Oxygen Profiles in Salty Crust

The salinity responses of cyanobacteria, anoxygenic phototrophs, sulfate reducers, and methanogens from the laminated endoevaporitic community in the solar salterns of Eilat, Israel, were studied in situ with oxygen microelectrodes and in the laboratory in slurries. The optimum salinity for the sulfate reduction rate in sediment slurries was between 100 and 120‰, and sulfate reduction was strongly inhibited at an in situ salinity of 215‰. Nevertheless, sulfate reduction was an important respiratory process in the crust, and reoxidation of formed sulfide accounted for a major part of the oxygen budget. Methanogens were well adapted to the in situ salinity but contributed little to the anaerobic mineralization in the crust. In slurries with a salinity of 180‰ or less, methanogens were inhibited by increased activity of sulfate-reducing bacteria. Unicellular and filamentous cyanobacteria metabolized at near-optimum rates at the in situ salinity, whereas the optimum salinity for anoxygenic phototrophs was between 100 and 120‰.

Gypsum crusts containing multicolored stratified microbial populations grow in the evaporation ponds of a commercial saltern in Eilat, Israel. These crusts contain two prominent cyanobacterial layers, a bright purple layer of anoxygenic phototrophs, and a lower black layer with active sulphate reduction. We explored the diel dynamics of oxygen and sulphide within the crust using specially constructed microelectrodes, and further explored the crust biogeochemistry by measuring rates of sulphate reduction, stable sulphur isotope composition, and oxygen exchange rates across the crust–brine interface. We explored crusts from ponds with two different salinities, and found that the crust in the highest salinity was the less active. Overall, these crusts exhibited much lower rates of oxygen production than typical organic‐rich microbial mats. However, this was mainly due to much lower cell densities within the crusts. Surprisingly, on a per cell‐volume basis, rates of photosynthesis were similar to organic‐rich microbial mats. Due to relatively low rates of oxygen production and deep photic zones extending from 1.5 to 3 cm depth, a large percentage of the oxygen produced during the day accumulated into the crusts. Indeed, only between 16% to 34% of the O 2 produced in the crust escaped, and the remainder was internally recycled, used mainly in O 2 respiration. We view these crusts as potential homologs to ancient salt‐encrusted microbial ecosystems, and we compared them to the 3.45 billion‐year‐old quartz barite deposits from North Pole, Australia, which originally precipitated gypsum.

The vertical distribution of phototrophic and non-phototrophic microorganisms was examined in 2 saltern evaporation ponds with salinities of 156 and 206 g l-1. The biogeochemistry of these 2 ponds was examined using microsensors for oxygen, pH and sulfide. These measurements showed that net rates of oxygen production/consumption were significantly higher at a salinity of 156 than at 206 g l-1. The distribution of phototrophic microorganisms was studied by microscopy, which revealed several differences between the 2 crusts. The relative amounts of Bacteria, Archaea, sulfate reducers and methanogens were studied by real-time quantitative PCR amplification of genes for 16S rRNA, dissimilatory sulfite reductase (DSR), and methyl coenzyme M reductase (MCR). Sulfate reducers and methanogens were detected only in the deepest part of the phototrophic zone and below. Sulfate reducers were most abundant in the zone just below the phototrophic layer, where the DSR gene copy number w̃1.5% that of the 16S rRNA gene copy number. Methanogens were much less abundant than sulfate reducers, and the number of MCR gene copies never exceeded 0.1% of the number of 16S rRNA gene copies. Methanogens were less abundant at a salinity of 206 than at 156 g l -1. Inter-pond and vertical variations in the composition of methanogenic and sulfate reducing communities were further characterized by DGGE analysis. The detected sulfate reducers were affiliated with 4 different phylogenetic groups that included members of the Desulfovibrionales, relatives of Desulfotomaculum, and 2 deeply branching groups with no close cultured relatives. The detected phylotypes were distributed in a distinct pattern in the crust according to both biogeochemical regimes and salinity. Methanogens were all affiliated with the known halophilic genera Methanohalophilus and Methanohalobium. © Inter-Research 2009.