In the Gulf of Mexico, recurring algal blooms caused by Karenia brevis (formerly known as Gymnodinium breve) have significant adverse health and economic impacts. K. brevis is one member of a small group of dinoflagellates, related morphologically and by DNA-based phylogenetic analysis, that synthesize the carotenoid, gyroxanthin diester, in place of the more widely distributed peridinin. While this novel photopigment has been proposed as a biomarker, especially for remote-sensing imaging technologies, to detect the emergence of K. brevis blooms, other chemicals such as sterols and triglycerides, respectively, with potential to report the distribution and physiological condition of K. brevis are required. Recent work from our laboratories characterizing the lipids of dinoflagellates has confirmed that K. brevis, together with those few close relatives lacking peridinin, possesses a relatively simple sterol profile comprised of two unusual primary 4-methyl sterols, designated ED and NED, each with an ergosterol-type side chain. A recent dinoflagellate bloom in the waters of the northwest Gulf of Mexico near the Gulf Breeze EPA laboratory provided an opportunity to examine the usefulness of these sterols and other lipids as indicators of K. brevis in phytoplankton communities. Lipid extracts of filtered bloom samples, fractionated to separate free and esterified sterols, were examined by GC/MS of trimethylsilyl ether derivatives. ED and NED were the major sterols found in all bloom samples. Fatty acids found in lipid fractions containing membrane phospholipids, chloroplast-associated glycolipids, and storage triglycerides, respectively, differed significantly. The glycolipid fraction was found to contain octadecapentaenoic acid [18:5(n-3)], a fatty acid commonly associated with dinoflagellates. The phospholipid fraction was found to contain small amounts of the recently described highly unsaturated fatty acids, octacosaoctaenoic acid [28:8(n-3)] and octacosaheptaenoic acid [28:7(n-6)]. Fatty acids from the triglyceride fraction were more abundant than those associated with glycolipids or phospholipids.

Key words: dinoflagellate, fatty acid, lipid, sterol