Articles | Volume 13, issue 10
Research article
01 Jun 2016
Research article |  | 01 Jun 2016

Evidence for methane production by the marine algae Emiliania huxleyi

Katharina Lenhart, Thomas Klintzsch, Gerald Langer, Gernot Nehrke, Michael Bunge, Sylvia Schnell, and Frank Keppler

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.

Short summary
In this study we investigated marine algae as a source of CH4 in oxic surface waters of oceans. Algae-derived CH4 may explain the CH4 oversaturating state within the surface mixed layer, sometimes also termed the "oceanic methane paradox". This finding of an overlooked source of CH4 in marine environments will be of considerable importance to scientists in many disciplines because algae play a crucial role in organic matter cycling in marine and freshwater ecosystems.
Final-revised paper