SESQUITERPENOIDS IN SEDIMENTS OF A HYPERSALINE LAGOON - A POSSIBLE ALGAL ORIGIN

Core samples were collected in Lagoa Vermelha, a hypersaline lagoon lo cated about 100 km east of Rio de Janeiro (Brazil). The sediment compo sition is predominantly carbonate in amounts up to 93%. Analysis of de lta(13)C Of the total organic matter in the sediments showed that mari ne organic matter predominates throughout the core (delta(13)C ranges from -15.84 to -22.64 parts per thousand vs. PDB). Organic carbon cont ents (TOC) ranged from 0.81 to 13.28%. A series of cadinane-type sesqu iterpenoids can be recognized in the gas chromatography-mass spectrome try data. Essentially the same components are present in all the sampl es, with variations only in their relative abundances. The most abunda nt compounds are alpha- and beta-cubebene, alpha- and beta-cedrene, ca dinenes (different isomers), alpha-curcumene and calamenene, with mino r amounts of calarene, humulene, calacorene and cadalene. Since this l agoon is surrounded by dunes with only minor vegetation typical of thi s environment (grasses, small non-resinous shrubs and no forest) with no potential source for sesquiterpenoids, a terrestrial origin for the se compounds is excluded and an algal origin is more consistent with t he locale and the recognition of sesquiterpenoids (including cadinol) in microbial mats from the lagoon. Only the natural product precursor sesquiterpenoids are present in the microbial mats with no detectable diagenetic derivatives (e.g. calamenene and cadalene). This indicates that the compounds in the mats are from recent input and those found i n the sediments are most likely derived from former algal biomass in t his lagoon, a fact confirmed by the recognition of a series of diagene tic aromatic components in the sediments. Surface sediments contain n- alkanes with no even-to-odd predominance indicating that microbial act ivity is higher in shallower sediments. Moreover, mass fragmentograms (m/z 191) of biomarkers revealed the presence of 17 alpha(H),21 beta(H )-hopanes, the mature isomers, together with their beta beta precursor s and low amounts of the intermediates with the beta alpha configurati on (moretanes). This indicates a contribution of mature organic matter to these immature sediments. (C) 1997 Elsevier Science Ltd.