THE CARBON ISOTOPE SYSTEMATICS AND PHOSPHOLIPID FATTY ACID PROFILES OF MICROBIALITE-ASSOCIATED COMMUNITIES

Abstract

<p>Modern microbialites provide the opportunity to explore the influences of biology on microbialite formation and understand how biosignatures can be preserved in these structures. In this study, we used the isotopic compositions (δ¹³C) of phospholipid fatty acids (PLFAs) and their structurally-defined profiles, in conjunction with calcium carbonate isotopic compositions and imaging to evaluate microbial autotrophic and heterotrophic processes associated with freshwater microbialites from Kelly Lake, British Columbia. This was done to determine what types of metabolism may have been influencing microbialite growth and whether a biosignature of this process was preserved. In addition, PLFA profiles from a microbialite-derived pure culture were analyzed under various growth conditions to assess environmental influences on microbial PLFA composition.</p> <p>Although the majority of the δ¹³C values of Kelly Lake microbialite surface carbonates fell within the range predicted for equilibrium precipitation, samples collected from 26 m were found to have enriched δ¹³C_carb values and are likely a biosignature of autotrophy at this depth. PLFA profiles and δ¹³C_PLFAvalues also supported the predominance of autotrophy, however, they indicated that heterotrophic organisms were also present. This data suggests that autotrophic metabolisms have influenced the local geochemistry in the past, at least at 26 m, and are likely substantial contributors to microbialite growth.</p> <p>Changes in temperature, pH, NaCl concentrations, and cell densities were found to induce variations in the PLFA profiles of the <em>Exiguobacterium</em> strain RW2. The degree of PLFA unsaturation changed in each of the different culture conditions, and was predominantly adjusted through alterations in the branched monoenoic PLFAs, particularly i-17:1Δ⁵. These results highlight the difficulties associated with applying PLFA profiles as evidence for shifts in a microbial community composition, since altered growth conditions can induce intra-specific PLFA changes.</p>