A geolipid characterization of Organic Lake — a hypersaline meromictic Antarctic lake

An organic geochemical study of the benthic sediments taken over a transect of Organic Lake is reported. Fatty acids, hydroxyacids, and sterols were the three major geolipid classes isolated from the total solvent extracts. The fatty acid chain lengths range between C12 and C28 carbons and show two maxima viz. at C16–18 and at C24–26. The first maximum is consistent with a microbial origin which in the context of Organic Lake includes mainly bacteria and some algae. The C24–26 maximum is also found in other Antarctic lakes and is likely to be bacterial in origin. Organic Lake differs from other lakes in the region in that sulphate-reducing bacteria and their signature biolipids are absent in the lake sediments, but dimethyl sulphide is present. The salinity is about 19% thus the lake is hypersaline and the biological communities present are very simplified, providing a rare opportunity to examine a biomarker array in an extreme environment with limited species diversity. The general distribution of fatty acids and sterols in Organic Lake is common to other lakes of Antarctica. Organic Lake differs, however, in the presence of unusual hydroxy acid isomers and of Δ7-sterols. The fatty acid total abundances are much higher and the sterol abundances lower than expected for Antarctic lakes of comparable salinity. The gross analyses of the three geolipid classes suggest that the collection sites broadly represent eco-niches across this meromictic lake. The deepest site is both anoxic and hypersaline and appears to be preserving organic matter. Analysis of the phospholipid fatty acids reveals, however, environmental differences in the benthic sediments and by using signature fatty acids it has proven possible to suggest how these benthic bacterial communities differ with depth and oxicity.