Green Skinned Animals and Other Unlikely Things

[mishalak]

Recently I finally got a clue as to why animals with chlorophyll in their skins to produce "free" energy and oxygen are unlikely. In theory it would work out just dandy to have sort of an onboard symbiotic relationship with some little chloroplasts or some simple algae as I've heard is the case with some sea organisms. Cyanophyta (Blue-Green Bacteria) in some sea squirts and the more common zooxanthellae (brown algae) in corrals.

Easy enough to extend that thought out to aliens of bioengineered cows, right? Well it turns out that chlorophyll is a pretty simple little molecule. In fact that's a good thing because the lower its molecular weight the less light it needs to get excited enough to do its clever little trick with water and carbon dioxide. But this also means that it doesn't have an excluder device to get rid of the oxygen and if it accidentally grabs an oxygen instead of a carbon dioxide it can run the process backwards. That's the job of the cell. Get rid of oxygen and let in carbon dioxide, but that right there should give a clue as to the problem.

Animals need oxygen and try to get rid of carbon dioxide as fast as possible. In order to have the two states in the same organism you'd need specialized skin cells that push the O2 out and get the CO2 right next door to cells that do the opposite. Possible but not as simple as just adding chloroplasts I would think. Especially since there is a bit of a bug where at warmer temperatures oxygen dissolves easier in water than carbon dioxide.

I'm not saying it's impossible. Just rather complex and so while in theory it all works out in practice it is probably a better engineering solution to have plants that do their thing and animals that do their thing. Unless they come from an environment where it is the only way to live like for corals.

Comments

[jinian.livejournal.com]

C4 plant respiration (http://en.wikipedia.org/wiki/C4_plants) might interest you. Plants using it separate respiration in just the way you're talking about. Of course, plants still need oxygen to run their cells too; the entire ATP manufacture from glucose is exactly the same as in animals et al. C4 and CAM respiration methods are only preferable to the usual in hot, dry conditions.

[mishalak.livejournal.com]

Yeah. Looking up stuff about the C4 channel is what got me started on this. Someone had told me it was more efficient and I learned that wasn't quite true. It just was more efficient under conditions of high temperature. Which since warm-blooded animals maintain at temperatures above 35C I would assume that it would be necessary there too.

Perhaps it photosynthetic skin would work out better for cold-blooded animals, but still I suspect there would be problems. Much easier to just eat a bit more I would think. Looking at the only animals that do have chlorophyll in them they are all sedentary. Which also makes sense. Why go anywhere if you get your food from the sun?

[scottscidmore.livejournal.com]

There is this group of critters

http://www.seaslugforum.net/display.cfm?id=391
http://www.seaslugforum.net/display.cfm?id=1969
http://www.plantphysiol.org/cgi/content/full/123/1/29

Note that while some species 'farm' algae, others actually extract the chloroplasts and keep them running in their own cells. This gives the animal the ability to control the chloroplast's O2 uptake.

C4 and CAM cycle is more a way of dealing with high water loss environments than with hot ones; it's hot and dry locales that favor C4/CAM. They are less energy efficient than C3, but most hot+dry climates have high light flux. (many CAM plants switch to C3 during the wet season, BTW)

A part of the problem is concentrating the CO2 at the locations where the chloroplasts are - the skin of the animal. This makes it most likely to work in animals that use skin respiration - small, simple animals living in moist environments or in water.

[mishalak.livejournal.com]

Well I would think that with animals they would need to seriously limit the chance of photorespiration to maximize benefit from the complex structures that would probably be required. More complex animals would have the advantage of a circulatory system so unlike plants they wouldn't need to just loose water by having open stoma/pours. But the simple animals living in moist environments or water certainly seems to be an accurate assessment of where it would work best. And lo, that's where they are found.

I should post on things like this more often. Lots of useful links, thanks.