Ultraviolet Radiation Drives Emission of Climate-Relevant Gases From Marine Phytoplankton

Marine phytoplankton contribute about one half of global primary production and play a key role in global biogeochemical cycles. High cell densities in extensive phytoplankton blooms are expected to be modified by global changes in ocean circulation and stratification, acidification and carbonation, solar radiation, temperature, and eutrophication. Although photochemical gas production from chromophoric dissolved organic matter (CDOM) has been widely studied, ultraviolet (UV) effects on emissions from phytoplankton cells themselves have not been fully explored. We therefore investigated UV-driven emissions of carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), ethene (C2H4), ethane (C2H6), and nitrous oxide (N2O) from cell suspensions of 16 phytoplankton species and their filtrates under controlled experimental conditions. These gases make direct or indirect contributions to radiative forcing of the atmosphere or contribute to atmospheric chemistry including stratospheric ozone (O3) depletion. We observed production of CH4, CO, CO2, C2H4, and N2O from cell suspensions and CO, CO2, and N2O after 0.45 μm-filtration to remove phytoplankton cells. CH4 production was only observed with cells present, whereas N2O was still produced after filtration. Production of CO from filtrates was 30%–90% of that from cell suspensions in all but two species with a CO2:CO mole ratio from filtrates always below one. Our results clearly demonstrate a need to quantify the production potentials of these climate-relevant gases in situ under natural sunlight using key phytoplankton species, especially those forming blooms which are predicted to change in prevalence and distribution with future global change scenarios.

Plain Language Summary
The oceans play an important role in the global exchange of gases with the atmosphere including the greenhouse gases carbon dioxide and methane that contribute to global warming. Several recent studies have shown that phytoplankton metabolism can produce methane during growth. Other studies also demonstrated that sunlight can degrade dissolved organic matter and plastic particles in seawater to produce methane and other gases. However, few studies have examined whether the ultraviolet radiation in sunlight can drive production of methane and other climate-relevant gases from the cells of phytoplankton. In this study, we exposed cell suspensions of 16 phytoplankton species to controlled levels of ultraviolet radiation in the laboratory. We observed production of methane, carbon dioxide, carbon monoxide, ethene and also the important greenhouse gas nitrous oxide from cell suspensions. Nitrous oxide was still produced after phytoplankton cells were removed by filtration, whereas methane was only produced with cells present. These gases make direct or indirect contributions to global warming and atmospheric chemistry. Blooms of phytoplankton cells may cover extensive areas of the ocean. Consequently, it is important to evaluate the role of the ultraviolet component of sunlight to emissions of these gases under future global change scenarios.

Key Points
Ultraviolet radiation drives emission of climate-relevant gases, including methane and nitrous oxide, from marine phytoplankton cells

Methane was only produced with cells present, whereas nitrous oxide was also produced after phytoplankton cells were removed by filtration

Global change impacts on phytoplankton blooms may result in increased UV-driven emissions of these climate-relevant gases