Sunday, 14 July 2013

Fiddling with photosynthesis (Alien Plants IV)

'Alien Plants IV'? Where are the other 'alien plant' posts? Well, 'Alien plants I' and 'II' were published a long time ago, and 'Alien Plants III' was not labelled as such: that would be the post 'The black, black grass of home...' posted one year ago. That one was more serious than the first two, and  dealt with the colour of plants on Earth. To be succinct: green does not equal photosynthesis.

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
The illustration above has nothing to do with photosynthesis itself, but illustrates that there are many pigments in vision that are sensitive to varying wavelengths and to varying ranges of wavelengths. The pigment of the nectar-varying bat is interesting in that it is sensitive to a very broad range of light with a broad peak in the green area. A pigment like that, used for photosynthesis, would result in plants using light best where there is most of it, without throwing the rest away. Such plants would probably be a boring dark purplish grey.   

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
Of course, apart from biochemistry different biomechanical design principles will also result in differently looking plants. To see whether that approach yields interesting choices, we may need to travel back to the Silurian and Devonian and have a look at designs principles that came into being when land plants first struggled against gravity. Changing designs and changing plant biochemistry ought to result in enough 'alienosity' to please anyone. We'll see...


Christmas Snow said...

I wonder if the relatively inefficient photosynthesis on Earth is due to type of photosynthesis, or lack of enough carbon-dioxide. After all, the plants cannot store light for use when enough CO2 is captured.
An experiment I saw a few years ago clearly shows that greenhouses enriched with CO2 allowed plants to grow faster, even with the same amount of light.

Jan said...

I have found some pretty fictional plants here
And I hope to see more about Furaha and Nereus plants

Ronan said...

Christmas Snow: Increasing the CO2:O2 ratio increases the efficiency of RuBisCO/decreases the incidence of photorespiration, so in that sense you're spot on. Photorespiration is only part of the efficiency problem, though, as witness the second diagram in this post.

...And Jan, my goodness, those are beautiful. I had no idea anyone had devoted that much thought and attention to xenoflora; that puts my own efforts (melanin photosynthesis using ionizing radiation, large scale plant growth based on budding and spreading à la Pando) to utter shame.

Ronan said...

It occurs to me (inspired by one of the earlier posts here regarding the pressures driving erect vs. sprawled limb postures, and the effect of gravity and air pressure on both), that while alien plants are often given bulky, bulbous shapes--which, given that many artists depicting such plants hail from temperate areas with spindly plants, makes sense--plants growing on a world with either low gravity or low winds/air pressure might evolve bizarre, elongated forms, suspending gigantic leaves at the tops of huge, spindly stems. Many speculative plants are crafted to look alien by making them look bulky and bulbous, like animals--but suppose one made them look alien by emphasizing the traits that already differentiate plants from animals, rather than trying to erase those traits?

Sigmund Nastrazzurro said...

Christmas Snow: CO2 is indeed a limiting factor as Ronan also mentioed, but not the only one. By the way, in view of global warming it would be nice if vegetation did more to soak up the additional CO2 that humanity has been pumping into the atmosphere. It may be limiting the increase a bit, but not by much.

Jan: thanks for the link; they are indeed nice. You may see more Furahan plants in the future; as for Nereus, that's up to Evan.

Ronan: I agree. I too have designed quite a few fat plants in the past out of a wish to make them alien. On Earth, plants with fat stems usually use them to store water. As you said, mechanical needs should play a role, and gravity and wind stresses are indeed major influences. It would be interesting to compare wind resistance (springiness?) with weight loading (gravity) in a 2x2 table: the low-gravity fast wind plants would be intriguing. I was playing with the idea of devoting a future 'plant post' to just such issues.

Jan said...

It would be interesting to see some inspirations from the constructons of scyscrapers
Although the circulation of the fluids is probably more important for the possibities of plant structure

Michel Van said...

what about a evolutionary solution ?

Plants start as algae in earth ocean, first purple and red , then blue green and finally green algae.
the Algae used the light they could receive underwater, filtered sunlight !

Jan, big THX for the link

here links to PDF on Color of alien plans by Nancy Y Klang.

Sigmund Nastrazzurro said...

Jan: Thank you. The idea of multiple tubes ties together has also been used in submarines for much the same reasons: a submarine skin buckling is probably similar to withstanding lateral stresses in a building. Some of my ideas for different tree biomechanics boil down to multiple stems (see the main Furaha site).

Michel Van: could you explain the order some order? But the effect of filtering by water is in fact considered as a factor in why earth plants are green: they evolved for green algae in water.
Thanks very much for the Kiang papers. I found the first one very illuminating, where she discusses how the actual colour of Earth plants may not only be an inefficient evolutionary leftover, but may have some features that are well balanced. But she also stresses the weaknesses of Rubisco and states that some green light on Earth is wasted (p.244).

Michel Van said...

Sigmund Nastrazzurro
>Michel Van: could you explain the order some order?

That i got from a article form Scientific America from 2008
"The Color of Plants on other worlds"
with first purple and red , then blue green and finally green algae.

is that not correct ?

Petr said...

another awesome post!

Indeed I find it very hard to come up with aline-looking plants...