Effect of Salinity on the Stable Carbon Isotopic Composition of Microbial Mats and Associated Lipid Biomarkers

Abstract:

Modern microbial mats are commonly used as analogs for understanding early ecosystem evolution. A primary feature of benthic ecosystems is the fixation of CO₂ in the photic zone and transfer and/or mineralization of organic carbon through a diverse tropic structure. Generally, the resulting δ¹³C values for organic matter relative to DIC is small which indicates that these mats are CO₂-limited. Microcoleus mat flux measurements indicate that small amounts of CO₂ fixed during the day are lost to the overlying water at night as C¹³-enriched DIC (Des Marais and Canfield 1994). This loss results in depletion in mat organic matter and is thought to occur as a result of remineralized DIC fixation by chemo- and/or photoautotrophs.

We have examined the fate of fixed carbon in hypersaline mats by analysis of the stable C-isotopes of bulk organic carbon and lipid compounds in a laminated-Microcoleus mat (90 ‰, pH 8.6) and in a gypsum-endoevaporitic, Halothece mat (163‰, pH 7.4). In the surface photic zone of Microcoleus mat, the δ¹³C TOC was –10.0 ‰ with membrane fatty acids (i-15, n-16, n-18, cy-19) ranging from –17.2 to –18.0‰. Cyanobacterial alkanes were similarly depleted (n-17 = –19.5‰). Eurkaryotic sterols and bacterial hopanoids were somewhat enriched with δ¹³C values ~–15‰. For Microcoleus mat, little variation occurred over 70 mm of core depth. The TOC and FA values for surface, endoevaporitic Halothece mat were generally similar to those of Microcoleus. However, below the surface gypsum crust, discrimination increased. Mass balance calculations for surface FA = –18.5‰ while the FA from layers below the crust = –25.2‰. The δ¹³C values for cyanobacterial alkanes (~–20‰) were similar from all layers while hopanoids from below crust were slightly depleted relative to those from surface. An apparent increased discrimination during fixation of remineralized DIC presumably by anoxygenic phototrophs present below the surface gypsum crust (Jahnke et al. 2014) resulted in a dramatic C¹³-depletion to TOC and associated lipid biomarkers.