Elevated pCO2 and temperature levels modulate the ratios of the photosynthetic methane production to CO2 fixation in the coccolithophorid Emiliania huxleyi

Most phytoplankton species have been shown to release methane (CH₄) during photosynthesis; however, little has been documented on how changed levels of CO₂ at different temperatures affect their CH₄ production along with photosynthetic C fixation. Here, we examined CH₄ production and photosynthetic performance in the most cosmopolitan coccolithophorid, Emiliania huxleyi, grown under high (1000 μatm, HC) and ambient (415 μatm, LC) pCO₂ levels at five temperatures (16, 20, 22, 24 and 27 °C). The HC treatment slightly lowered the optimal temperature for growth and CH₄ production, and temperature changes significantly affected both carbon fixation and CH₄ production. Under suboptimal temperatures, increasing temperature from 16 to 20 °C led to about 96 % increase in CH₄ production per POC and HC treatment further enhanced this increase by an additional 9 %. In contrast, under super-optimal temperatures, a temperature rise by 4 °C reduced the microalgal CH₄ production per POC under HC treatment by about 24 % compared to the control. The calculated CH₄ production quotient (MPQ, CH₄ released vs. CO₂ fixed) ranged between 2 × 10⁻⁵−6 × 10⁻⁵, and showed a decreasing trend with increasing temperature under both pCO₂ levels, implying that the CH₄ production by this microalga is being affected by global ocean changes, and the CH₄ produced by phytoplankton should be quantified and included in assessing the feedback of marine phytoplankton to climate change.