Spatial and Temporal Dynamics of Microbial Sulfur Cycling During the Onset of Meromixis in Mono Lake, California

Mono Lake is a closed-basin soda lake below the eastern slope of the Sierra Nevada in California with high alkalinity (pH 9.8), salinity (≥80 g/L), and sulfate concentration (110 mM). Large influxes of freshwater driven by high precipitation and ice melt can cause the lake to stratify and enter a period of meromixis, thus making the lake an ideal ecosystem for studying the response of microbial biogeochemistry to changes in water chemistry. We have captured the most recent development of meromixis between May 2017 and June 2018, a period following the 2016-17 winter that produced one of the largest snowpacks in California's recorded history. Previous work reported the temporal relationship between microbial sulfur cycling and lake biogeochemistry from May through September 2017 - documenting the onset of stratification. In May and June 2018, we revisited a site in the deepest part of the lake to investigate the changes in water chemistry, microbial diversity, and the sulfur cycle with the advancement of lake stratification. Our results show that from September 2017 to May 2018 the chemocline and oxycline shifted 10m and 7m deeper respectively, representing an increase in freshwater influx and advective mixing of the upper water column. During September 2017, particulate organic sulfur fractionation was detectable below the chemocline (δ³⁴S_POS:15.70‰ to 5.80‰) and became more pronounced in May 2018 with δ³⁴S_POS= 0.26‰ at 17m depth. Additionally, we noted the appearance of hydrogen sulfide below the chemocline with mean sulfide concentration of 0.37µM and δ³⁴S_sulfide ranging from -21.75‰ to -29.68‰ with depth. Lipid biomarkers revealed an increase over time in polyunsaturated fatty acids indicative of Picocystis,a unicellular green alga, and a unique hydroxy fatty acid with a single double bond possibly created by bacteria. In contrast to September 2017, the microbial community in May 2018 clearly reflects advanced stratification, with high abundance of several genera of sulfate-reducing bacteria and likely fermentative organisms below the chemocline, suggesting a potential syntrophic relationship between them. Our data illustrate the latest stage of the lake's meromictic conditions and suggest that microbial sulfur cycling is undergoing rapid changes impacting microbial diversity and water column chemistry.