Light-dependent methane production by a coccolithophorid may counteract its photosynthetic contribution to carbon dioxide sequestration

Many phytoplankton produce methane, a potent greenhouse gas. However, little is known about the relationship between their methane production and photosynthesis, which drives carbon sequestration in the oceans. Here, by ruling out the possibility of classical methanogenesis, we show that the bloom-forming marine microalga Emiliania huxleyi released methane during photosynthesis (did not generate it in darkness) while grown under different light levels, the amount of methane released correlated positively with photosynthetic electron transfer and carbon fixation. Under growth-saturating light, E. huxleyi produces methane at a maximal rate of about 6.6 ×10⁻¹¹ μg cell⁻¹ d⁻¹ or 3.9 μg g⁻¹ particulate organic carbon d⁻¹. The microalga released up to 7 moles methane while fixing about 10⁵ moles of carbon dioxide. Considering the higher global warming potential of methane than that of carbon dioxide and complicated processes involved in methane air-sea fluxes, the warming potential of phytoplankton methane production should be broadly evaluated.