Population Dynamics of a Single-Stage Sulfidogenic Bioreactor Treating Synthetic Zinc-Containing Waste Streams

Waste streams from industrial processes such as metal smelting or mining contain high concentrations of sulfate and metals with low pH. Dissimilatory sulfate reduction carried out by sulfate-reducing bacteria (SRB) at low pH can combine sulfate reduction with metal-sulfide precipitation and thus open possibilities for selective metal recovery. This study investigates the microbial diversity and population changes of a single-stage sulfidogenic gas-lift bioreactor treating synthetic zinc-rich waste water at pH 5.5 by denaturing gradient gel electrophoresis of 16S rRNA gene fragments and quantitative polymerase chain reaction. The results indicate the presence of a diverse range of phylogenetic groups with the predominant microbial populations belonging to the Desulfovibrionaceae from δ-Proteobacteria. Desulfovibrio desulfuricans-like populations were the most abundant among the SRB during the three stable phases of varying sulfide and zinc concentrations and increased from 13% to 54% of the total bacterial populations over time. The second largest group was Desulfovibrio marrakechensis-like SRB that increased from 1% to about 10% with decreasing sulfide concentrations. Desulfovibrio aminophilus-like populations were the only SRB to decrease in numbers with decreasing sulfide concentrations. However, their population was <1% of the total bacterial population in the reactor at all analyzed time points. The number of dissimilatory sulfate reductase (DsrA) gene copies per number of SRB cells decreased from 3.5 to 2 DsrA copies when the sulfide concentration was reduced, suggesting that the cells' sulfate-reducing capacity was also lowered. This study has identified the species present in a single-stage sulfidogenic bioreactor treating zinc-rich wastewater at low pH and provides insights into the microbial ecology of this biotechnological process.     

The effect of sulfate reduction on the thermophilic (55°C) methane fermentation process

Summary The continuously operated suspended growth anaerobic contact system was utilized to estimate the effect of sulfate reduction on the thermophilic (55°C) methane fermentation process. Results indicated that reduction in methanogenesis in the presence of sulfate was due to two separate, but related, processes;i.e. competitive and sulfide inhibition. Although prevention of competitive inhibition would be difficult under normal fermenter operation, sulfide inhibition could be minimized by environmental selection of sulfide tolerant microbial populations through biomass recycle and pH control. Stable fermenter operation was achieved at soluble sulfide concentrations as high as 330 mg/l soluble sulfide. Using batch fermenters, a maximum thermophilic sulfate reduction rate of 3.7 mg SO₄ ^2––S/g volatile solids (VS)-day was estimated. The importance of reporting sulfate reduction rates on a biomass basis is demonstrated by a simple population adjustment kinetic model.This research study was conducted at the Department of Agricultural Engineering, Cornell University, Riley Robb Hall, Ithaca, NY 14853, U.S.A.     

Biodegradation of Pentachlorophenol in a Continuous Anaerobic Reactor Augmented with Desulfitobacterium frappieri PCP-1

In this work, a strain of anaerobic pentachlorophenol (PCP) degrader, Desulfitobacterium frappieri PCP-1, was used to augment a mixed bacterial community of an anaerobic upflow sludge bed reactor degrading PCP. To estimate the efficiency of augmentation, the population of PCP-1 in the reactor was enumerated by a competitive PCR technique. The PCP-1 strain appeared to compete well with other microorganisms of the mixed bacterial community, with its population increasing from 10⁶ to 10¹⁰ cells/g of volatile suspended solids within a period of 70 days. Proliferation of strain PCP-1 allowed for a substantial increase of the volumetric PCP load from 5 to 80 mg/liter of reaction volume/day. A PCP removal efficiency of 99% and a dechlorination efficiency of not less than 90.5% were observed throughout the experiment, with 3-Cl-phenol and phenol being observable dechlorination intermediates.     

