Mercury Gradient Research Articles

Mercury Bioaccumulation in Benthic Invertebrates: From Riverine Sediments to Higher Trophic Levels

Riverine sediments are important sites of mercury methylation and benthic invertebrates may be indicators of Hg exposure to higher organisms. From 2014 to 2018, sediments and invertebrates were collected along a mercury gradient in the Toce River (Northern Italy) and analyzed for THg and MeHg. Concentrations in invertebrates, separated according to taxon and to Functional Feeding Group, ranged from 20 to 253 µg kg−1 dry weight (d.w.) for THg, increasing from grazers (Leuctra, Baetis, Serratella) to predators (Perla). MeHg ranged from 3 to 88 µg kg−1 d.w. in biota, representing 6–53% of THg, while in sediments it was mostly below LOD (0.7 µg kg−1), accounting for ≤3.8% of THg. The Biota-Sediment Accumulation Factor (BSAF, ranging 0.2–4.6) showed an inverse relation to exposure concentrations (THg in sediments, ranging 0.014–0.403 µg kg−1 d.w.) and to organic carbon. THg in invertebrates (up to 73 µg kg−1 wet weight), i.e., at the basal levels of the aquatic trophic chain, exceeded the European Environmental Quality Standard for biota (20 µg kg−1 w.w.), posing potential risks for top predators. Concentrations in adult insects were close to those in aquatic stages, proving active mercury transfer even to terrestrial food chains.

Characteristics of archaea and bacteria in rice rhizosphere along a mercury gradient

Several strains of archaea have the ability to methylate or resist mercury (Hg), and the paddy field is regarded to be conducive to Hg methylation. However, our knowledge of Hg-methylating or Hg-resistant archaea in paddy soils is very limited so far. Therefore, the distribution of archaea and bacteria in the rhizosphere (RS) and bulk soil (BS) of the rice growing in Xiushan Hg-mining area of southwest China was investigated. Bacterial and archaeal 16S rRNA gene amplicon sequencing of the rice rhizosphere along the Hg gradient was conducted. THg concentrations in RS were significantly higher than that in BS at site S1 and S2, while MeHg concentrations in RS was always higher than that in BS, except S6. Bacterial species richness estimates were much higher than that in archaea. The bacterial α-diversity in high-Hg sites was significant higher than that in low-Hg sites based on ACE and Shannon indices. At the genus level, Thiobacillus, Xanthomonas, Defluviicoccus and Candidatus Nitrosoarchaeum were significantly more abundant in the rhizosphere of high-Hg sites, which meant that strains in these genera might play important roles in response to Hg stress. Hg-methylating archaea in the paddy field could potentially be affiliated to strains in Methanosarcina, but further evidence need to be found. The results provide reference to understand archaeal rhizosphere community along an Hg gradient paddy soils.

Structural and Functional Responses of Macrobenthic Communities to Mercury Contamination

This study focuses on the impact of mercury contamination on the structure and functioning of the macrobenthic communities of Ria de Aveiro (Portugal), using secondary production as an indicator of population fitness. Through the implementation of this new approach which combines the static and dynamic variables of a population, it is possible to better understand ecosystems’ responses to external stressors. For that, a monthly monitoring programme was implemented at three different contaminated sites and data collected was analysed combining structural and functional levels. A permutational multivariate analysis of variance (PERMANOVA) detected significant differences in macrobenthic assemblages between the three sampling sites along the mercury gradient. The most contaminated area was characterized by low biomass and species richness, while the least contaminated one presented slightly higher values. A significant negative correlation was observed between mercury concentration in the sediments and biomass of macrobenthic species (r = −0.50; P < 0.05). However, species diversity (heterogeneity) and total secondary production presented irregular patterns in response to mercury contamination. In the present study, the highest values of total biomass and secondary production were, curiously, observed in the intermediate contaminated area, followed by the least contaminated site and finally the most contaminated one. This response seems to be related to the hormesis phenomenon. Moreover, distinct taxonomic and trophic groups showed different responses to mercury contamination, while body size was the trait that best responded to the contaminant stressor.

