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Molecular information about the bacterial composition of a coculture capable of sulfate reduction after exposure to oxic and microoxic conditions was used to identify and subsequently to isolate the components of the mixture in pure culture. PCR amplification of 16S ribosomal DNA fragments from the coculture, analyzed by denaturing gradient gel electrophoresis, resulted in two distinct 16S ribosomal DNA bands, indicating two different bacterial components. Sequencing showed that the bands were derived from a Desulfovibrio strain and an Arcobacter strain. Since the phylogenetic positions of bacteria are often consistent with their physiological properties and culture requirements, molecular identification of the two components of this coculture allowed the design of specific culture conditions to separate and isolate both strains in pure culture. This approach facilitates the combined molecular and physiological analysis of mixed cultures and microbial communities.

Biogeochemical, isotope geochemical and microbiological investigation of Lake Svetloe (White Sea basin), a meromictic freshwater was carried out in April 2014, when ice thickness was ∼0.5 m, and the ice-covered water column contained oxygen to 23 m depth. Below, the anoxic water column contained ferrous iron (up to 240 μμM), manganese (60 μM), sulfide (up to 2 μM) and dissolved methane (960 μM). The highest abundance of microbial cells revealed by epifluorescence microscopy was found in the chemocline (redox zone) at 23-24.5 m. Oxygenic photosynthesis exhibited two peaks: the major one (0.43 μmol C L day ) below the ice and the minor one in the chemocline zone, where cyanobacteria related to Synechococcus rubescens were detected. The maximum of anoxygenic photosynthesis (0.69 μmol C L day ) at the oxic/anoxic interface, for which green sulfur bacteria Chlorobium phaeoclathratiforme were probably responsible, exceeded the value for oxygenic photosynthesis. Bacterial sulfate reduction peaked (1.5 μmol S L day ) below the chemocline zone. The rates of methane oxidation were as high as 1.8 μmol CH L day at the oxi/anoxic interface and much lower in the oxic zone. Small phycoerythrin-containing Synechococcus-related cyanobacteria were probably involved in accumulation of metal oxides in the redox zone.

A chemostat culture of the sulfate-reducing bacterium Desulfovibrio oxyclinae isolated from the oxic layer of a hypersaline cyanobacterial mat was grown anaerobically and then subjected to gassing with 1% oxygen, both at a dilution rate of 0.05 h ؊1 . The sulfate reduction rate under anaerobic conditions was 370 nmol of SO 4 2؊ mg of protein ؊1 min ؊1 . At the onset of aerobic gassing, sulfate reduction decreased by 40%, although viable cell numbers did not decrease. After 42 h, the sulfate reduction rate returned to the level observed in the anaerobic culture. At this stage the growth yield increased by 180% compared to the anaerobic culture to 4.4 g of protein per mol of sulfate reduced. Protein content per cell increased at the same time by 40%. The oxygen consumption rate per milligram of protein measured in washed cell suspensions increased by 80%, and the thiosulfate reduction rate of the same samples increased by 29% with lactate as the electron donor. These findings indicated possible oxygen-dependent enhancement of growth. After 140 h of growth under oxygen flux, formation of cell aggregates 0.1 to 3 mm in diameter was observed. Micrometer-sized aggregates were found to form earlier, during the first hours of exposure to oxygen. The respiration rate of D. oxyclinae was sufficient to create anoxia inside clumps larger than 3 m, while the levels of dissolved oxygen in the growth vessel were 0.7 ؎ 0.5 M. Aggregation of sulfate-reducing bacteria was observed within a Microcoleus chthonoplastes-dominated layer of a cyanobacterial mat under daily exposure to oxygen concentrations of up to 900 M. Desulfonema-like sulfate-reducing bacteria were also common in this environment along with other nonaggregated sulfatereducing bacteria. Two-dimensional mapping of sulfate reduction showed heterogeneity of sulfate reduction activity in this oxic zone.The photic zone of cyanobacterial mats is characterized by extreme diurnal shifts in oxygen and sulfide concentrations from oxygen supersaturation during the light period to anoxic, sulfide-enriched conditions in the dark (25). These changes are a challenge for the organisms living in such a habitat. Daily exposures to high oxygen concentrations may be deleterious to obligate anaerobes such as sulfate-reducing bacteria (SRB). Nonetheless, high numbers of SRB were found in oxic zones of microbial mats (5,7,12,22,23,32), indicating that these organisms can deal with temporal exposures to elevated oxygen concentrations of even up to 1.5 mM. Previous research demonstrated the abundance of members of the genera Desulfovibrio (18, 26) and Desulfonema (22,23,32) in the upper layers of hypersaline cyanobacterial mats. Moreover, high rates of sulfate reduction were found in the oxic layers of these microbial mats (8, 11). Attempts were made to isolate oxygentolerant SRB from these environments (13a, 16, 17). However, all SRB isolated to date required anoxic conditions for growth. Although the notion that SRB are strictly anaerobic was recently reconsidered (14,19), the evidence of...