Development of Sulfidogenic Sludge from Marine Sediments and Trichloroethylene Reduction in an Upflow Anaerobic Sludge Blanket Reactor

The importance of microbial sulfate reduction relies on the various applications that it offers in environmental biotechnology. Engineered sulfate reduction is used in industrial wastewater treatment to remove large concentrations of sulfate along with the chemical oxygen demand (COD) and heavy metals. The most common approach to the process is with anaerobic bioreactors in which sulfidogenic sludge is obtained through adaptation of predominantly methanogenic granular sludge to sulfidogenesis. This process may take a long time and does not always eliminate the competition for substrate due to the presence of methanogens in the sludge. In this work, we propose a novel approach to obtain sulfidogenic sludge in which hydrothermal vents sediments are the original source of microorganisms. The microbial community developed in the presence of sulfate and volatile fatty acids is wide enough to sustain sulfate reduction over a long period of time without exhibiting inhibition due to sulfide. This protocol describes the procedure to generate the sludge from the sediments in an upflow anaerobic sludge blanket (UASB) type of reactor. Furthermore, the protocol presents the procedure to demonstrate the capability of the sludge to remove by reductive dechlorination a model of a highly toxic organic pollutant such as trichloroethylene (TCE). The protocol is divided in three stages: (1) the formation of the sludge and the determination of its sulfate reducing activity in the UASB, (2) the experiment to remove the TCE by the sludge, and (3) the identification of microorganisms in the sludge after the TCE reduction. Although in this case the sediments were taken from a site located in Mexico, the generation of a sulfidogenic sludge by using this procedure may work if a different source of sediments is taken since marine sediments are a natural pool of microorganisms that may be enriched in sulfate reducing bacteria.     

Consortium diversity of a sulfate‐reducing biofilm developed at acidic pH influent conditions in a down‐flow fluidized bed reactor

Sulfate reduction is an appropriate approach for the treatment of effluents with sulfate and dissolved metals. In sulfate‐reducing reactors, acetate may largely contribute to the residual organic matter, because not all sulfate reducers are able to couple the oxidation of acetate to the reduction of sulfate, limiting the treatment efficiency. In this study, we investigated the diversity of a bacterial community in the biofilm of a laboratory scale down‐flow fluidized bed reactor, which was developed under sulfidogenic conditions at an influent pH between 4 and 6. The sequence analysis of the microbial community showed that the 16S rRNA gene sequence of almost 50% of the clones had a high similarity with Anaerolineaceae. At second place, 33% of the 16S rRNA phylotypes were affiliated with the sulfate‐reducing bacteria Desulfobacca acetoxidans and Desulfatirhabdium butyrativorans, suggesting that acetotrophic sulfate reduction was occurring in the system. The remaining bacterial phylotypes were related to fermenting bacteria found at the advanced stage of reactor operation. The results indicate that the acetotrophic sulfate‐reducing bacteria were able to remain within the biofilm, which is a significant result because few natural consortia harbor complete oxidizing sulfate‐reducers, improving the acetate removal via sulfate reduction in the reactor.     

Acidophilic degradation of methanol by a methanogenic enrichment culture

Abstract An acidophilic methanogenic enrichment culture was obtained in a continuous up-flow anaerobic sludge blanket reactor operated at pH 4.2 with methanol as the sole carbon source. The specific methylotrophic methanogenic activity of the enriched reactor sludge at pH 5 was 3.57 g COD g⁻¹ volatile suspended solids day⁻¹ and the apparent doubling time of the biomass was 15.8 h. Acidic conditions were obligatory, since the enrichment culture was not able to produce methane or to grow at pH 7. Based on morphological characteristics, the dominant methanogenic species in the enrichment culture was a Methanosarcina .     

Operational temperature regulates anodic biofilm growth and the development of electrogenic activity

The operational temperature of microbial fuel cell reactors influences biofilm development, and this has an impact on anodic biocatalytic activity. In this study, we compared three microbial fuel cell (MFC) reactors acclimated at 10°C, 20°C and 35°C to investigate the effect on biomass development, methanogenesis and electrogenic activity over time. The start-up time was inversely influenced by temperature, but the amount of biomass accumulation increased with increased temperatures, the 10°C, 20°C and 35°C acclimated biofilms resulted in 0.57, 0.82 and 5.43 g biomass (volatile suspended solids) per litre respectively at 56 weeks of operation. Biofilm build-up on the 35°C anode was further demonstrated by scanning electron microscopy, which showed large aggregations of biomass accumulating on the anode when compared to 10°C and 20°C biofilms. Biomass accumulation had a direct impact on biocatalytic performance, with the maximum power at 35°C after 60 weeks of operation being 2.14 W m⁻³ and power densities for the 10°C and 20°C reactors being and 4.29 W m⁻³. Methanogenic activity was also shown to be higher at 35°C, with a rate of 10.1 mmol CH₄ biofilm per gram of volatile suspended solid (VSS) per day, compared to 0.28 mmol CH₄ per gram of VSS per day produced at 20°C. These results demonstrate that higher MFC operating temperatures could be detrimental to the biocatalytic performance of electrochemically active bacteria in anodic biofilms due to biomass accumulation with enhanced development of non-electrogenic communities (e.g. methanogens and fermenters), meaning that, over time, psychro- or mesophilic operation can have beneficial effects for the development of electrogenically active populations in the reactor.     

