Photosynthesis in frog foam
Although Brazil’s been making biofuels for decades, the rest of the world has quickly got interested over the last few years, due to concerns about climate change, as well as the rising price of oil. Unfortunately, it’s none too easy: plants tend to store a lot of the energy in molecules that are hard to break down, like cellulose. To make matters worse, the land needed to grow these plants is land that can’t be used to grow food, nor left to nature.
One option is to copy photosynthesis, and run the process without the plants. With a little help from a South American frog, it seems that might work. Some scientists in Ohio managed to assemble a set of enzymes to produce glucose from carbon dioxide and sunlight. Here’s what they used:
A surfactant protein, Rsn-2, from the Túngara frog, to make the foam. The frog itself uses the foam to protect its eggs. The experiments found that the system worked better using this foam than making foam with an artificial detergent, a testament to the power of evolution.
- Bacteriorhodopsin (from Archaea) to capture light energy and pump protons.
- An ATP synthase from a bacterium, which could use those protons to make ATP, the standard form of energy in a cell. This and the bacteriorhodopsin were embedded in lipid vesicles—little spheres made from a thin layer of fats enclosing some water, not unlike a soap bubble. The bacteriorhodopsin reduced the pH inside the vesicle, and that difference drove the ATP synthase.
- The enzymes of the Calvin cycle, including RUBISCO, which collectively fix carbon dioxide and produce carbohydrates.
- Several more enzymes, to convert the carbohydrates from the Calvin cycle into glucose, which we can easily use.
By my count, there are no fewer than four kingdoms of life contributing to this: an animal, an archaeon, a bacterium, and a plant (or several plants). The importance of the frog foam is to spread the enzymes out, so that light and carbon dioxide can easily get to them. I wonder, though, if they would use up the CO2 in the bubbles? Perhaps it could be continually reprocessed, the carbohydrates taken out, and the foam re-formed with new CO2.
The researchers validated that each step worked by itself, then plugged them all together to prove that the system really could make glucose from sunlight. They suggest that it could be used to produce a fuel called DMF, and calculate that, in ideal conditions, it could produce as much as 34.5 tonnes per hectare per year. They say that compares favourably to production from plants, although I can’t find any mention of DMF in the reference they give for comparison.
One possible application is on rooftops, where we don’t generally try to grow food. Since bacteriorhodopsin is purple, this raises the tantalising thought of one day covering my roof with a thick layer of purple foam, all in the name of the environment, but sadly I don’t think it will work quite like that. They also suggest that it could be used at sources of carbon dioxide, such as fossil fuel power plants.