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A graph I saw in high school appeared to show the Earth breathing.
It was a graph that plotted carbon dioxide in the atmosphere over the course of the 20th century and into the 21st. CO₂ had risen steadily, and then more rapidly, but it hadn’t gone up in a straight line. Each year it had fallen sharply before rising to a new peak, increasing over time in an upwards zig-zag.
What explained this annual, temporary fall in CO₂, the gas that is overwhelmingly responsible for climate change? The answer was photosynthesis, my physics teacher explained – the miracle by which plants turn light and CO₂ into food.
This is how our planet has regulated atmospheric carbon for longer than our species has existed. Fossil fuels are disrupting this equilibrium in several ways.
Spring is dawning in the northern hemisphere, where most of the planet’s green land is situated. Trees are unfurling leaves that will soak up carbon in the air and turn it into new bark, roots and branches. On a global scale, it’s like a gigantic inhalation of carbon. In autumn, when trees shed their leaves, Earth will exhale again.
The air we all breathe is increasingly polluted by fossil fuels. That includes products of fossil fuels, like plastic, which is now so ubiquitous that research suggests simply breathing can introduce microscopic fragments into your brain.
Something similar is happening in plants – and it could have global consequences.
Plants are losing their appetite
“Microplastics are hindering photosynthesis, the process by which plants convert energy from the sun into the fruit and vegetables we eat,” says Denis J. Murphy, an emeritus professor of biotechnology at the University of South Wales.
“This threatens massive losses in crop and seafood production over the coming decades that could mean food shortages for hundreds of millions of people.”
These are the conclusions of a recent study by researchers in China, Germany and the US. Murphy wasn’t involved, but his own research with plant cells – which the tiniest microplastics can infiltrate, and damage the organs involved in photosynthesis – has him worried.
“Given the potential (albeit speculative) risk to global food production, more priority should be given to rigorous scientific research of microplastics and their effects on both crops and the marine life that supports fish and seafood stocks,” he says.
Not so long ago, people wondered if our fossil fuel habit might actually benefit plant photosynthesis. After all, plants eat CO₂. Flooding the atmosphere with more of it each year could only whet their appetites, right?
“The amount of CO₂ used by photosynthesis and stored in vegetation and soils has grown over the past 50 years, and now absorbs at least a quarter of human emissions in an average year,” say ecologists Amanda Cavanagh (University of Essex) and Caitlin Moore (University of Western Australia).
Most of this extra carbon absorption has come from crops and young trees, the pair say, less from mature forests where a lot of the world’s carbon is stored. Cavanagh and Moore say this carbon pump is slowing down, as the other necessary ingredients for photosynthesis – soil nutrients and water – have fallen or stayed the same.
Microplastics could slow the rate at which plants remove carbon further. And then there are the effects of climate change, like drought, fires and floods, which will intensify as long as we continue burning fossil fuels.
After monitoring forests and shrublands in Australia for 20 years, Moore and a team of six colleagues concluded that these ecosystems are at risk of losing their ability to bounce back, and continue absorbing carbon, after successive climate disasters.
Hacking photosynthesis
We may have done plenty to reduce global photosynthesis, but a team of scientists at the University of Oxford and the Fraunhofer Society in Germany is trying to turn things around. How? By hacking plants to help them get more out of the process.
“You would be forgiven for thinking nature has perfected the art of turning sunlight into sugar,” say Jonathan Menary, Sebastian Fuller and Stefan Schillberg.
“But that isn’t exactly true. If you struggle with life goals, it might reassure you to know even plants haven’t yet reached their full potential.”
The team say that plants tend to convert less than 5% of sunlight into new tissue – often as little as 1%. That’s because of a mistake plants regularly make, in which an enzyme involved in photosynthesis latches on to oxygen instead of CO₂.
“If we could prevent this mistake, it would leave plants more energy for photosynthesis,” they say.
Cyanobacteria are Earth’s most ancient photosynthesisers. Menary, Fuller and Schillberg say these microscopic organisms could possess useful genes for better sunlight management that might benefit crops like rice and potato plants. Another technique involves helping plants recover from high light exposure quicker.
More efficient photosynthesis, with the help of gene editing and other tools, is not “a silver bullet”, the team stress. Certainly not while fossil fuels continue to drown our green planet in carbon it cannot metabolise.
However, this work is likely to prove useful as farmers seek to grow more in an increasingly volatile environment, while sparing enough land for nature.
“This research is about making sure we can grow enough food to feed ourselves,” the team say.