Microbial diversity and activity in seafloor brine lake sediments (Alaminos Canyon block 601, Gulf of Mexico)

The microbial communities thriving in deep‐sea brines are sustained largely by energy rich substrates supplied through active seepage. Geochemical, microbial activity, and microbial community composition data from different habitats at a Gulf of Mexico brine lake in Alaminos Canyon revealed habitat‐linked variability in geochemistry that in turn drove patterns in microbial community composition and activity. The bottom of the brine lake was the most geochemically extreme (highest salinity and nutrient concentrations) habitat and its microbial community exhibited the highest diversity and richness indices. The habitat at the upper halocline of the lake hosted the highest rates of sulfate reduction and methane oxidation, and the largest inventories of dissolved inorganic carbon, particulate organic carbon, and hydrogen sulfide. Statistical analyses indicated a significant positive correlation between the bacterial and archaeal diversity in the bottom brine sample and inventories. Other environmental factors with positive correlation with microbial diversity indices were DOC, H₂S, and DIC concentrations. The geochemical regime of different sites within this deep seafloor extreme environment exerts a clear selective force on microbial communities and on patterns of microbial activity.     

High-Rate two-phase process for the anaerobic degradation of cellulose, employing rumen microorganisms for an efficient acidogenesis

A novel two-stage anaerobic process for the microbial conversion of cellulose into biogas has been developed. In the first phase, a mixed population of rumen bacteria and ciliates was used in the hydrolysis and fermentation of cellulose. The volatile fatty acids (VFA) produced in this acidogenic reactor were subsequently converted into biogas in a UASB-type methanogenic reactor.A stepwise increase of the loading rate from 11.9 to 25.8 g volatile solids/L reactor volume/day (g VS/L/day) did not affect the degradation efficiency in the acidogenic reactor, whereas the methanogenic reactor appeared to be overloaded at the highest loading rate. Cellulose digestion was almost complete at all loading rates applied. The two-stage anaerobic process was also tested with a closed fluid circuit. In this instance total methane production was 0.438 L CH(4)g VS added, which is equivalent to 98% of the theoretical value. The application of rumen microorganisms in combination with a high-rate methane reactor is proposed as a means of efficient anaerobic degradation of cellulosic residues to methane. Because this newly developed two-phase system is based on processes and microorganisms from the ruminant, it will be referred to as "Rumen Derived Anaerobic Digestion" (RUDAD-) process.     

Characterization of sulfate-reducing bacteria dominated surface communities during start-up of a down-flow fluidized bed reactor

An anaerobic down-flow fluidized bed reactor was inoculated with granular sludge and started-up with sulfate containing synthetic wastewater to promote the formation of a biofilm enriched in sulfate-reducing bacteria (SRB), to produce biogenic sulfide. The start-up was done in two stages operating the reactor in batch for 45 days followed by 85 days of continuous operation. Low-density polyethylene was used as support. The biofilm formation was followed up by biochemical and electron microscopy analyses and the composition of the community was examined by 16S rDNA sequence analysis. Maximum immobilized volatile solids (1.2 g IVS/L_support) were obtained after 14 days in batch regime. During the 85 days of continuous operation, the reactor removed up to 80% of chemical oxygen demand (COD), up to 28% of the supplied sulfate and acetate was present in the effluent. Sulfate-reducing activity determined in the biofilm with ethanol or lactate as substrate was 11.7 and 15.3 g COD/g IVS per day, respectively. These results suggested the immobilization of sulfate reducers that incompletely oxidize the substrate to acetate; the phylogenetic analysis of the cloned 16S rDNA gene sequences showed high identity to the genus Desulfovibrio that oxidizes the substrates incompletely. In contrast, in the granular sludge used as inoculum a considerable number of clones showed homology to Methanobacterium and just few clones were close to SRB. The starting-up approach allowed the enrichment of SRB within the diverse community developed over the polyethylene support.     

