Microstructure variability in freshwater microbialites, Pavilion Lake, Canada

• Freshwater calcite microbialites are widespread in Pavilion Lake, BC.
• Microbialites were studied using a variety of microscopy techniques.
• Microstructure of microbialites varies as a function of depth.
• Both phototrophy and heterotrophy are involved in carbonate precipitation.
• Microbial biosignatures are preserved in carbonate microbialites.

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.