Mineralization in the Ems-Dollard estuary effects on carbon and oxygen budgets

Summary The Ems estuary is a tidal system, showing typical estuaries characteristics, such as gradients of salinity and suspended matter in the water phase. The tidal amplitude is 2.5–3 m. Tidal flats cover 40% of the total area in the outer part and 75% in the innermost part of the estuary, the Dollard. In the sediment aerobic heterotrophic bacteria are concentrated in the upper 2 cm, the numbers rapidly decreasing with depth.Oxygen production and consumption rates in the sediment, and oxygen consumption in the water are measured, together with environmental parameters and numbers of aerobic heterotrophic bacteria. Using a conversion factor of 12/32, oxygen measurements are translated to organic carbon. Input of organic carbon from external sources (particulate carbon from the river Ems and the North Sea, and organic waste discharge) is calculated for the Dollard. An organic carbon budget for the Dollard, using these calculated figures from one year's measurements, showed that most of the organic carbon entering the systems, originated from external sources.Mineralization was quantitatively more important in the sediment than it was in the water phase. The calculated import and production of organic carbon in the system was larger than the calculated consumption. An explanation for this difference is probably the export of soluble organic carbon, which was not measured, to the adjoining Waddensea, which must be considerable. It was concluded that heterotrophic bacteria must play an important role in carbon fluxes in the Dollard and that studies of growth yield under in situ conditions are necessary for a better understanding of their role in the ecosystem.     

Variation of a benthic heterotrophic bacteria community with different respiratory metabolisms in Coyuca de Benítez coastal lagoon (Guerrero, Mexico)

The fluctuations of the number, biomass and composition of the heterotrophic community were studied daily for two days, according to depth, pH, Eh, O2 and organic carbon concentration within a zone of the canal between the Coyuca de Benitez lagoon (Guerrero, Mexico) and the coastal waters. At the three moments of the day studied (6 am, 2 pm and 10 pm), the oxygen concentrations in the overlying water and in the superficial sediment layer were near air-saturation in the diurnal samplings (582 microM at 6 am and 665 microM at 2 pm), and sub-satured during the night (158 microM). In the sediments, the models of vertical distribution of Eh and organic carbon distributions were very irregular due to the bio-perturbation of the benthic, meio- and macrofauna, whose activity allows the superficial organic carbon to migrate towards sediment deeper layers. Vertical distribution of the different viable bacteria populations seems to be related to the hydrodynamic patterns of the communicating canal and sediments heterogeneity. In the sediment column, the heterotrophic bacteria total number varied from 6.8 to 20.3 x 108 cells cm(-3). The highest heterotrophic bacterial biomass values were encountered during the diurnal samplings (39.2 microgC.l(-1) at 6 am and 34.4 microgC.(l(-1) at 2 pm) and the lowest during the night (9.7 microgC.l(-1). The fluctuations of viable heterotrophic bacteria populations with different respiratory metabolisms (aerobic, microaerophilic and anaerobic) can be explained by the existence of suboxic microniches that appear when particles of sediment are resuspended due to the water circulation and the benthic infauna excavating activity, that allows the supernatant water oxygen to penetrate through its galleries towards deeper sediment zones. The statistical analysis (Multiple lineal regression model r2 > or = 0.5) showed that the on the whole, the hydrological parameters are not influence over the bacterial number and bacterial biomass distribution (r2 < or = 0.5), Nevertheless, the variations of the heterotrophic bacteria community observed in the two days sampling, seem to be governed (with F-values of 0.6 to 0.9) by the irregular flows of bio-available organic material and the sediment porosity.     

