Photosynthetic isotope biosignatures in laminated micro-stromatolitic and non-laminated nodules associated with modern, freshwater microbialites in Pavilion Lake, B.C.

The influence of microbial activity on carbonate precipitation was investigated within micro-stromatolitic nodules associated with modern, freshwater microbialites located in Pavilion Lake, B.C. Observed carbonate δ13C values enriched by up to +3.6‰ as compared to predicted abiotic carbonate δ13C values from measured dissolved inorganic carbon (mean −1.2‰, n=13) were consistent with microbial photosynthetic influence on in situ precipitation within the nodule microenvironment. Estimated carbonate precipitation temperatures within the nodules based on δ18O were consistent with recorded summertime temperatures, indicative of precipitation during the period of highest levels of photosynthetic activity. Low δ13C values of organic matter within the nodules (−30.6 to −21.1‰) and an average inorganic to organic carbon Δδ13C value of 26.8‰ reflected the preferential uptake of 12C during non-CO2 limited photosynthesis, supporting the generation of 13C-enriched DIC. Microelectrode profiles through the nodules showed oxygen supersaturation of up to ∼275%, elevated pH compared to ambient water and a lack of any observable dissolved sulphide, Mn or Fe further indicated that photosynthetic activity was the predominant metabolic process within the nodule during light exposure. Microbial phospholipid fatty acid profiles of the nodule communities were indicative of bacteria rather than eukaryotes and PLFA δ13C values were depleted relative to the bulk cell by 2.6–6.6‰, consistent with a predominance of photosynthetic microbes. Scanning electron microscopy images of the relationship between carbonate minerals and filaments indicated that carbonate precipitation had occurred in situ due to microbial influences on the geochemistry within the nodule microenvironment rather than due to cell surface effects or trapping and binding. The observation of photosynthetically induced 13C-enrichment of in situ precipitated carbonate within the nodule microenvironment is thus a biosignature of the activity of these surface communities and is consistent with the hypothesized role of biology in the formation of microbialites.