Study of the mineralization effect on the distribution of lipids in sediments from the Cretan Sea: Evidence for hydrocarbon degradation and starvation stress

Abstract

Sedimentary diagenetic processes alter the composition and distribution of different lipid compounds. In the present study alterations mediated by microbial communities were investigated along a bathymetric gradient (100 m at 35°23′N–25°09′E, 617 m at 35°33′N–25°08′E, 1494 m at 35°44′N–25°08′E) over the continental margin of northern Crete (Greece, Eastern Mediterranean Sea). Bacterial abundances and distribution were studied using phospholipid linked fatty acids (PLFA), in the range of C 8–C 22, released from intact phospholipids. Lipid components (aliphatic hydrocarbons, free fatty acids, glycerides and glycolipids) were studied over a 2-month incubation period. Carbon mineralization rates at all stations indicated an uneven distribution of active aerobic bacteria with values decreasing towards the deeper stations. PLFA homologue profiles denoted that aerobic gram negative and sulfur oxidizing bacteria dominated microbial communities while the anaerobic, gram positive and sulfate reducing bacteria occurred only in traces. The n-alkane (NA) composition revealed a strong predominance of homologues with odd carbon numbers suggesting an important terrestrial contribution to the sediments. The estimated descriptive ratios of NA, the sum of short chain NA (C 15–C 20) and long chain NA (C 21–C 36) to 17 α( H),21 β( H)-C 30-hopane, before and after a two-month incubation period, indicated the occurrence of hydrocarbon degradation processes. Increased ratios of saturated to unsaturated fatty acids were also recorded after the incubation indicating the starvation of bacterial communities by the end of the experiments.