PS 8-103 - CANCELLED - Seasonal variation in bacterial diversity throughout a hypersaline water column

Abstract: The Salton Sea is a hypersaline, terminal lake that sits in Riverside and Imperial Counties in Southern California. Though the Sea is shallow and actively shrinking due to ongoing desiccation and water diversion, stratification within the seawater occurs every summer. Rising temperatures in the summer months warm the surface water of the sea, reducing its density and resulting in lake stratification (Reese et al. 2008). As summer progresses, high wind speeds cause an upwellings, bringing sulfide to the surface and oxygen to the hypolimnion (i.e., the bottom layer; Ma et al., 2020; Reese et al., 2008; Tiffany et al., 2007).This mixing leads to rapid oxidation of sulfide to sulfate, consuming the available dissolved oxygen throughout the lake (Reese et al. 2008). While the connection between seasonality and biogeochemistry in the Salton Sea has been established, the involvement of microorganisms in the nutrient cycling of this lake has been ignored.

The goal of this work was to establish the connection between seasonality, biogeochemistry, and microbial assembly throughout the water column within this extreme environment. Nutrient availability is responsible for shaping microbial communities and their respective functional traits (Nemergut et al. 2013, Louca et al. 2016, 2017, 2018), thus we hypothesized that bacterial taxonomic and functional diversity would vary with seasonal and geochemical changes in the Salton Sea water. Seawater was collected at 0m, 3m, 4m, 5m, 7m, 9m, 10m, and 11m depths at the same GPS coordinates in June 2021, August 2021, December 2021, and April 2022. Dissolved oxygen concentration, oxidation-reduction potential, and temperature (C) measurements were taken at these depths for each timepoint. DNA was extracted from the seawater and submitted for amplicon sequencing (targeting the 16S rRNA marker gene) as well as shotgun metagenome sequencing. Preliminary results illustrate that bacterial composition within the water column significantly varies across seasons (p=0.001). Additionally, oxidative-reduction potential (p=0.001) as well as temperature (p=0.001) significantly drive bacterial community assembly. Lastly, sulfur metabolic pathways were found in the metagenome-assembled genomes isolated from the seawater. These findings suggest that microbial metabolic strategies in the water column could be responsible for the relationship between microbial composition and seasonal biogeochemistry. Previous research has shown that low redox potential ( < 200 mV) is linked to microbial sulfur metabolism in stratified lakes (Søndergaard 2009). Collectively, these results support that nutrient availability and environmental conditions select for the function, and thus the structure, of microbial community diversity.