Survival and Activity of Bacteria in a Deep,Aged Lake Sediment (Lake Constance)

Abstract Viable counts and potential activities of different bacteria were determined as a function of depth in the deep profundal sediment of Lake Constance, Germany. The sediment layer at the bottom of the lake had a total depth of about 7 m and was deposited in the time after the last ice age, i.e., over the past 13,000 years. The high clay content of the sediment prevents seepage. Below 25 cm all of the viable heterotrophic bacteria were present as heat-resistant spores. Numbers of viable spores of both aerobic and anaerobic heterotrophic bacteria decreased exponentially with sediment depth and were below the detection limit (5–55 cells ml⁻¹) at 4–6 m, i.e., in about 8,900-year-old sediment. Absence of viable heterotrophic bacteria in deeper sediment layers demonstrated that aseptic sampling conditions were achieved. The decrease of viable spores with depth may be interpreted as time-dependent death of spores resulting in a death rate of about 0.0013–0.0025 year⁻¹. Viable units of specific metabolic groups of bacteria were detected only in the upper sediment layers (0–50 cm). Nitrifying bacteria could not be detected below 30 cm. Methane-oxidizing bacteria were present in the sediment down to >30 cm, but were in a dormant state. Nitrate reduction activity decreased by a factor of 6 within the upper 25 cm of the sediment, but was still detected at 50 cm. Sulfate reduction, on the other hand, could not be detected at depths of 20 cm and below. By contrast, methanogenesis and methanogenic bacteria could be detected down to 50 cm. These observations indicate that bacteria eventually become nonviable in aged sediments. Received: 5 March 1996; Accepted: 12 March 1996     

Distribution of aerobic bacteria,protozoa, algae,and fungi in deep subsurface sediments

The distribution of microorganisms in deep subsurface profiles was determined at three sites at the Savannah River Plant, Aiken, South Carolina. Acridine orange direct counts (AODC) of bacteria were highest in surface soil samples and declined to the 10⁶ to 10⁷ per gram range in the subsurface, but then did not decline further with depth. In the subsurface, AODC values varied from layer to layer, the highest being found in samples from sandy aquifer formations and the lowest in clayey interbed layers. Sandy aquifer sediments also contained the highest numbers of viable bacteria as determined by aerobic spread plate counts (CFU) on a dilute heterotrophic medium. In some of these samples bacterial CFU values approached 100% of the AODC values. Viable protozoa (amoebae and flagellates, but no ciliates) were found in samples with high bacterial CFU values. A variety of green algae, phytoflagellates, diatoms, and a few cyanobacteria were found at low population densities in samples from two of the three boreholes. Low numbers of fungi were evenly distributed throughout the profiles at all three sites. Microbial population density estimates correlated positively with sand content and pore‐water pH, and negatively with clay content and pore‐water metal concentration. A large diversity of prokaryotic and eukaryotic microorganisms was found in samples with high population densities. A survey of bacterial strains isolated from subsurface samples revealed associations of gram‐positive bacteria with high clay sediments and gram‐negative bacteria with sandy sediments. The ability to deposit lipophilic storage material (presumably poly‐ß‐hydroxybutyrate) was found in a high proportion of isolates from sandy sediments, but only rarely in isolates from high clay sediments.     

Temporal Variations in Heterotrophic Mesophilic Bacteria from a Marine Shallow Hydrothermal Vent off the Island of Vulcano (Eolian Islands, Italy)

Abstract The fluctuations of the total microbial abundance, the culturable heterotrophic bacterial population, and the composition of heterotrophic bacteria were investigated in relation to environmental parameters in a shallow, marine hydrothermal vent off the Island of Vulcano (Eolian Islands, Italy). Standing stock dynamics were studied by measuring the total population of picoplankton by direct count and the population of viable heterotrophic bacteria in water and sediment samples collected monthly. The environmental factors most strongly linked to the total microbial abundance and heterotrophic bacterial populations were pH and H₂S content in water and C/N ratio in sediment samples. The pattern of variation of microbial populations associated with water was different from those associated with sediment. Assessment of the qualitative composition of aerobic heterotrophic bacterial communities was based on 30 morphological and biochemical characteristics for each strain. Numerical analysis was used for an initial survey of the similarity among the isolates. The data were successively used to determine the structure and the metabolic potential of water and sediment bacteria. Metabolic properties varied between water- and sediment-isolated bacteria. Bacteria from water were structurally more diverse, and active in the use of carbohydrates, than those from sediment. Moreover, most of the sediment bacteria were able to grow at a high temperature (60 and 70°C). The fluctuations of bacterial characteristics in relation to environmental parameters present an evident temporal variation in water, but not in the sediment habitat. Received: 13 January 1997; Accepted: 7 August 1997     

