Assessment of Changes in Biodiversity when a Community of Ultramicrobacteria Isolated from Groundwater Is Stimulated to Form a Biofilm

With the continuing increase of ultraviolet-B radiation (UVBR: 280-320 nm) fluxes toward the Earth's surface, there is concern regarding a possible negative impact on heterotrophic bacterioplankton. The effects of enhanced UVBR on a natural bacterioplankton community were studied during a 7-day experiment conducted in mesocosms (1500 L). Four light regimes were tested: natural light, 280 to 313 nm excluded UVBR, and two levels of UVBR enhancement. During the first 3 days of the experiment characterized by high inorganic nutrient concentrations (nitrates > 1 μmol L-1 and ammonium > 0.1 μmol L-l), UVBR had no effect on both bacterial abundances and activities. From day 4 to the end of the experiment, nitrate concentrations remained low (<1 μmol L-1) and those of ammonium varied with a general tendency of decrease. During this period, bacterial abundances increased more rapidly in the UVBR enhanced treatments, reaching on the last day of the experiment values that were 39 to 73% higher than those observed in the natural UVBR treatment. 3H-Thymidine (TdR) incorporation rarely showed a significant inhibiting effect of UVBR. However, when expressed per bacterium, TdR incorporation decreased by approximately 40% with the UVBR enhancement above natural levels. Two explanations are possible. First, we know that UVBR reduced protozooplankton bacterivory, leading to an increase in the bacterial abundance. It may be that this increase in community abundance compensated for the UVBR inhibition of bacterial activity at the cellular level. Alternatively, community production may have been set by constant nutrient supply rates; UVBR "inhibition" was then a result of accumulating dead cells, a taxonomic shift, or increased competition among the more abundant cells.     

In situ ecological development of a bacteriogenic iron oxide-producing microbial community from a subsurface granitic rock environment

The initial development and diversity of an in situ subsurface microbial community producing bacteriogenic iron oxides (BIOS) were investigated at the initiation of biofilm growth (2‐month period) and after a 1‐year period of undisturbed growth. Water chemistry data, samples of iron encrusted biofilm material and groundwater were collected from BRIC (BIOS reactor, in situ, continuous flow) apparatuses installed 297 m below sea level at the Äspö Hard Rock Laboratory (HRL) in south eastern Sweden. Comparisons between the BIOS BRIC system and an anaerobic control (AC) BRIC revealed that water mixing at the inflow leads to profuse development of BIOS related to a slightly elevated level of O₂ (up to 0.3 mg L^?1 at the transition zone between BIOS development and non‐development) and elevated Eh (>120 mV) in the first 70 mm of water depth. Decreases in dissolved and particulate iron were connected to the visible appearance of BIOS biofilms. The basic phylogenetic diversity of this site was evaluated using amplified ribosomal DNA restriction enzyme analysis (ARDRA), denaturing gradient gel electrophoresis (DGGE) and partial sequencing of 16S rDNA. From 67 clones that were positive for 16S rDNA inserts, a total of 42 different ARDRA profiles were recognized, representing four bacterial phyla and 14 different metabolic lifestyles. DGGE profiles indicated that there are differences in the representative bacteria when considering either BIOS biofilms or groundwater. DGGE also indicated that the DNA extraction protocols and any polymerase chain reaction biases were consistent. Bacterial metabolic groups associated with indirect metal adsorption and reduction along with bacteria utilizing many alternative electron acceptors were strongly represented within the clones. This study indicates that the microbial diversity of BIOS is greater than previously thought.     

Activity of an isothiazolone biocide against Hormoconis resinae in pure and mixed biofilms

Biofilms containing single or mixed cultures of the fungus Hormoconis resinae and anaerobic sulphate-reducing bacteria (SRB) on stainless steel were incubated with an isothiazolone biocide (Kathon FP) at 28°C for 24 h. H. resinae within the biofilm was enumerated by immunofluorescence microscopy using specific antiserum, and SRB were assayed by culture. Fungal numbers in mixed biofilms were considerably reduced in comparison with those in pure biofilms. The biocide was shown to be effective against H. resinae in pure biofilms at 50 and 100 ppm, but in mixed biofilms only at the higher concentration. This concentration also reduced the sessile SRB numbers by 99%.P.S. Guiamet is with the Sección Biolectroquimica, INIFTA, Suc. 4, C.C. 16, 1900 La Plata, Argentina. C.C Gaylarde is with the Departamento de Solos, Fac. de Agronomia, UFRGS, Av. Bento Gonçalves, 7712, 91540-000 Porto Alegre, RS, Brazil     

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.

