Microbial response to organic matter enrichment in the Oligotrophic Levantine Basin (Eastern Mediterranean)

The response of a deep-water benthic microbial community to organic matter (OM) enrichment was studied in the unexplored region of the SW Cretan margin (E. Mediterranean). A food pulse of 0.5 g C m-2 was simulated by adding 13C-labelled diatoms to sediment cores retrieved from 1079 m depth. The diatom addition resulted in a significant increase in the sediment community oxygen consumption (SCOC). After 6 days, 50 mg C m-2 of the added material was processed by the microbial community. The major carbon sink was respiration, which accounted for 96% of the total processed material. The carbon uptake rate (12 mg C m-2 d-1) was considerably lower than previously published values in the E. Mediterranean at similar depths. The microbial community in our study site is distinct, as revealed by the unusually high presence of branched phospholipid fatty acids (PLFA). Previous studies have revealed that the slope under investigation may act as a conduit of OM from the shallow shelf to the deep basins resulting in the prevalence of relatively refractory OM at mid-slope depths. We postulate that sedimentary processes affect the amount of bioavailable sedimentary OM and consequently the structure and physiological state of bacterial community in our study site. The distinct microbial community composition at our station compared to more stable adjacent slopes could explain the limited response of the microbial community to the addition of labile OM. Supplemental materials are available for this article. Go to the publisher's online edition of Geomicrobiology Journal to view the free supplemental file.