Friday 8 May 2020

It's a plant! It's an animal! It's a bitroph!

Click to enlarge; Source: wikipedia

Several years ago, a species of sea slug had its day of fame on internet sites specialising in scientific news. Those sites all showed a bright green flattened blob. like the image above. This sea slug was green because it performed photosynthesis, which animals are generally not supposed to do.

I guess everyone interested in speculative biology sat up straight, because a lifeform that is part animal and part plant exudes ‘alienness’ through every pore. But was the flow of alienness coming out of those pores accompanied by oxygen, as in plants, or by carbon dioxide, something more befitting an animal? 

The slugs of the genus Elysia get their photosynthetic ability by feeding on algae. Algae, as the well-informed readers of this blog will know, perform photosynthesis in intracellular organelles called chloroplasts. The slugs eat the algae, but rather than simply digesting the chloroplasts too, they envelop then through phagocytosis, and keep them alive, in their own bodies. From then on the chloroplasts are called ‘kleptoplasts’, or ‘stolen plasts’.

It turns out that the photosynthetic slugs can live quite well in the dark, so they do not critically rely on photosynthesis. They do use photosynthesis as an auxiliary power source, mostly when they are starved anyway. When the slugs are kept in the dark AND starved, the number of kleptoplasts decreases, so the slugs then apparently disassemble the then useless chloroplasts and get a final energy boost from the hapless organelles (Cartaxana et al  2017).

Plant-animal combinations are not novel in speculative biology. Actually, there is a group of creatures  on Furaha called, for the time being, ‘mixomorphs’. They probably share characteristics with plants as well as with animals. The ‘probably’ is in there because I always had the uneasy feeling that a plant-animal combination might not work. After all, Earth is not filled with such creatures, doing whatever it is ‘plantanimals’ do when they are not just sitting in the sun. Does their absence mean that they do not make sense?

The concept of animals performing their own photosynthesis certainly sounds like a good idea. Earth plants take in carbon dioxide (CO2), water (H2O) and sunlight and turn them into carbohydrates. Because they turn nonbiological material into carbohydrates, they are called ‘autotroph’. Animals cannot do that and require some ready-made carbohydrates as a source of carbon, making them ‘heterotroph’. By breaking up those carbohydrates animals get materials for their own bodies, producing H2O, CO2 and energy. An animal is a plant in metabolic reverse, in a way.

Why not do what the slug does and cut out the middle man? This plant-animal chimaera could use photosynthesis as an auxiliary and cheap way to store free energy in carbohydrates, giving it an edge over animals that have to hunt, chew and digest to get any carbohydrates. They would even have an edge over plants in that a major problem with photosynthesis for plants is that there is so little CO2 in the air. The animal part of a chimaera would produce more then enough CO2 to boost photosynthesis of the plant part.

Click to enlarge; source: wikipedia

Autotroph + heterotroph = bitroph
There is a nice scheme on Wikipedia explaining the full nomenclature of how lifeforms get energy and carbohydrates. There are three big two-by-two divisions, shown above. These result in six fragments of phrases: hetero- vs. auto-, chemo- vs. photo-, and organo- vs. litho-. There are eight possible combinations. Our garden-variety plants (sorry for that pun...) are ‘photo-litho-auto-troph’, while ordinary animals are ‘chemo-organo-hetero-troph’.

This nice scheme seems to cover all the possibilities, creating a challenge for speculative biology lovers: where should we classify animals that can photosynthesise? Note that there already are lifeforms that cannot build their own carbohydrates and yet use photosynthesis: photo-litho- and photo-organo-heterotrophs. However, they are all bacteria, and to increase the ‘alienness’ level we want creatures we can see without a microscope, and that we can stroke, or supply with compost. Or both. Also, as these creatures would run both energy pathways, they do not fit in the scheme. They might be labelled ‘autoheterotroph’; I can't say I much like the term ‘plantanimal’. Let’s introduce ‘bitroph’ to emphasize the dual energy principle (without also adding 'photo-organo-litho-chemo-').            

