Bacterial and cyanobacterial flux to the deep NE atlantic on sedimenting particles

Long term (17 month) moored sediment trap studies during 1989/90 in the NE Atlantic reveal that heterotrophic bacteria and cyanobacteria are transported into the deep-sea in large numbers (up to 32 × 10 ⁹ cells/m ²/day and 37 × 10 ⁷ cells/m ²/day, respectively) by attachment to, or incorporation in, rapidly sedimenting particles. The fluxes of these microorganisms follow the seasonal mass and POC flux patterns with two major flux events at 3100 and 4465 m each year and another flux event present only in the deeper trap during early 1990. Bacteria contributed 0.5-3.5% (mean = 2.0%) of POC flux at 3100 m, and at 4465 m they contributed 1-40% (mean = 11%) of the POC. Cyanobacteria contributed up to 0.13% of POC at both trap depths. The substantial seasonal and interannual variations in both bacterial and cyanobacterial fluxes may be due to different seasonal and interannual growth patterns of the heterotrophic and phototrophic bacteria and/or different mechanisms of inclusion in sinking particles in the upper ocean. The proportions of bacterial and cyanobacterial production in surface waters arriving at 3100 m during the maximum flux of microorganisms were 1.5% and 0.26%, respectively, and will be lower at other times of year. There was a significant positive correlation between % bacterial carbon and % particulate organic carbon of the mass flux which may indicate that the higher the bacterial concentration on sedimenting material the higher the concentration of POC. Significant negative correlations also occurred between C/N molar ratio and bacterial carbon flux. The material comprising the second major flux event in 1989 contained extremely high concentrations of bacteria and cyanobacteria. These observations may indicate that aggregates may escape solubilization in the upper water column and arrive relatively fresh on the deep-sea bed, 74% of the annual total of bacterial flux and 77% of the annual cyanobacterial flux, occurred within this short period (12%) of the investigation. There is evidence that the deeper trap received bacteria from resuspended sediments and colonized settled detritus and that there may be enhanced growth of these deep sea bacteria through stimulation from a major mass flux event. The annual bacterial flux was 1.1 × 10 ¹² cells/m ²/annum which is equivalent to 56.3 mg bacterial carbon/m ²/annum, around 4, 5 and 27.0 mg DNA, RNA and protein/m ²/annum and 27.5 × 10 ¹² plasmid encoded phenotypic genes/m ²/annum. The supply of DNA and mechanisms of transfer are potentially available for genetic exchange to occur between populations previously assumed to be genetically isolated, that of the bacteria in the surface waters and those in the deep-sea.