Biosignatures Associated with Freshwater Microbialites

Abstract

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.

Molecular and isotopic evidence reveals the end-Triassic carbon isotope excursion is not from massive exogenous light carbon

“…Microbial mats are known to produce lipids with greater 13 C-depletion compared to those of phytoplankton (60). Investigations into the isotopic composition of phospholipid fatty acids in modern freshwater microbialites have shown lipids greatly depleted in 13 C (δ 13 C values ranging between −31.4 ± 0.6 and 44.1 ± 0.5‰) (61,62), and biomarkers of sulfate-reducing bacteria (important contributors in microbial-mat communities) in the carbonate matrix of concretions are also considerably 13 C-depleted (δ 13 C values ranging between −40.5 and 42.0‰) (63). Furthermore, the recycling of 13 C-depleted autotrophic biomass by lower-residing heterotrophic layers within freshwater microbialites is required to explain changes in microbialite vertical δ 13 C profiles and calcium carbonate precipitation equations whereby the local δ 13 C DIC becomes increasingly negative as a result of microbial respiration (64,65).…”