Sulfate reduction and sulfide oxidation in extremely steep salinity gradients formed by freshwater springs emerging into the Dead Sea

Abstract

Abundant microbial mats, recently discovered in underwater freshwater springs in the hypersaline Dead Sea, are mostly dominated by sulfur-oxidizing bacteria. We investigated the source of sulfide and the activity of these communities. Isotopic analysis of sulfide and sulfate in the spring water showed a fractionation of 39-50‰ indicative of active sulfate reduction. Sulfate reduction rates (SRR) in the spring sediment (< 2.8 nmol cm(-3) day(-1)) are too low to account for the measured sulfide flux. Thus, sulfide from the springs, locally reduced salinity and O2 from the Dead Sea water are responsible for the abundant microbial biomass around the springs. The springs flow is highly variable and accordingly the local salinities. We speculate that the development of microbial mats dominated by either Sulfurimonas/Sulfurovum-like or Thiobacillus/Acidithiobacillus-like sulfide-oxidizing bacteria, results from different mean salinities in the microenvironment of the mats. SRR of up to 10 nmol cm(-3) day(-1) detected in the Dead Sea sediment are surprisingly higher than in the less saline springs. While this shows the presence of an extremely halophilic sulfate-reducing bacteria community in the Dead Sea sediments, it also suggests that extensive salinity fluctuations limit these communities in the springs due to increased energetic demands for osmoregulation.

Microbial Mat Compositional and Functional Sensitivity to Environmental Disturbance

“…However, it is possible that, while sulfide oxidation is thermodynamically favorable for osmoadaptation, increased levels of salinity or other environmental factors may have suppressed the activity of the sulfide oxidizing community. Evidence for a possible salinity-driven suppression of sulfide oxidation was observed in a study examining the sulfur cycle along a salinity gradient from freshwater into the Dead Sea ( Häusler et al, 2014 ). This study observed that increasing salinity selected for different sulfide oxidizing communities with the highest salinity restricting growth to only halotolerant or halophilic taxa.…”

Microbial sedimentary imprint on the deep Dead Sea sediment

“…This sulphidation is most probably the result of microbial sulphate reduction in the Dead Sea sediment. Such a process has been identified in the present Dead Sea (Häusler et al ., ) where it can benefit from very sharp salinity gradients formed, for example, by underwater fresh water springs.…”

“…Second, this pathway would depend on the complete degradation of organic matter through microbial activity, which is not supported by the data presented here. Instead, this study supports either a slow and/or incomplete sulphate reduction, as previously demonstrated in the Dead Sea environment (Bishop et al ., ; Häusler et al ., ), which would have a limited effect on the alkalinity of the sedimentary micro‐environment (Dupraz & Visscher, ). The impact of calcium carbonate generated via sulphate reduction on the Dead Sea environment should then be minor compared to the huge amount of aragonite needles precipitating in the water column: 15 mol m −2 y −1 in Lake Lisan (Stein et al ., ), and 1·4 mol m −2 y −1 after heavy flooding during winter, 1992 (Barkan et al ., ).…”

“…Sulphate reduction rates in the sediment are, however, supposed to be very slow because of the high salinity and low input of fresh carbon (Häusler et al ., ). It means that the production of sulphide from an allegedly infinite sulphate source in most of the deep Dead Sea sediment (as observed from gypsum saturation), is limited.…”

Microbial Mat Functional and Compositional Sensitivity to Environmental Disturbance

“…However, it is possible that, while sulfide oxidation is thermodynamically favorable for osmoadaptation, increased levels of salinity or other environmental factors may have suppressed the activity of the sulfide oxidizing community. Evidence for a possible salinity-715 driven suppression of sulfide oxidation was observed in a study examining the sulfur cycle along a salinity gradient from freshwater into the Dead Sea (Häusler et al, 2014). This study observed that increasing salinity selected for different sulfide oxidizing communities with the highest salinity restricting growth to only halotolerant or halophilic taxa.…”