Role of Growth Rate and Grazing Mortality in Regulating the Dynamics of Photosynthetic Prokaryotic Picoplankton in the Sargasso Sea

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Two closely related unicellular cyanobacteria, Prochlorococcus and Synechococcus, are responsible for a significant fraction of total oceanic primary production. Although these two picoplankton groups are ubiquitous and abundant in most parts of the ocean, their distribution in time and space is often contrasting. This pattern is particularly evident over the course of the seasonal cycle in the NW Sargasso Sea, where Prochlorococcus dominates during the summer (under highly stratified, low nutrient conditions), and Synechococcus typically dominates during the winter (under well-mixed conditions). The fact that the summed biomass of these two groups remains approximately constant has led to the suggestion that together these organisms fill some sort of "picophytoplankton niche" in this environment. Explanations for these contrasting patterns of Prochlorococcus and Synechococcus distribution are most often framed in the context of growth rate regulation: dominance by one or the other group is assumed to reflect faster growth by that group under the environmental conditions prevailing at the time. However, the abundance of a natural population must reflect the relative balance between both growth and mortality terms, and there is no firm theoretical requirement that growth rate and standing stock be correlated. In fact, the few studies to date that have simultaneously examined the growth rates of both Prochlorococcus and Synechococcus in the field have found no such correlation. These results suggest that factors other than differential growth rate limitation are important in regulating the relative abundance of these two picophytoplankton groups in oceanic environments.This project seeks to examine the relationship between growth rate, grazing mortality, and relative abundance of Prochlorococcus and Synechococcus, over the course of the well-defined seasonal cycle in the abundance of these two groups in the Sargasso Sea. A suite of overlapping and complementary approaches for measuring these processes in natural picoplankton populations will be applied during four research cruises. Specifically, the following questions will be addressed: Are seasonal changes in the relative abundance of Prochlorococcus and Synechococcus correlated with changes in the relative growth and grazing mortality rates of these populations? Are there significant seasonal changes in micro-grazer abundance or activity? Do such changes influence the seasonal cycle of picophytoplankton standing stock? Are there detectable differences in the functional response of the protozoan community to Prochlorococcus and Synechococcus abundance? Do these differences influence the seasonal cycle of picophytoplankton? Do cell cycle-based estimates of in situ growth rate coincide with estimates from dilution experiments? Are estimates of grazing mortality based on fluoresecently-labeled prey consistent with estimates from these experiments?Unraveling the mechanisms by which the relative dominance of these two picophytoplankton groups (and their summed biomass) is regulated would lead to a better understanding of the more general, but much less tractable, issue of regulation of overall phytoplankton community structure.