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.

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.