Field transplantation of the bivalve Scrobicularia plana along a mercury gradient in Ria de Aveiro (Portugal): Uptake and depuration kinetics

The bioaccumulation and depuration capabilities of mercury by the edible bivalve Scrobicularia plana was studied in a coastal lagoon (Ria de Aveiro, Portugal) through a transplantation experiment. Little information on this topic is available in the literature, especially concerning different tissues' responses to contaminant exposure, but the present study is one of the few works that can surpass this knowledge gap. Organisms from a reference area were transplanted to two different contaminated areas in the Ria de Aveiro. In both areas, the bivalves (i.e., entire organism, digestive gland and the rest of the organism) presented a similar saturation model of mercury accumulation, the digestive gland being the tissue that reached the highest concentrations after 25days of exposure to the contaminant. During this short uptake period, the transplanted organisms reached 20–30% of the concentrations observed in resident contaminated organisms. After the exposure period, the organisms were transplanted to a clean area for more than 25days of depuration. At the end of the transplantation period, organisms lost approximately 50% of their mercury body burden (60%: the entire organism and digestive gland; 35%: gills and 40%: the rest of the organism) and the ones from the least contaminated site almost reached the concentrations recorded in the reference area. So, the results suggest that S. plana is a promising biomonitoring species, since it accumulates the contaminant in a considerable extent quite rapidly and at the same time it has a low metal retention capacity (low biological half-life) when exposed to clean sediments.

Biotic methylation of mercury by intestinal and sulfate-reducing bacteria and their potential role in mercury accumulation in the tissue of the soil-living Eisenia foetida

Monomethylmercury as one of the most toxic mercury species influences the health and development of higher organisms and tends to accumulate in the tissue of animals and humans. The aim of this study was to explore the mercury methylating capability of (1) intestinal microbiota of the soil-living earthworm Eisenia foetida (E. foetida) and (2) intestinal sulfate reducing-bacteria in pure cultures. After exposing animals to inorganic mercury chloride (4 mg kg−1 Hg2+) in soil and sterile soil for ten days, the amount of methylmercury in tissue was measured. Despite sterilization of soil, the accumulation of the organic mercury species in tissue was 51 ng g−1. To elucidate the potential of mercury methylation by the intestinal bacterial community, the methylation of inorganic mercury (0.1 mg L−1 Hg2+) in growth media inoculated with soil, gut content and gut epithelia was compared. The media with microorganisms of gut content and gut epithelia reached 710–980 pg mL−1 methylmercury while the methylmercury amount in samples inoculated with soil was below the detection limit. This lead to the assumption that, in this experiment, the soil microbiota is not the main source for methylmercury. Furthermore, we investigated the distribution of mercury in tissue thin-sections of worms kept in soil containing mercury chloride using laser ablation inductively coupled plasma mass spectrometry. Thereby, a mercury gradient with highest signal intensities in the intestines and lowest intensities in the epidermis was measured. These results indicated that the biotic production of methylmercury in the gut of the soil-living earthworm E. foetida is mainly related to the intestinal and, to a much lesser extent, to the soil microbiota in their habitat. Moreover, we demonstrated the capability of intestinal sulfate-reducing bacteria in pure cultures to be a source for methylmercury. Ten of 14 intestinal sulfate-reducing bacteria were found to be able to produce methylmercury in growth medium. In the presence of 0.1 mg L−1 inorganic HgCl2 bacterial cultures produced 100–1200 pg mL−1 methylmercury in 12 h. The mercury methylating species were Desulfovibrio spp., Desulfovibrio piger, Desulfovibrio giganteus, Desulfovibrio termitidis, Desulfotomaculum ruminis and Desulfobulbus propionicus, and two enrichment cultures, Zy1 and EF4, originating from the gut of a damselfly and E. foetida, respectively. To the best of our knowledge, this is the first study concerning biotic methylation including several pure and enrichment strains of intestinal microorganisms and it is a new insight that mercury methylating capability was found within the genus Desulfotomaculum.

Changes in zooplankton communities along a mercury contamination gradient in a coastal lagoon (Ria de Aveiro, Portugal)

The main objective of this paper was to evaluate the impact of mercury on the zooplankton communities’ structure and functioning and their bioaccumulation patterns along a contamination gradient in a temperate coastal lagoon. Our results demonstrated that total abundance was not negatively affected by Hg contamination, since the most contaminated areas presented the highest values, being the copepod Acartia tonsa the dominant species, which means that it is a very well adapted and tolerant species to mercury. Nevertheless, negative effects were observed in terms of species diversity, since the most contaminated areas presented the lowest values of species richness, evenness and heterogeneity. Moreover, the spatial mercury gradient was reflected on the bioaccumulation patterns of the zooplankton communities. This reinforces the idea that zooplankton can be considered as an important vehicle of mercury transfer through the food pelagic web since it constitutes a primordial food resource for several commercial fish species.