Microbial leaching of metals from solid industrial wastes

Biotechnological applications for metal recovery have played a greater role in recovery of valuable metals from low grade sulfide minerals from the beginning of the middle era till the end of the twentieth century. With depletion of ore/minerals and implementation of stricter environmental rules, microbiological applications for metal recovery have been shifted towards solid industrial wastes. Due to certain restrictions in conventional processes, use of microbes has garnered increased attention. The process is environmentally-friendly, economical and cost-effective. The major microorganisms in recovery of heavy metals are acidophiles that thrive at acidic pH ranging from 2.0–4.0. These microbes aid in dissolving metals by secreting inorganic and organic acids into aqueous media. Some of the well-known acidophilic bacteria such as Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, Leptospirillum ferrooxidans and Sulfolobus spp. are well-studied for bioleaching activity, whereas, fungal species like Penicillium spp. and Aspergillus niger have been thoroughly studied for the same process. This mini-review focuses on the acidophilic microbial diversity and application of those microorganisms toward solid industrial wastes.     

Inorganic Species Distribution and Microbial Diversity within High Arctic Cryptoendolithic Habitats

Cryptoendolithic habitats in the Canadian high Arctic are associated with a variety of microbial community assemblages, including cyanobacteria, algae, and fungi. These habitats were analyzed for the presence of metal ions by sequential extraction and evaluated for relationships between these and the various microorganisms found at each site using multivariate statistical methods. Cyanobacteria-dominated communities exist under higher pH conditions with elevated concentrations of calcium and magnesium, whereas communities dominated by fungi and algae are characterized by lower pH conditions and higher concentrations of iron, aluminum, and silicon in the overlying surfaces. These results suggest that the activity of the dominant microorganisms controls the pH of the surrounding environment, which in turn dictates rates of weathering or the possibility for surface crust formation, both ultimately deciding the structure of microbial diversity for each cryptoendolithic habitat.     

Evaluation of performance and microbial community in a two-stage UASB reactor pretreating acrylic fiber manufacturing wastewater

A two-stage UASB reactor was employed to pretreat acrylic fiber manufacturing wastewater. Mesophilic operation (35 ± 0.5 °C) was performed with hydraulic retention time (HRT) varied between 28 and 40 h. Mixed liquor suspended solids (MLSS) in the reactor was maintained about 8000 mg/L. The results showed COD and sulfate removal could be kept at 51% and 75%, respectively, when the HRT was no less than 38 h. Sulfate reduction mainly occurred in the acidification-stage reactor while methane production mainly occurred in the methane-stage reactor. The size of granule formed in the acidification-stage reactor ranged between 1 and 5 mm while the largest size of granule in the methane-stage reactor ranged from 0.5 to 2 mm. Compared to microbial populations in the acidification-stage reactor, the microbial diversity in methane-stage reactor was more abundant. In the acidification-stage reactor, the Syntrophobacter sulfatireducens devoted to both sulfate reduction and acetate production.     

Stimulation of conversion rates and bacterial activity in a silage-fed two-phase biogas process by initiating liquid recirculation

The effects of liquid recirculation on a liquefaction-acidogenic reactor in an anaerobic two-phase digesting system operating with grass-clover silage was studied during 40 days after initiating recirculation of effluent from the methanogenic reactor to the liquefaction-acidogenic reactor. An increase in alkalinity and, thus, an increase in pH from 5.2 to 6.0 occurred in the liquefaction-acidogenic reactor. During the same period, a 10-fold increase (from 0.2 to 1.9 g·l^–1·h^–1) in the degradation rate of mannitol and an almost 9-fold increase in the activity of hydrogenotrophic methanogens was observed. The estimated number of these bacteria increased by one order of magnitude. The average degradation rate of lactate increased 3-fold, probably as a consequence of the more efficient hydrogen consumption by the hydrogenotrophic methanogens. An observed increase in net mineralization of organic nitrogen compounds was probably the main reason for an enhanced net production of organic acids (from 0.2 to 0.9 g·l^–1·d^–1). The liquefaction of cellulose and hemicellulose was low from the start of recirculation (3% and 20% reduction, respectively) and did not seem to be affected by the liquid recirculation. This was in accordance with the low number of cellulose degraders (4.0·10² counts·ml^–1) observed. The results from this investigation show that the initiation of liquid recirculation in silage-fed two-phase biogas processes will stimulate the activity of hydrogenotrophic methanogens in the liquefaction-acidogenic reactor. This will lead to more thermodynamically favourable conditions for acidification reactions which are dependent upon interspecies transfer of reducing equivalents.Abbreviations COD chemical oxygen demand - CSTR completely stirred tank reactor - HRT hydraulic retention time - LA-reactor liquefaction-acidogenic reactor - M-reactor methanogenic reactor - MPN most probable number - OLR organic loading rate - SMA specific methanogenic activity - SRT solids retention time - TKN total Kjeldahl nitrogen - ts total solids - tss total suspended solids - vs volatile solids - vss volatile suspended solids     

