Capturing sunlight in droplets

RubiscoLife on Earth, including ourselves, relies totally on photosynthesis. Photosynthesis pulls carbon from carbon dioxide in the air to make the molecules of which plants are made of. Then we eat these plants, and, if we are not vegan, the products of animals that eat these plants. Photosynthesis, like everything else in biology, is the product of evolution. Very simply speaking there are two schools of thought on evolution. The first is that it is an incredible process that has produced marvels such as a soaring eagle with eyesight keen enough to see a rabbit a kilometre away. The second is that it is a blind process that gradually cobbles together just-about-working solutions to the problem of living and reproducing.

Perhaps surprisingly, a key enzyme that pulls carbon from carbon dioxide in photosynthesis, is a poster child for the second viewpoint. The enzyme is called rubisco, and is notoriously inefficient. It is a bit weird that most life on earth relies on such an inefficient enzyme, but that is, quite literally, life.

Various photosynthesising organisms fight the inefficiency of rubisco in various ways, such as concentrating large amounts of rubisco in small volumes, and controlling the environment of the rubisco as much as they can. Rubisco becomes more inefficient, the higher the local oxygen content is (a problem given the Earth’s atmosphere is 20% oxygen), so by concentrating large amounts of rubisco in a high carbon dioxide, low oxygen, environment, rubisco becomes more efficient.

One way to concentrate molecules in cells is in liquid droplets inside cells — if you shopw at Waitrose think a dark balsamic vinegar droplet in olive oil, an engine oil droplet in water is a more blue-collar example. A couple of years of Freeman Rosenzweig and coworkers showed that a type of algae concentrated their rubisco in liquid droplets. I think previous to that, we thought that the rubisco was in more rigid crystalline structures, not dynamic, occasionally flowing droplets.

As a species we don’t eat much algae, or even things that eat algae, so I guess that very few of the carbon atoms in your body were pulled from atmospheric carbon dioxide inside rubisco droplets inside algae. But the inefficiency of rubisco has inspired attempts to engineer this protein to be better. The idea is to give, say, wheat, a better rubisco and so make it grow faster and yield more. As far as I know these attempts have not succeeded. An alternative approach would be to give wheat or some other crop rubisco droplets, and see if they are more efficient. The human population is predicted to max out at around 10 billion. This is a lot of mouths to feed, and photosynthesis is ultimately where all their food will come from.

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