Ocean acidification plays havoc with diatoms

Diatoms – single-celled algae which occupy much of the world’s water – were thought to be more resistant to acidification than some of their aquatic counterparts, with their silicon-containing shells more impervious to acid than the carbonates in shellfish and corals.

But according to research published in Nature, acidification threatens diatoms as well.

“With an overarching analysis of field experiments and observational data, we wanted to find out how ocean acidification affects diatoms on a global scale,” says first author Dr Jan Taucher, a marine biologist at the Helmholtz Centre for Ocean Research Kiel, Germany.

“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. These findings can be deceptive if the complexity of the Earth system is not taken into account.”

The researchers examined five experiments done in different ocean waters, from 2010 to 2014. They then designed five similar experiments, simulating ocean water from 2100.

Person on a boat lowering cylindrical net several metres in diametre connected to floats into the ocean
Biological oceanographer Jan Taucher is working on a Mesocosm. Mesocosms are a type of large-volume, oversized test tube in the ocean 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. Credit: Ulf Riebesell / GEOMAR

More acidic water, the researchers found, causes the silicon shells of diatoms to thicken. This makes them sink and dissolve, meaning their silicon falls to the bottom of the ocean and becomes silica, where it can’t get into new diatoms. This causes an overall decline.

“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,” says Taucher.

“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.”

On current trajectories, the oceans of 2100 do not look welcoming to diatoms.

“Already by the end of this century, we expect a loss of up to 10% of diatoms,” says Taucher. “That’s immense when you consider how important they are to life in the ocean and to the climate system.

“However, it is important to think beyond 2100. Climate change will not stop abruptly, and global effects in particular take some time to become clearly visible.

“Depending on the amount of emissions, our model in the study predicts a loss of up to 27% silica in surface waters and an ocean-wide decline in diatoms of up to 26% by the year 2200 – more than a quarter of the current population.”

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This research sits in contrast with previous studies, which assumed the main risk of ocean acidification was calcium-dependent, not silicon-dependent, organisms.

“This study once again highlights the complexity of the Earth system and the associated difficulty in predicting the consequences of man-made climate change in its entirety,” says co-author Professor Ulf Riebesell, also a marine biologist at the Helmholtz Centre.

“Surprises of this kind remind us again and again of the incalculable risks we run if we do not counteract climate change swiftly and decisively.”

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