Biokinetics in acidogenesis of highly suspended organic wastewater by adenosine 5' triphosphate analysis

In this paper, we pointed out the problems of using conventional volatile suspended solids (VSS) and chemical oxygen demand (COD) to evaluate biokinetic coefficients, especially for the treatment of highly suspended organic wastewater. We also introduced a novel approach to evaluate biokinetic coefficients by measurement of adenosine 5'-triphosphate (ATP) of microorganisms. The concept of using ATP analysis in biokinetic evaluations with highly suspended wastewater was shown to be effective. This study also showed that the conventional VSS and COD methods were strongly affected by incoming suspended organics in the wastewater and by biokinetics of microorganisms. A cheese-processing wastewater was used in evaluating the biokinetics of mesophilic acidogens. The concentration of COD and total suspended solids in the wastewater was 63.3 g/L and 12.4 g/L, respectively. The TSS was 23.6% of total solids concentration. A high ratio of VSS to total suspended solids of 96.7% indicated that most of the suspended particles were organic materials. Lactose and protein were the major organic components contributing COD in the wastewater, and a total of 94.2% of the COD in the wastewater was due to the presence of lactose and protein. Two different physiological conditions where the maximum rates of acetate and butyrate production occurred were tested. These were pH 7 (condition A for acetate production) and pH 7.3 (condition B for butyrate production) at 36.2C, respectively. Based on the molecular structures of the major organic substances and microbial ATP analysis, the residual substrate and microbial concentrations were stoichiometrically converted to substrate COD (SuCOD) and microbial VSS (MVSS), respectively, using correlation coefficients reported previously. These SuCOD and MVSS were simultaneously used to evaluate the biokinetic coefficients using Monod-based mathematical equations. The nonlinear least squares method with 95% confidence interval was used to evaluate biokinetic coefficients. The maximum microbial growth rate, mu(max) and half saturation coefficient, K(s), for conditions A and B were determined to be 9.9 +/- 0.3 and 9.3 +/- 1.0 day(-1) and 134.0 +/- 58.3 and 482.5 +/- 156.5 mg SuCOD/L, respectively. The microbial yield coefficient, Y, and microbial decay rate coefficient, k(d) for conditions A and B were determined to be 0.29 +/- 0.03 and 0.20 +/- 0.05 mg MVSS/mg SuCOD, and 0.14 +/- 0.05 and 0.25 +/- 0.05 day(-1), respectively. Specific substrate utilization rate at condition B was 43.8 +/- 20.6 mg SuCOD/mg MVSS/day, which was 31% higher than that at condition A.     

Control of microbial attachment by inhibition of ATP and ATP‐mediated autoinducer‐2

In this study, 2,4‐dinitrophenol (DNP), a typical chemical uncoupler, was employed to investigate the possible roles of ATP and autoinducer‐2 (AI‐2) of suspended microorganisms in attachment onto nylon membrane and glass slide surfaces. Results showed that DNP could disrupt ATP synthesis, subsequently led to a reduced production of AI‐2 which is a common signaling molecule for cellular communication. Attachment of suspended microorganisms exposed to DNP was significantly suppressed as compared to microorganisms without contact with DNP. These suggest that an energized state of suspended microorganisms would favor microbial attachment to both nylon membrane and glass slide surfaces. The extent of microbial attachment was found to be positively related to the AI‐2 content of microorganisms. This study offers insights into the control of biofouling by preventing initial microbial attachment through inhibition of energy metabolism. Biotechnol. Bioeng. 2010;107: 31–36. © 2010 Wiley Periodicals, Inc.     