Growth of legionella and other heterotrophic bacteria in a circulating cooling water system exposed to ultraviolet irradiation

The effect of ultraviolet irradiation on the growth and occurrence of legionella and other heterotrophic bacteria in a circulating cooling water system was studied. Water of the reservoir was circulated once in 28 h through a side-stream open channel u.v. radiator consisting of two lamps. Viable counts of legionellas and heterotrophic bacteria in water immediately after the u.v. treatment were 0—12 and 0·7—1·2% of those in the reservoir, respectively. U.v. irradiation increased the concentration of easily assimilable organic carbon. In the u.v. irradiated water samples incubated in the laboratory the viable counts of heterotrophic bacteria reached the counts in reservoir water within 5 d. The increase in viable counts was mainly due to reactivation of bacterialcells damaged by u.v. light, not because of bacterial multiplication. Despite u.v. irradiation the bacterial numbers in the reservoir water, including legionellas, did not decrease during the experimental period of 33 d. The main growth of bacteria in the reservoir occurred in biofilm and sediment, which were never exposed to u.v. irradiation.     

Seasonal seawater temperature as the major determinant for populations of culturable bacteria in the sediments of an intact mangrove in an arid region

Mangroves are highly productive marine ecosystems where bacteria (culturable and non-culturable) actively participate in biomineralization of organic matter and biotransformation of minerals. This study explores spatial and seasonal fluctuations of culturable heterotrophic bacteria and Vibrio spp. in the sediments of an intact mangrove ecosystem and determines the dominant environmental factors that govern these fluctuations. Sediment samples were collected monthly from three stations in the mangroves of Laguna de Balandra, Baja California Sur, Mexico, through an annual cycle. Physicochemical parameters included seawater temperature, salinity, and concentration of dissolved oxygen. Viable counts of culturable heterotrophic bacteria and Vibrio spp. were made. In one sample (March 2003), nutrient concentrations (ammonium, nitrites, nitrates, and phosphates), organic matter, pH and sediment texture were also determined. General cluster analyses, analysis of variance of specific variables, and several principal component analyses demonstrated that seawater temperature is the principal determinant of seasonal distribution of culturable heterotrophic bacteria and Vibrio spp. in mangrove sediments. Soil texture, concentration of nutrients, and organic matter concentration contribute to heterogenicity to a lesser extent. A large spatial variation in bacterial populations was observed over short distances ( approximately 1 m) in sampling areas within the same site. These analyses show that the culturable bacterial distribution in sediments of mangroves has high spatial and temporal heterogeneity.     

Long-term investigations on the changes of the MPN values of bacterial communities participating in the sulphur cycle in Lake Velencei,Hungary

Lake Velencei is a shallow lake with a fairly variable water quality due to the effect of the earlier basin reconstruction and external factors, such as the weather and the organic matter loading from the catchment areas. For studying the changes in sediment of Lake Velencei, MPN technique, based bacteriological investigations, as well as measurements of water chemical parameters were performed. Microbial communities of aerobic thiosulphate-oxidising, anaerobic phototrophic, and sulphate-reducing bacteria taking part in the sulphur cycle were monitored between 1993 and 2001. In the western part of the lake the bacteriological results and the chemical parameters verified the presence of sulphuretum. In the first low water period of the studies the quantity of the bacteria contributing to the sulphur cycle was low in the sediment. After the increase of the water level the distribution of these microbes became somewhat homogeneous in the sediment. However, the different sediment regions might be characterised with dissimilar MPN values of the studied bacterial communities.

