Microscale distribution of bacterioplankton in relation to phytoplankton: Results from 100-nl samples

The microspatial distribution of marine bacterioplankton was investigated by comparing the bacterial abundance in 100-nl subsamples of natural seawater. For sampling, staining with DAPI, and filtering, we designed capillary microtips that allowed pipetting of 100 nl with a sampling error '10%. Highly significant correlations were obtained between the 100-nl method and the conventional direct-counting method. Frequency distributions of bacterial cells were determined in experiments with natural seawater during bloom conditions as well as with seawater enriched with diatom cells from cultures of Chaetoceros muelleri. Bacteria in 100-nl subsamples were normally distributed in each single experiment and in the Formalin-fixed control. Mean bacterial abundance in subsamples containing at least one phytoplankton cell was never significantly different from samples lacking algal cells. Based on our results we conclude that bacteria are homogeneously distributed in subsample volumes of 100 nl, and therefore we could not verify the current concept that bacteria may cluster around phytoplankton cells.