Methane production during zooplankton grazing on marine phytoplankton

Zooplankton produced CH4 while grazing on marine phytoplankton during laboratory experiments. We consider anaerobic microniches within zooplankton digestive tracts to be the most likely site for such methanogenesis. Methane production appeared to be zooplankton species-specific rather than dependent on the phytoplankton species being grazed. The amount of CH4 produced (4–20 nmol copepod−1 d−1) was sufficient under experimental conditions to make a significant contribution to the formation and maintenance of oceanic subsurface CH4 maxima. The methanogenic substrates monomethylamine and trimethylamine were constituents of the diatom, dinoflagellate, and flagellate phytoplankton species used in the CH4 production feeding experiments. Dimethylamine was present in two of the three algal species. Trimethylamine was the most abundant methylamine in all cases and was present in sufficient quantity to account for the CH4 production observed during copepod feeding experiments. Methanogens in these experiments would need to convert only 4–12% of the phytoplankton methylamines available to copepods during grazing to produce CH4 concentrations observed in the experimental flasks.