     

Bacterial activities in the sediment of Lake Velencei,Hungary

Lake Velencei (Hungary) is one of the westernmost shallow soda lakes, extending from Eastern Europe to the Carpatian basin. The spatial and temporal distribution of the sediment microbiota, the metabolic potential of bacterial communities and the species composition of the genera Bacillus and Clostridium, as well as sulphate-reducing bacteria (SRB) were investigated regarding the close interactions between the lake sediment and the overlaying water column, the special water chemical parameters, and the extensive reed coverage of the lake. Aerobic microbial activities were tested with community-level physiological profiling (CLPP) using BIOLOG microplates. The quantification of the anaerobic fermentative and sulphate-reducing bacteria was done by the MPN (Most Probable Number) method. The cultivation of bacteria adapted to the special physico-chemical characteristics of the lake was carried out employing selective media. Multivariate analysis of CLPP data indicated that the microbial communities of the sediment separated from that of the water and showed seasonal variations of the utilised carbon sources. The results of the MPN demonstrated that the counts of the fermentative and sulphate-reducing bacteria in the reed rhizosphere were about one order higher than in the sediment. Among the isolated bacterial strains, a large number were characterised as facultative or obligate alkaliphilic and also moderately halophilic. The partial sequencing of 16S rDNA of the selected representatives resulted in species of aerobic bacteria, such as Bacillus pseudofirmus, B. halmapalus, B. cohnii, B. (Marinibacillus) marinus, and anaerobes, such as Clostridium putrificum – sporogenes, C. scatologenes, C. bifermentans, Desulfotomaculum guttoideum, Desulfovibrio alcoholivorans, and Desulfovibrio burkinensis.

     

Bacterial Activity in a Reservoir Determined by Autoradiography and its Relationships to Phyto- and Zooplankton

In the drinking water reservoir Římov (Southern Bohemia) bacterioplankton was studied during 1983. Special attention was given to the relationships between parameters of bacterial abundance, total and individual activity. Bacterial counts and biomass was assessed and autoradiographic determinations of the proportion of active bacteria incorporating thymidine (Th) and mixture of amino acids (AA) and total uptake rate of AA were made over a year in the surface layer and during summer stratification from the thermocline and 15 m depth. Specific activity of metabolically active bacteria (SAMAB) and specific activity per unit of biomass (SAUB) were negatively correlated with counts of metabolizing cells and with bacterial biomass, respectively. Total and individual heterotrophic activity and counts of bacteria coincided with the changes of phytoplankton biomass, whereas bacteria incorporating Th were more tightly correlated with primary production. The most significant relation of metabolically active bacteria was found to cladoceran biomass. Thus, this part of heterotrophic bacterial activity seems to be stimulated by leakage of dissolved organic matter from phytoplankton being disrupted and incompletely digested by cladocerans rather than from healthy photosynthetizing cells.     

The ecology of mercury-resistant bacteria in Chesapeake Bay

Total ambient mercury concentrations and numbers of mercury resistant, aerobic heterotrophic bacteria at six locations in Chesapeake Bay were monitored over a 17 month period. Mercury resistance expressed as the proportion of the total, viable, aerobic, heterotrophic bacterial population reached a reproducible maximum in spring and was positively correlated with dissolved oxygen concentration and sediment mercury concentration and negatively correlated with water turbidity. A relationship between mercury resistance and metabolic capability for reduction of mercuric ion to the metallic state was established by surveying a number of HgCl₂-resistant cultures. The reaction was also observed in microrganisms isolated by differential centrifugation of water and sediment samples. Mercuric ion exhibited an average half-life of 12.5 days in the presence of approximately 10⁵ organisms/ml. Cultures resistant to 6 ppm of mercuric chloride and 3 ppm of phenylmercuric acetate (PMA) were classified into eight generic categories.Pseudomonas spp. were the most numerous of those bacteria capable of metabolizing both compounds; however, PMA was more toxic and was more selective forPseudomonas. The mercury-resistant generic distribution was distinct from that of the total bacterial generic distribution and differed significantly between water and sediment, positionally and seasonally. The proportion of nonglucose-utilizing mercury-resistantPsuedomonas spp. was found to be positively correlated with total bacterial mercury resistance. It is concluded from this study that numbers of mercury-resistant bacteria as established by plate count can serve as a valid index ofin situ Hg²⁺ metabolism.     

