Fatty acids, hydrocarbons, sterols and alkenones of microbial mats from coastal ecosystems of the ebro delta

Abstract

Fatty acids, hydrocarbons, ketones, alcohols and sterols have been analyzed in microbial mats collected in a coastal environment of the Ebro delta. In general, cyanobacteria, diatoms and haptophyceae contribute significantly to the organic matter of these mats. Dinoflagellates are also important. Evaluation of the long-term redox status of the mats based on the sterol composition, e.g. Δ⁵-stenol, 5α(H)- and 5β(H)- stanol ratios, shows a good correspondence with the degree of stationary growth of purple phototrophic bacteria (as indicated by the ratio between cyclopropylnonadecanoic and n-octadec-11(Z)-enoic acids) and the relative proportion of sulphate reducing bacteria remains (iso- and anteiso-pentadecanoic acids).

Comparison of the composition of these mats to that of mats developed in hypersaline environments, namely carbonate and gypsum deposition, from the same area shows that diatoms, dinoflagellate and haptophyceae inputs are also very relevant in the former. In the gypsum mats these inputs are negligible except for the C₃₇-C₃₈ alkenones from haptophyceae.

Cyclopropylnonadecanoic, n-octadec-11(Z)-enoic and iso- and anteiso-pentadecanoic acids are major constituents of the fatty acid distributions of the carbonate and gypsum mats, suggesting a higher efficiency of the phototrophic bacteria from hypersaline than coastal mats for the uptake of the hydrogen sulphide generated by the sulphate reducers. In addition, the cyclopropylnonadecanoic/n-octadec-11(Z)-enoic acid ratio in the carbonate and gypsum mats is higher than in the coastal mat, indicating a lower degree of stationary growth in the latter.