Phytoplankton photosynthesis: an unexplored source of biogenic methane emission from oxic environments

Abstract

Abstract Recent studies show that all evaluated Cyanobacteria, diatoms and haptophytes, major constituents of freshwater and marine phytoplankton, emit methane during photosynthesis. This is independent of external methylated precursors, whose demethylation by prokaryotic phytoplankton (and other microorganisms) has been already shown to be a potent methane source. Methane is the second most significant anthropogenically influenced greenhouse gas with a global warming potential 85 times higher than carbon-dioxide, over a period of 20 years. Atmospheric methane has been steadily increasing, yet its natural sources are not well constrained, with the largest uncertainties occurring in aquatic ecosystems. Already for several decades, increasing evidence has been pointing out that methane produced under oxic conditions through non-traditional pathways (i.e. not archaeal methanogenesis) significantly contributes to the methane flux from marine and freshwater environments. This paper highlights the potential significance of phytoplankton as methane producers, particularly through the ubiquitous process of photosynthesis. Currently, available data serve as a proof of principle for the process and its potential rates, yet further extensive research is required to successfully provide emission estimates at a global scale. Therefore, studies investigating the taxonomic breadth of the process and the correlation of the methane emission rate with environmental factors should be conducted.