BACTERIAL AND CYANOBACTERIAL FLUX TO THE DEEP NE ATLANTIC ON SEDIMENTING PARTICLES

Long term (17 month) moored sediment trap studies during 1989/90 in th e NE Atlantic reveal that heterotrophic bacteria and cyanobacteria are transported into the deep-sea in large numbers (up to 32 x 10(9) cell s/m(2)/day and 37 x 10(7) cells/m(2)/day, respectively) by attachment to,or incorporation in, rapidly sedimenting particles. The fluxes of t hese microorganisms follow the seasonal mass and POC flux patterns wit h 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 contr ibuted 0.5-3.5% (mean = 2.0%) of POC flux at 3100 m, and at 4465 m the y 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 h eterotrophic 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 si gnificant positive correlation between % bacterial carbon and % partic ulate organic carbon of the mass flux which may indicate that the high er 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 compri sing the second major flux event in 1989 contained extremely high conc entrations of bacteria and cyanobacteria, These observations may indic ate that aggregates may escape solubilization in the upper water colum n and arrive relatively fresh on the deep-sea bed, 74% of the annual t otal of bacterial flux and 77% of the annual cyanobacterial flux, occu rred within this short period (12%) of the investigation. There is evi dence that the deeper trap received bacteria from resuspended sediment s 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 x 10(12) cells/m(2)/annum wh ich 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 x 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 be tween populations previously assumed to be genetically isolated, that of the bacteria in the surface waters and those in the deep-sea.