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 δ C biosignatures. The δ C 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. δ C of individual fatty acids from intact polar lipids (IPFA) in TO were similar to δ C of dissolved inorganic carbon (DIC) in the overlying water column, indicating limited discrimination by cyanobacteria during CO fixation. δ C of the underlying G was 3‰ greater than that of TO. The most δ C-depleted acetogenic lipids in the upper horizons were the cyanobacterial biomarkers C n-alkanes and polyunsaturated fatty acids. Bishomohopanol was 4 to 7‰ enriched, relative to alkanes and intact polar fatty acids (IPFA), respectively. Acyclic C isoprenoids were depleted by 14‰ relative to bishomohopanol. Significantly, ∆[δ C  - δ C ] increased from 6.9‰ in TO to 14.7‰ in OB. This major trend might indicate that C-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 C-depleted relative to upper crust by 7 and 4‰, respectively, and C isoprenoids were somewhat C-enriched. Synthesis of alkanes in P was evidenced only by C-depleted n-octadecane and 8-methylhexadecane. In OB, the marked increase of total inorganic carbon δ C (δ C ) of >6‰ perhaps indicated terminal mineralization. This δ C increase is consistent with degradation of the osmolyte glycine betaine by methylotrophic methanogens and loss of C-depleted methane from the mat.