Ultrastructure, aggregation-state, and crystal growth of biogenic nanocrystalline sphalerite and wurtzite

In this study, we investigated the size, submicrometer-scale structure, and aggregation state of ZnS formed by sulfate-reducing bacteria (SRB) in a SRB-dominated biofilm growing on degraded wood in cold (Tsimilar to8degreesC), circumneutral-pH (7.2-8.5) waters draining from an abandoned, carbonate-hosted Pb-Zn mine. High-resolution transmission electron microscope (HRTEM) data reveal that the earliest biologically induced precipitates are crystalline ZnS nanoparticles 1-5 nm in diameter. Although most nanocrystals have the sphalerite structure, nanocrystals of wurtzite are also present, consistent with a predicted size dependence for ZnS phase stability. Nearly all the nanocrystals are concentrated into 1-5 mum diameter spheroidal aggregates that display concentric banding patterns indicative of episodic precipitation and flocculation. Abundant disordered stacking sequences and faceted, porous crystal-aggregate morphologies are consistent with aggregation-driven growth of ZnS nanocrystals prior to and/or during spheroid formation. Spheroids are typically coated by organic polymers or associated with microbial cellular surfaces, and are concentrated roughly into layers within the biofilm. Size, shape, structure, degree of crystallinity, and polymer associations will all impact ZnS solubility, aggregation and coarsening behavior, transport in groundwater, and potential for deposition by sedimentation. Results presented here reveal nanometer- to micrometer-scale attributes of biologically induced ZnS formation likely to be relevant to sequestration via bacterial sulfate reduction (BSR) of other potential contaminant metal(loid)s, such as Pb2+, Cd2+, As3+ and Hg2+, into metal sulfides. The results highlight the importance of basic mineralogical information for accurate prediction and monitoring of long-term contaminant metal mobility and bioavailability in natural and constructed bioremediation systems. Our observations also provoke interesting questions regarding the role of size-dependent phase stability in biomineralization and provide new insights into the origin of submicrometer- to millimeter-scale petrographic features observed in low-temperature sedimentary sulfide ore deposits.

Performance of a substratum-irradiated photosynthetic biofilm reactor for the removal of sulfide from wastewater

The performance of a sulfide-removal system based on biofilms dominated by green sulfur bacteria (GSB) has been investigated. The system was supplied with radiant energy in the band 720-780 nm, and fed with a synthetic wastewater. The areal net sulfide removal rate and the efficacy of the incident radiant energy for sulfide removal have been characterized over ranges of bulk sulfide concentration (1.6-11.5 mg L-1) and incident irradiance (0.21-1.51 W m(-2)). The areal net sulfide removal rate increased monotonically with both increasing incident irradiance and increasing bulk sulfide concentration. The efficacy of the radiant energy for sulfide removal (the amount of sulfide removed per unit radiant energy supplied) also increased monotonically with rising bulk sulfide concentration, but exhibited a maximum value with respect to incident irradiance. The maximum observed values of this net removal rate and this efficacy were, respectively, 2.08 g m(-2) d(-1) and 2.04 g W-1 d(-1). In-band changes in the spectral composition of the radiant energy affected this efficacy only slightly. The products of sulfide removal were sulfate and elemental-S. The elemental-S was scarcely released into the liquid, however, and reasons for this, such as sulfur reduction and polysulfide formation, are considered. Between 1.45 and 3.85 photons were needed for the net removal of one electron from S-species. Intact samples of the biofilm were characterized by microscopy, and their thicknesses lay between 39 +/- 9 and 429 +/- 57 mum. The use of the experimentally determined rates and efficacies for the design of a pilot-scale system is illustrated. (C) 2004 Wiley Periodicals, Inc.

Effects of acetate and propionate on the performance of a photosynthetic biofilm reactor for sulfide removal

