Abstract
Modern microbial mats constitute rich repositories of biogeochemical information regarding the community structure and carbon flow within the mat ecosystem through time and provide valuable insights into Earth’s earliest communities. Using a hypersaline microbial mat core from Guerrero Negro, Mexico, we analysed the lipid constituents from the active microbial layers and the underlying sediments. Our comprehensive lipid survey considered the free (extractable) lipid assemblages as well as the distributions and abundances of bound lipid structures released from fragmentation of insoluble macromolecular organic matter (IMOM). A wide variety of lipid types, including abundant steroids and hopanoids, were recovered from both organic phases, reflecting the rich microbial diversity within this mat ecosystem. A major contribution from photosynthetic mat organisms was preserved by binding into the IMOM phase; including hydrocarbons derived from phytol and long-chain alkanes from fragmentation of algaenans. The stable carbon isotopic composition (δ13Corg) of the mat organic matter was faithfully recorded, even in the deepest sedimentary layers that had experienced the highest degree of diagenetic alteration. The consistently 13C-enriched signatures for bulk organic matter observed through the core (ranging from −8.9‰ to −11.5‰) reflected low net isotopic fractionation associated with autotrophic carbon fixation in this alkaline pond setting. A major finding was the rapid dominance of IMOM as the major organic matter phase (>95%) after only a few years of diagenesis and burial. Such studies on modern microbial ecosystems aid toward a better understanding of the molecular taphonomic controls on the recent and ancient biomarker records.
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