Why do coccolithophores calcify? Does the calcium carbonate shell serve as protection against viral infection and predation?

Coccolithophores are an important group of marine phytoplankton that are characterized by their ability to precipitate calcium carbonate. The single cells form small calcite plates (coccoliths), which are arranged on the cell surface in form of a spherical coating, called coccosphere.
Coccolithophores account for a significant proportion of the marine primary production and are among the most important calcifying organisms in the ocean, thus having a significant impact on the marine carbon cycle. However, it is not known in what way their ability to calcify contributes to their ecological success, as the question of why coccolithophores calcify remains unanswered.
Probably the most obvious theory is that the coccosphere provides protection against natural predators, the most relevant of which are viruses, microzooplankton (unicellular protists), and mesozooplankton (metazoan predators).
By means of laboratory experiments, this work investigated whether the coccosphere of certain species provides protection against infection with a virus, a phagotrophic protozoan, and a copepod.
The results show that the coccosphere does not prevent infection with the specific virus and reveal complex infection dynamics in the investigated host-virus system. It is further shown that the coccosphere does not protect against grazing by a certain copepod species. Experiments with a phagotrophic protozoan showed that the coccosphere affects food uptake and growth of the predator, but does not cause the grazer to avoid ingesting coccolithophores and to select non-calcified cells that were offered at the same time.
The results raise the questions whether other microzooplankton predators that naturally co-occur with coccolithophores are able to select against calcifying cells, and whether the effect of calcification on the growth of nonselective protozoa provides a benefit for coccolithophores.