Visible light-induced oxidation of lipid components of purple sulfur bacteria: a significant process in microbial mats

Visible light-induced degradation processes were studied in vitro in senescent cells of two purple sulfur bacteria (Thiohalocapsa halophila and Halochromatium salexigens) isolated from microbial mats from Camargue (France). These reactions act on the phytyl side chain of bacteriochlorophyll-a and on palmitoleic and cis-vaccenic acids. The experiments also confirmed some previous results, i.e. the regiospecific enzymatic oxygenation of the allylic carbon 10 of cis-vaccenic acid in senescent cells of T. halophila. The subsequent detection of substantial amounts of photoproducts from the phytyl chain and mono-unsaturated fatty acids in the lipid extracts of the microbial mats from which these two strains were isolated clearly established the importance of photodegradation processes in this particular ecosystem. The presence of photoproducts of cis-vaccenic acid showed that under natural conditions photochemical reactions act extensively on senescent phototrophic bacteria. Significant amounts of 10-oxooctadecanoic and 10-hydroxy-11-oxooctadecanoic acids were also detected in the lipid extracts. The detection of these compounds, which result from the enzymatic oxidation of carbon 10 of cis-vaccenic acid, supports a strong T. halophila contribution to the microbial mats.