Bacterial carbon dynamics on marine snow

We studied the biomass and production of heterotrophic bacteria on several types of marine snow including those composed predominantly of &atoms, fecal pellets, larvacean house mucus, or miscellaneous detrital components at 8 stations in the Pacific Ocean off California. We concurrently measured photoautotrophic biomass and production, and particulate organic carbon (POC) on marine snow to examine the quantitative significance of bacterial processes in carbon flow pathways in different types of marine snow Although a typical marine snow floc contained about 106 bacteria, the bacterial carbon (BOC) and phytoplankton carbon (PhytoC) were each < 13 % of total POC. Most of the floc carbon therefore consisted of detritus, unlike whole seawater samples (in other studies) where BOC + PhytoC generally comprises about 50 % of the POC. PhytoC on marine snow and in the surrounding water was generally similar to BOC (except at one station). Floc POC turnover time based on bacterial carbon demand. assuming a 30 O/ O carbon assimilation efficiency, was 20 to 100 d. This was comparable to the POC residence time reported for the Southern California Bight. Bacterial specific growth rates on flocs varied greatly between stations (0.06 to 0.96 d-l) with highest values at stations with diatom flocs where they were comparable to growth rates of free-living bacteria in mesotrophic waters. The ratio of bacterial carbon production to primary production on marine snow was typical of seawater samples from other studies. Bacterial carbon production was measured by the leucine incorporation method while bacterial cell production (and growth rate) was measured by the thymidlne incorporation method simultaneously on the same marine snow floc. We could thus calculate bacterial carbon per-new-cell (C,) and compare it with the microscopy-based average carbon per cell (C,,) to test the hypothesis that new cells reflect the average carbon content of the parent assemblage (bacteria on marine snow were much larger than those in the surrounding water). In 73 % of our samples the carbon content of the new daughter cells was within a factor of 1.5 of the carbon content of the parent cells.