Metagenomic Exploration of the Mono Lake Water Column: How Microbes Thrive despite High Salt and Arsenic Concentrations

Mono Lake is a hypersaline, As rich, closed basin lake that undergoes extended meromixis after winters with heavy snowfall. In order to understand the microbial community of this unusual environment, Metagenome Assembled Genome (MAG) bins and 16S rRNA samples were collected over 2017-2019 during the most recent meromictic event that began in spring 2017. Notable prokaryotic bins include a Cyanobium abundant below the photic zone, a likely photosynthetic member of the understudied class Gemmatimonadetes, and multiple members of the Nitriliruptor genus within the understudied Actinobacteria. One Nitriliruptor bin was among the most abundant prokaryotic bins across all depths and times as stratification developed and persisted. The cultured representative of this genus is known to grow on recalcitrant nitrile compounds, which other clades cannot use as a N source. Our analysis showed that this bin lacks the genes to break nitrile bonds. Instead it is distinguished by its ability to grow on a wider range of carbohydrate compounds than other members of its clade, possibly giving it an advantage in an OM rich system like Mono Lake. To further investigate the lake's N cycle, depth-sorted metagenomic reads were BLAST searched against reference databases of conserved domains present in N cycle functional genes; amoA, hao, narG, nirS, nirK, nor, nosZ, nifH, nrfA, ureC, and nitrile hydratase beta were present. hao, nifH, and nrfA increased with depth in the Mono Lake water column. Abundant ureC and nitrile hydratase beta subunit sequences in metagenomic reads highlight the probable importance of urea and recalcitrant DON in the lake's N cycle. Relationships between the lake's unusual chemistry and biology were investigated using a Canonical Correspondence Analysis (CCA) which combined multi-year CTD, nutrient (NO₃^-, NO₂^-, NH₄⁺, PO₄^3-, and Si) and TAG sequencing data. Trends were seen with dissolved O₂ and NH₄⁺. Mono Lake's N cycle is highly unusual due to its elevated DON and accumulated NH₄⁺ during stratification. While further research is needed into the nutrient cycles of unusual environments like Mono Lake, our results show that DON is likely an important N source and that one of the most abundant organisms is a generalist heterotroph capable of aerobically or anaerobically metabolizing a wide range of carbohydrates.