Bacterial [methyl-3H]thymidine incorporation in substrate-amended estuarine sediment slurries

Abstract Five different bacterial communities were enriched in substrate-amended slurries of sediment from the Tay Estuary, Scotland. During incubation of the slurries, concentrations of volatile fatty acids, sulphate, sulphide and methane were monitored to clearly define the activity of the stimulated populations. An aerobic population, a ‘microaerophilic’ population and three anaerobic populations (fermentative heterotrophs, sulphate-reducing bacteria and methanogens plus acetogens) were established to reflect community growth and metabolism both in surface oxic and deeper anoxic layers. Similar numbers of cells involved in division were observed in all five slurries, demonstrating the potential for bacterial production. Thymidine incorporation rates in glucose-stimulated slurries under both aerobic and fully anaerobic conditions were similar, confirming the ability of fermentative anaerobic heterotrophs to incorporate [ methyl -³H]thymidine into DNA during growth. Although anaerobic communities of sulphate-reducing, acetogenic plus methanogenic bacteria were stimulated and actively growing, they did not incorporate [ methyl -³H]thymidine into DNA. Since the thymidine technique does not measure the growth of these important groups, calculated productivity values based upon thymidine incorporation within anoxic sediment systems will be substantially underestimated, even if growth substrates are not limiting.     

Environmental factors affecting the occurrence of mycobacteria in brook sediments

The occurrence of mycobacteria was studied in aerobic brook sediments from 39 drainage areas in Finland. The culturable counts of mycobacteria were related to climatic conditions, characteristics of the drainage area, chemical characteristics of the sediment and water, culturable counts of other heterotrophic bacteria, and microbial respiration rate in the sediment. The counts of mycobacteria varied from 1·1 × 10² to 1·5 × 10⁴ cfu g⁻¹ dry weight of sediment. They correlated positively with the proportion of the drainage area consisting of peatland, total content of C, content of Pb, microbial respiration rate in the sediment, and chemical oxygen demand of the water. The correlations of the mycobacterial counts with pH of sediment and alkalinity of water were negative. The results of the present sediment study and of the forest soil study published earlier strongly suggest that an increase in acidity increases the counts of mycobacteria and decreases the counts and activity of other heterotrophic bacteria. Mycobacterial counts were more than 100 times higher (per dry weight) in forest soils with pH 3·5–4·3 than in sediments with pH 4·5–6·3.     

Aerobic metabolic potential of microbial populations indigenous to deep subsurface environments

Subsurface sediment samples, collected from three boreholes ranging in depths from 0.1 to 260 m, were used in substrate mineralization studies to examine the aerobic metabolic potential of microbial populations indigenous to the deep subsurface. Mineralization was measured by quantifying the amount of ¹⁴CO₂ released from radiolabeled acetate, phenol, or 4‐methoxybenzoate added to subsurface sediments at 10 μg g^‐1. Mineralization of the three compounds was observed in all but a few of the subsurface samples and did not decrease with depth. In addition, mineralization data collected from similar geologic formations from the different boreholes indicated that there was significant lateral continuity of microbial activity. Regression analyses were performed to determine which environmental factors were related to microbial metabolic potential. Mineralization was positively correlated with heterotrophic abundance as measured by plate counts. Other parameters that appeared to influence metabolic potential included pH and clay content.     