Mercury bioaccumulation and the population dynamics of Mesopodopsis slabberi (Crustacea: Mysidacea) along a mercury contamination gradient

The mercury bioaccumulation and population dynamics of the mysid Mesopodopsis slabberi was assessed along a mercury gradient in Ria de Aveiro (Portugal). M. slabberi is one of the most important mysid species in European temperate coastal shallow waters playing a key ecological role. Nevertheless, no references were found concerning the possible consequences of the Hg on the trophodynamics of these coastal ecosystems. M. slabberi showed a clear bioaccumulation along the Hg gradient and through life, with mature females reaching the highest concentrations. In terms of population structure, higher densities and biomasses of M. slabberi were assessed in the most contaminated areas contrarily to the least polluted areas. Despite the mercury accumulation in its tissues no strong negative effects on the structure and population dynamics of the species were observed. However, mysids might be important in the transfer of metals from the sediments and zooplankton to higher trophic levels such as fishes, most of them with commercial interest.

Impact of mercury contamination on the population dynamics of Peringia ulvae (Gastropoda): Implications on metal transfer through the trophic web

The effects of mercury contamination on the population structure and dynamics of the gastropod Peringia ulvae (also known as Hydrobia ulvae) and its impact on the trophic web were assessed along a mercury gradient in Ria de Aveiro (Portugal). The gastropod was revealed to be a tolerant species to the contaminant, since the highest densities, biomasses and growth productivity values were recorded at the intermediate contaminated area followed by the most contaminated one and finally the least contaminated area. P. ulvae was however negatively affected by mercury in terms of growth and life span. So, in the most contaminated area the population was characterised mainly by the presence of juveniles and young individuals. The intermediate contaminated area showed a greater equilibrium in terms of groups' proportion, being the adults the dominant set. The least contaminated area presented intermediate values. P. ulvae life spans were shortest in the most contaminated area (7–8 mo), followed by the least contaminated area (10–11 mo) and finally, the intermediate one (11–14 mo).P. ulvae revealed to be an important vehicle of mercury transfer from sediments to the trophic web, incorporating approximately 15 g of Hg, annually, in the inner area of the Laranjo Bay (0.6 Km2). Therefore, despite P. ulvae being revealed to be not a good bio-indicator of mercury contamination, since it did not suffer profound modifications in its structure and functioning, it is a crucial element in the mercury biomagnification processes throughout the food web.

Mercury’s mitochondrial targeting with increasing age in Scrobicularia plana inhabiting a contaminated lagoon: Damage-protection dichotomy and organ specificities

This study aimed to understand bivalves’ (Scrobicularia plana) adaptive strategies, with emphasis on mitochondria as a target organelle, in response to mercury-contaminated environment. Inter-age and organ-specific approaches were applied using different annual size classes (2+, 3+, 4+ and 5+ years old) and assessing specific organs (gill, digestive gland), respectively. Bivalves were collected from moderately (M) and highly (H) contaminated sites at Laranjo basin - Ria de Aveiro (Portugal), where a mercury gradient was identified, and compared with those from a reference (R) site. Total antioxidants capacity (TAC) was measured in mitochondria, whereas the lipid peroxidation was assessed as a marker of damage. S. plana age determined the clams’ vulnerability towards mercury induced mitochondrial peroxidative damage depending upon the mercury accumulation: younger animals were more vulnerable than older. Clams showed a decreasing trend of TAC with increasing age. This decrease was found to be statistically significant in 4+ and 5+ years at M; whereas, at H, with depleted TAC, remained same until they have grown 5+ years. The organ specificity was evident for antioxidant response and peroxidative damage. A clear pattern of overall TAC increase in digestive gland (at M) and decrease in gill (at H) was observed, while only gills were found to be susceptible to peroxidative damage. Overall, mitochondria proved to be a sensitive fraction for the effects of mercury in S. plana inhabiting mercury contaminated area.