A chemostat coculture of the sulfate-reducing bacterium Desulfovibrio oxyclinae together with a facultative aerobe heterotroph tentatively identified as Marinobacter sp. strain MB was grown under anaerobic conditions and then exposed to a stepwise-increasing oxygen influx (0 to 20% O 2 in the incoming gas phase). The coculture consumed oxygen efficiently, and no residual oxygen was detected with an oxygen supply of up to 5%. Sulfate reduction persisted at all levels of oxygen input, even at the maximal level, when residual oxygen in the growth vessel was 87 M. The portion of D. oxyclinae cells in the coculture decreased gradually from 92% under anaerobic conditions to 27% under aeration. Both absolute cell numbers and viable cell counts of the organism were the same as or even higher than those observed in the absence of oxygen input. The patterns of consumption of electron donors and acceptors suggest that aerobic incomplete oxidation of lactate to acetate is performed by D. oxyclinae under high oxygen input. Both organisms were isolated from the same oxic zone of a cyanobacterial mat where they have to adapt to daily shifts from oxic to anoxic conditions. This type of syntrophic association may occur in natural habitats, enabling sulfate-reducing bacteria to cope with periodic exposure to oxygen.The photic zone of cyanobacterial mats is characterized by extreme shifts in oxygen and sulfide concentrations from oxygen supersaturation during the light period to anoxic, sulfideenriched conditions in the dark (24). These changes are a challenge for the organisms living in such a habitat. Daily exposure to high oxygen concentrations may be deleterious to obligately anaerobic organisms, such as sulfate-reducing bacteria (SRB) (13,18,24). High numbers of SRB were found in the oxic zone of a cyanobacterial mat from the Solar Lake (Sinai) throughout the diurnal cycle (10,21,22,25,27), indicating that these organisms can deal with exposure to elevated oxygen concentrations of even up to 1.5 mM. Moreover, high rates of sulfate reduction were found in the oxic layers of these microbial mats (4, 5, 9), as well as in microbial mats from Baja California, Mexico (2).None of the SRB isolated so far can either grow aerobically or reduce sulfate under high concentrations of oxygen. Desulfovibrio vulgaris Hildenborough was found to be capable of slow linear aerobic growth under very low concentrations of oxygen. Oxygen concentrations of only 0.07% were toxic to this organism (14). The oxygen-tolerant SRB Desulfovibrio oxyclinae isolated in our laboratory (16) was demonstrated to persist in continuous culture gassed with 1% O 2 , although flocculation occurred after exposure to oxygen (25).It was previously demonstrated that SRB in the upper layer of cyanobacterial mats can form close associations with oxygen-scavenging bacteria (12, 26). Such consortia may be responsible, at least in part, for the observed in situ sulfatereducing activity under aerobic conditions.In this report, we present the effects of exposure to oxygen on the ...