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.