The oceanic methane paradox

Abstract

The upper ocean is supersaturated with methane with respect to the atmosphere but its source remains unclear. The biogenic production of methane in the ocean occurs primarily by anaerobic methanogens and, to a lesser extent, by aerobic bacteria. The presence of supersaturated methane in the surface ocean has been termed the ‘oceanic methane paradox’ because most of the oxygenated water in the upper ocean presumably should prevent methanogens from generating methane. This thesis examines potential sources for the in situ production of methane and the environmental factors responsible for variability in dissolved methane concentration in the upper waters (≤ 300 m) of the southwest Pacific Ocean, east of New Zealand. Dissolved methane concentrations and environmental variables were measured during spring and summer in two water masses of contrasting biogeochemistry, subtropical water (STW) and sub-Antarctic water (SAW). Higher levels of methane and methane production were found in STW compared to SAW. In STW in summer, there was indirect but no conclusive evidence of a link between phytoplankton and methane from field measurements. The presence of more negative δ13CCH4 in the surface mixed layer (MXL) and below the pycnocline indicated that in situ biogenic production of methane was dominant over methane loss processes. A high-resolution study of the evolution of a phytoplankton bloom in STW showed that methane production and depletion in 13CCH4 was associated with a DMSP-containing Phaeocystis globosa and Synechococcus spp. bloom. And an increase in methane below the MXL was associated with the export of particles after the peak of the bloom. The presence of viable oceanic methane producers was indicated by potential rates of methane production (pRMP) in aerobic incubations of particles from photic and aphotic zones. Molecular techniques indicated up to ten unique phylotypes of potential methanogen populations and three DNA sequences were closely related to Methanosarcina methanogens in photic and aphotic zones of STW. Phylogenetic analysis of sequences from serially isolated cultures revealed a potentially new group of methane-producing Archaea that was distantly related to known methanogens. Prokaryotic cell aggregates resembling small particles that autofluoresced under UV illumination in all cultures were indicative of the presence of the methanogenic coenzyme and hydride carrier for methane production, F420. Methane production, associated with the coccolithophore E. huxleyi, was attributed to prokaryotes (including cyanobacterial phototrophs). The presence of methanogens and methane-producing Archaea in particles from the upper STW and pRMP associated with particles and microalgae represents an important link to the observed methane supersaturations in the southwest Pacific Ocean.