     

Propionate-driven sulphate-reduction by oil-field bacteria in a pressurised porous rock bioreactor

The occurrence of high concentrations (up to 4900 mg/l) of anions of acetic, propionic and butyric acids in co-produced water from oil reservoirs represents a large pool of potential electron donors for bacterial sulphate reduction. Enrichment cultures in defined media, isolated from a variety of oil-filed environments demonstrated the wide distribution of acetate-and propionate-utilising sulphate-reducing bacteria (SRB). A propionate-utilising enrichment culture consisting predominantly of SRB, tentatively identified as species of Desulfobulbus, was used to inoculate a pressurised porous rack bioreator operating under simulated reservoir conditions. Using a flood velocity of 6.3 cm/h with an inlet propionate concentration of 168 mg/l, a sulphide generation rate of 2.5 g/ml of rock per hour was achieved at 30° C and 20 MPa. This rate indicates that a sphere of reservoir rock 9.3 m in radius, colonised with propionate-utilising SRB, could produce 50 kg sulphide day. The rates of propionate-driven bacterial sulphate reduction observed in the porous rock bioreactor could sustain the H₂S production rates observed from wells in souring reservoirs. Correspondence to: I. Vance     

Related assemblages of sulphate-reducing bacteria associated with ultradeep gold mines of South Africa and deep basalt aquifers of Washington State

We characterized the diversity of sulphate-reducing bacteria (SRB) associated with South African gold mine boreholes and deep aquifer systems in Washington State, USA. Sterile cartridges filled with crushed country rock were installed on two hydrologically isolated and chemically distinct sites at depths of 3.2 and 2.7 km below the land surface (kmbls) to allow development of biofilms. Enrichments of sulphate-reducing chemolithotrophic (H2) and organotrophic (lactate) bacteria were established from each site under both meso- and thermophilic conditions. Dissimilatory sulphite reductase (Dsr) and 16S ribosomal RNA (rRNA) genes amplified from DNA extracted from the cartridges were most closely related to the Gram-positive species Desulfotomaculum thermosapovorans and Desulfotomaculum geothermicum, or affiliated with a novel deeply branching clade. The dsr sequences recovered from the Washington State deep aquifer systems affiliated closely with the South African sequences, suggesting that Gram-positive sulphate-reducing bacteria are widely distributed in the deep subsurface.     

Methanogenic and Sulphate-Reducing Microbial Communities in Deep Groundwater of Crystalline Rock Fractures in Olkiluoto,Finland

The long-term safety of final disposal of spent nuclear fuel in the deep geosphere is dependent on stability of biogeochemical conditions at the disposal site. Microbial processes, such as sulphate reduction and methanogenesis, may have profound effects on site biogeochemistry. In this study, sulphate-reducing bacteria and methane-producing archaea were investigated at depths ranging from 68 to 545 m in crystalline rock fractures at an intended spent nuclear fuel disposal site in Olkiluoto, Finland. Denaturing gradient gel electrophoresis detected diverse sulphate-reducing bacterial communities in all samples. Although the number of dsrB gene copies was below 10³ copies ml^?1 in all analyzed samples according to real-time quantitative PCR, their abundance was highest in samples that had the highest sulphate concentrations. Several distinct mcrA gene fragments were also recovered from most of the analyzed samples by cloning, although the number of methanogens was lower than that of sulphate-reducing bacteria when measured by mcrA-targeted quantitative PCR. The detected gene fragments were most closely related to sequences obtained from aquatic and deep subsurface environments. Results imply that sulphate reduction, methanogenesis, and anaerobic methane oxidation may all take place in the Olkiluoto deep geobiosphere.     

