Annual sulfur cycle in a warm monomictic lake with sub-millimolar sulfate concentrations

Abstract

Background:We studied the annual variability of the concentration and isotopic composition of main sulfur species and sulfide oxidation intermediates in the water column of monomictic fresh-water Lake Kinneret. Sulfate concentrations in the lake are <1 mM and similar to concentrations that are proposed to have existed in the Paleoproterozoic ocean. The main goal of this research was to explore biogeochemical constrains of sulfur cycling in the modern low-sulfate fresh-water lake and to identify which processes may be responsible for the isotopic composition of sulfur species in the Precambrian sedimentary rocks. Results:At the deepest point of the lake, the sulfate inventory decreases by more than 20% between March and December due to microbial sulfate reduction leading to the buildup of hydrogen sulfide. During the initial stages of stratification, sulfur isotope fractionation between sulfate and hydrogen sulfide is low (11.6 ‰) and sulfur oxyanions (e.g. thiosulfate and sulfite) are the main products of the incomplete oxidation of hydrogen sulfide. During the stratification and at the beginning of the lake mixing (July-December), the inventory of hydrogen sulfide as well as of sulfide oxidation intermediates in the water column increases and is accompanied by an increase in sulfur isotope fractionation to 30 ± 4 ‰ in October. During the period of erosion of the chemocline, zero-valent sulfur prevails over sulfur oxyanions. In the terminal period of the mixing of the water column (January), the concentration of hydrogen sulfide decreases, the inventory of sulfide oxidation intermediates increases, and sulfur isotope fractionation decreases to 20 ± 2 ‰. Conclusions:Sulfide oxidation intermediates are present in the water column of Lake Kinneret at all stages of stratification with significant increase during the mixing of the water column. Hydrogen sulfide inventory in the water column increases from March to December, and sharply decreases during the lake mixis in January. Sulfur isotope fractionation between sulfate and hydrogen sulfide as well as concentrations of sulfide oxidation intermediates can be explained either by microbial sulfate reduction alone or by microbial sulfate reduction combined with microbial disproportionation of sulfide oxidation intermediates. Our study of sulfur cycle in Lake Kinneret may be useful for understanding the range of biogeochemical processes in low sulfate oceans over Earth history.

Isotopic evidence for the origin of dimethylsulfide and dimethylsulfoniopropionate-like compounds in a warm, monomictic freshwater lake

“…The uniform values of d 34 S DMSPwc (þ13.3 AE 1.0 %) under oxic conditions in the water column during the mixed period (March 2014) and early stratification (June 2014) had similar values to those of d 34 S SO4 (þ13.1 AE 0.6 %) measured by Knossow et al in the epilimnion of Lake Kinneret. [36] The sulfur isotope fractionation during assimilatory sulfate reduction is small, À0.9 to À2.8 %, [13] which results in d 34 S of bulk organic matter slightly 34 S depleted relative to sulfate values (Fig. 8a,b).…”

“…Stable sulfur isotopes have the potential to provide information on the natural sources and sinks of compounds like DMS without experimental manipulation. Sulfur has four stable isotopes ( 32 S, 34 S, 33 S and 36 S) of which the most abundant are 32 S and 34 S. Usually during microbial processes there is a discrimination against the heavy isotope, leaving the product isotopically enriched in the light one and the residual pool enriched in the heavy one. Anaerobic dissimilatory sulfate reduction (respiratory electron accepting process) is the main sulfur metabolism process on Earth and is characterised by large isotopic fractionation (up to 72 per mille (%)) with the light isotope favoured in the H 2 S product.…”

High Sulfur Isotope Fractionation Associated with Anaerobic Oxidation of Methane in a Low-Sulfate, Iron-Rich Environment

“…Freshwater systems low in sulfate are frequently used as early Earth analogs to study components of early ocean biogeochemistry (Canfield et al, 2010;Crowe et al, 2011;Knossow et al, 2015). One such site is the anoxic sediment of Lake Ørn, with sulfate concentrations <200 µmol L −1 , maximum methane concentrations of 1.4 mmol L −1 and reactive iron oxide concentrations of up to 65 µmol cm −3 (NorDi et al, 2013).…”

Stratification of microbiomes during the holomictic period of Lake Fuxian, an alpine monomictic lake

“…Meanwhile, Lake Fuxian has an unusual geological background, as the run-off from the surrounding mountains carries large quantities of sulfur into the waters (sulfur concentration in some inlets reaches 37.6 mg L −1 ; Cai et al 2002). In terms of electron acceptors, chemoorganoheterotrophic can oxidize many different types of organic compounds by transferring electrons from organic compounds to nitrate or sulfate when oxygen is limited, which helps in the degradation of organic materials in the hypolimnion (Knossow et al 2015;Wenk et al 2016). The analysis of functional genes and metabolic pathways also supported this hypothesis.…”