Picocyanobacterial cellular physiology and trophic interaction with a heterotrophic nanoflagellate

Abstract

Prochlorococcus and Synechococcus are two closely related picocyanobacteria that together are responsible for a large proportion of the total primary production in open ocean environments. Relatively little is known about the growth rates and trophic interactions of these organisms in the field. This dissertation focuses on characterizing the growth physiology of representative strains of Prochlorococcus (MIT9312) and Synechococcus (WH8103), and exploring the potential role of these organisms as prey for a model heterotrophic nanoflagellate (Paraphysomonas imperforata). Two proposed approaches for assessing picocyanobacterial growth rates in the field would use either cellular RNA content or population DNA distributions as the basis for estimating growth rate. In this study I show that biomass-normalized RNA content is linearly related to growth rate in Synechococcus WH8103. Re-analysis of previously published data suggests that many (though not all) other Synechococcus strains behave similarly, and therefore that cellular RNA may represent a reasonable approach for estimating in situ growth rates in natural Synechococcus populations. However, the non-linear relationship observed here (and in previous studies) for Prochlorococcus indicates that application of the approach may be problematic in this case. The cell cycle study in this dissertation is the first to systematically characterize the relationship between growth rate and cell cycle behavior in Prochlorococcus. I show that Prochlorococcus and Synechococcus display a notable degree of similarity with respect to this behavior. The combined duration of the replication and post-replication phases varied with growth rate in both species, suggesting that typical strategies for calculating growth rates from DNA data may need to be modified. Furthermore, I found that cell mass at the start of DNA replication decreased with increasing growth rate, indicating that the initiation of chromosome replication may not be a simple function of cell biomass, as previously suggested. Regarding picocyanobacteria as potential prey items for heterotrophic flagellates, I found that P. imperforata could graze and grow upon both these strains, but that Synechococcus was the preferred prey. This preference was flexible, however, and could be modulated by the ratio of prey types and/or the overall concentration of available prey.