IDENTIFICATION OF METHANOTROPHIC LIPID BIOMARKERS IN COLD-SEEP MUSSELGILLS - CHEMICAL AND ISOTOPIC ANALYSIS

A lipid analysis of the tissues of a cold-seep mytilid mussel collecte d from the Louisiana slope of the Gulf of Mexico was used in conjuncti on with a compound-specific isotope analysis to demonstrate the presen ce of methanotrophic symbionts in the mussel gill tissue and to demons trate the host's dependence on bacterially synthesized metabolic inter mediates. The gill tissue contained large amounts of group-specific me thanotrophic biomarkers, bacteriohopanoids, 4-methylsterols, lipopolys accharide-associated hydroxy fatty acids, and type I-specific 16:1 fat ty acid isomers with bond positions at Delta 8, Delta 10, and Delta 11 . Only small amounts of these compounds were detected in the mantle or other tissues of the host animal. A variety of cholesterol and 4-meth ylsterol isomers were identified as both free and steryl esters, and t he sterol double bond positions suggested that the major bacterially d erived gill sterol [11.0% 4 alpha-methyl-cholesta-8(14),24-dien-3 beta -ol] was converted to host cholesterol (64.2% of the gill sterol was c holest-5-en-3 beta-ol). The stable carbon isotope values for gill and mantle preparations were, respectively, -59.0 and -60.4 parts per thou sand for total tissue, -60.6 and -62.4 parts per thousand for total li pids, -60.2 and -63.9 parts per thousand for phospholipid fatty acids, and -71.8 and -73.8 parts per thousand for sterols. These stable carb on isotope values revealed that the relative fractionation pattern was similar to the patterns obtained in pure culture experiments with met hanotrophic bacteria (R. E. Summons, L. L. Jahnke, and Z. Roksandic, G eochim. Cosmochim. Acta 58:2853-2863, 1994) further supporting the con version of the bacterial methylsterol pool.