Uncertain fate of pelagic calcifying protists: a cellular perspective on a changing ocean.

Pelagic calcifying protists such as coccolithophores and foraminifera represent an important microbial component of the marine carbon cycle. Although their calcitic shells are preserved in oceanic sediments over millennia, their resilience in the future decades is uncertain. We review current literature describing the response of calcifying protists to ocean acidification and temperature warming. We examine these key ecological and biogeochemical processes through the cellular perspective, exploring the physiological, metabolic, and molecular responses of calcifying protists. Ocean acidification is a chemical process that takes place in the seawater outside the cell, whereas protists calcify inside a modified cellular microenvironment. The function of these calcification compartments depends on cellular response to ocean acidification, such as maintaining pH homeostasis. The response of calcifying protists to ocean acidification and temperature warming is species-specific, with no unifying trends but rather a range of sensitivity levels. Coccolithophores and foraminifera display physiological sensitivity that may hamper their ecological success in comparison to non-calcifying species. Yet, certain species may be more adaptable, especially when comparing to highly vulnerable calcifying molluscs as pteropods. As the molecular machinery mediating cellular calcification is not fully resolved, as well as the functional role of the calcitic shell, our ability to predict the fate of calcifying microorganisms in a warmer, more acidic ocean is limited. We propose the urgent need to expand the study of these model systems by advancing cell biology approaches, to better understand the impact of climate change on microbial food webs in the ocean.