The response of the microbial community of marine sediments to organic carbon input under anaerobic conditions

Cyanobacterial biomass was added to anaerobic sediment to simulate the natu ral input of complex organic substrate that occurs in nature after algae bl ooms. Sediments were incubated at 0 degrees C, 8 degrees C and 24 degrees C for 13 days. Community dynamics were measured by fluorescence in situ hybr idisation (FISH), denaturing gradient gel electrophoresis (DGGE), and seque ncing of 16S rDNA PCR products. Metabolic changes were followed by the anal ysis of total carbon mineralisation, sulfate reduction, and ammonium produc tion rates. The addition of organic material resulted in significant change s in the composition of the microbial community at all temperatures tested. Sulfate reduction was the main mineralisation process detected. However, n ot sulfate-reducers but rather members of the Cytophaga-Flavobacterium phyl ogenetic cluster showed the highest increase in the bacterial cells as dete cted by FISH. We conclude that these organisms play an important role in th e anaerobic decomposition of complex organic material perhaps because they are the main catalysts of macromolecule hydrolysis and fermentation. The mo lecular methods also indicated a stimulation of ribosome synthesis. The det ection of a large number of rRNA-rich cells belonging to the Cytophaga-Flav obacterium phylogenetic cluster further supports the importance of their ro le in the degradation of complex organic material in anaerobic marine sedim ents. Their detection in high numbers in the field may indicate recent depo sition events.