The effects of acetate and propionate on the performance of a recently proposed and characterized photosynthetic biological sulfide removal system have been investigated with a view to predicting this concept's suitability for removing sulfide from wastewater undergoing or having undergone anaerobic treatment. The concept relies on substratum-irradiated biofilms dominated by green sulfur bacteria (GSB), which are supplied with radiant energy in the band 720 - 780 nm. A model reactor was fed for 7 months with a synthetic wastewater free of volatile fatty acids (VFAs), after which time intermittent dosing of the wastewater with acetate or propionate was begun. Such dosing suppressed the areal net sulfide removal rate by similar to50%, and caused the principal net product of sulfide removal to switch from sulfate to elemental-S. Similarly suppressed values of this rate were observed when the wastewater was dosed continuously with acetate, and this rate was not significantly affected by changes in the concentration of ammonia-N in the feed. The main net product of sulfide removal was again elemental-S, which was scarcely released into the liquid, however. Sulfate reduction and sulfur reduction were observed when the light supply was interrupted and were inferred to be occurring within the irradiated biofilm. A preexisting conceptual model of the biofilm was augmented with both of these reductive processes, and this augmented model was shown to account for most of the observed effects of VFA dosing. The implications of these findings for the practicality of the technology are considered. (C) 2004 Wiley Periodicals, Inc.

Sources of hydrogen sulfide in groundwater on reclaimed land

Fisherman Islands is an area of reclaimed land at the mouth of the Brisbane River in Queensland, Australia. Ongoing groundwater monitoring has found elevated concentrations of hydrogen sulfide (H2S) in the groundwater on the island. The presence of H2S on Fisherman Islands is of concern because of its toxic nature, the potential for acid sulfate soil formation, and its noxious odor. The aim of this study was,to, identify the sources of H2S within the groundwater on Fisherman Islands. It was hypothesized that the H2S is being formed by sulfate reducing bacteria acting on sulfate from seawater, rather than the introduction of sulfide with the dredge sediments. Groundwater and soil samples were collected and analyzed for sulfide, sulfate, and organic carbon contents. Elevated concentrations of sulfides coincide with,elevated concentrations of sulfate in the groundwater and elevated concentrations of organic carbon in the sediments, supporting the hypothesis that sulfide formation is the result of heterotrophic, sulfate reducing organisms.

Isotopes content, mineralogical composition, Uranium and Thorium contents and determined activity ratios from METEOR cruise M74/3 in 2007

Authigenic carbonate deposits have been sampled with the remotely operated vehicle 'MARUM-QUEST 4000 m' from five methane seeps between 731 and 1823 m water depth along the convergent Makran continental margin, offshore Pakistan (northern Arabian Sea). Two seeps on the upper slope are located within the oxygen minimum zone (OMZ; ca. 100 to 1100 m water depth), the other sites are situated in oxygenated water below the OMZ (below 1100 m water depth). The carbonate deposits vary with regard to their spatial extent, sedimentary fabrics, and associated seep fauna: Within the OMZ, carbonates are spatially restricted and associated with microbial mats, whereas in the oxygenated zone below the OMZ extensive carbonate crusts are exposed on the seafloor with abundant metazoans (bathymodiolin mussels, tube worms, galatheid crabs). Aragonite and Mg-calcite are the dominant carbonate minerals, forming common early diagenetic microcrystalline cement and clotted to radial-fibrous cement. The delta18O carbonate values range from 1.3 to 4.2 per mil V-PDB, indicating carbonate precipitation at ambient bottom-water temperature in shallow sediment depth. Extremely low delta13Ccarbonate values (as low - 54.6per mil V-PDB) point to anaerobic oxidation of methane (AOM) as trigger for carbonate precipitation, with biogenic methane as dominant carbon source. Prevalence of biogenic methane in the seepage gas is corroborated by delta13C methane values ranging from - 70.3 to - 66.7per mil V-PDB, and also by back-calculations considering delta 13C methane values of carbonate and incorporated lipid biomarkers.

Data on the biogeochemical cycle of methane in the ocean

Geological, mineralogical and microbiological aspects of the methane cycle in water and sediments of different areas in the oceans are under consideration in the monograph. Original and published estimations of formation- and oxidation rates of methane with use of radioisotope and isotopic methods are given. The role of aerobic and anaerobic microbial oxidation of methane in production of organic matter and in formation of authigenic carbonates is considered. Particular attention is paid to processes of methane transformation in areas of its intensive input to the water column from deep-sea hydrothermal sources, mud volcanoes, and cold methane seeps.

