Microbial assemblages associated with sinking particles in the Porcupine Abyssal Plain (NE Atlantic Ocean)

Downward fluxes of microbial assemblages associated with sinking particles sampled in sediment traps deployed at nominal depths of 1000 m (trap A), 30 00 m (trap B) and 4700 m (trap Q were measured between October 1995 and Aug ust 1998 on the Porcupine Abyssal Plain (PAP, NE Atlantic). The goal of the study was to provide detailed information on the microbial contributions t o the particulate organic carbon and DNA fluxes. Bacterial fluxes associate d with settling particles in the PAP area were generally low and significan tly lower than bacteria] fluxes reported from the same area during 1989-90. Marked seasonal pulses in the microbial assemblages were observed in all y ears that were associated with particle flux maxima in April-June. No signi ficant differences were found in microbial fluxes between 1000 and 4700 m d epth, but both the bacterial biomass flux and the frequency of dividing bac teria increased with depth, suggesting that organic matter turnover and con version into bacterial biomass increased in the deeper traps. The structure of microbial assemblages displayed clear changes with increasing depth;, t he ratios of bacteria to both flagellates and cyanobacteria increased up to 4-fold between 1000 and 4700 m, showing a marked increase in bacterial dom inance in the deeper layers of the water column. A parallel increase of the bacterial contribution to particulate organic carbon (POC) and DNA fluxes was observed. Total microbial contribution to the POC flux in the PAP area was about 2%, whereas the contribution of cyanobacteria was negligible. Flu xes of microbial assemblages were significantly correlated with DNA fluxes and on average the bacteria accounted for 5% of DNA, fluxes. Data reported here confirm that the "rain" of particulate bacterial DNA may represent an important source of nucleotides for deep-sea bacteria, but also suggests th at a much larger pool of detrital DNA is potentially available to deep-sea micro-organisms. (C) 2001 Elsevier Science Ltd. All rights reserved.