Abstract. Methane (CH4) production within the oceanic mixed layer is a widespread phenomenon, but the underlying mechanisms are still under debate. 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 E. huxleyi, other widespread haptophytes, i.e., Phaeocystis globosa and Chrysochromulina sp. We performed CH4 production and stable carbon isotope measurements and provide unambiguous evidence that all three investigated marine algae are involved in the production of CH4 under oxic conditions. Rates ranged from 1.9±0.6 to 3.1±0.4 µg of CH4 per gram of POC (particulate organic carbon) per day, with Chrysochromulina sp. and E. huxleyi showing the lowest and highest rates, respectively. Cellular CH4 production rates ranged from 16.8±6.5 (P. globosa) to 62.3±6.4 ag CH4 cell−1 d−1 (E. huxleyi; ag = 10−18 g). In cultures that were treated with 13C-labeled hydrogen carbonate, δ13CH4 values increased with incubation time, resulting from the conversion of 13C–hydrogen carbonate to 13CH4. The addition of 13C-labeled dimethyl sulfide, dimethyl sulfoxide, and methionine sulfoxide – known algal metabolites that are ubiquitous in marine surface layers – resulted in the occurrence of 13C-enriched CH4 in cultures of E. huxleyi, clearly indicating that methylated sulfur compounds are also precursors of CH4. By comparing the algal CH4 production rates from our laboratory experiments with results previously reported in two field studies of the Pacific Ocean and the Baltic Sea, we might conclude that algae-mediated CH4 release is contributing to CH4 oversaturation in oxic waters. Therefore, we propose that haptophyte mediated CH4 production could be a common and important process in marine surface waters.

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Methane (CH 4) production in the ocean surface mixed layer is a widespread but still largely unexplained phenomenon. In this context marine algae have recently been described as a possible source of CH 4 in surface waters. In the present study we investigated the effects of temperature and light intensity (including daylength) on CH 4 formation from three widespread marine algal species Emiliania huxleyi, Phaeocystis globosa, and Chrysochromulina sp. Rates of E. huxleyi increased by 210% when temperature increased in a range from 10°C to 21.5°C, while a further increase in temperature (up to 23.8°C) showed reduction of CH 4 production rates. Our results clearly showed that CH 4 formation of E. huxleyi is controlled by light: When light intensity increased from 30 to 2,670 μmol m −2 s −1 , CH 4 emission rates increased continuously by almost 1 order of magnitude and was more than 1 order of magnitude higher when the daylength (light period) was extended from 6/18 hr light-dark cycle to continuous light. Furthermore, light intensity is also an important factor controlling CH 4 emissions of Chrysochromulina sp. and P. globosa and could therefore be a species-independent regulator of phytoplankton CH 4 production. Based on our results, we might conclude that extensive blooms of E. huxleyi could act as a main regional source of CH 4 in surface water, since blooming of E. huxleyi is related to the seasonal increase in both light and temperature, which also stimulate CH 4 production. Under typical global change scenarios, E. huxleyi will increase its CH 4 production in the future. Plain Language Summary Methane is a gas that affects the Earth's climate and is typically produced by microbes in the absence of oxygen or through geological processes. Surprisingly, methane is also produced in oceanic surface waters that are well oxygenated, known as the ocean-methane paradox. Marine phytoplankton has recently been discovered as a methane source, which might help to explain the paradox. Environmental factors such as light and temperature might be important for controlling methane production from marine algae. In order to understand how environmental factors affect methane formation from phytoplankton, we performed several experiments under laboratory conditions. We find that temperature, light intensity, and day length strongly control methane production of phytoplankton. The field blooms of marine algae, which are often strongly related to the seasonal increase of light and temperature, could act as an important regional source of methane in oceanic surface waters. Under typical global change scenarios, marine algae might increase their methane production in the 21th century.

<p><strong>Abstract.</strong> The world’s oceans are considered to be a minor source of methane (CH<sub>4</sub>) to the atmosphere although the magnitude of total net emissions is highly uncertain. In recent years the origin of the frequently observed in situ CH<sub>4</sub> production in the ocean mixed layer has received much attention. Marine algae might contribute to the observed CH<sub>4</sub> oversaturation in oxic waters, but so far direct evidence for CH<sub>4</sub> production by marine algae has only been provided for the coccolithophore <i>Emiliania huxleyi</i>. In the present study we investigated, next to <i>Emiliania huxleyi</i>, other widespread haptophytes, i.e. <i>Phaeocystis globosa</i> and <i>Chrysochromulina sp.</i> for CH<sub>4</sub> formation. Our results of CH<sub>4</sub> production and stable carbon isotope measurements provide unambiguous evidence that all three investigated marine algae produce CH<sub>4</sub> per se under oxic conditions and at rates ranging from 1.6 ± 0.5 to 2.7 ± 0.7 µg CH<sub>4</sub> per g POC (particulate organic carbon) d<sup>−1</sup> at a temperature of 20 °C with <i>Chrysochromulina sp.</i> and <i>E. huxleyi</i> showing the lowest and highest rates, respectively. In cultures that were treated with <sup>13</sup>C-labelled hydrogen carbonate δ<sup>13</sup>CH<sub>4</sub> values increased with incubation time, clearly resulting from the conversion of <sup>13</sup>C-hydrogen carbonate to <sup>13</sup>CH<sub>4</sub>. The addition of <sup>13</sup>C 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 <sup>13</sup>C-enriched CH<sub>4</sub> in cultures of <i>Emiliania huxleyi</i> clearly indicating that methylated sulphur compounds are also precursors of CH<sub>4</sub>. We propose that CH<sub>4</sub> production could be a common process among marine haptophytes likely contributing to CH<sub>4</sub> oversaturation in oxic waters.</p>

Aquatic ecosystems play an important role in global methane cycling and many field studies have reported methane supersaturation in the oxic surface mixed layer (SML) of the ocean and in the epilimnion of lakes. The origin of methane formed under oxic condition is hotly debated and several pathways have recently been offered to explain the “methane paradox.” In this context, stable isotope measurements have been applied to constrain methane sources in supersaturated oxygenated waters. Here we present stable carbon isotope signatures for six widespread marine phytoplankton species, three haptophyte algae and three cyanobacteria, incubated under laboratory conditions. The observed isotopic patterns implicate that methane formed by phytoplankton might be clearly distinguished from methane produced by methanogenic archaea. Comparing results from phytoplankton experiments with isotopic data from field measurements, suggests that algal and cyanobacterial populations may contribute substantially to methane formation observed in the SML of oceans and lakes.