Relative abundance of anaerobic methanotrophic archaea in bacterial mat Cascadia margin off Oregon

The microbially mediated anaerobic oxidation of methane (AOM) is the major biological sink of the greenhouse gas methane in marine sediments (doi:10.1007/978-94-009-0213-8_44) and serves as an important control for emission of methane into the hydrosphere. The AOM metabolic process is assumed to be a reversal of methanogenesis coupled to the reduction of sulfate to sulfide involving methanotrophic archaea (ANME) and sulfate-reducing bacteria (SRB) as syntrophic partners which were describes amongst others in Boetius et al. (2000; doi:10.1038/35036572). In this study, 16S rRNA-based methods were used to investigate the distribution and biomass of archaea in samples from sediments above outcropping methane hydrate at Hydrate Ridge (Cascadia margin off Oregon) and (ii) massive microbial mats enclosing carbonate reefs (Crimea area, Black Sea). Sediment samples from Hydrate Ridge were obtained during R/V SONNE cruises SO143-2 in August 1999 and SO148-1 in August 2000 at the crest of southern Hydrate Ridge at the Cascadia convergent margin off the coast of Oregon. The second study area is located in the Black Sea and represents a field in which there is active seepage of free gas on the slope of the northwestern Crimea area. Here, a field of conspicuous microbial reefs forming chimney-like structures was discovered at a water depth of 230 m in anoxic waters. The microbial mats were sampled by using the manned submersible JAGO during the R/V Prof. LOGACHEV cruise in July 2001. At Hydrate Ridge the surface sediments were dominated by aggregates consisting of ANME-2 and members of the Desulfosarcina-Desulfococcus branch (DSS) (ANME-2/DSS aggregates), which accounted for >90% of the total cell biomass. The numbers of ANME-1 cells increased strongly with depth; these cells accounted 1% of all single cells at the surface and more than 30% of all single cells (5% of the total cells) in 7- to 10-cm sediment horizons that were directly above layers of gas hydrate. In the Black Sea microbial mats ANME-1 accounted for about 50% of all cells. ANME-2/DSS aggregates occurred in microenvironments within the mat but accounted for only 1% of the total cells. FISH probes for the ANME-2a and ANME-2c subclusters were designed based on a comparative 16S rRNA analysis. In Hydrate Ridge sediments ANME-2a/DSS and ANME-2c/DSS aggregates differed significantly in morphology and abundance. The relative abundance values for these subgroups were remarkably different at Beggiatoa sites (80% ANME-2a, 20% ANME-2c) and Calyptogena sites (20% ANME-2a, 80% ANME-2c), indicating that there was preferential selection of the groups in the two habitats.

Microbialites and lipid biomarker from coral reefs off the volcanic islands Tahiti and Vanuatu and from the nonvolcanic sites Belize and the Maldives

The occurrence of microbialites in post-glacial coral reefs has been interpreted to reflect an ecosystem response to environmental change. The greater thickness of microbialites in reefs with a volcanic hinterland compared to thinner microbial crusts in reefs with a non-volcanic hinterland led to the suggestion that fertilization of the reefal environment by chemical weathering of volcanic rocks stimulated primary productivity and microbialite formation. Using a molecular and isotopic approach on reef-microbialites from Tahiti (Pacific Ocean), it was recently shown that sulfate-reducing bacteria favored the formation of microbial carbonates. To test if similar mechanisms induced microbialite formation in other reefs as well, the Tahitian microbialites are compared with similar microbialites from coral reefs off Vanuatu (Pacific Ocean), Belize (Caribbean Sea, Atlantic Ocean), and the Maldives (Indian Ocean) in this study. The selected study sites cover a wide range of geological settings, reflecting variable input and composition of detritus. The new lipid biomarker data and stable sulfur isotope results confirm that sulfate-reducing bacteria played an intrinsic role in the precipitation of microbial carbonate at all study sites, irrespective of the geological setting. Abundant biomarkers indicative of sulfate reducers include a variety of terminally-branched and mid chain-branched fatty acids as well as mono-O-alkyl glycerol ethers. Isotope evidence for bacterial sulfate reduction is represented by low d34S values of pyrite (-43 to -42 per mill) enclosed in the microbialites and, compared to seawater sulfate, slightly elevated d34S and d18O values of carbonate-associated sulfate (21.9 to 22.2 per mill and 11.3 to 12.4 per mill, respectively). Microbialite formation took place in anoxic micro-environments, which presumably developed through the fertilization of the reef environment and the resultant accumulation of organic matter including bacterial extracellular polymeric substances (EPS), coral mucus, and marine snow in cavities within the coral framework. ToF-SIMS analysis reveals that the dark layers of laminated microbialites are enriched in carbohydrates, which are common constituents of EPS and coral mucus. These results support the hypothesis that bacterial degradation of EPS and coral mucus within microbial mats favored carbonate precipitation. Because reefal microbialites formed by similar processes in very different geological settings, this comparative study suggests that a volcanic hinterland is not required for microbialite growth. Yet, detrital input derived from the weathering of volcanic rocks appears to be a natural fertilizer, being conductive for the growth of microbial mats, which fosters the development of particularly abundant and thick microbial crusts.