中梁山岩溶槽谷区荒草地土壤微生物群落对隧道建设的响应

隧道工程建设给地方交通和经济发展带来便利的同时,也造成地下水漏失和地下水文流场改变,可能引起土壤微环境和土壤微生物群落的改变,进而严重影响上覆区域生态系统平衡。以重庆市中梁山岩溶槽谷为例,选取隧道影响区和无隧道影响区典型荒草地,通过16S rDNA高通量测序对比微生物群落结构差异,研究微生物群落多样性对土壤pH、含水率和土壤养分变化的响应。结果表明：隧道影响区Alpha多样性反映的土壤微生物总丰度和多样性大于无隧道影响区,Beta多样性反映的土壤微生物群落结构存在显著差异;Wilcoxon检验说明norank_o__iii1-15、norank_c__Gemmatimonadetes、norank_o__MND1是隧道影响区土壤中显著增加的微生物物种,其相对丰度为无隧道影响区土壤的2倍以上;RDA分析表明土壤pH和含水率是驱动土壤微生物群落结构变化的主要因素。隧道建设使其影响区内土壤水分垂直漏失比无隧道影响区更严重,土壤含水率下降,导致pH上升、土壤养分下降,利于土壤中norank_o__iii1-15、norank_c__Gemmatimonadetes、norank_o__MND1等适应低含水率、高pH、贫营养环境的优势菌群生长繁殖。     

In Situ Enrichment of a Diverse Community of Bacteria from a 4–5 km Deep Fault Zone in South Africa

The extreme environment of South Africa's ultra-deep gold mines offers an opportunity to discover novel microorganisms (e.g., Alkaliphilus transvaalensis), including extremophiles that may provide insight into the origins of life on earth and offer industrial applications because of their thermophilic enzymatic properties. This study employed culture-independent methods to examine the bacterial diversity in water (T = 55° C, pH = 9, Cl^? = 1000 ppm and age = 4–53 Ma) emanating from an exploration borehole in a South African Au mine that intersected a quartzite-hosted fault at 3.3 km below land surface. The more adhesive strains of sulfate reducing bacteria were effectively selected and enriched from the planktonic community by forcing water from a flowing borehole through a sand/agar cartridge that was installed within the borehole. The cartridge's agar contained sulfate and lactate that diffused from the agar into the cartridge. DNA was extracted from the sand, after which a bacterial 16S rDNA gene clone library was generated. Analysis of clone sequences indicated that the groundwater bacterial community was quite diverse, including members of the α-, β-, and γ-Proteobacteria (20%), Actinobacteria (6%), Firmicutes (57%), Chloroflexi (3%), and Deinococcus-Thermus (14%) phyla. One of the most frequently detected clone types was associated with Desulfotomaculum (a known SRB). Another predominant clone type was closely related to Meiothermus cerbereus. A proportion of Proteobacteria clones were closely related to Ralstonia, Alishewanella, Hydrogenophaga, or Methylobacterium species. Some of the Firmicutes clones were closely related to Alkaliphilus transvaalensis, which was isolated from a nearby South African Au mine, or to Clostridium thermocellum. Of the total 21 OTUs identified from the cartridge sand, 6 most likely represent novel species of Firmicutes given their dissimilarity to other 16S rDNA sequences in the GenBank database.     

Biological sulphate reduction using food industry wastes as carbon sources

Biological treatment with dissimilatory sulphate-reducing bacteria has been considered the most promising alternative for decontamination of sulphate rich effluents. These wastewaters are usually deficient in electron donors and require their external addition to achieve complete sulphate reduction. The aim of the present study was to investigate the possibility of using food industry wastes (a waste from the wine industry and cheese whey) as carbon sources for dissimilatory sulphate-reducing bacteria. The results show that these wastes can be efficiently used by these bacteria provided that calcite tailing is present as a neutralizing and buffer material. A 95 and 50 % sulphate reduction was achieved within 20 days of experiment by a consortium of dissimilatory sulphate-reducing bacteria grown on media containing waste from the wine industry or cheese whey respectively. Identification of the dissimilatory sulphate-reducing bacteria community using the dsr gene revealed the presence of the species Desulfovibrio fructosovorans, Desulfovibrio aminophilus and Desulfovibrio desulfuricans. The findings of the present study emphasise the potential of using wastes from the wine industry as carbon source for dissimilatory sulphate-reducing bacteria, combined with calcite tailing, in the development of cost effective and environmentally friendly bioremediation processes.     

