Nitrate availability modulates induced defenses in Phaeocystis globosa against protozoan grazers

Colony formation by Phaecoystis has been widely considered an induced defense mechanism against various grazers and contributes to the occurrence of algal blooms in nitrate-enriched waters. However, the effects of nitrate availability on these induced defenses against protozoan grazers remain unclear. Four P. globosa strains isolated from 4 off-coast areas of the South China Sea were exposed to grazing cues associated with the ciliate Euplotes vannus and the heterotrophic dinoflagellate Oxyrrhis marina under nitrate-sufficient and nitrate-deficient conditions to evaluate whether nitrate availability modulates the expression and costs of induced defenses. Except for one strain exposed to ciliate cues under nitrate-deficient conditions, all strains expressed defensive responses to grazing cues. Colony numbers consistently increased in the presence of grazing cues regardless of nitrate availability, although the amplitude of the increase was greater under nitrate-deficient conditions. Negative linear relationships were observed between colony numbers and population growth rates for 2 strains under nitrate-deficient conditions, suggesting increased energetic costs of defenses. The other 2 strains that did not exhibit negative relationships between colony numbers and growth nevertheless exhibited decreased cellular particulate organic carbon, particulate organic nitrogen, and chlorophyll a content in addition to smaller cellular diameters, suggesting that P. globosa can save energetic costs by adjusting physiological processes and cell sizes. These results suggest that nitrate availability strongly modulates the expression of induced grazing defenses and substantially influences the partitioning of cells into colonial or solitary forms in Phaeocystis , thereby also modulating their ecological roles in local food webs.