Microbial community composition in sediments of the Hydrate Ridge (Cascadian Margin) collected during RV SONNE cruises SO143 (1999) and SO148-1 (2000)

Cold seep environments such as sediments above outcropping hydrate at Hydrate Ridge (Cascadia margin off Oregon) are characterized by methane venting, high sulfide fluxes caused by the anaerobic oxidation of methane, and the presence of chemosynthetic communities. This investigation deals with the diversity and distribution of sulfate-reducing bacteria, some of which are directly involved in the anaerobic oxidation of methane as syntrophic partners of the methanotrophic archaea. The composition and activity of the microbial communities at methane vented and nonvented sediments are compared by quantitative methods including total cell counts, fluorescence in situ hybridization (FISH). Bacteria involved in the degradation of particulate organic carbon (POC) are as active and diverse as at other productive margin sites of similar water depths. The availability of methane supports a two orders of magnitude higher microbial biomass (up to 9.6×10**10cells/cm**3). Sediment samples were obtained during RV SONNE cruises SO143-2 and SO148-1 at the crest of southern Hydrate Ridge at the Cascadia convergent margin off the coast of Oregon. Sediment cores of 20 - 40 cm length were obtained using a video-guided multiple corer from gas hydrate bearing sediments and from reference sites not enriched in methane in the surface sediments. Samples for total cell counts were obtained from 1 cm core slices, fixed with 2% formaldehyde and stored cold (4°C) and the quantification of aggregates was done via epifluorescence microscopy after staining the sediments with Acridine Orange Direct Counts (AODC) according to the method of Meyer- Reil (1983, doi:10.1007/BF00395813). Total cell counts were defined as the sum of single cells plus the aggregated cells in the syntrophic consortia. DAPI staining was used to measure ANME2/DSS aggregate sizes via epifluorescence microscopy of FISH-treated samples. For FISH, subsamples of sediment cores were sliced into 1 cm intervals and fixed for 2-3 h with 3% formaldehyde (final concentration), washed twice with 1×PBS (10 mM sodium phosphate; 130 mM NaCl), and finally stored in 1×PBS/EtOH (1:1) at -20°C.

Hydrochemical and microbiological properties of groundwater from pristine aquifer sampled at the Lower Rhine Embayment

The hydrochemistry and the microbial diversity of a pristine aquifer system near Garzweiler, Germany next to the open-pit lignite mine Garzweiler 1, were characterized. Hydrogeochemical and isotopic data indicate a recent activity of sulfate-reducing bacteria in the Tertiary marine sands. The community structure in the aquifer was studied by fluorescence in situ hybridization (FISH). Up to 7.3 x 10**5 cells/ml were detected by DAPIstaining. Bacteria (identified by the probe EUB338) were dominant, representing 51.9% of the total cell number (DAPI). Another 25.7% of total cell were affiliated with the domain Archaea as identified by the probe ARCH915. Within the domain Bacteria, the beta-Proteobacteria were most abundant (21.0% of total cell counts). Using genusspecific probes for sulfate-reducing bacteria (SRB), 2.5% of the total cells were identified as members of the genus Desulfotomaculum. This reflects the predominant role these microorganisms have been found to play in sulfatereducing zones of aquifers at other sites. Previously, all SRB cultured from this site were from the spore-forming genera Desulfotomaculum and Desulfosporosinus. Samples were taken after pumping for >= 40 min and after parameters such as temperature, pH, redox potential, oxygen and conductivity of the groundwater had remained stable for >= 15 min due to recharge of aquifer water. Hybridization and microscopy counts of hybridized and 4',6'-diamidino-2-phenylindole (DAPI)- stained cells were performed as described in Snaidr et al., (1997, http://aem.asm.org/content/63/7/2884.full.pdf). Means were calculated from 10 to 20 randomly chosen fields on each filter section, corresponding to 800-1000 DAPI stained cells. Counting results were always corrected by subtracting signals observed with the probe NON338. Formamide concentrations and oligonucleotide probes used please see further details.