The effects of mercury on the dynamics of the peracarida community in a temperate coastal lagoon (Ria de Aveiro, Portugal)

This study focuses on the impact of mercury on the dynamics and bioaccumulation patterns of suprabenthic peracarida community along a mercury gradient. Suprabenthic peracarida was negatively affected in terms of biomass, diversity and productivity. Mysids including Mesopodopsis slabberi dominated the community, presenting a regular distribution along the Hg gradient, so are considered tolerant species to mercury. Then, isopods, the second most abundant group, dominated in the most contaminated areas being considered also a tolerant group. Contrarily, amphipods were negatively correlated with higher Hg concentrations, dominating in the intermediate areas, thus are considered more sensitive species. In addition, isopods and amphipods were the taxa with higher mercury body burden, followed by mysids. This paper highlights the importance of suprabenthic peracarida as a vehicle of mercury transference through the estuarine trophic web since it is a main link between benthic and pelagic levels and an essential food resource for numerous fish species.

Mercury-Induced Chromosomal Damage in Wild Fish (Dicentrarchus labrax L.) Reflecting Aquatic Contamination in Contrasting Seasons

Ria de Aveiro (mainly Laranjo basin, Portugal) has been subjected to mercury contamination from a chlor-alkali plant, currently presenting a well-described mercury gradient. This study aimed to assess mercury genotoxicity in this area by measuring the frequency of erythrocytic nuclear abnormalities (ENA) in the European sea bass (Dicentrarchus labrax), addressing the relation with total mercury concentration in the blood and the modulatory role of seasonal variables. Fish were collected, in warm and cold periods, at three locations differing in their distances to the main mercury source: reference (R), moderately (M), and highly (H) contaminated sites. Genotoxicity was detected in both degrees of contamination (M and H) and in both periods of the year (warm and cold), which is in line with the greater levels of mercury measured in fish blood. No significant seasonal variations were observed for mercury bioaccumulation or ENA frequency. The apparent low imperviousness of ENA frequency to seasonal factors reinforced its consistency as a genotoxicity biomarker, thus enabling a clearer identification of cause-and-effect relationships. Overall, the results reflected a serious environmental risk to native ichthyofauna at Laranjo basin due to mercury contamination, showing a potential of mercury to induce genetic damage in fish blood cells through clastogenic and/or aneugenic actions.

Lipid peroxidation vs. antioxidant modulation in the bivalve Scrobicularia plana in response to environmental mercury—Organ specificities and age effect

This study aimed at the assessment of mercury burden and its association to damage vs. antioxidant protection in the bivalve Scrobicularia plana environmentally exposed to mercury. Inter-age and organ-specific approaches were applied by using different annual size classes (2+, 3+, 4+ and 5+ year old) and assessing specific organs (gills, digestive gland), respectively. Bivalves were collected from moderately and highly contaminated sites at Laranjo basin – Ria de Aveiro (Portugal), where a mercury gradient was identified, and compared with those from a reference site. Besides total and organic mercury accumulation, endpoints combining lipid peroxidation (LPO), as a damage sign, and antioxidant protection (catalase, CAT; glutathione peroxidase, GPX; glutathione S-transferase, GST, activities) were determined. The whole-body accumulation as total mercury changed according to the environmental gradient for all age classes, while in terms of organic mercury, only 5+-year-old animals showed increased accumulation with increased environmental mercury level. Mercury induced peroxidative damage, showing that antioxidative mechanisms were insufficient. The adaptive capacity to pro-oxidant challenge, expressed as antioxidant induction and lesser vulnerability to enzyme inhibition, increased with age. The specific analyses of gills and digestive gland revealed that both organs were able to mirror external levels of exposure in the accumulation of total and organic mercury. Nevertheless, gills displayed higher potential to accumulate organic forms. The organ specificity was evident for antioxidant response with a clear pattern of overall increase in gills and decrease in digestive gland. In addition, only gills displayed statistical correlations between oxidative stress responses and mercury accumulation. Besides the contribution to understand mercury toxicodynamics, specific organs approach is strongly recommended in order to avoid misinterpretations. The use of whole-body analyses can be particularly compromising when oxidative stress responses (rather than mercury accumulation) are addressed.

