Acidic oceans could drastically reduce one of the world’s largest oxygen producers

The tiny floating organisms that provide our world with up to a fifth of its oxygen will be in dire straits as our oceans acidify, new research suggests.

The creatures, called diatoms, will be deprived of the silica building blocks they need to build their protective shells, which come in all sorts of dazzling opaline forms.

This could reduce their numbers by up to 26% by the end of the next century, researchers have found.

“Diatoms are one of the most important plankton groups in the ocean,” says marine biologist Jan Taucher of the GEOMAR Helmholtz Center for Ocean Research Kiel (GEOMAR).

“Their decline could result in a significant change to the marine food web or even a change to the ocean as a carbon sink.”

These single-celled algae make up 40% of the photosynthetic biomass in the ocean, making them one of the main components of the biological pump that absorbs CO₂ out of our atmosphere, storing it in the depths of the ocean.

They are one of the reasons why the oceans have managed to absorb much of the excess CO₂ we humans produce.

(Samarpita Basu/Katherine RM Mackey/Wiki/CC BY-SA 4.0)

Above: Role of phytoplankton in the biological carbon pump.

But as our excess CO₂ dissolves in seawater, it reacts to form more hydrogen ions, increasing the acidity of the water. This altered ocean chemistry has already resulted in a 10% decrease in carbonate concentrations since industrialization.

Less carbonate means it is harder for calcium carbonate to form; it is a vital molecule for most marine animals as it is part of their shells and exoskeletons.

If the carbonate concentration drops too low, the calcium carbonate dissolves. Some animals now experience the dissolution of their shell.

In contrast, it was thought that diatoms, which build their complex glass houses out of completely different materials, would be relatively unaffected by ocean acidification and might even benefit from increased CO₂.

These phytoplankton build their outer shells, called frustules, from the silica that floats in the surface waters of the ocean.

But the new research identifies a factor that has been missed by previous studies. It turns out that as the pH of the water drops, these vital silica elements will start to dissolve more slowly, which means more of it will sink deeper into the depths of the ocean. before it becomes light enough to stay afloat.

This leads to more silica at the bottom of the ocean, well out of reach of diatoms floating in the light which they use to transform CO₂ in oxygen, water and carbohydrates, which hinders their ability to build their frustule homes.

An opaline silica frustule under 1500x magnification. (Massimo brizzi/Wikipedia/CC BY-SA 4.0)

Taucher and his fellow researchers discovered this by using giant ocean “test tubes” (mesocosms), where they added different concentrations of CO₂ to simulate future warming scenarios.

They then assessed samples from different depths – analyzing the sediments filled with dead diatoms that they captured. This, together with modelling, supported by previous studies of the silica chemistry of diatoms, revealed a staggering decline in floating silica, suggesting diatoms could decline by up to a quarter by 2200.

Such a huge loss of these phytoplankton will have dramatic ramifications for other life forms on our planet, given that these organisms are one of the primary primary producers of the ocean.

“[A]The associated consequences on ecosystem functioning and the carbon cycle are more difficult to assess,” the team states in their paper, explaining that they failed to take into account many physiological and ecological processes that could trigger an effect. domino over the rest of the food web.

Either way, the results show how unexpected feedback mechanisms in Earth systems can radically alter environmental and biological changes that we may think we understand – revealing that we still have a lot to learn about how our planet works. and its life forms are intertwined.

“This study highlights once again the complexity of the Earth system and the associated difficulty in predicting the consequences of human-induced climate change in its entirety,” says GEOMAR marine biologist Ulf Riebesell.

“Surprises like this remind us again and again of the incalculable risks we face if we don’t tackle climate change quickly and decisively.”

This research was published in Nature.