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Open AccessArticle

Biosignatures Associated with Freshwater Microbialites

1
Department of Plant Pathology, Washington State University, Pullman, WA 99163, USA
2
RAW Molecular Systems (RMS) LLC, Spokane, WA 99218, USA
3
Australian Centre for Astrobiology, University of New South Wales, Sydney, NSW 2052, Australia
4
School of Geography and Earth Sciences, McMaster University, Hamilton, ON L8S 4K1, Canada
5
School of Earth and Environmental Sciences, University of Queensland, QLD 4072, Australia
6
NASA Ames Research Center, Moffett Field, CA 94035, USA
*
Author to whom correspondence should be addressed.
These authors equally contributed.
Life 2020, 10(5), 66; https://doi.org/10.3390/life10050066
Received: 31 March 2020 / Revised: 11 May 2020 / Accepted: 11 May 2020 / Published: 15 May 2020
Freshwater microbialites (i.e., lithifying microbial mats) are quite rare in northern latitudes of the North American continent, with two lakes (Pavilion and Kelly Lakes) of southeastern BC containing a morphological variety of such structures. We investigated Kelly Lake microbialites using carbon isotope systematics, phospholipid fatty acids (PLFAs) and quantitative PCR to obtain biosignatures associated with microbial metabolism. δ13CDIC values (mean δ13CDIC −4.9 ± 1.1‰, n = 8) were not in isotopic equilibrium with the atmosphere; however, they do indicate 13C-depleted inorganic carbon into Kelly Lake. The values of carbonates on microbialite surfaces (δ13C) fell within the range predicted for equilibrium precipitation from ambient lake water δ13CDIC (−2.2 to −5.3‰). Deep microbialites (26 m) had an enriched δ13Ccarb value of −0.3 ± 0.5‰, which is a signature of photoautotrophy. The deeper microbialites (>20 m) had higher biomass estimates (via PLFAs), and a greater relative abundance of cyanobacteria (measured by 16S copies via qPCR). The majority of PLFAs constituted monounsaturated and saturated PLFAs, which is consistent with gram-negative bacteria, including cyanobacteria. The central PLFA δ13C values were highly depleted (−9.3 to −15.7‰) relative to δ13C values of bulk organic matter, suggesting a predominance of photoautotrophy. A heterotrophic signature was also detected via the depleted iso- and anteiso-15:0 lipids (−3.2 to −5.2‰). Based on our carbonate isotopic biosignatures, PLFA, and qPCR measurements, photoautotrophy is enriched in the microbialites of Kelly Lake. This photoautotrophy enrichment is consistent with the microbialites of neighboring Pavilion Lake. This indication of photoautotrophy within Kelly Lake at its deepest depths raises new insights into the limits of measurable carbonate isotopic biosignatures under light and nutrient limitations. View Full-Text
Keywords: Kelly Lake; Pavilion Lake; microbialites; biosignatures Kelly Lake; Pavilion Lake; microbialites; biosignatures
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White, R.A., III; Soles, S.A.; Brady, A.L.; Southam, G.; Lim, D.S.; Slater, G.F. Biosignatures Associated with Freshwater Microbialites. Life 2020, 10, 66.

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