|Click to enlarge. Copyright 2007 University of Chicago. From: Nature's palette by David Lee|
As can be seen from the absorption spectrum of chlorophyll above, photosynthesis does not use the green portion of the spectrum, so that portion gets reflected for us to see. In doing so plants ignore much energy potentially available to them, as green is right in the part of the spectrum where the sun emits a lot of light. You might think that photosynthesis would evolve to make the most of the light falling on it, and, if so, you would predict that Earth plants should be purple (see the 'black grass' post for speculations why some bacteria are purple but plants are not).
Some people wonder whether we can predict the colour of plants on a planet by looking at the spectrum of its sun. Earth's example definitely suggests that we cannot, so I personally see no problems with filling hypothetical planets with plants of just about any colour; well, as long as the absorbed colour is present in that sun's spectrum, of course. A perfect photosynthesis process would be able to use light of every frequency equally well, with the effect that such plants would be grey or black.
After writing the 'black grass' post I returned to the question why it is difficult to come up with alien-looking plants. Intuition suggested that there would be only so much you could do with plant shapes: flat leaves fixed to the ends of a branching structure seem so sensible that they are probably universal, so plants everywhere would look similar. Perhaps so, but intuition is not a reliable predictor in science, so some old-fashioned studying was called for. I recommend 'The Life of a Leaf' by Steven Vogel, who also wrote a fine book on biomechanics.
The fun part will be designing new plant shapes, if possible, but before we get to that there is some work to do, I am afraid. This post starts with photosynthesis on Earth, to find out if it can be tweaked to produce plants with a high degree of 'alienosity'.
|Click to enlarge; Based on Long SP et al. Can improvement in photosynthesis increase crop yields? Plant, cell , and environment 2006; 29: 315-330|
1. Efficiency of photosynthesis
The job of photosynthesis is to take water, CO2 and light, and turn out carbohydrates to use as energy sources and building materials, with O2 as a leftover waste product. Although the total energy capture by photosynthesis outranks human power consumption by far, photosynthesis is less efficient than the photovoltaic process used in solar panels. Photosynthesis is surprisingly inefficient. The image above is based on analyses done by scientists looking for ways to improve crop yield. The 'black grass' post explained that only a portion of sunlight is used for photosynthesis, and the papers show that portion to be about half of the available energy. The graph above states the efficiency of each step, which is which fraction of energy gets passed on to the next step. The efficiency of the first step is 0.5: of 100% light to start with, 50% is left. That's a big loss.
The efficiency of the next step is 0.9. In terms of the original amount of light 45% goes on to the next step. And so it goes on, multiplying all the efficiency factors in turn, step by step, until only about 5% of the original energy is left at the end. As I said, not impressive at all. I should add that this holds for the so-called C3 photosynthesis type. The C4 type does better, managing to end up at 6 to 6.5%. That does not seem like a big improvement, but it is still up to 30% better than C3 photosynthesis.
One biochemical step deserves additional mention: 'photorespiration'. The reactions that take in H2O, CO2 and light to turn them into sugars and O2 are not exactly simple; an important enzyme capturing CO2 is ribulose-1,5-bisphosphate carboxylase oxygenase (no wonder that it is called 'Rubisco'). Rubisco deserves to be known, if only because it is probably the most common protein on Earth. Its job is to speed up the reaction binding CO2 that ultimately ends in O2. Oddly, Rubisco binds quite readily with O2, driving a process in the wrong direction! This backwards process is called 'photorespiration' and has puzzled biologists a lot. Its presence suggested that it might have some use, but apparently plants do quite well in artificial atmospheres without any O2 at all, so photorespiration seems to be a gigantic and puzzling waste.
2. Bright light: photosynthesis saturation
As if the above series of limitations is not enough, there is another one: photosynthesis saturates. Photosynthesis normally increases with the level of light but only up to a point. If light intensity increases beyond that point, photosynthesis cannot increase with it (it may apparently even decrease to protect the plant). Whether this is an important limitation depends on where you are: to catching the maximum amount of light to reach the Earth's surface, you will have to stand at the equator, at noon, on a clear day. The C3 type of photosynthesis can only use about a quarter of the light there! If you were to add that step to the image above, the scheme would start with a giant loss of 75% right at the start. Seen in that light (pun intended) the overall efficiency of 5% becomes an even less impressive 1.25%.
Then again, it is a bit unfair to set light at noon in the tropics on a cloudless day as the standard. Living at higher latitudes, clouds and shadows from mountains or leaves will limit the amount of light that reaches a plant, so in many cases the saturation point will never be reached. That is fine for those plants, but the tropics are still there, and photosynthesis could do a lot more for tropical plants if their saturation point would lie at a higher intensity.
3. Shadows: the photosynthesis compensation point
Plant cells burn molecules with the help of oxygen to free stored energy and use that for their metabolic needs, exactly like animal cells. This process is called cellular respiration and does the opposite of photosynthesis. As the amount of light decreases, photosynthesis will be less effective and produce less oxygen, while cellular respiration keeps using it a stable rate. At some shadowy light intensity the two processes are matched: the compensation point. When light levels drop beyond that point, plants become net users of oxygen and energy instead of producers. Plants can survive that state and in fact do so every night, but over time there must be a net profit. There are many places, such as the floor of dense forests, where it permanently too dark for photosynthesis to work.
"It's photosynthesis, Jim, but not photosynthesis as we know it".
With all this in mind there seems to be ample opportunity to tinker with the process and design an alien photosynthesis. Mind you, photosynthesis could well be even less efficient on an alien planet than on Earth, and that possibility should not be dismissed out of hand. World builders have a strong tendency to design super-organisms, better than what Earth has to offer, but that is not very realistic. For once I will follow the flow and aim to improve on Earth's state of affairs. The following list concerns my suggestions how to improve on off-the-shelf photosynthesis:
Alien photosynthetic to-do list
- Have your photosynthesis process use a larger portion of the light falling on it
- Increase its affinity for CO2 (abolish photorespiration!) and improve reaction speed
- Increase its saturation point so it can use intense light
- Lower the compensation point so it can work with less light.
This 'to-do list' assumes that there are numerous biochemical pathways that can take in CO2, H2O and light and produce carbohydrates. Such processes may be centred on completely different pigments, sensitive to other wavelengths.
|Click to enlarge; copyright University of Chicago. From: Nature's palette by David Lee|
You may well ask whether all this biochemical tinkering will make plants look different. If they still look like Earth plants but grow faster the exercise loses much of its appeal, doesn't it? I think they would look different: if leaves can use all light falling on them, that will have consequences for any leaves underneath; simple blobs or needles might replace complex leaves; the ability to have fewer leaves might induce trees to grow higher; plants might continue to grow through winter, etc., etc.
|Click to enlarge; by Ghedoghedo|