Found in lakes and rivers worldwide, single-celled creatures like these Paramecium bursaria can both eat and photosynthesize.

Ocean microbes could indicate climate change tipping point

Image credit: Daniel J. Wieczynski, Duke University

Global warming could cause ocean plankton to transform from carbon-absorbing to carbon-emitting organisms, scientists have discovered.

Scientists may have identified a new indicator of a climate change tipping point – marine microorganisms.  

A team of scientists from Duke University and the University of California, Santa Barbara, used a computer simulation to study how global warming impacts the metabolism of the world’s ocean plankton and other aquatic single-celled creatures.

Their findings showed that when temperatures crossed a certain threshold, these cells stop absorbing carbon and instead start emitting it.

“They're like ‘switches’ that could either help reduce climate change or make it worse,” said co-author Holly Moeller, an assistant professor at the University of California. 

The team focused on a group of tiny organisms called mixotrophs, which forms most of the plankton in the ocean. They’re also common in lakes, peatlands, damp soils and beneath fallen leaves.

These aquatic organisms are relevant because they are able to obtain nutrients from sunlight as well as from hunting food. Depending on what type of nutrition they choose to pursue, these microbes can either capture or emit carbon dioxide.

For this reason, the researchers believed that the change in the behaviour of these organisms could be used to anticipate the climate change tipping point. 

“Right before a tipping point, their abundances suddenly start to fluctuate wildly,” said Daniel Wieczynski, a postdoctoral associate at Duke. “If you went out in nature and you saw a sudden change from relatively steady abundances to rapid fluctuations, you would know it’s coming.”

The researchers developed a mathematical model to predict changes in the mixotrophs' metabolism and monitor climate change, using it to test how the microbes would react to increasing global temperatures.  

The analysis showed that the warmer it gets, the more mixotrophs rely on eating food rather than making their own via photosynthesis, therefore increasing the amount of carbon they produce and thus, in turn, worsening climate change. 

The scientists theorised that these microbes could reach a tipping point when they flip from carbon sink to carbon source, having a net warming effect instead of a cooling one. Once they cross that threshold, it would take significant cooling – more than 1°C – for them to go back to capturing carbon. 

Nonetheless, the researchers' ability to detect early warning signs of this change could be affected by water pollution. 

When the team included higher amounts of certain nutrients like nitrate and phosphate in their models, they found that the signal starts to sink until it suddenly disappears and the tipping point arrives with no apparent warning.

These nutrients are present in lakes, rivers and coastal waters due to discharges from wastewater treatment facilities, runoff from farms and lawns laced with chemical fertilisers and animal waste. 

“Tipping points can be short-lived and thus hard to catch,” said, Jean P. Gibert, a researcher at Duke University. “This paper provides us with a search image, something to look out for, and makes those tipping points - as fleeting as they may be - more likely to be found.”

The team's findings have been published in the journal Functional Ecology.

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