Autotrophic and heterotrophic associated biosignatures in modern freshwater microbialites over seasonal and spatial gradients

Phospholipid fatty acid (PLFA) profiles and isotopic biosignatures associated with autotrophic and heterotrophic microbial processes in freshwater microbialites exposed to seasonal and spatial gradients in Pavilion Lake, British Columbia were investigated. The PLFA biosignature profiles of the microbialite associated microbial communities were dominated by saturated and monoenoic PLFAs and showed no resolvable response to variation in light or temperature down to a water depth of 33 m and across seasons. Microbialite mean δ13Corg values (−26.0 ± 3.8‰) and Δδ13CDIC-org discrimination of ca. 25‰ supported non-CO2 limited photosynthesis. More abundant and 13C-depleted PLFAs (Δδ13C 7-14‰ vs. bulk organic matter) were indicative of autotrophic microbes. Less abundant and 13C-depleted PLFAs (Δδ13C 3-4‰) were indicative of heterotrophic organisms, particularly branched (iso/anteiso15:0 and 10me16:0) PLFAs. Dark coloured microbialites from the bottom of the lake (below 46 m water depth) had comparatively low biomass and a higher proportion of branched PLFAs, including biomarkers for sulfate reducing bacteria. Bulk δ13Ccarb values of microbialite carbonate at 6 and 11 m water depth were up to ca. 2‰ more 13C enriched than the value predicted for precipitation from ambient dissolved inorganic carbon and had increased biomass in summer, indicating a preserved biosignature of photosynthetic activity. Other δ13Ccarb values were generally within the range predicted for equilibrium precipitation. Estimated precipitation temperature values from δ18Ocarb were consistent with measured late summer water values. While both autotrophic and heterotrophic processes occurred at all depths, preservation of an enriched 13C biosignature was only detected at shallow depths where photosynthetic activity and biomass production were relatively high.