The epsomitic phototrophic microbial mat of Hot Lake, Washington: community structural responses to seasonal cycling

Abstract

Phototrophic microbial mats are compact ecosystems composed of highly interactive organisms in which energy and element cycling take place over millimeter-to-centimeter-scale distances. Although microbial mats are common in hypersaline environments, they have not been extensively characterized in systems dominated by divalent ions. Hot Lake is a meromictic, epsomitic lake that occupies a small, endorheic basin in north-central Washington. The lake harbors a benthic, phototrophic mat that assembles each spring, disassembles each fall, and is subject to greater than tenfold variation in salinity (primarily Mg2+ and SO2−4) and irradiation over the annual cycle. We examined spatiotemporal variation in the mat community at five time points throughout the annual cycle with respect to prevailing physicochemical parameters by amplicon sequencing of the V4 region of the 16S rRNA gene coupled to near-full-length 16S RNA clone sequences. The composition of these microbial communities was relatively stable over the seasonal cycle and included dominant populations of Cyanobacteria, primarily a group IV cyanobacterium (Leptolyngbya), and Alphaproteobacteria (specifically, members of Rhodobacteraceae and Geminicoccus). Members of Gammaproteobacteria (e.g., Thioalkalivibrio and Halochromatium) and Deltaproteobacteria (e.g., Desulfofustis) that are likely to be involved in sulfur cycling peaked in summer and declined significantly by mid-fall, mirroring larger trends in mat community richness and evenness. Phylogenetic turnover analysis of abundant phylotypes employing environmental metadata suggests that seasonal shifts in light variability exert a dominant influence on the composition of Hot Lake microbial mat communities. The seasonal development and organization of these structured microbial mats provide opportunities for analysis of the temporal and physical dynamics that feed back to community function.

Trade-offs between microbiome diversity and productivity in a stratified microbial mat

“…Microbial mats were sampled from the previously described Hot Lake field site (Lindemann et al, 2013). Whole-mat samples and underlying sediment were transferred under dark, cool (4-10°C) conditions.…”

“…The depth-wise cryosectioning, DNA extraction and DNA sequencing methods were performed as previously described (Lindemann et al, 2013). Briefly, triplicate mat samples were embedded in sucrose, flash-frozen and cryosectioned into 0.5 mm vertical subsamples used for triplicate for genomic DNA extractions.…”

“…Hence, we infer that Pseudanabaenaceae may be light sensitive and/or capable of operating as an aerobic heterotroph just below the photic zone. Another possibility is that it and other taxa are non-motile and thereby trapped within the microcosm that that does not experience the high irradiances possible from solar noon (~2000 μmol photons m − 2 sec − 1 ) in the natural habitat (Lindemann et al, 2013).…”

“…The goal was to determine how vertical gradients in community composition and associated micro-environmental conditions translate to productivity. The experimental microcosm was derived from a benthic microbial mat native to epsomitic Hot Lake; located in Washington State, USA (Anderson, 1958;Lindemann et al, 2013;Zachara et al, 2016). Productivity was assessed by two spatially resolved, complementary metrics: the gross rate of oxygenic photosynthesis and %C replacement of biomass over time.…”

Autonomous Device for Evaluating the Field Performance of Microbial Fuel Cells in Remote Areas

“…Indeed, the concentration of sulfate ions in Hot Lake increases significantly during the dry season, during which the RSMFCTs were deployed, for example, reaching approximately 1.8 mM on September 1 st and October 20 th of 2011. 27 The presence of competing reactions on the cathode (such as sulfate reduction, which has a standard reduction potential of −39 mV vs. Ag/AgCl 36 ) could also explain the relatively low cathode potential compared to similar SMFCs deployed in freshwater 2,25 and ocean water sediments. 32,37 We should note that sulfate reduction is possible in the presence of a thick biofilm in which the top layers consume oxygen and generate anoxic conditions for the sulfate reducers at the bottom of the biofilm.…”