A model system elucidating calcification functions in the prymnesiophyte Emiliania huxleyi reveals dependence of nitrate acquisition on coccoliths

Abstract

Emiliania huxleyi has been the most abundant coccolithophore in the world’s ocean for the last 70,000 yr, forming extensive blooms in vast oceanic areas. Although coccolithophores in general and E. huxleyi in particular have been extensively studied, the selective advantage furnished by coccoliths is yet to be pinpointed.This study utilizes two substrains of E. huxleyi 1516, the original calcifying strain, and its isogenic twin which lost the capacity to calcify, in an attempt to determine the ecological advantages of producing coccoliths.Under standard laboratory conditions, the noncalcifyers outcompeted the calcifyers in growth. However, exposure of the two substrains to a combination of environmental stressors revealed that coccoliths mitigated stress imposed by a mechanical perturbation, reducing cell lysis and supporting greater cell concentrations in the presence of moderate turbulence. Noncalcifyers appeared to be susceptible to growth media with a high P:N ratio, with their nitrate acquisition capacity hampered under mechanical perturbation. We suggest that the stabilized microenvironment provided by coccoliths is essential for high-affinity nitrate transport and its regulation. Our results demonstrate that introduction of more natural stressors is instrumental in and sometimes essential to unveil the ecological advantages of coccolithophore calcification.