Bitrophy in practice

'Bitrophism' needs consideration of energy requirements. The first question is how much energy you get from a leaf, or a standardised area performing photosynthesis.  Luckily, that information was already available on my bookshelf, in ‘Energy for animal life’ by the late R. McNeill Alexander (if you want to give your speculative biology a scientific edge, get his books). 
In bright sunlight the flux of light on the surface of the Earths is about 1000 Watt per square meter, and with that light intensity the rate of photosynthesis reaches a maximum of 21 Watt per square meter. This ratio of 21 to 1000 shows, again, how inefficient photosynthesis is. Mind you, this light flux is the maximum value in Alexander's biome, which was England. Just outside the atmosphere you get 1370 Watt per square meter. Obviously, seasons, clouds, latitude, and the time of day all influence the amount of sunlight the surface actually gets. For now, let’s go with that value of 21 Watt per square meter.

The next question is how much energy an animal actually needs. That also depends on many things, such as its activity, but it's minimum level is largely fixed: the ‘minimal metabolic rate’ describes the energy requirement of an animal doing nothing, except being alive. This rate depends on two factors.

The first is the type of animal: warm-blooded animals such as birds and mammals burn energy at much higher rates than other groups, such as lizards, fishes, etc. For two animals that have the same mass, a mammal uses almost 5 times the energy of a lizard (even one warmed up to 37 °C), and 12 times the energy of a crustacean at 20 °C.

The second factor is mass: a 100 kg animal will use more energy than a 10 kg one. However, it needs less than 10 times as much. As Alexander remarked: ”Weight for weight, it is a great deal cheaper to feed elephants than mice.”  The relationship between minimal metabolic rate (MMR) is an exponential one, and has the form

MMR = a (body mass) ^ b

(formatting is difficult here; the '^b' part means 'to the power of b'

The exponent ‘b’ differs somewhat between animal groups, but lies close to 0.75. The fact that it is less than 1 explains why large animals have a lower metabolic rate per kg than small ones. The factor ‘a’ is the one that differs between animal groups (it is 3.3. for mammals, 0.68 for warm lizards, and 027 for crustaceans.

Click to enlarge; copyright Gert van Dijk
The image above provides the Minimal Metabolic Rate the rate for mammals, (warm) lizards and crustaceans, all ranging from 0.1 to 1 kg. The crustaceans burn the least energy, and bigger animals need more energy than small ones.

But we wanted to get to photosynthesis; remember that one square meter of photosynthetic area provides 21 Watts, so I provided an additional y-axis on the right, which is simply the left y-axis divided by 21. The right one tells you how many square meters of photosynthetic area we need for each point on the graph. A 1 kg mammal will need about 0.16 square meters of ‘leaf’. That corresponds to a square with sides of 40 cm. Examples of 1 kg mammals are seven-banded armadillos, muskrat, pine martens, platypuses, meerkats and European hedgehogs. Just picture one of those them with a 40 cm by 40 cm parasol to catch sunlight. A large fruit-eating bat may also have a mass of 1 kg; it needs a large wing area anyway; hmmm...

Anyway, as I found it difficult to imagine how large that actually is, I assembled a mock animal with a mass of 1 kg (the volume can be calculated because the animal consists of spheres and cylinders; its density is 1.05). I used mammal characteristics to calculate the disc it needs to provide the energy for its MMR.

Click to enlarge; copyright Gert van Dijk

The image above shows such a 'Disneius solamor'. The small squares on the ground are 1x1 cm, and the larger ones 5x5 cm. The animal is 21 cm long, and the radius of its dark green 'sun disc' ('antenna'? 'leaf'?) is 22 cm. It needs that to power its MMR. A general human provides additional scale. Hm; the animal does not look very elegant, and that large 'leaf' looks rather vulnerable.

But we are not done yet. The calculations so far used maximum light settings, which is not realistic. And how about the effect of mass? How about animals that are thriftier with energy than mammals? How about more efficient photosynthesis? I suspect that this post may already have passed the 'maximum allowed complexity per unit of enjoyment ratio' (MACPUOER), so I will stop here. But I will very likely return to this theme.

PS. Although I welcome the large number of questions the blog has recently received, many had nothing to do with the post under which they were asked, and many could easily have been answered by using the blog's search options. So from now on I will be less likely to answer such questions.  Surely you would prefer me to spend my time working on The Book or on writing posts?