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

Many phytoplankton produce methane (CH4), a potent greenhouse gas. However, little is known about the relationship between their CH4 production and photosynthesis, the biggest biological reaction driving carbon  sequestration in the oceans. Here, by ruling out the possibility of classical methanogenesis in the cultures, we show that the bloom-forming marine microalga Emiliania huxleyi released CH4 during photosynthesis (did not generate CH4 in darkness) while grown under different light levels, the amount of CH4 released correlated positively with photosynthetic electron transfer and carbon fixation. Under growth-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.  We estimated that the microalga released 7 moles CH4 while fixing about 105 moles of CO2. Considering the higher global warming potential of CH4 than that of CO2 and complicated processes involved in CH4 fluxes from the surface oceans, the warming potential of phytoplankton CH4 production should be broadly evaluated.

Methods
We used Counter Particle counter and size analyzer (Z2 Coulter, Beckman, U.S.A.), Picarro G2308 (Picarro Inc., U.S.A.), Multi-color PAM (Walz, Germany), liquid scintillation counter (Beckman, LS6500, Germany) and a CHNS elemental analyzer (Vario EL cube Elementar, Germany) to measure the samples and collect our data.

CO2SYS software (V.2.3) was used to calculate parameters of the seawater carbonate system, and SPSS statistics (V.21) was used to analyzed the data.