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Abstract: Calcite microbialites in Pavilion Lake, British Columbia, exhibit a diverse range in macro-morphology, biomass abundance, porosity, and mineral content. To evaluate the role of microorganisms in their formation, samples collected from a range of depths were examined by scanning electron microscopy (SEM) and synchrotron radiation-based micro-X-ray fluorescence (μ-XRF) spectroscopy to characterize both their outer surfaces as well as internal structures. Observed trends in both surface colonization as well as microbialite framework with increasing lake depth include decreasing microbial abundance on outer surfaces as well as increasing ratios of carbonate:biomass in the microbialites. Microscopic investigations of the interiors show bacteria and algae entrapped within calcite, with this calcite exhibiting micropores and casts similar in size and shape to microorganisms. Based on these observations, it is hypothesized that microbialite development in Pavilion Lake initiates calcite precipitation in phototrophic microbial mats, i.e., combined phototrophy and heterotrophy, followed by heterotrophic oxidation of organic matter leading to eventual carbonate infilling of the microbial–mineral matrix. In addition, an observed shift from cyanobacteria to algae with increasing lake depth suggests variability in contemporary conditions controlling microbialite growth and diagenesis. High photosynthetic growth rates at shallower depths result in significant porosity and friability due to biomass accumulation outpacing carbonate precipitation. At intermediate depths, lower light levels and slower growth rates of phototrophs lead to a greater proportion of the microbialite matrix being in-filled by carbonate. Carbonates precipitate initially within the bacteria-EPS matrix, with abundant uncalcified algae maintaining microbialite porosity. In the deepest waters, the presence of only sparse algal colonization as well as fine-grained, laminated metal-rich sediments covering microbialites suggests that present-day insolation levels are too low to support the development of photosynthetic microbial mats. As a consequence, heterotrophic carbonate precipitation has progressively in-filled these microbialite interiors to create lithified calcite fabrics that exhibit minimal porosity but preserve the casts of microorganisms as biosignatures. While the origin of microbialites in Pavilion Lake remains unknown, current observations provide valuable information in evaluating how environmental conditions influence microbialite growth in a freshwater, lacustrine environment. [Copyright &y& Elsevier]

Abstract: 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 δ 13C values enriched by up to +3.6‰ as compared to predicted abiotic carbonate δ 13C 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 δ 18O were consistent with recorded summertime temperatures, indicative of precipitation during the period of highest levels of photosynthetic activity. Low δ 13C values of organic matter within the nodules (−30.6 to −21.1‰) and an average inorganic to organic carbon Δδ 13C 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 δ 13C 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. [Copyright &y& Elsevier]