Decline in diatoms due to ocean acidification

Diatoms are the main producers of plant biomass in the ocean and help transport carbon dioxide (CO2) from the atmosphere to the deep ocean and thus regulate our climate. Because diatoms rely on silica rather than calcium carbonate to build their shells, they were previously thought to benefit from ocean acidification — a chemical change in seawater caused by the increasing uptake of CO2 that complicates calcification. In a study published today in Nature, scientists at the GEOMAR Helmholtz Center for Ocean Research Kiel show that diatoms, a type of plankton, are also affected. Analyzes of data from field experiments and model simulations suggest that ocean acidification could drastically reduce diatom populations.

While calcifying organisms such as oysters and corals struggle to form their shells and skeletons in more acidic seawater, diatoms were considered less susceptible to the effects of ocean acidification — a chemical change caused by the uptake of carbon dioxide (CO2). The globally widespread small diatoms use silica, a compound of silicon, oxygen and hydrogen, as building material for their shells. That diatoms are nevertheless threatened has now been demonstrated for the first time by researchers from GEOMAR Helmholtz Center for Ocean Research Kiel, the Institute of Geological and Nuclear Sciences Limited New Zealand and the University of Tasmania in a study published in Nature. For the study, researchers linked an overarching analysis of different data sources to modeling of the Earth’s system. The findings provide a new assessment of the global impact of ocean acidification.

Due to the acidification of the ocean, the silicon shells of diatoms dissolve more slowly. This is not an advantage – it causes diatoms to sink into deeper water layers before they chemically dissolve and convert back to silica. As a result, this nutrient is exported more efficiently to the deep ocean and thus becomes scarcer in the light-flooded surface layer, where it is needed to form new shells. This leads to a decrease in diatoms, the scientists say in their recent publication. Diatoms contribute 40 percent to the production of plant biomass in the ocean and form the basis of many marine food webs. They are also the main driver of the biological carbon pump that produces CO. transports2 to the deep ocean for long-term storage.

dr. Jan Taucher, marine biologist at GEOMAR and lead author of the study says: “With an overarching analysis of field experiments and observational data, we wanted to discover how ocean acidification affects diatoms on a global scale. Our current understanding of ecological effects of ocean changes is largely based on small-scale experiments, that is, of a particular place at a particular time. These findings can be deceptive if the complexity of the Earth system is not taken into account. Our research uses diatoms as an example to show how small-scale impacts can lead to ocean-wide changes with unforeseen and far-reaching consequences for marine ecosystems and matter cycles. 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 meta-analysis examined data from five mesocosm surveys from 2010 to 2014, from different ocean regions ranging from arctic to subtropical waters. Mesocosms are a type of large-volume, oversized ocean test tube with a capacity of tens of thousands of liters, in which changes in environmental conditions can be studied in a closed but otherwise natural ecosystem. For this purpose, the water in the mesocosm was enriched with carbon dioxide to meet future scenarios of moderate to high increase in atmospheric CO2 levels. For the current study, the chemical composition of organic matter from sediment traps was evaluated as it sank through the water in the experimental containers over the course of several weeks of experiments. In combination with measurements from the water column, an accurate picture of biogeochemical processes within the ecosystem was created.

The findings of the mesocosm surveys can be confirmed using global open ocean observation data. In line with the results of the meta-analysis, they show a lower decomposition of the silicon shells at a higher acidity of the seawater. With the resulting datasets, simulations were performed in a Earth system model to assess the ocean-wide consequences of the observed trends.

“Already by the end of this century we expect a loss of up to ten percent of diatoms. That’s immense when you consider how important they are to ocean life and to the climate system,” continued Dr. taucher. “However, it is important to think beyond 2100. Climate change does not stop abruptly, and global effects in particular take some time to become apparent. Depending on the amount of emissions, our model in the study predicts a loss of up to 27 percent silica in surface waters and a decrease in ocean-wide diatoms of up to 26 percent by the year 2200 – more than a quarter of the current population.”

This finding of the study is in stark contrast to the previous state of ocean research, in which calcifying organisms are seen as losers and diatoms are less affected by ocean acidification. Professor Ulf Riebesell, marine biologist at GEOMAR and head of the mesocosm experiments added: “This study once again highlights the complexity of the Earth’s system and the associated difficulty in predicting the impacts of human-induced climate change as a whole. These kinds of surprises remind us time and again of the incalculable risks we face if we do not tackle climate change quickly and decisively.”

Reference:

  1. Jan Taucher, Lennart T. Bach, AE Friederike Prowe, Tim Boxhammer, Karin Kvale, Ulf Riebesell. Improved silica export in a future ocean causes global decline in diatoms. Nature, 2022; 605 (7911): 696 DOI: 10.1038/s41586-022-04687-0
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