Photosynthetic bacteria -- Prochlorococcus and Synechococcus -- are important components of phytoplankton biomass and primary production in the most warm open ocean. To determine the relative importance of Prochlorococcus and Synechococcus to primary production in various oceanic ecosystems, appropriate growth rate methods were required to permit species-specific estimates. Cell cycle analysis has been proven a reliable tool for estimating growth rates of Prochlorococcus spp. Applying a modified model to Prochlorococcus in the equatorial and subtropical North Pacific Ocean revealed that Prochlorococcus grew equally well in oligotrophic and mesotrophic conditions at about one doubling per day in the surface mixed layer. However, the contribution by Prochlorococcus to primary production was significantly higher in the oligotrophic subtropical North Pacific than in the mesotrophic central equatorial Pacific.

Attempts to estimate the growth rate of Synechococcus from cell cycle analyses were negatively impacted by preliminary results which showed that natural Synechococcus populations are less tightly synchronized to the daily photocycle and some populations possess irregular cell cycle patterns. Applying the relationship between tS+G2 and chemostat growth rate of Synechococcus WH7803 to field samples collected from the Arabian Sea provided unrealistic results. Therefore, an alternative method was developed in which Synechococcus growth rate could be calculated from diel variations in population abundances based on the fact that Synechococcus cell division occurred mostly during daytime.

Using this approach for Synechococcus and the diel cell cycle analysis for Prochlorococcus, I estimated their growth and mortality rates and productions in Arabian Sea during SW and NE Monsoons. Prochlorococcus growth rates were typically less than 1 doubling per day, although growth rate in excess of one doubling per day were observed. Synechococcus spp. grew much faster than Prochlorococcus in the upper water column at almost every station during both seasons, but the depth range of its maximum growth rate was shallower and its growth and abundance decreased sharply in deeper waters. Maximum growth rates > 2 d-1 were observed at onshore stations during both seasons. Synechococcus growth rate increased with nutrient availability whereas Prochlorococcus growth rates did not vary dramatically with nutrient conditions. Although there was no significant difference in Synechococcus growth rates between the late SW and early NE Monsoon seasons, the estimated carbon production and relative contribution to primary production was greater during the early NE Monsoon owing to the larger biomass of Synechococcus and lower total primary production. Maximum Prochlorococcus production was found only in the most oligotrophic regions and Prochlorococcus was not a major contributor of primary production for the most part of the Arabian Sea during the SW and NE Monsoons. Maximum Synechococcus production occurred in mesotrophic (nitrate concentration 0.1-3 uM) areas during both SW and NE Monsoon, but decreased at offshore and coastal stations. Overall, I have demonstrated that an inverse relationship between the importance of Prochlorococcus and Synechococcus to primary production exist. This relationship holds among all studied oceanic regions with nutrient conditions ranging from oligotrophic open ocean to nutrient-rich coastal upwelling water.