(Table 1) Stable carbon isotope ratios of methane and carbon dioxide from ODP Leg 169S holes

Sites 1033 and 1034 of ODP Leg 169S in Saanich Inlet have an unusual diagenetic system, that has the appearance of being depth reversed, i.e. a bacterial methane accumulation zone underlain by a sulphate reduction zone. During the late Pleistocene grey, undifferentiated, glacio-marine clays were deposited with low Corg contents (<0.4 wt.%), and interstitial fluids replete in SO4 (ca. 27 mM), devoid of CH4 and low in nutrients. This indicates oxic conditions are present, reflecting the open exchange of waters with Haro Strait during the Pleistocene before the Saanich Peninsula emerged. In the earliest Holocene (ca. 11,000 years BP) the inlet was formed, severely restricting water circulation, and leading to the presence of anoxic bottom waters. The sediments are laminated and show a dramatic rise to high Corg, Norg and Stot contents (up to 2.5, 0.4, 1.4 wt.%, respectively) over a period of ca. 1000 years. The nutrient concentrations are especially high (TA, NH4, PO4 up to 115 meq/l, 20 mM and 400 µM, respectively), SO4 is exhausted and CH4 is prolific. Stable carbon isotope ratio measurements of CH4 and co-existing CO2 indicate that methanogenesis is via carbonate reduction (delta13C-CH4 ca. -60 to - 70 per mil, delta13C-CO2 ca. +10 per mil). At the sulphate-methane interfaces, both at the near-surface and at 50 mbsf (Site 1033) and 80 mbsf (Site 1034) methane consumption by sulphate reducing bacteria is intensive.

Microbial community in the sediment of the arctic Smeerenburgfjorden

Fluorescence in situ hybridization (FISH) with 16S rRNA-targeted oligonucleotide probes were used to investigate the phylogenetic composition of a marine Arctic sediment (Svalbard). Hybridization and microscopy counts of hybridized and 4',6'-diamidino-2-phenylindole (DAPI)-stained cells were performed as described previously from Snaidr et al. (1997, http://aem.asm.org/content/63/7/2884.full.pdf). Means were calculated from 10 to 20 randomly chosen fields on each filter section, corresponding to 800 to 1,000 DAPI-stained cells. Counting results were always corrected by subtracting signals observed with the probe NON338. Formamide concentrations are given in further details. FISH resulted in the detection of a large fraction of microbes living in the top 5 cm of the sediment. Up to 65.4% ± 7.5% of total DAPI cell counts hybridized to the bacterial probe EUB338, and up to 4.9% ± 1.5% hybridized to the archaeal probe ARCH915. Besides delta-proteobacterial sulfate-reducing bacteria (up to 16% 52) members of the Cytophaga-Flavobacterium cluster were the most abundant group detected in this sediment, accounting for up to 12.8% of total DAPI cell counts. Furthermore, members of the order Planctomycetales accounted for up to 3.9% of total cell counts. In accordance with previous studies, these findings support the hypothesis that these bacterial groups are not simply settling with organic matter from the pelagic zone but are indigenous to the anoxic zones of marine sediments. Members of the gamma-proteobacteria also constituted a significant fraction in this sediment (6.1% ± 2.5% of total cell counts). A new probe (GAM660) specific for sequences affiliated with free-living or endosymbiotic sulfur-oxidizing bacteria was developed. A significant number of cells was detected by this probe (2.1% ± 0.7% of total DAPI cell counts), showing no clear zonation along the vertical profile. Gram-positive bacteria and the beta-proteobacteria were near the detection limit in all sediments.

Geochemistry and geomicrobiology profiles of ferruginous sediment, Lake Towuti, Indonesia

