Phytoplankton produces methane during photosynthesis in the oxygen-rich water column. I Photo: Solvin Zankl
Last year, Mina Bizic and colleagues showed that cyanobacteria in water and on land emit methane during photosynthesis (in Science Advances, 2020). Since then, this has also been demonstrated for diatoms and haptophytes in freshwaters and oceans. The data currently available serves as proof of principle for the process and its potential effects.
Natural sources of methane are a black box
The current global methane budget does not separately account for emissions from natural oxic processes in aquatic environments. These are combined with other sources under the categories "wetlands" and "other natural sources". The amount of methane released from "other natural sources" is reported with a wide uncertainty ranging between 143 and 306 teragrams per year. "Methane emissions from fossil fuel production and agriculture can be quantified fairly accurately and are about 113 to 154 and191 to 223 teragrams per year, respectively. The large uncertainty in emissions from natural sources is due to the large variability and knowledge gaps in contributing sources," explained Mina Bizic.
Researchers assume that aquatic ecosystems, and specifically inland waters, are a major contributor to methane emissions. However, the knowledge on the different processes contributing to these emissions, in particular when it comes to methane production in oxic environment, are not fully understood and have not been quantified. For example, the amount of photosynthesis-associated methane produced or that resulting from the demethylation of methylphosphonates while scavenging phosphorus.
Worrying feedback loop of photosynthesis, methane and global warming
The researcher shows how climate change processes could further drive methane emissions in a self-reinforcing process: Eutrophication is thought to be the main cause of recent increases in phytoplankton blooms. The emission of photosynthesis-associated methane by these blooms could increase global warming, which in turn would increase the frequency, intensity and duration of the blooms, resulting in even more methane being produced and released into the atmosphere. In addition, phytoplankton blooms can increase the occurrence of anoxic regions and dead zones, which can increase the emission of methane through classical methane formation under oxygen depletion. "For climate impact research, it is essential to be able to better quantify methane emissions by phytoplankton in their natural environment. Therefore, I have identified three important future research fields. For example, studies are needed on methane emission by phytoplankton-taxa that are predicted to dominate phytoplankton blooms in the future. Additionally, research of oxic methane production should be extended to cover other environments such as rivers which have been so far neglected. And the investigations should take into account different environmental conditions such as temperature or light", said Mina Bizic.
