Baseline microbial biogeochemistry of a terminal lake in transition, Walker Lake, Nevada

Background/Question/Methods

Walker Lake, an alkaline terminal lake in west-central Nevada, has undergone severe desiccative stress since the late 1800s; resulting in a >75% decrease in volume and >5-fold increase in salinity. These changes have consequently led to the catastrophic decline of endemic fisheries and opportunities for migratory birds. Walker Lake is characterized by high alkalinity (~1,500 mg/L) and pH (~9.35); N-limitation; high dissolved organic C content (~40 mg/L); and elevated levels of solutes (e.g. As at >1,400 ppb [~20 micromolar]). Our group conducted a survey of water column and sediment microbial biogeochemistry of Walker Lake in 2007/2008 and recently (beginning in November of 2013) has commenced a new phase of research. Thus, this study has witnessed the passage of an ecological tipping point: from an initial state that supported freshwater fisheries towards an increasingly saline (e.g. Mono Lake-like) future. To better constrain the role of microorganisms in biogeochemical transformations at Walker and to develop an understanding of the baseline biogeochemistry over a range of conditions, microbial and microalgal populations are being assessed using cultivation- and molecular DNA-based approaches; both considered within the context of environmental and aqueous chemical variables.   

Results/Conclusions

During this study, salinity increased (16.5 g/L to 18.5 g/L) and a diverse bacterial community, mostly affiliated with saline lake alkaliphiles, was documented. A microbially-driven arsenic cycle was indicated by both an abundance of As oxidizers and reducers (up to 10e6 and 10e4 per mL), and complete transformation of arsenate to arsenite and then thioasenates in the hypolimnion. Other abundant physiotypes included: aerobic heterotrophs and fermenters, and nitrate-, iron-, sulfur- and manganese reducers. In the hypolimnion, sulfate reducer abundance (~10e3) and stable isotopic evidence for sulfate reduction did not result in elevated H₂S. Rather, the dominance of a cyanobacterium (Synechococcus spp. ~7 x 10⁵) is consistent with anoxygenic photosynthesis. Typical of regional terminal lakes with anoxic hypolimnia (e.g. Mono, Soda), Walker remained severely N-limited (PO₄^- ~16 mM-P vs. NO₃^- ~70 nM-N), but hypolimnetic ammonia accumulated dramatically during stratification (e.g. ~25 mM-N). As the lake’s depth declined (from 22 m to 19.5 m), however, the tendency to form a hypolimnion was correspondingly reduced. We hypothesize that this switch (e.g. from monomictic to holomictic) will drastically impact lake function; for example, by abolishing hypolimnetic blooms of anoxytrophic cyanobacteria supported by internal N-loading and increasing the relative importance of N-fixing forms like Nodularia spp.).