Seasonal shifts in community composition and proteome expression in a sulphur-cycling cyanobacterial mat

Seasonal changes in light and physicochemical conditions have strong impacts on cyanobacteria, but how they affect community structure, metabolism, and biogeochemistry of cyanobacterial mats remains unclear. Light may be particularly influential for cyanobacterial mats exposed to sulphide by altering the balance of oxygenic photosynthesis and sulphide-driven anoxygenic photosynthesis. We studied temporal shifts in irradiance, water chemistry, and community structure and function of microbial mats in the Middle Island Sinkhole (MIS), where anoxic and sulphate-rich groundwater provides habitat for cyanobacteria that conduct both oxygenic and anoxygenic photosynthesis. Seasonal changes in light and groundwater chemistry were accompanied by shifts in bacterial community composition, with a succession of dominant cyanobacteria from Phormidium to Planktothrix, and an increase in diatoms, sulphur-oxidizing bacteria, and sulphate-reducing bacteria from summer to autumn. Differential abundance of cyanobacterial light-harvesting proteins likely reflects a physiological response of cyanobacteria to light level. Beggiatoa sulphur oxidation proteins were more abundant in autumn. Correlated abundances of taxa through time suggest interactions between sulphur oxidizers and sulphate reducers, sulphate reducers and heterotrophs, and cyanobacteria and heterotrophs. These results support the conclusion that seasonal change, including light availability, has a strong influence on community composition and biogeochemical cycling of sulphur and O₂ in cyanobacterial mats.     

Abundance and Diversity of Sulfate-Reducing Bacteria in High Arsenic Shallow Aquifers

The abundance, diversity, and relative distribution of sulfate-reducing bacteria (SRB) in high arsenic (As) groundwater aquifers of Hangjinhouqi County in the Hetao Basin, Inner Mongolia was investigated using denaturing gradient gel electrophoresis (DGGE) and quantitative polymerase chain reaction (qPCR) analysis of dsrB genes (encoding dissimilatory sulfite reductase beta-subunit). DGGE results revealed that SRB populations were diverse, but were mainly composed of Desulfotomaculum, Desulfobulbus, Desulfosarcina, and Desulfobacca. The abundance of Desulfobulbus was positively correlated with the ratio of Fe(II)/Fe(III). Although qPCR results showed that the dsrB gene abundance in groundwater samples ranged from below detection to 4.9 × 10⁶ copies/L, and the highest percentage of dsrB gene copies to bacterial 16S rRNA gene copies was 2.1%. Geochemical analyses showed that As(III) content and the ratio of Fe(II) to Fe(III) increased with total As, while sulfate concentrations decreased. Interestingly, the dsrB gene abundance was positively correlated with As concentrations. These results indicate that sulfate reduction occurs simultaneously with As and Fe reduction, and might result in increased As release and mobilization when As is not incorporated into iron sulfides. This study improves our understanding of SRB and As cycling in high As groundwater systems.     

Microbial community in a geothermal aquifer associated with the subsurface of the Great Artesian Basin, Australia

To investigate the biomass and phylogenetic diversity of the microbial community inhabiting the deep aquifer of the Great Artesian Basin (GAB), geothermal groundwater gushing out from the aquifer was sampled and analyzed. Microbial cells in the groundwater were stained with acridine orange and directly counted by epifluorescence microscopy. Microbial cells were present at a density of 10⁸–10⁹ cells per liter of groundwater. Archaeal and bacterial small-subunit rRNA genes (rDNAs) were amplified by PCR with Archaea- and Bacteria-specific primer sets, and clone libraries were constructed separately. A total of 59 clones were analyzed in archaeal and bacterial 16S rDNA libraries, respectively. The archaeal 16S rDNA clones were divided into nine operated taxonomic units (OTUs) by restriction fragment length polymorphism. These OTUs were closely related to the methanogenic genera Methanospirillum and Methanosaeta, the heterotrophic genus Thermoplasma, or miscellaneous crenarchaeota group. More than one-half of the archaeal clones (59% of total 59 clones) were placed beside phylogenetic clusters of methanogens. The majority of the methanogen-related clones (83%) was closely related to a group of hydrogenotrophic methanogens (genus Methanospirillum). The bacterial OTUs branched into seven phylogenetic clusters related to hydrogen-oxidizing thermophiles in the genera Hydrogenobacter and Hydrogenophilus, a sulfate-reducing thermophile in the genus Thermodesulfovibrio, chemoheterotropic bacteria in the genera Thermus and Aquaspirillum, or the candidate division OP10. Clones closely related to the thermophilic hydrogen-oxidizers in the genera Hydrogenobacter and Hydrogenophilus were dominant in the bacterial clone library (37% of a total of 59 clones). The dominancy of hydrogen-users strongly suggested that H₂ plays an important role as a primary substrate in the microbial ecosystem of this deep geothermal aquifer.     

