Phytoplankton defenses: Do Emiliania huxleyi coccoliths protect against microzooplankton predators?

The calcium carbonate plates (coccoliths) surrounding most coccolithophorid cells are strikingly reminiscent of armor, and defense against predators has been hypothesized as a selective advantage provided by these mineral structures. Although microzooplankton are the main predators of small phytoplankton such as coccolithophores, few putative phytoplankton defenses have been tested against this group. In this study, predation by the heterotrophic dinoflagellate Amphidinium longum on three calcifying strains of Emiliania huxleyi was compared with predation on cells acid-treated to remove coccoliths. As a control for acidification effects on other cell properties, we also compared predation on acid-treated and untreated non-calcifying (haploid) E. huxleyi strains. We found no systematic support for the defense hypothesis in this predator-prey system. Acid-treated cells experienced predation rates that were higher, the same as, and lower than rates on untreated cells, depending on E. huxleyi strain. Other cell properties showing no relationship to predation included size, dimethylsulfoniopropionate (DMSP) content, and DMSP release. For untreated cells, however, hydrogen peroxide (H2O2) concentration in the cell suspension was positively related to predation, suggesting that H2O2 acts as a signal promoting prey detection, or that H2O2 co-varies with other (unmeasured) predation-enhancing cell properties. A. longum uses two different prey capture and ingestion strategies, depending on prey type. This plasticity suggests co-evolution of predation and defense strategies, facilitating a generalist feeding strategy compatible with the diversity of potential phytoplankton prey.