Grazing interactions between Oxyrrhis marina and Synechococcus strains grown in single nitrogen sources

The goal of this study was to assess the interaction between abiotic and biotic factors on diverse Synechococcus strains isolated from the coastal California Current (CC9311, CC9605, CC9902) and the oceanic Sargasso Sea (WH8102 and mutants: JMS40 and SIO7B). Previous research has demonstrated that abiotic factors, such as nutrient source or concentration, can alter cellular structure and chemistry. These cell characteristics in turn influence biotic factors such as predation by protozoan grazers. Synechococcus strains isolated from coastal and open ocean waters were grown to nitrogen (N) depletion in N-reduced medium. After reaching stationary phase, strains were transferred to media containing nitrate, ammonium, urea, proline, alanine, glycine, or glutamine to assess the growth rates for each strain on these individual N sources. Compared to growth rates prior to N-limited stationary phase, all strains increased their growth rate in the single N source media. Synechococcus strains appear to have diverse abilities to grow on a broad range of N sources; however, the pattern of N use was not related to coastal or oligotrophic clade association. The majority of strains showed maximal growth on glycine, rather than on nitrate, ammonium, or urea. However, coastal strain CC9902 and mutants of the Sargasso Sea strain WH8102 either did not grow on or were actively inhibited by several amino acids. Further analysis of cell size, shape, and carbon:nitrogen (C:N) ratios of N source-grown coastal strain CC9311 and oceanic strain WH8102 demonstrated that cell physiological and morphological characteristics, in addition to growth rates, varied among N sources within a strain, as well as between strains. Coastal strain CC9311 and oceanic strain WH8102 were used in 30-minute grazing experiments with the heterotrophic dinoflagellate Oxyrrhis marina. Overall, grazing on coastal strain CC9311 was consistently higher than grazing on open ocean strain WH8102. However, within each strain grazing behavior also varied depending on N sources for strain growth. Physiological and morphological analysis of prey, in concert with grazing experiments, suggested that N source alters prey morphology and physiology, and the predator O. marina responds to these cell alterations. While many characteristics such as C and N content, cell size, and cell shape were inter-related, grazing on coastal strain CC9311 was strongly linked to cell shape (highest on more rounded cells) and C and N content (higher on cells with higher nutrient content). In contrast to coastal strain CC9311, few clear relationships could be discerned between ocean strain WH8102 N source-grown cell characteristics and the feeding behavior of the heterotrophic dinoflagellate, O. marina. While previous work has shown that O. marina readily eats coastal strain CC9311, this study showed O. marina grazing rate is also affected by prey growth condition, reflected in the physiology and morphology of the cell. Further studies expanding the breadth of protozoan predators and Synechococcus strains would aid in the understanding of the microzooplankton's role in top-down control of Synechococcus populations under different nutrient regimes and in more general issues of how resource use might affect predation.