Study of parameters implicated in the biodeterioration of mild steel in the presence of different species of sulphate-reducing bacteria

Two different species of sulphate-reducing bacteria, strain classified by NCIMB as Desulfovibrio desulfuricans subspecies desulfuricans New Jersey (8313) isolated from the corroding heat exchanger, and SRB species recovered from a corroding ship hull anchored off the Indonesian coast (Indo isolate) were grown as laboratory batch cultures. Several factors such as the surface finish of substratum, metabolic activity of planktonic and sessile bacterial populations, initial attachment of cells to surfaces and subsequent formation of biofilms on the process of biodeterioration of mild steel in the presence of these two different species of SRB were investigated. The corrosion rates of mild steel were estimated by weight loss measurements and correlated with the density of sessile SRB population. The yield and composition of extracellular polymers released into the bulk phase of culture media were determined and the amount of dissolved hydrogen sulphide was monitored. The results revealed differences between SRB species in their aggressiveness towards mild steel under identical growth conditions, emphasising the importance of biochemistry and physiology of SRB for the biocorrosion process. Biochemical and genetic characterisation of SRB isolates chosen for this study are currently in progress.     

Microbial biodiversity in glacier-fed streams

While glaciers become increasingly recognised as a habitat for diverse and active microbial communities, effects of their climate change-induced retreat on the microbial ecology of glacier-fed streams remain elusive. Understanding the effect of climate change on microorganisms in these ecosystems is crucial given that microbial biofilms control numerous stream ecosystem processes with potential implications for downstream biodiversity and biogeochemistry. Here, using a space-for-time substitution approach across 26 Alpine glaciers, we show how microbial community composition and diversity, based on 454-pyrosequencing of the 16S rRNA gene, in biofilms of glacier-fed streams may change as glaciers recede. Variations in streamwater geochemistry correlated with biofilm community composition, even at the phylum level. The most dominant phyla detected in glacial habitats were Proteobacteria, Bacteroidetes, Actinobacteria and Cyanobacteria/chloroplasts. Microorganisms from ice had the lowest α diversity and contributed marginally to biofilm and streamwater community composition. Rather, streamwater apparently collected microorganisms from various glacial and non-glacial sources forming the upstream metacommunity, thereby achieving the highest α diversity. Biofilms in the glacier-fed streams had intermediate α diversity and species sorting by local environmental conditions likely shaped their community composition. α diversity of streamwater and biofilm communities decreased with elevation, possibly reflecting less diverse sources of microorganisms upstream in the catchment. In contrast, β diversity of biofilms decreased with increasing streamwater temperature, suggesting that glacier retreat may contribute to the homogenisation of microbial communities among glacier-fed streams.     

Intrinsic bioremediability of an aromatic hydrocarbon-polluted groundwater: diversity of bacterial population and toluene monoxygenase genes

The functional and phylogenetic biodiversity of bacterial communities in a benzene, toluene, ethylbenzene and xylene (BTEX)-polluted groundwater was analysed. To evaluate the feasibility of using an air sparging treatment to enhance bacterial degradative capabilities, the presence of degrading microorganisms was monitored. The amplification of gene fragments corresponding to toluene monooxygenase (tmo), catechol 1,2-dioxygenase, catechol 2,3-dioxygenase and toluene dioxygenase genes in DNA extracted directly from the groundwater samples was associated with the presence of indigenous degrading bacteria. Five months of air injection reduced species diversity in the cultivable community (as calculated by the Shannon-Weaver index), while little change was noted in the degree of biodiversity in the total bacterial community, as characterised by denaturing gradient gel electrophoresis (DGGE) analysis. BTEX-degrading strains belonged to the genera Pseudomonas, Microbacterium, Azoarcus, Mycobacterium and Bradyrhizobium. The degrading capacities of three strains in batch liquid cultures were also studied. In some of these microorganisms different pathways for toluene degradation seemed to operate simultaneously. Pseudomonas strains of the P24 operational taxonomic unit, able to grow only on catechol and not on BTEX, were the most abundant, and were present in the groundwater community at all stages of treatment, as evidenced both by cultivation approaches and by DGGE profiles. The presence of different tmo-like genes in phylogenetically distant strains of Pseudomonas, Mycobacterium and Bradyrhizobium suggested recent horizontal gene transfer in the groundwater.     

