Jun 24 2022

Food Without Photosynthesis

Yesterday I wrote about the fact that as resources become limited, people have typically found ways around that limitation through technology and ingenuity. Specifically, as land-based resources of the metals we need for our technology, such as batteries, becomes limited, we may turn to the sea which has vastly greater reserves of many of those metals. There is essentially an inexhaustible supply of lithium in seawater, for example, which will be critical for a battery-powered future. Developing the technology to essentially “mine” elements from seawater, in other words, is a game changer.

We have a similar situation with food production. As the human population grows we need to be able to grow enough food to feed everyone. Around a century ago our food system was limited by the nitrogen cycle – we could only get so much nitrogen into the soil, mainly through manure, and that limited the amount of food we could produce. Then came the Haber Bosch process, an industrial-scale process for turning nitrogen from the atmosphere (combined with hydrogen) into fertilizer. This was a game-changer, brining the green revolution which allowed human populations increase dramatically. And now we are facing a similar problem. We have used up most of the reasonably arable land and so our best option for increasing food production is to increase the amount of food produced per acre.

There are several options to achieve this. One is developing new crop cultivars (through genetic engineering and other methods) that produce more food per acre. Another is to optimize our food production globally, making sure that each acre of land is used for its best purpose. This will likely involved decreasing the proportion of meat we consume, but not eliminating it, as some land is optimal for grazing and not growing crops. (We also can use the manure to feed back into the system – currently about half of fertilizer is manure.) Hydroponic food production for some crops can also be massively land efficient (and water efficient) with towers of food production not limited by the properties of the land. There is also sea-based food production, growing food (such as algae) in vats, and insect farming which is highly land efficient. There are lots of options and we will likely be exploiting all of them increasingly in the future.

I would like to add one more game-changing technology to the list – bypassing photosynthesis altogether. Much of the food we eat derives ultimately from photosynthesis, the conversion of water and CO2 into sugars and oxygen, powered by light. Essentially photosynthesis is the process by which light energy is converted into chemical energy. This is a key process for life on Earth, because we are bathed in lots of solar energy, but we can’t use it directly. It needs to be converted into a form that life can use – chemical energy. The limiting factor of photosynthesis is that it is only about 1% efficient. Researchers are looking into using genetic engineering to make crops with photosynthesis that is slightly more efficient, increasing yield. But was if we could bypass photosynthesis altogether, and use a method for converting solar energy into chemical energy at a higher efficiency?

Such a process would not need to make food directly. Once we have high-energy chemical compounds (a way of storing chemical energy) that can become feedstock for food production. For example, remember the Haber Bosch process above, which requires nitrogen and hydrogen. Free hydrogen is a high energy substance and is therefore useful in many industrial reactions, including making fertilizer. Right now a lot of our hydrogen comes from fossil fuel, stripped off of methane, for example. One of the major research programs for our green future is figuring out how to make lots of hydrogen in a green process, using sunlight to split hydrogen from water, for example.

But green hydrogen is only part of the equation. We also need to convert CO2 (which is a relatively stable low energy compound) into high energy organic compounds. That is what photosynthesis does. It doesn’t really matter what those high energy organic compounds are – they can likely be used in subsequent reactions to make food. This is exactly what researchers have done using a new process:

Here a two-step CO2 electrolyser system was developed to produce a highly concentrated acetate stream with a 57% carbon selectivity (CO2 to acetate), allowing its direct use for the heterotrophic cultivation of yeast, mushroom-producing fungus and a photosynthetic green alga, in the dark without inputs from biological photosynthesis.

They can make CO2 into acetate, which can then be used to grow yeast, mushrooms, and algae. In addition they found: “An evaluation of nine crop plants found that carbon from exogenously supplied acetate incorporates into biomass through major metabolic pathways.” In other words, they can feed acetate to certain crop plants which they can use as food rather than growing their own food with sunlight. If you power the process with photovoltaic panels, the researchers estimate that the entire process produces four times as much food from the sunlight as does photosynthesis. This could translate into a four-fold increase in food produced per acre of sunlight. This efficiency will increase as photovoltaic technology improves.

That four-fold increase is huge, but that is for the entire light-to-food chain. The core process itself is 18 times as efficient as photosynthesis in terms of converting solar energy to chemical energy, so there is some room for even further net efficiency gain.

We should not become complacent about our food supply, and the effect producing so much food is having on the planet. Making the process more efficient (in energy conversion, land use, and water use) is good for everyone. We will likely come to the day when most of our calories do not come ultimately from growing plants in large tracks of land.

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