Lake Towuti is a tectonic basin, surrounded by ultramafic rocks. Lateritic soils form through weathering and deliver abundant iron (oxy)hydroxides but very little sulfate to the lake and its sediment. To characterize the sediment biogeochemistry, we collected cores at three sites with increasing water depth and decreasing bottom water oxygen concentrations. Microbial cell densities were highest at the shallow site - a feature we attribute to the availability of labile organic matter and the higher abundance of electron acceptors due to oxic bottom water conditions. At the two other sites, OM degradation and reduction processes below the oxycline led to partial electron acceptor depletion. Genetic information preserved in the sediment as extracellular DNA provides information on aerobic and anaerobic heterotrophs related to Actinobacteria, Nitrospirae, Chloroflexi and Thermoplasmatales. These taxa apparently played a significant role in the degradation of sinking organic matter. However, extracellular DNA concentrations rapidly decrease with core depth. Despite very low sulfate concentrations, sulfate-reducing bacteria were present and viable in sediments at all three sites, as confirmed by measurement of potential sulfate reduction rates. Microbial community fingerprinting supported the presence of taxa related to Deltaproteobacteria and Firmicutes with demonstrated capacity for iron and sulfate reduction. Concomitantly, sequences of Ruminococcaceae, Clostridiales and Methanomicrobiales indicated potential for fermentative hydrogen and methane production. Such first insights into ferruginous sediments show that microbial populations perform successive metabolisms related to sulfur, iron and methane. In theory, iron reduction could reoxidize reduced sulfur compounds and desorb OM from iron minerals to allow remineralization to methane. Overall, we found that biogeochemical processes in the sediments can be linked to redox differences in the bottom waters of the three sites, like oxidant concentrations and the supply of labile OM. At the scale of the lacustrine record, our geomicrobiological study should provide a means to link the extant subsurface biosphere to past environments.

(Table 4) Distribution of bacterial populations in sediments of ODP Hole 128-798B

Sediment whole-round cores from a dedicated hole (798B) were obtained for detailed microbiological analysis, down to 518 m below the seafloor (mbsf). These sediments have characteristic bacterial profiles in the top 6 mbsf, with high but rapidly decreasing bacterial populations (total and dividing bacteria, and concentrations of different types of viable heterotrophic bacteria) and potential bacterial activities. Rates of thymidine incorporation into bacterial DNA and anaerobic sulfate reduction are high in the surface sediments and decrease rapidly down to 3 mbsf. Methanogenesis from CO2/H2 peaks below the maximum in sulfate reduction and although it decreases markedly down the core, is present at low rates at all but one depth. Consistent with these activities is the removal of pore-water sulfate, methane gas production, and accumulation of reduced sulfide species. Rates of decrease in bacterial populations slow down below 6 mbsf, and there are some distinct increases in bacterial populations and activities that continue over considerable depth intervals. These include a large and significant increase in total heterotrophic bacteria below 375 mbsf, which corresponds to an increase in the total bacterial population, bacterial viability, a small increase in potential rates of sulfate reduction, and the presence of thermogenic methane and other gases. Bacterial distributions seem to be controlled by the availability of terminal electron acceptors (e.g., sulfate), the bioavailability of organic carbon (which may be related to the dark/light bands within the sediment), and biological and geothermal methane production. Significant bacterial populations are present even in the deepest samples (518 mbsf) and hence it seems likely that bacteria may continue to be present and active much deeper than the sediments studied here. These results confirm and extend our previous results of bacterial activity within deep sediments of the Peru Margin from Leg 112, and to our knowledge this is the first comprehensive report of the presence of active bacterial populations from the sediment surface to in excess of 500 mbsf and sediments > 4 m.y. old.

Anthropogenic impact on diazotrophic diversity in the mangrove rhizosphere revealed by nifH pyrosequencing

Diazotrophs in the mangrove rhizosphere play a major role in providing new nitrogen to the mangrove ecosystem and their composition and activity are strongly influenced by anthropogenic activity and ecological conditions. In this study, the diversity of the diazotroph communities in the rhizosphere sediment of five tropical mangrove sites with different levels of pollution along the north and south coastline of Singapore were studied by pyrosequencing of the nifH gene. Bioinformatics analysis revealed that in all the studied locations, the diazotroph communities comprised mainly of members of the diazotrophic cluster I and cluster III. The detected cluster III diazotrophs, which were composed entirely of sulfate-reducing bacteria, were more abundant in the less polluted locations. The metabolic capacities of these diazotrophs indicate the potential for bioremediation and resiliency of the ecosystem to anthropogenic impact. In heavily polluted locations, the diazotrophic community structures were markedly different and the diversity of species was significantly reduced when compared with those in a pristine location. This, together with the increased abundance of Marinobacterium, which is a bioindicator of pollution, suggests that anthropogenic activity has a negative impact on the genetic diversity of diazotrophs in the mangrove rhizosphere.