The distribution and diversity of culturable aerobic heterotrophic benthic bacteria in the continental slope of the Bay of Bengal: Linked abiotic factors, including a tsunami

The culturable aerobic heterotrophic benthic bacterial population and community structure in relation to the physico-chemical parameters in the continental slope of the Bay of Bengal was studied. In addition, diversity indices were calculated and pretsunami (in 2004) and post-tsunami (in 2005) diversity values were compared. Sediment samples were collected from two cruises in the depth zone of 214–1000 m (10°36′ N–20°01′ N and 79°59′ E–87°30′ E). The vertical distribution of the total heterotrophic bacterial population during both cruises was higher in the top section (0–3 cm) of the sediment. The average total heterotrophic bacterial population was in the range of 0.42–37.38 × 10⁴ CFU/g to 1.66–19.73 × 10⁴ CFU/g dry sediment weight during the two cruises, respectively. The limiting physico-chemical factors were sediment pH, sediment temperature, TOC, porosity, and clay as revealed from multiple regression (r = 0.75) and BIOENV (Partial Correlation ρω = 0.447) analyses. The shannon-Wiener index (H′ log e), Simpson index (D), Margalef index (d) and Pielou’s evenness index (J′) were found to be higher in the 1000 m depth stations. Cluster analysis showed that the 500 m depth stations clustered either with the 200 m or with the 1000 m stations. The 200 m depth stations never formed a cluster with the 1000 m stations. Pre-tsunami diversity indices at two depth ranges (200 m and 1000 m) were higher than those of the post-tsunami indices, which was quite evident from the cluster analysis as well. This study confirms the effect of the tsunami surge in the sediments of the continental slope of the Bay of Bengal in the marine ecosystem, which is also attributed to the temporal variation of the heterotrophic bacterial population and diversity.     

The bacteria of the sulphur cycle

This paper concentrates on the bacteria involved in the reductions and oxidations of inorganic sulphur compounds under anaerobic conditions. The genera of the dissimilatory sulphate-reducing bacteria known today are discussed with respect to their different capacities to decompose and oxidize various products of fermentative degradations of organic matter. The utilization of molecular hydrogen and formate by sulphate reducers shifts fermentations towards the energetically more favourable formation of acetate. Since acetate amounts to about two-thirds of the degradation products of organic matter, the complete anaerobic oxidation of acetate by several genera of the sulphate-reducing bacteria is an important function for terminal oxidation in sulphate-sufficient environments. The results of pure culture studies agree well with ecological investigations of several authors who showed the significance of sulphate reduction for the complete oxidation of organic matter in anaerobic marine habitats. In the dissimilatory sulphur-reducing bacteria of the genus Desulfuromonas the oxidation of acetate is linked to the reduction of elemental sulphur. Major characteristics of the anaerobic, sulphide-oxidizing phototrophic green and purple sulphur bacteria as well as of some facultative anoxygenic cyanobacteria, are given. By the formation of elemental sulphur and sulphate, these bacteria establish sulphur cycles with the sulphide-forming bacteria. In view of the morphological diversity of the sulphate-reducing bacteria and question of possible evolutionary relations to phototrophic sulphur bacteria is raised.     

Distribution of indicator bacteria and Vibrio parahaemolyticus in sewage-polluted intertidal sediments.

