GROWTH OF PROCHLOROCOCCUS, A PHOTOSYNTHETIC PROKARYOTE, IN THE EQUATORIAL PACIFIC-OCEAN

The cell cycle of Prochlorococcus, a prokaryote that accounts for a si zable fraction of the photosynthetic biomass in the eastern equatorial Pacific, progressed in phase with the daily light cycle. DNA replicat ion occurred in the afternoon and cell division occurred at night. Gro wth rates were maximal (about one doubling per day) at 30 meters and d ecreased toward the surface and the bottom of the ocean. Estimated Pro chlorococcus production varied between 174 and 498 milligrams of carbo n per square meter per day and accounted for 5 to 19 percent of total gross primary production at the equator. Because Prochlorococcus multi plies close to its maximum possible rate, it is probably not severely nutrient-limited in this region of the oceans.