Abstract. The world’s oceans are considered to be a minor source of methane (CH4) to the atmosphere although the magnitude of total net emissions is highly uncertain. In recent years the origin of the frequently observed in situ CH4 production in the ocean mixed layer has received much attention. Marine algae might contribute to the observed CH4 oversaturation in oxic waters, but so far direct evidence for CH4 production by marine algae has only been provided for the coccolithophore Emiliania huxleyi. In the present study we investigated, next to Emiliania huxleyi, other widespread haptophytes, i.e. Phaeocystis globosa and Chrysochromulina sp. for CH4 formation. Our results of CH4 production and stable carbon isotope measurements provide unambiguous evidence that all three investigated marine algae produce CH4 per se under oxic conditions and at rates ranging from 1.6 ± 0.5 to 2.7 ± 0.7 µg CH4 per g POC (particulate organic carbon) d−1 at a temperature of 20 °C with Chrysochromulina sp. and E. huxleyi showing the lowest and highest rates, respectively. In cultures that were treated with 13C-labelled hydrogen carbonate δ13CH4 values increased with incubation time, clearly resulting from the conversion of 13C-hydrogen carbonate to 13CH4. The addition of 13C labelled dimethyl sulfide, dimethyl sulfoxide and methionine sulfoxide – known algal metabolites that are ubiquitous in marine surface layers - enabled us to clearly monitor the occurrence of 13C-enriched CH4 in cultures of Emiliania huxleyi clearly indicating that methylated sulphur compounds are also precursors of CH4. We propose that CH4 production could be a common process among marine haptophytes likely contributing to CH4 oversaturation in oxic waters.
Infectious titres of Emiliania huxleyi
virus 86 are reduced by exposure to millimolar dimethyl sulfide and acrylic acid