Distribution of anaerobic methane-oxidizing and sulfate-reducing communities in the G11 Nyegga pockmark,Norwegian Sea

Pockmarks are seabed geological structures sustaining methane seepage in cold seeps. Based on RNA-derived sequences the active fraction of the archaeal community was analysed in sediments associated with the G11 pockmark, in the Nyegga region of the Norwegian Sea. The anaerobic methanotrophic Archaea (ANME) and sulfate-reducing bacteria (SRB) communities were studied as well. The vertical distribution of the archaeal community assessed by PCR-DGGE highlighted the presence of ANME-2 in surface sediments, and ANME-1 in deeper sediments. Enrichments of methanogens showed the presence of hydrogenotrophic methanogens of the Methanogenium genus in surface sediment layers as well. The active fraction of the archaeal community was uniquely composed of ANME-2 in the shallow sulfate-rich sediments. Functional methyl coenzyme M reductase gene libraries showed that sequences affiliated with the ANME-1 and ANME-3 groups appeared in the deeper sediments but ANME-2 dominated both surface and deeper layers. Finally, dissimilatory sulfite reductase gene libraries revealed a high SRB diversity (i.e. Desulfobacteraceae, Desulfobulbaceae, Syntrophobacteraceae and Firmicutes) in the shallow sulfate-rich sediments. The SRB diversity was much lower in the deeper section. Overall, these results show that the microbial community in sediments associated with a pockmark harbour classical cold seep ANME and SRB communities.     

Stratified Microbial Structure and Activity in Sulfide- and Methane-Producing Anaerobic Sewer Biofilms

Simultaneous production of sulfide and methane by anaerobic sewer biofilms has recently been observed, suggesting that sulfate-reducing bacteria (SRB) and methanogenic archaea (MA), microorganisms known to compete for the same substrates, can coexist in this environment. This study investigated the community structures and activities of SRB and MA in anaerobic sewer biofilms (average thickness of 800 μm) using a combination of microelectrode measurements, molecular techniques, and mathematical modeling. It was seen that sulfide was mainly produced in the outer layer of the biofilm, between the depths of 0 and 300 μm, which is in good agreement with the distribution of SRB population as revealed by cryosection-fluorescence in situ hybridization (FISH). SRB had a higher relative abundance of 20% on the surface layer, which decreased gradually to below 3% at a depth of 400 μm. In contrast, MA mainly inhabited the inner layer of the biofilm. Their relative abundances increased from 10% to 75% at depths of 200 μm and 700 μm, respectively, from the biofilm surface layer. High-throughput pyrosequencing of 16S rRNA amplicons showed that SRB in the biofilm were mainly affiliated with five genera, Desulfobulbus, Desulfomicrobium, Desulfovibrio, Desulfatiferula, and Desulforegula, while about 90% of the MA population belonged to the genus Methanosaeta. The spatial organizations of SRB and MA revealed by pyrosequencing were consistent with the FISH results. A biofilm model was constructed to simulate the SRB and MA distributions in the anaerobic sewer biofilm. The good fit between model predictions and the experimental data indicate that the coexistence and spatial structure of SRB and MA in the biofilm resulted from the microbial types and their metabolic transformations and interactions with substrates.     

Influence of cathodic protection of mild steel on the growth of sulphate‐reducing bacteria at 35°C in marine sediments

In order to protect metallic structures from marine corrosion, cathodic protection using sacrificial anodes or impressed current is widely used. In aerated seawater steel is considered to be protected when a cathodic potential of — 800 mV/SCE (Saturated Calomel Electrode) is applied. However, in many cases, this potential must be lowered due to the presence and activity of microorganisms such as acid‐producing bacteria or sulphate‐reducing bacteria (SRB). SRB are obligate anaerobes using sulphate as an electron acceptor with resultant production of sulfides. Some SRB are able to use hydrogen as an electron donor causing thereby depolarization of steel surfaces.

An experiment was performed in marine sediments to determine the relationship between cathodically produced hydrogen and growth of SRB in marine sediments both at ambiant temperature (Therene, 1988) and at 35°C. Results concerning the latter experiments are reported here.

Analytical techniques included microbiological analyses, lipid biomarker studies and electrochemical measurements including AC impedance spectroscopy. Results indicated a change in the bacterial community structure both on the steel and sediment as a function of time and potential. The results also showed that cathodically‐produced hydrogen promoted the growth of SRB with the Desulfovibrio genus predominating.