Photosynthesis drives methane production in a coccolithophorid, creating a negative feedback for carbon sequestration.

Abstract Phytoplankton produces methane (CH4), a potent greenhouse gas. Little is known about the relationship between their CH4 production and photosynthesis, the predominant biological pathway of CO2 sequestration in the ocean. We show that CH4 released by the widespread, bloom-forming marine microalga Emiliania huxleyi grown under different light levels correlated positively with photosynthetic electron transfer and carbon fixation. We ruled out the possibility of classical methanogenesis in the cultures and showed that under saturating light E. huxleyi produces CH4 at a maximal rate of about 6.6 ×10− 11 µg CH4 cell− 1 d− 1 or 3.9 µg CH4 g− 1 particulate organic carbon d− 1, and cannot generate CH4 in darkness. Accounting for the CH4 released vs CO2 fixed, the stronger global warming potential of CH4, and estimates of CO2 that reaches the ocean interior, we conclude that E. huxleyi’s contribution to the marine biological carbon pump may be attenuated by up to 13% due to its CH4 release.