The coccosphere of the heavy calcifying coccolithophore Coccolithus braarudii provides defense against bacteria.

Coccolithophores produce 40-60% of marine calcium carbonate, largely through biomineralization of plates which encase cells in a 'coccosphere'. Despite the importance of coccolithophore calcification in ocean biogeochemistry, its function remains unresolved. A hypothesis suggesting it acts as a physical deterrent has been investigated in grazers and viruses, but not in bacteria. Here we show bacterial pathogenicity in heavily-calcified C. braarudii treated with Gephyrocapsa huxleyi bacterial-pathogen, Phaeobacter inhibens, is only observed with decalcified algae, leading to algal-cell death within as little as 15 hours. Decalcified algal cell mortality is P. inhibens-specific and likely requires close proximity, since treatment with bacterial supernatant or growth-inhibiting concentrations of indole-3-acetic acid shows no detrimental effect. Additionally, scanning electron microscopy shows visible bacterial attachment only on decalcified C. braarudii. These findings provide the first experimental evidence that the coccosphere can act as a barrier against specific bacteria, highlighting its defensive role in coccolithophores.