THE IMPORTANCE OF PROCHLOROCOCCUS TO COMMUNITY STRUCTURE IN THE CENTRAL NORTH PACIFIC-OCEAN

Assessments of plankton community structure in the oligotrophic oceans based solely on microscopy may overstate the importance of heterotrophic bacterial biomass. Using flow cytometry to distinguish heterotrophic bacteria from the photosynthetic procaryotes Prochlorococcus spp., we found that Prochlorococcus contributed 31% of total bacterial counts in the upper 100 m at station ALOHA (22-degrees 45'N, 158-degrees-W). In terms of carbon, procaryotic biomass was the largest component (greater-than-or-equal-to 80%) of the microbial community, but almost half of this was photosynthetic biomass contributed by Prochlorococcus. Overall, the total 200-m integrated photosynthetic biomass exceeded heterotrophic bacterial biomass (55 vs. 45%). We suggest that the relative proportion of photosynthetic to heterotrophic bacterial biomass varies among oligotrophic regions of the ocean and that dominance by heterotrophic bacteria is not typical.