Calvin Cycle 2.0

An article recently came out about a a team which used a series of enzymes in a test tube to do something similar to what plants do in carbon fixation. However, the process is not entirely the same in that it uses different enzymes from a variety of sources, including animals and bacteria, to complete the reaction chain. This chimera test tube of enzymes is apparently 25% more efficient than its natural competitor, RuBisCo. Engineering a system more efficient than this shouldn’t come as too much of a surprise because it is actually one of the least efficient enzymes in nature, as I have written previously (see also):

It is indisputable that CO2 concentrations in the atmosphere are increasing, and the burning of fossil fuels causes some or most of it.  However, CO2 is a natural part of the life cycle. Plants fixate CO2 from the atmosphere in order to grow. RuBisCo is the enzyme which fixes gaseous carbon into simple sugars in plants. This is arguably the single most important enzyme in existence. In addition to plants themselves, all animals and fungi, and most bacteria, are dependent on this enzyme working. It creates the food for those organisms. It also happens to be one of the least efficient enzymes. That is, it doesn’t work very well at doing its job, surprisingly. For one thing, it is very slow. RuBisCo is also capable of catalyzing oxygenation of its substrate rather than fixing a carbon dioxide molecule and it does so at fairly high rates. When oxygenation occurs, the energy is completely wasted because the byproduct isn’t useful for the plant. Moreover, energy has to be expended to reverse the process to make the substrate available for carbon fixation again. Some plants have even evolved special CO2 concentrating mechanisms to try to combat this problem. Increasing the carbon dioxide concentration of the air via burning fossil fuels should make plants better able to use this enzyme because increased concentration of the CO2 substrate increases the enzyme’s efficiency. For example, by increasing the likelihood that CO2 will be fixed rather than oxygen molecules. In other words, the expected result of increased carbon concentrations should be bigger plants, faster growing plants, and/or larger numbers of plants. Both agricultural and wild plants could be expected to benefit from this.

Intelligently designing a better system than RuBisCo, then, is seemingly one of the lowest bars in advanced genetics to cross. Not that that makes it easy in an absolute sense, only that it is easier than, say, designing human geniuses. Getting all these enzymes lined up physically and working well together in a chloroplast is no small barrier.

The obvious purpose of this work and research is to combat climate change. Personally, I am very skeptical that climate change as caused by released CO2 is actually something to worry about. I am inclined to think CO2 hysteria is more an expression of crypto-theology. So, the motivations for this work are suspect. That said, I could still see it being useful. Imagine the kinds of crops we could get if we made them 25% more efficient? What if we could use it to generate a very cheap source of organic fuels and/or starting chemical reagents. Even ignoring overblown warnings about an apocalypse, there is still potential use in this technology. I see no reason not to switch from fossil fuels to better sources if they are in fact better and cheaper.

However, there are other environmental considerations than climate change to at least think about. The first thing that comes to mind is what would be the consequences of introducing vastly more efficient plants into the wild, whether deliberate or inadvertent? Such a plant would presumably be at least a little more evolutionarily fit than its wild counterparts and potentially vastly more fit. If so, it could potentially disrupt entire ecosystems on a massive scale in a relatively short time. Out-competed plants would die out and all the life dependent on those plants would follow shortly thereafter, if they couldn’t adapt to the new composition of their environment. The quest to stop climate change could have unintended consequences far outstripping the largely imagined climate apocalypse. However, even in this case I have little doubt life as a whole would adapt and move on even if the disruption is quite severe. I am reminded of this rather charming documentary (called Cane toads: the conquest if the link goes bad) about the introduction of Cane Toads to Australia and all the havoc that caused. Unintended consequences are real, friends, and leftists are masters at generating them.

All this of course is assuming that this new system would actually work as well as they hope it might (and that the plants it was introduced in were otherwise capable of fierce ecological competition in addition to the new fixation system). This is possible. Evolution is subject to path dependence. Once the initial system of carbon fixation evolved, it would be stuck with the basic mechanism and could only adapt from that in minute steps. It would be very difficult to transfer to a completely different system naturally via small steps. In other words, it might be possible to tweak RuBisCo towards more efficiency, but nearly impossible to substitute a whole different enzyme which was much better overall. A newly evolved system, even if potentially better after additional evolution, would likely start off as less efficient as the already long extent and well adapted one and thus would have a hard time sticking around long enough to become a proper better alternative. Therefore, it is quite possible that better systems than RuBisCo are possible yet still unevolved. Some things are quite difficult to evolve.

On the other hand, it is also quite possible that there are good biological reasons for this inefficiency that we don’t know about. If so, other considerations may prevent the newly developed system from working well and/or resulting in a net loss in fitness due to side effects. In which case it won’t work and there is nothing to worry about. Either way, great care should be taken before committing to the introduction of a vastly different system of doing things, and that applies to biology as much as to government.

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3 thoughts on “Calvin Cycle 2.0

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