Diel characteristics of Prochlorococcus and Synechococcus populations in the western Sargasso Sea
Blythe, Brad James
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Prochlorococcus and Synechococcus are ubiquitous marine icocyanobacteria that contribute significantly to oceanic primary production. The tightly phased patterns of cell growth and division in these groups provide a basis for evaluating their in situ growth and mortality. In particular, the daily progression of Prochlorococcus populations through their cell cycle can be used to estimate the growth rate (and by inference, the loss rate) of these populations. In this dissertation, the diel dynamics of cell abundance, cellular growth, and cell cycle progression are presented for Prochlorococcus and Synechococcus populations in the Western Sargasso Sea. I use these data to critically evaluate two methods of calculating in situ growth rates of Prochlorococcus populations in this environment. I also utilize concurrently sampled incubation bottles to test the efficacy of commonly used incubation techniques to accurately simulate the dynamics of natural picoplankton populations. Finally, I report on a numerical simulation model of Prochlorococcus based on the current state of knowledge of the biology and ecology of these organisms. My results demonstrate that while as a group Prochlorococcus growth rates estimated by the two cell cycle-based approaches do not vary systematically, individual estimates can differ significantly between these two approaches. Growth rates varied widely over depth and between stations; no significant seasonality was detected. Regarding the bottle incubations, the physiology of in situ Prochlorococcus populations (as reflected in growth rate) appears to be reasonably reflected in the bottles. In contrast, grazing on Prochlorococcus was dramatically reduced in the bottles, relative to the in situ rates. Thus, traditional bottle incubations may not accurately replicate the ecological dynamics of in situ populations. Finally, the biology-based cell cycle model produced diel patterns in cell size and cell cycle dynamics that were qualitatively similar to those observed in natural populations. The model predicted strong day to day variability in these dynamics, and in the resultant growth rates, suggesting that estimates based on 24 h sampling may not accurately reflect the true growth rate of these populations on ecologically relevant time scales.