Evidence for methane production by marine algae (Emiliana huxleyi) and its implication for the methane paradox in oxic waters

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 13 C-labeled carbon substrates, namely bicarbonate and a position-specific 13 C-labeled methionine (R-S-13 CH 3). 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 13 C-labeled substrate, the isotope label was observed in headspace CH 4 . Moreover, the absenceof 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