Fixing photosynthesis: the innovation that could feed the world

Innovation, not restraint, is why famine has all but disappeared and why more land is returning to nature

Matt Ridley is a science writer and a former member of the House of Lords.

An economy is also an ecology. Society is fuelled mostly by hydrocarbons, which are derived from carbohydrates synthesised by plants and turned into coal, oil and gas. A typical coal seam a metre thick was formed by 2,000 summers of plant photosynthesis 300 million years ago.

Photosynthesis is therefore central to everything we do. It makes our food and it made our fuel. And it is, by common consent, a catastrophically inefficient procedure. Of the sunlight that falls on a leaf, often just 1% of the energy is captured as biomass. At best, if they pull their fingers out, some kinds of plants can reach 4%. The rest of the energy is lost. Contrast this with a photovoltaic panel, which captures around 10% of the sunlight that falls on it as electricity.

So we – by which I mean all life on earth – are stuck with an embarrassingly inefficient engine at the very heart of our genetic technologies. The first steam engine was also 1% efficient when Thomas Newcomen made it in 1712. Along came James Watt with a separate condenser in 1776 and the efficiency jumped. Now, 250 years later, we have steam turbines with efficiencies in the 60% range. Why not do the same to photosynthesis?

Ever since this inefficiency was discovered, scientists have dreamed of improving it. Imagine a wheat or maize crop with five times the energy conversion productivity, say 5% instead of 1%. It would yield five times as much or take one-fifth as long to ripen, perhaps allowing several harvests a year from the same field. Either would enable a farm to feed many more people, sparing a vast acreage for rewilding or golf courses. Even a small change would have a huge effect.

Other limiting factors have already been tackled to generate improvements in farm yields. Thanks to diesel, we don’t need to divert a chunk of the harvest to feeding horses to do the cultivation. Thanks to our ability to use natural gas to fix nitrogen from the air and turn it into fertiliser, we have trebled yields per acre. Thanks to the Industrial Revolution, we have enriched the air with carbon dioxide, further boosting yields and re-greening arid areas all over the world. But we did all this without touching the inefficiency of photosynthesis itself. Could that change?

There are several ways in which photosynthesis wastes energy. It cannot use infrared, it reflects away green light (hence the colour of plants) and it wastes much of the energy in blue photons. But the big inefficiency comes with the design of a critical protein machine called rubisco. This 18,000-atom molecular device at the heart of photosynthesis struggles to distinguish oxygen from carbon dioxide. So almost half the time it undoes its own work, unfixing carbon and wasting energy.

The reason it does this is probably a historical accident. When rubisco first emerged three billion years ago there was no oxygen in the atmosphere, so “photorespiration” was not a problem. And the reason life has failed to solve it since is that all ways of making the enzyme better at discriminating between the two gases tend to slow down the reaction. Most plants, known as C3 plants, simply tolerate photorespiration and fine-tune rubisco to be as discriminating as possible without slowing things down too much.

Photo by Francesco Gallarotti on Unsplash

About 35 million years ago, some plants came up with a neat workaround, called the carbon-concentrating mechanism, whereby they gather carbon dioxide up in special compartments and feed it to rubisco. This is a bit like James Watt’s separate condenser for the steam engine: it wastes some energy, but not as much. These plants are known as C4 plants, after the four-carbon sugar they make as an intermediate step, and they thrive in warmer, drier conditions.

Various versions of the C4 mechanism were invented around 65 times independently. Maize and sugar cane are good examples: they grow faster than wheat and rice but only in warm weather. Around 3% of plant species are C4 plants, but they so dominate tropical ecosystems that they account for nearly a quarter of all plant photosynthesis. An Oxford-led international project is exploring how to turn rice into a C4 plant, with the goal of increasing rice yields by 50%, but progress has been slow.

Why not redesign rubisco itself? Short answer: because if it were easy, Mother Nature would have found a way. She does not like leaving opportunities like that lying around any more than we do. But a longer answer is that she sometimes gets stuck in the “path dependence” of evolution, a captive of history. You can’t get there from here by evolution – but maybe you can by innovation.

So it is not crazy to think that we human beings could succeed where nature has failed. There are some promising ideas out there for transforming rubisco’s efficiency. For example, why not pair it with an enzyme called carbonic anhydrase that can trap carbon dioxide and feed it directly to the rubisco? Such synthetic biology experiments, boosted by AI, might one day succeed.

The most optimistic graph I know is the one that shows the acreage devoted to agriculture in the world falling steadily over the last 20 years while the quantity of food produced by agriculture marches rapidly upwards. Incredibly, we now feed eight billion people from a smaller acreage of land than we needed to feed six billion in 2000.

That is why famine has all but died out, in sharp contrast to the predictions of most eco-pessimists. “By 1985 enough millions will have died to reduce the earth’s population to some acceptable level, like 1.5 billion people,” wrote doom-monger-in-chief Paul Ehrlich in 1968, sounding almost gleeful at the prospect. It is why more people can afford to eat plentiful protein as well as carbs. And it is why we can increasingly afford to return land to nature through rewilding or just do nice things like grow wildflower meadows.

Innovation saves the planet far more effectively than restraint and austerity ever could.