highlight the need of further investigations focused on mercury and PCBs in this area taking the seasonal temperature influence into account for a better understanding of the pollution distribution and the possible threat on the eel population from the Adour basin.

Simultaneous use of strontium:calcium and barium:calcium ratios in otoliths as markers of habitat: Application to the European eel ( Anguilla anguilla) in the Adour basin, South West France

Sr:Ca and Ba:Ca ratios in water from the Adour estuary show a clear relationship with the salinity of the surrounding water for salinities <20, while ratios are almost constant above this level of salinity. A positive relationship was observed for the Sr:Ca ratio, whereas it was inverse for the Ba:Ca ratio. These two elemental ratios were measured in the otoliths of the European eels ( Anguilla anguilla L.) using femtosecond laser ablation linked to an ICP-MS (fs-LA–ICP-MS). There was a direct relationship between the elemental ratios recorded in eel otoliths and those found in water from fresh and marine areas, suggesting that Sr:Ca and Ba:Ca ratios in eel otoliths can be used as markers of habitat in this estuary. Continuous profiling allowed the determination of three behaviour patterns in terms of habitat: freshwater, estuary and migratory individuals. Finally, the above results support the simultaneous use of both ratios for a better understanding of the migratory contingents and also as a relevant method to avoid a misidentification of environmental migratory history due to the presence of vaterite crystal in the otolith matrix.

Analysis and visualization of the glass eel behavior (Anguilla anguilla) in the Adour estuary and estimate of its upstream migration speed

The studies carried out on the Adour estuary on the migration of glass eels lead to the characterisation of their swimming behavior. Individuals migrate passively with the flood tide current behind the dynamic tide front, and their movements into the water column are linked to the surrounding luminosity and water turbidity. A numerical model was built from observations gathered on glass eel densities during scientific surveys. It uses the outputs of a hydrodynamic model accounting for the variations in the river flow and tide coefficient. It allows the simulation of the displacement of a particle in the longitudinal axis of the estuary. The variation of the vertical movements through the water column takes into account the cloud cover, the moon phases, the alternation of days and nights and the water turbidity. The results allow displaying the migration speed of groups of glass eels entering the Adour estuary. These simulations are validated by the comparison with in situ observations and the outputs of the behavioral model make possible, on the one hand, to simulate the migration speed of glass eels according to hydrological data and, on the other hand, to define the environmental conditions that stop or slow down their displacement into the estuary.

Mercury methylation by a microbial community from sediments of the Adour Estuary (Bay of Biscay, France)

In order to study the influence of microorganisms on the mercury biogeochemistry, the metal content and the structure of microbial communities were determined in sediments from stations along the Adour Estuary. The comparison of the bacterial communities and their distribution in function of the environmental parameters by Canonical Correspondence Analysis (CCA) revealed the influence of metals on the bacterial communities structure. Sediments where the bacterial communities are mostly influenced by methylmercury were incubated in slurries with or without mercury, under oxic and anoxic conditions. Methylmercury production was detected in the anoxic biotic slurries with a net methylation yield of 0.3% after 24 h. CCA based on T-RFLP profiles revealed the impact of mercury addition on the bacterial communities structure. In addition, 17 bacterial strains, mainly sulphate-reducing bacteria involved in mercury methylation, were isolated and identified.

Characterization of Desulfomicrobium salsuginis sp. nov. and Desulfomicrobium aestuarii sp. nov., two new sulfate-reducing bacteria isolated from the Adour estuary (French Atlantic coast) with specific mercury methylation potentials

Three strains of sulfate-reducing bacteria (ADR21, ADR26 and ADR28) were isolated from Adour estuary sediments (French South Atlantic coast). Cells of these isolates were rod-shaped, motile and stained Gram-negative. The 16S rRNA and dsrAB genes sequence analyses indicated that these three strains belonged to the genus Desulfomicrobium within the delta Proteobacteria, with Desulfomicrobium escambiense strain DSM10707 T as their closest relative. According to phenotypic characteristics, strains ADR21 and ADR28 could be considered as members of the same species. The relatedness values, based on DNA–DNA hybridization studies, between strains ADR21/DSM10707 T, ADR26/DSM10707 T and ADR21/ADR26 ranged between 30.6–40.8%, 45.2–43.0% and 19.0–26.4%, respectively. Strains ADR21 and ADR28 grew well on lactate, fumarate, malate, formate, ethanol and H 2/acetate in the presence of sulfate as an electron acceptor. Thiosulfate, nitrate, fumarate and DMSO were alternative electron acceptors. Malate was well fermented but pyruvate and fumarate only poorly. Strain ADR26 could not grow on ethanol or fumarate and was unable to use DMSO or fumarate as electron acceptors. The three new strains exhibited differences compared to the type strain of D. escambiense, such as temperature optima, substrate utilization and mercury methylation capacities. On the basis of both genetic and phenotypic evidences, strain ADR21 is proposed as the type strain of the species Desulfomicrobium salsuginis sp. nov., and strain ADR26 as the type strain of the species Desulfomicrobium aestuarii sp. nov.