Evidence for methane production by the marine algae Emiliania huxleyi

Abstract. Methane (CH₄), an important greenhouse gas that affects radiation balance and consequently the earth's climate, still has uncertainties in its sinks and sources. The world's oceans are considered to be a source of CH₄ to the atmosphere, although the biogeochemical processes involved in its formation are not fully understood. Several recent studies provided strong evidence of CH₄ production in oxic marine and freshwaters, but its source is still a topic of debate. Studies of CH₄ dynamics in surface waters of oceans and large lakes have concluded that pelagic CH₄ supersaturation cannot be sustained either by lateral inputs from littoral or benthic inputs alone. However, regional and temporal oversaturation of surface waters occurs frequently. This comprises the observation of a CH₄ oversaturating state within the surface mixed layer, sometimes also termed the "oceanic methane paradox". In this study we considered marine algae as a possible direct source of CH₄. Therefore, the coccolithophore Emiliania huxleyi was grown under controlled laboratory conditions and supplemented with two ¹³C-labeled carbon substrates, namely bicarbonate and a position-specific ¹³C-labeled methionine (R-S-¹³CH₃). The CH₄ production was 0.7 µg particular organic carbon (POC) g⁻¹ d⁻¹, or 30 ng g⁻¹ POC h⁻¹. After supplementation of the cultures with the ¹³C-labeled substrate, the isotope label was observed in headspace CH₄. Moreover, the absence of methanogenic archaea within the algal culture and the oxic conditions during CH₄ formation suggest that the widespread marine algae Emiliania huxleyi might contribute to the observed spatially and temporally restricted CH₄ oversaturation in ocean surface waters.