The dissolution of anthropogenic CO2 into surface seawater increases the ambient CO2 concentration and decreases both the pH and the carbonate ion concentration. This presentation will focus on the effects of this “ocean acidification” on marine phytoplankton, the microorganisms responsible for the photosynthetic formation of the organic matter that fuels marine ecosystems. The maintenance of appropriate internal H+ and CO2 concentrations in phytoplankton requires pumping of these solutes across external membranes. In eukaryotic microalgae such as diatoms, the cost of H+ pumping is small and the current and anticipated decrease in pH has a negligible effect on growth. To compensate for the low CO2 affinity of the Rubisco enzyme, these organisms maintain at the site of carbon fixation a CO2 concentration much higher than in seawater. The result is a modest beneficial effect of the CO2 increase on photosynthesis and growth. In coccolithophores, the precipitation of CaCO3(s) occurs in intracellular vesicles where the chemistry is actively controlled such that calcification is little affected by acidification. A quite different response to ocean acidification is observed in Trichodesmium, an important cyanobacterium that fixes N2 in the surface ocean. The internal pH of this organism is poorly buffered and decreases along with that of seawater. The net result is a reduced activity of enzymes, including N2 fixation by nitrogenase, an effect that is magnified at low Fe. In the most common situation when Trichodesmium colonies are organized in single trichomes (end to end assemblages of cells) the net result is a significant decrease in growth rate despite the over-production of nitrogenase. If this result is generalizable to other marine diazotrophs, ocean acidification could have a significant effect on the nitrogen cycle and productivity in seawater.

Effect of ocean acidification on marine phytoplankton

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.