Microbial community changes in methanogenic granules during the transition from mesophilic to thermophilic conditions

Upflow anaerobic sludge blanket (UASB) reactor is one of the most applied technologies for various high-strength wastewater treatments. The present study analysed the microbial community changes in UASB granules during the transition from mesophilic to thermophilic conditions. Dynamicity of microbial community in granules was analysed using high-throughput sequencing of 16S ribosomal RNA gene amplicons, and the results showed that the temperature strictly determines the diversity of the microbial consortium. It was demonstrated that most of the microbes which were present in the initial mesophilic community were not found in the granules after the transition to thermophilic conditions. More specifically, only members from family Anaerolinaceae managed to tolerate the temperature change and contributed in maintaining the physical integrity of granular structure. On the contrary, new hydrolytic and fermentative bacteria were quickly replacing the old members in the community. A direct result from this abrupt change in the microbial diversity was the accumulation of volatile fatty acids and the concomitant pH drop in the reactor inhibiting the overall anaerobic digestion process. Nevertheless, by maintaining deliberately the pH levels at values higher than 6.5, a methanogen belonging to Methanoculleus genus emerged in the community enhancing the methane production.

     

Biodegradation of pentachlorophenol in a continuous anaerobic reactor augmented with Desulfitobacterium frappieri PCP-1.

In this work, a strain of anaerobic pentachlorophenol (PCP) degrader, Desulfitobacterium frappieri PCP-1, was used to augment a mixed bacterial community of an anaerobic upflow sludge bed reactor degrading PCP. To estimate the efficiency of augmentation, the population of PCP-1 in the reactor was enumerated by a competitive PCR technique. The PCP-1 strain appeared to compete well with other microorganisms of the mixed bacterial community, with its population increasing from 10(6) to 10(10) cells/g of volatile suspended solids within a period of 70 days. Proliferation of strain PCP-1 allowed for a substantial increase of the volumetric PCP load from 5 to 80 mg/liter of reaction volume/day. A PCP removal efficiency of 99% and a dechlorination efficiency of not less than 90.5% were observed throughout the experiment, with 3-Cl-phenol and phenol being observable dechlorination intermediates.     

Microbial communities of ultramafic soils in maquis and rainforest at Mont Do,New Caledonia

We analysed variation in microbial community richness and function in soils associated with a fire‐induced vegetation successional gradient from low maquis (shrubland) through tall maquis to rainforest on metal‐rich ultramafic soils at Mt Do, New Caledonia. Random amplified polymorphic DNA fingerprinting was used to determine the extent of genetic relatedness among the microbial communities and indicated that the open and tall maquis microbial communities were more similar to each other than they were to the rainforest community. Sole‐source carbon utilization indicated variation in the microbial communities, again with greater diversity in rainforest soils. Plate counts showed that both rainforest and maquis soils contained bacteria that can grow in the presence of up to 20 mmol L^?1 nickel and 10 mmol L^?1 chromium. Understanding microbial community composition and dynamics in these ultramafic soils may lead to a better understanding of the processes facilitating vegetation succession from shrubland to forest on these high‐metal substrates, and of approaches to successful revegetation following mining for metals including nickel, chromium and cobalt.     

Diversity decoupled from sulfur isotope fractionation in a sulfate‐reducing microbial community

The extent of fractionation of sulfur isotopes by sulfate‐reducing microbes is dictated by genomic and environmental factors. A greater understanding of species‐specific fractionations may better inform interpretation of sulfur isotopes preserved in the rock record. To examine whether gene diversity influences net isotopic fractionation in situ, we assessed environmental chemistry, sulfate reduction rates, diversity of putative sulfur‐metabolizing organisms by 16S rRNA and dissimilatory sulfite reductase (dsrB) gene amplicon sequencing, and net fractionation of sulfur isotopes along a sediment transect of a hypersaline Arctic spring. In situ sulfate reduction rates yielded minimum cell‐specific sulfate reduction rates < 0.3 × 10^?15 moles cell^?1 day^?1. Neither 16S rRNA nor dsrB diversity indices correlated with relatively constant (38‰–45‰) net isotope fractionation (ε³⁴S_{sulfide‐sulfate}). Measured ε³⁴S values could be reproduced in a mechanistic fractionation model if 1%–2% of the microbial community (10%–60% of Deltaproteobacteria) were engaged in sulfate respiration, indicating heterogeneous respiratory activity within sulfate‐reducing populations. This model indicated enzymatic kinetic diversity of Apr was more likely to correlate with sulfur fractionation than DsrB. We propose that, above a threshold Shannon diversity value of 0.8 for dsrB, the influence of the specific composition of the microbial community responsible for generating an isotope signal is overprinted by the control exerted by environmental variables on microbial physiology.