The macrobenthic community along a mercury contamination in a temperate estuarine system (Ria de Aveiro, Portugal)

Mercury is a widely distributed environmental pollutant and a toxic element to all living organisms. This study represents an attempt to evaluate its correlation with the macrobenthic community structure in a temperate estuarine system, the Ria de Aveiro (Portugal). The macrobenthic community structure showed significant differences along the mercury gradient. Overall, the increase of mercury contamination was associated with reduced total abundance, lower species richness, and dominance of tolerant taxa. The polychaetes Hediste diversicolor and Alkmaria romijni, and the isopod Cyathura carinata were associated with high levels of mercury in sediment, while the bivalves Scrobicularia plana, Cerastoderma edule and Abra alba showed higher density in lesser contaminated sites. Furthermore, highly contaminated sites showed substantially lower abundances of surface-deposit feeders and herbivores, and higher abundances of subsurface-deposit feeders and omnivores. Salinity was also a significant factor affecting the community, being responsible for seasonal variations in the macrobenthic assemblages.

Long-term effects of mercury in a salt marsh: Hysteresis in the distribution of vegetation following recovery from contamination

During four decades, the Ria de Aveiro was subjected to the loading of mercury from a chlor-alkali industry, resulting in the deposition of several tons of mercury in the sediments. The present study evaluates the impact of this disturbance and the recovery processes, temporally and spatially, by means of examining the richness of the species of salt marsh plants and mercury concentrations in sediments over the last fifty years. The temporal assessment showed that the mercury loading induced a shift in the species composition of the salt marsh from a non-disturbed salt marsh with higher species richness to an alternative state dominated by Phragmites australis. The horizontal assessment, through a mercury gradient, presents the same trend, indicating that P. australis is the species most tolerant to higher mercury concentrations, comparative to Halimione portulacoides, Arthrocnemum fruticosum, Triglochin maritima, Juncus maritimus and Scirpus maritimus. After the reduction of mercury discharges in 1994, the salt marsh shows a slowly return path recovery response. The hysteresis in the response results in the temporal gap between the reduction in mercury concentrations in the sediment and the salt marsh species richness response, comparatively to the existing diversity in the local reference marsh.

Erythrocytic nuclear abnormalities in wild and caged fish ( Liza aurata) along an environmental mercury contamination gradient

Laranjo basin (Aveiro, Portugal) has been subjected to mercury contamination from a chlor-alkali plant, presenting a well-described mercury gradient. This study aims the assessment of mercury genotoxicity in this area by measuring erythrocytic nuclear abnormalities (ENA) frequency in the mullet Liza aurata, and its relation with total mercury concentration (Hg t) in blood. Wild fish were seasonally analysed, and, complementarily, fish were caged for 3 days at three locations differing on their distances to the mercury source. The results from Laranjo were compared with those from a reference area (S. Jacinto). Wild fish from Laranjo showed elevated ENA frequency in summer and autumn in concomitance with increased blood Hg t. Surprisingly, no ENA induction was found in winter, despite the highest blood Hg t, which may be explained by haematological dynamics alterations, as supported by a decreased immature erythrocytes frequency. Caged fish displayed ENA induction only at the closest site to the contamination source, also showing a correlation with blood Hg t.

Pattern and annual rates of Scrobicularia plana mercury bioaccumulation in a human induced mercury gradient (Ria de Aveiro, Portugal)

Due to the lack of knowledge regarding annual bioaccumulation rates in estuarine and marine fauna, the main aim of this work was to study the annual mercury bioaccumulation in the well-documented bivalve species Scrobicularia plana along a human induced mercury gradient in the Ria de Aveiro coastal lagoon (Portugal) and in a nearby, non-polluted system (Mondego estuary), parallel to the risks associated with its consumption by humans. Minimum total mercury concentration was as low as 0.019 mg kg −1 (wwt) in 4+ year old organisms in the reference site, where a significant negative correlation ( p < 0.05) was found between total mercury concentrations and size, resulting in negative bioaccumulation rates (detoxification). On the other hand, values reached 1.8 mg kg −1 (wwt) in 3+ year old bivalves from the most contaminated area, where a strong positive correlation with size was found ( p < 0.01) and annual bioaccumulation rates were as high as 0.25 mg kg −1 yr −1. Annual bioaccumulation rates were highly correlated with suspended particulate matter mercury concentrations. Even though the levels of organic mercury contents increased parallel to the contamination gradient, at each sampling station, no increment was found with age, which corresponded to a decrease in organic mercury percentage with age. In terms of ecological management and public health, the ratio of 0.01 consistently found between Scrobicularia plana annual mercury accumulation rates and SPM mercury levels for most sites may permit to roughly estimate S. plana contamination of commercial sized individuals (>2.5 cm) and, if verified and confirmed in other systems, be used as a simple management tool.

