Calcification increases carbon supply, photosynthesis, and growth in a globally distributed coccolithophore

Coccolithophores fix organic carbon and produce calcite plates (coccoliths) that ballast organic matter and facilitate carbon export. Photosynthesis consumes carbon dioxide, while calcification produces it, raising questions about whether coccolithophores are a net sink or source of carbon. We characterized the physiology of calcified and noncalcified (“naked”) phenotypes of Emiliania huxleyi (CCMP374) and investigated the relationship between calcification and photosynthesis across a gradient of light (25–2000 μmol photons m⁻² s⁻¹) spanning the euphotic zone. Growth and photophysiological parameters increased with light until reaching a mid-light (150 μmol photons m⁻² s⁻¹) maximum for both phenotypes. Calcified cells were characterized by enhanced photophysiology and less photoinhibition. Further, enhanced bicarbonate transport in calcified cells led to higher rates of particulate organic carbon fixation and growth compared to naked cells at mid-light to high light (150–2000 μmol photons m⁻² s⁻¹). Coccolith production was similarly high at mid and high light, but the rate of coccolith shedding was >3-fold lower at high-light (1.2 vs. 0.35 coccoliths cell⁻¹ h⁻¹). The cellular mechanims(s) of this differential shedding remain unknown and underly light-related controls on coccosphere maintenance. Our data suggest coccoliths shade cells at high light and that enhanced bicarbonate transport associated with calcification increases internal carbon supplies available for organic carbon fixation.