The impact of a sewage point source on the bacterial densities in an intertidal mud flat in Boston Harbor, Mass., was investigated. The area, Savin Hill Cove, acts as a receiving basin for a combined storm and sewage outlet (CSO). Preliminary examination of sediments and overlying water at high tide demonstrated that fecal coliforms were present in sediments at abundances 2 to 4 orders of magnitude higher than in the overlying water column. The following bacterial counts were determined from sediments along a sampling transect extending 460 m from the CSO: total bacteria by epifluorescent microscopy, heterotrophic bacteria by plate counts on nutrient-rich and nutrient-poor media, fecal coliforms and enterococci by membrane filtration, and Vibrio parahaemolyticus by a most-probable-number technique with a resuscitation step. Median sediment grain size, average tidal exposure, carbon/nitrogen ratio, and total organic carbon were also measured. All bacterial indices, except for V. parahaemolyticus, declined significantly with distance from the outfall. Multiple regression analysis indicated that tidal exposure (low tides) may affect densities of total bacteria. Fecal coliforms and enterococci were still present in appreciable numbers in sediments as far as 460 m away from the CSO. In contrast, V. parahaemolyticus densities did not correlate with the other bacterial counts nor with any of the environmental parameters examined. These results indicate that intertidal sediments which adjoin point sources of pollution are severely contaminated and should be considered as potentially hazardous reservoirs of sewage-borne diseases.     

Distribution of indicator bacteria and Vibrio parahaemolyticus in sewage-polluted intertidal sediments

The impact of a sewage point source on the bacterial densities in an intertidal mud flat in Boston Harbor, Mass., was investigated. The area, Savin Hill Cove, acts as a receiving basin for a combined storm and sewage outlet (CSO). Preliminary examination of sediments and overlying water at high tide demonstrated that fecal coliforms were present in sediments at abundances 2 to 4 orders of magnitude higher than in the overlying water column. The following bacterial counts were determined from sediments along a sampling transect extending 460 m from the CSO: total bacteria by epifluorescent microscopy, heterotrophic bacteria by plate counts on nutrient-rich and nutrient-poor media, fecal coliforms and enterococci by membrane filtration, and Vibrio parahaemolyticus by a most-probable-number technique with a resuscitation step. Median sediment grain size, average tidal exposure, carbon/nitrogen ratio, and total organic carbon were also measured. All bacterial indices, except for V. parahaemolyticus, declined significantly with distance from the outfall. Multiple regression analysis indicated that tidal exposure (low tides) may affect densities of total bacteria. Fecal coliforms and enterococci were still present in appreciable numbers in sediments as far as 460 m away from the CSO. In contrast, V. parahaemolyticus densities did not correlate with the other bacterial counts nor with any of the environmental parameters examined. These results indicate that intertidal sediments which adjoin point sources of pollution are severely contaminated and should be considered as potentially hazardous reservoirs of sewage-borne diseases.     

The ecology of mercury-resistant bacteria in Chesapeake Bay

Total ambient mercury concentrations and numbers of mercury resistant, aerobic heterotrophic bacteria at six locations in Chesapeake Bay were monitored over a 17 month period. Mercury resistance expressed as the proportion of the total, viable, aerobic, heterotrophic bacterial population reached a reproducible maximum in spring and was positively correlated with dissolved oxygen concentration and sediment mercury concentration and negatively correlated with water turbidity. A relationship between mercury resistance and metabolic capability for reduction of mercuric ion to the metallic state was established by surveying a number of HgCl₂-resistant cultures. The reaction was also observed in microrganisms isolated by differential centrifugation of water and sediment samples. Mercuric ion exhibited an average half-life of 12.5 days in the presence of approximately 10⁵ organisms/ml. Cultures resistant to 6 ppm of mercuric chloride and 3 ppm of phenylmercuric acetate (PMA) were classified into eight generic categories.Pseudomonas spp. were the most numerous of those bacteria capable of metabolizing both compounds; however, PMA was more toxic and was more selective forPseudomonas. The mercury-resistant generic distribution was distinct from that of the total bacterial generic distribution and differed significantly between water and sediment, positionally and seasonally. The proportion of nonglucose-utilizing mercury-resistantPsuedomonas spp. was found to be positively correlated with total bacterial mercury resistance. It is concluded from this study that numbers of mercury-resistant bacteria as established by plate count can serve as a valid index ofin situ Hg²⁺ metabolism.