Macroalgae response to a mercury contamination gradient in a temperate coastal lagoon (Ria de Aveiro, Portugal)

Primary producers represent an important pathway for mercury incorporation in aquatic food webs. With eutrophication processes occurring worldwide, macroalgae may represent a substantial pool of mercury, as a result of its high growth rate and capacity to bind trace metals. The main aim of this work was to evaluate the response of the macroalgae to a human-induced environmental mercury gradient in a temperate coastal lagoon, by assessing the total and organic mercury contamination levels of the dominant species ( Enteromorpha, Fucus and Gracilaria). Total mercury in the plant tissues ranged from 0.02 to 2.1 μg g −1 dwt. Fucus was the most contaminated algae, followed by Gracilaria and Enteromorpha. As a whole, organic mercury never exceeded 15% of total mercury content, but tended to increase with distance to metal source on all macroalgae indicating complex physiological responses from these primary producers in areas of high and low mercury concentrations. Sessile macroalgae may be important mercury immobilisation agents, while free-floating algae ( Enteromorpha) play an important role in mercury transport from contaminated areas (±10 g ha −1) to other areas of the lagoon and even to coastal waters. Based on the present results the use of macroalgal biomass from contaminated areas for direct or indirect human use (e.g. agricultural, industrial and food purposes) may result in health risks, due to the high bioaccumulation capacity (as high as 10 4 the dissolved mercury concentrations).

Distribution of anthropogenic mercury in French Guyana river sediments downstream from gold mining sites

Total mercury concentrations in French Guyana river sediments were determined near gold mining sites and compared to unaffected reference sites. In small meter-size rivers the anthropogenic impact is localized and characterised by high total mercury peak concentrations in the mg/kg dry weight range (St. Elie site). The mercury load is more diluted in larger water streams such as the Inini river system where a mercury gradient was observed along a 50 km flow path downstream with 510, 295, 261 ng/g total mercury concentrations in the sediments at 0, 35 and 50 km distance from the Dorlin mining site. Sediments situated upstream the gold mining sites compared to the geochemical background concentrations indicating that the impact related to atmospheric deposition is relatively insignificant. The mercury content of upstream sediments compare to the mercury load of unaffected soils and river sediments collected in distant virgin sites (less than 400 ng/g total mercury).

The effect of long-term mercury pollution on the soil microbial community.

The effect of long-term exposure to mercury on the soil microbial community was investigated in soil from three different sites along a pollution gradient. The amount of total and bioavailable mercury was negatively correlated to the distance from the center of contamination. The size of the bacterial and protozoan populations was reduced in the most contaminated soil, whereas there was no significant difference in fungal biomass measured as chitinase activity. Based on the number of colony morphotypes, moreover, the culturable bacterial population was structurally less diverse and contained a higher proportion of resistant and fast-growing forms. The profiles of amplified 16S rDNA sequences obtained from community DNA by denaturating gradient gel electrophoresis (DGGE) also reflected the altered community structure and decreased diversity along the mercury gradient as expressed in terms of the number and abundance of bands. The functional potential of the microbial population measured as sole carbon source utilization by Ecoplates((R)) differed between the soils, but there was no change in the number of substrates utilized. The observed changes in the different soil microbial populations are probably a combination of both direct and indirect effects of the mercury contamination.

The effect of long-term mercury pollution on the soil microbial community

The effect of long-term exposure to mercury on the soil microbial community was investigated in soil from three different sites along a pollution gradient. The amount of total and bioavailable mercury was negatively correlated to the distance from the center of contamination. The size of the bacterial and protozoan populations was reduced in the most contaminated soil, whereas there was no significant difference in fungal biomass measured as chitinase activity. Based on the number of colony morphotypes, moreover, the culturable bacterial population was structurally less diverse and contained a higher proportion of resistant and fast-growing forms. The profiles of amplified 16S rDNA sequences obtained from community DNA by denaturating gradient gel electrophoresis (DGGE) also reflected the altered community structure and decreased diversity along the mercury gradient as expressed in terms of the number and abundance of bands. The functional potential of the microbial population measured as sole carbon source utilization by Ecoplates ® differed between the soils, but there was no change in the number of substrates utilized. The observed changes in the different soil microbial populations are probably a combination of both direct and indirect effects of the mercury contamination.