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

Abstract

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 δ13 C biosignatures. The δ13 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. δ13 C of individual fatty acids from intact polar lipids (IPFA) in TO were similar to δ13 C of dissolved inorganic carbon (DIC) in the overlying water column, indicating limited discrimination by cyanobacteria during CO2 fixation. δ13 CTOC of the underlying G was 3‰ greater than that of TO. The most δ13 C-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, ∆[δ13 CTOC -δ13 C∑IPFA ] increased from 6.9‰ in TO to 14.7‰ in OB. This major trend might indicate that 13 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 13 C-depleted relative to upper crust by 7 and 4‰, respectively, and C20 isoprenoids were somewhat 13 C-enriched. Synthesis of alkanes in P was evidenced only by 13 C-depleted n-octadecane and 8-methylhexadecane. In OB, the marked increase of total inorganic carbon δ13 C (δ13 CTIC ) of >6‰ perhaps indicated terminal mineralization. This δ13 CTIC increase is consistent with degradation of the osmolyte glycine betaine by methylotrophic methanogens and loss of 13 C-depleted methane from the mat.