Study shows we’ve got photosynthesis wrong

Plants fix carbon from the atmosphere through one of two versions of photosynthesis, known as C3 and C4. The science surrounding these alternative pathways was long thought settled, but now research suggests the assumptions made might be wrong.

It has been held that the first version, C3, was the most sensitive to environmental carbon dioxide (CO₂) levels, and that the plants using it would produce more biomass as those levels rise.

Results of a 20-year experiment, however, have overturned such certainties, revealing the biomass boost to be only a short-term effect that reverses over longer time-scales.

The findings, published in the journal Science, carry important implications for our ability to predict the full ecological impact of rising CO₂ levels and ensuing climate change.

In the C3 photosynthesis pathway, the carbon from carbon dioxide is converted to sugar via a copper-containing enzyme called ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco). The vast majority of plant species, including most trees, and major crops such as rice and wheat, use C3.

C4 plants, in contrast, comprise just 3% of species. However, they account for 25% of global land biomass because they are so abundant. Corn, sugar-cane and many grasses are C4 plants. They also use rubisco, but supercharge the process by first building up a reservoir of CO2 in the vicinity of the enzyme, substantially increasing its efficiency.

Essentially, C4 plants saturate rubisco with a high concentration of carbon dioxide, while C3 plants don’t. For this reason it was thought that C3 plants should be more sensitive to atmospheric CO₂ levels, and that increases would cause more pronounced growth.

A great deal of experimental evidence supports this, says lead author of the latest study, Peter Reich of the University of Minnesota, US.

“It’s as close to a paradigm, or a truism, in plant ecology as you ever get,” he says.

However, the experiments that delivered the evidence were based only short time-scales. So, two decades ago Reich and his colleagues initiated what is now one of the longest running ecology experiments in the world.

Numerous plots of C3 and C4 grasses were grown, with some exposed to ambient CO₂  and the rest exposed to elevated levels. For the latter, pipes flooded the air with extra carbon dioxide, effectively exposing the plants to levels expected by the year 2080.

Nothing interesting happened at first. For 12 years, C3 plants responded robustly to higher CO₂  with a 20% increase in total biomass, while the C4 plants barely registered the difference.

However, in the past eight years, things switched. Under higher CO₂  C3 plants stopped growing better than their ambient counterparts, while C4 plants took on 24% more biomass.

“That is completely unprecedented,” says Reich.

Curiously, photosynthesis rates didn’t explain the reversal. Instead, C4 plants were making the soil more fertile, fuelling better growth, whereas C3 plants did the opposite.

Plants need nitrogen to grow, and microbes in the soil break down living and dead biomatter and provide nitrogen in a form that plants can use. Under long-term high CO₂  this process is boosted in the soil surrounding C4 plants and suppressed around C3 plants.

Reich doesn’t know what causes this, but intends to find out. He suspects that, over time, high CO₂ causes C4 plants and C3 plants to alter soil microbial communities differently and, depending which microbes are favoured, this accelerates nutrient cycles or slows them down.

Robert Furbank, director of the ARC Centre for Excellence for Translational Photosynthesis in Canberra, Australia, who was not involved in the study, says the findings are surprising.

“Something is happening below ground that’s having a big impact on how the plants are growing,” he says.

“The paper presents more questions about why this is happening than answers, but it’s very interesting.”