H.A. Nolla

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°45′N, 158°W). In terms of carbon, procaryotic biomass was the largest component (≥ 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.

DNA synthesis and cell division of Prochlorococcus are tightly synchronized to the daily light cycle, therefore, cell division rates can be estimated from the fraction of cells in each cell cycle stage during a 24 h sampling period. The total mortality rate of Prochlorococcus can also be estimated from the difference between the observed cell abundance and the expected cell number projected from growth rate in that sampling period, providing an estlmate of grazing impact. Growth and mortality rates of Prochlorococcus were investigated at 2 statlons In the equatorial Pacific, as well as Station ALOHA in the subtropical North Pacific Ocean. Growth rate of Prochlorococcus remained high at all sites independent of the nitrate concentration. The maximum growth rate (up to 1 doubling d-') occurred at 70 m depth at the western equatorial Pacific site (166' E) and at 40 to 45 nl at the eastern equatorial Pacific site (150' W) and at Station ALOHA (22" 45' N, 158' W). Total mortality rates were roughly balanced by Prochlorococcus growth at all stations. Because of the phased cell cycle and continuous (if not evenly distributed) mortality, the abundance of Prochlorococcus at each depth could vary up to 2-fold between afternoon and midnight. Prochlorococcus production was estimated to contribute 9 and 39% to the total gross primary production in the eastern and western equatorial Pacific, respectively, and up to 82% in the subtropical North Pacific Ocean at Station ALOHA. Our results suggest Prochlorococcus are not severely nutrient-limited in the oligotrophic environment. Rapid nutrient recycling by grazing activity permits Prochlorococcus to contribute a significant fraction of the total primary production.

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°45′N, 158°W). In terms of carbon, procaryotic biomass was the largest component (≥ 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.