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Carbon isotopic composition of lipid biomarkers from an endoevaporitic gypsum crust microbial mat reveals cycling of mineralized organic carbon

Microbial mats that inhabit gypsum deposits in ponds at Guerrero Negro, Baja California Sur, Mexico, developed distinct pigmented horizons that provided an opportunity to examine the fixation and flow of carbon through a trophic structure and, in conjunction with previous phylogenetic analyses, to assess the diagenetic fates of molecular δ13C biosignatures. The δ13C values of individual biomarker lipids, total carbon, and total organic carbon (TOC) were determined for each of the following horizons: tan‐orange (TO) at the surface, green (G), purple (P), and olive‐black (OB) at the bottom. δ13C of individual fatty acids from intact polar lipids (IPFA) in TO were similar to δ13C of dissolved inorganic carbon (DIC) in the overlying water column, indicating limited discrimination by cyanobacteria during CO2 fixation. δ13CTOC of the underlying G was 3‰ greater than that of TO. The most δ13C‐depleted acetogenic lipids in the upper horizons were the cyanobacterial biomarkers C17 n‐alkanes and polyunsaturated fatty acids. Bishomohopanol was 4 to 7‰ enriched, relative to alkanes and intact polar fatty acids (IPFA), respectively. Acyclic C20 isoprenoids were depleted by 14‰ relative to bishomohopanol. Significantly, ∆[δ13CTOC − δ13C∑IPFA] increased from 6.9‰ in TO to 14.7‰ in OB. This major trend might indicate that 13C‐enriched residual organic matter accumulated at depth. The permanently anoxic P horizon was dominated by anoxygenic phototrophs and sulfate‐reducing bacteria. P hosted an active sulfur‐dependent microbial community. IPFA and bishomohopanol were 13C‐depleted relative to upper crust by 7 and 4‰, respectively, and C20 isoprenoids were somewhat 13C‐enriched. Synthesis of alkanes in P was evidenced only by 13C‐depleted n‐octadecane and 8‐methylhexadecane. In OB, the marked increase of total inorganic carbon δ13C (δ13CTIC) of >6‰ perhaps indicated terminal mineralization. This δ13CTIC increase is consistent with degradation of the osmolyte glycine betaine by methylotrophic methanogens and loss of 13C‐depleted methane from the mat.