Diatoms are a type of plankton – also known as phytoplankton, the most common of the plankton types. According to University College London, these photosynthesising algae have a siliceous skeleton (frustule) and are found in almost every aquatic environment, including fresh and marine waters, soils.
Interestingly, they are also the most important producers of plant biomass in the ocean and help to transport carbon dioxide (CO2) from the atmosphere into the deep ocean and thus regulate our climate. Since diatoms rely on silica rather than calcium carbonate to build their shells, they were previously thought to benefit from ocean acidification. This is a chemical change in seawater that is triggered by the increasing uptake of CO2, which makes calcification more difficult, according to an article on Science Daily.
In a new study published in the journal Nature, scientists now show that diatoms are also affected by this chemical change. Analyses of data from field experiments and model simulations suggest that ocean acidification could drastically reduce diatom populations.
For this study, researchers from the GEOMAR Helmholtz Centre for Ocean Research Kiel, the Institute of Geological and Nuclear Sciences Limited New Zealand and the University of Tasmania, linked an overarching analysis of various data sources with Earth system modeling.
As a result of ocean acidification, the silicon shells of diatoms dissolve more slowly. This is not an advantage, a press note from GEOMAR explains. It causes the diatoms to sink into deeper water layers, before they chemically dissolve and are converted back into silica. The note adds: “Consequently, this nutrient is more efficiently exported to the deep ocean and thus becomes scarcer in the light-flooded surface layer, where it is needed to form new shells. This causes a decline in diatoms.”
Diatoms contribute 40 percent of the production of plant biomass in the ocean and are the basis of many marine food webs. They are also the main driver of the biological carbon pump that transports CO2 into the deep ocean for long-term storage, the note explains.
Jan Taucher, a marine biologist at GEOMAR and the study’s first author explains why these findings are important. “Our current understanding of ecological effects of ocean change is largely based on small-scale experiments, that is, from a particular place at a particular time,” says Taucher. “These findings can be deceptive if the complexity of the Earth system is not taken into account. Our study uses diatoms as an example to show how small-scale effects can lead to ocean-wide changes with unforeseen and far-reaching consequences for marine ecosystems and matter cycles,” Taucher explains in the press note, while adding that since diatoms are one of the most important plankton groups in the ocean, their decline could lead to a significant shift in the marine food web or even a change for the ocean as a carbon sink.
The findings of the study are in sharp contrast to the previous state of ocean research, which sees calcifying organisms – such as oysters and corals – having difficulty in forming their shells and skeletons in more acidic seawater, while diatoms have been considered less susceptible to the effects of ocean acidification.