LIPID BIOMARKERS AND THEIR CARBON ISOTOPIC SIGNATURES FOR A HYPERSALINE MICROBIAL MAT COMMUNITY

Bacterial lipids are valuable biomarkers for identification of specific functional groups in ancient organic-rich sedimentary rocks (geolipids) and contemporary microbial communities (membrane lipids). Use of compound specific isotope analysis adds additional refinement to taxonomic attribution and provides information related to carbon cycling within a mat community. Our study focuses on lipid biomarkers associated with a Microcoleus-microbial mat found in hypersaline ponds at Guerrero Negro, Baja California Sur. Specific biomarkers for diatoms, cyanobacteria, archaea, green nonsulfur (GNS), sulfur oxidizing, sulfate reducing, and methanotrophic bacteria have been identified. Analyses of the ester-bound fatty acids indicated a highly diverse bacterial community. The low abundance of ether-bound isoprenyl lipids suggests a relatively small archaeal population. The d¹³C for cyanobacterial biomarkers such as the monomethylalkanes (-26.0 to -27.6‰) and hopanoids (-16.8‰) was consistent with cyanobacterial biomass in the -15 to -22‰ range, considerably lighter than bulk mat at -10.5‰. A variety of C₂₈ to C₃₄ wax esters (WXE), a biomarker for GNS bacteria, were most abundant in an anaerobic zone just below Microcoleus. WXE abundance in the upper mat layers (~400 ng mg^-1 TOC) suggests a relatively small GNS population. A mass weighted d¹³C value of -17.0‰ for WXE relative to the d¹³C_DIC of -5 to -8‰ obtained for these ponds was generally consistent with incorporation of bicarbonate via the 3-hydroxypropionate pathway and supports previous evidence for photoautotrophic growth of marine GNS. A C30:1 squalane, a biomarker associated with hypersalinity, was the most abundant archaeal lipid recovered and the most enriched biomarker identified (-7.0 to -10.8‰). The d¹³C for potential methanogen biomarkers, sn-2-hydroxyarchaeol and tetraether dibiphytanes, were relatively heavy in the -11 to -16 ‰ range relative to associate CH₄ values which approached -60‰.