Photosynthetic Evolution: how 2 organisms gained or lost the ability to eat sunshine

9/13/2010 Update

New research has come out that changes the story told below. Wägele et al. sequenced cDNA transcripts from RNA produced by slugs dependent on their plastids alone, and did not find the RNA to produce the proteins for plastid use in any meaningful quantity. But the slugs aren’t just using the proteins they got upon original ingestion; we simply do not know what is happening.  For a thorough write-up, I highly recommend this post from The Spandrel Shop entitled “Solar Sea Slugs: part plant, part animal… or not?”

Original post (01/20/2010):

The variety of protist life -- from Finlay, B.J. & Esteban, G.F. Protist, 149, 155-165 (via the British Society of Protist Biology).

Biologists and taxonomists love to put organisms into categories to help us organize the complicated living world.  I grew up on the 5 kingdom system of classification: plants, animals, fungi, bacteria and protists. The first four categories seemed simple enough, but the term “protists” always confused me.  This kingdom seemed to be a dumping ground for all the single-celled organisms that we didn’t know what to do with, ones that had evolved so far from their ancestors that their origins were unknown.

I’ve stumbled upon two fascinating articles about such animals that are out of place.  The first is about microorganisms that were once photosynthetic — and thus evolved with the cyanobacteria and plants — but no longer go through photosynthesis.  The second is about a sea slug that has developed the ability to photosynthesize, or harvest energy from the sun.  Imagine stumbling upon these animals for the first time.  The former would be placed in the animal kingdom due to its heterotrophy, or tendency to get its nutrients from food.  The latter would be baffling: is it some freak highly-organized plant, or an anomalous, energy-producing animal?  Thanks to our increasingly understanding of evolution, scientists have been able to figure out where these strange creatures came from.

The first story is about evolution driven by competition for food between many species of microorganisms.  In terms of energy acquisition, I usually think about two categories of organisms: the heterotrophs, which get their energy from eating, and the autotrophs, which create their own energy from outside inputs such as the sun.  But there are also the mixotrophs, which are able to do both.  To be a mixotroph!  How wonderful would it be to get energy from standing under the sun if you wanted, but you could also be able to eat a hearty meal for the same gain!  On first thought, this appears to be the best life strategy, allowing an organism to get energy whatever way is easiest at the time.

Scientists from the University of Potsdam in Germany and the Austrian Academy of Scientists propose that, in some circumstances, having both strategies may be too much (open access paper here).  In order to be a mixotroph, the organism needs to fit all the machinery required for both processes inside its single cell, increasing its size.  A bigger mixotrophic cell not only loses access to smaller food items, such as ultramicobacteria, but also has to compete with larger heterotrophic organisms, which are solely dedicated to eating and thus can do so more efficiently.  These scientists created a model showing how, under low-light conditions, it would be energenically beneficial for mixotrophs to be rid of their chloroplasts and other organelles needed for photosynthesis so that they could become physically smaller and have access to the smaller foods out of the range of other heterotrophs.  Thus, we have animal-like organisms that evolved from plants.

The second story is about a sea slug, Elysia chlorotica, which has gained the ability to photosynthesize.  It did not evolve this trait in the traditional sense, but rather picked it up from another organism.  The slug’s green color is not self-made, but is present due to its collection of chloroplasts, the photosynthetic center of a cell, from its prey.  Due to an unknown mechanism, the slug is able to hoard only the chloroplasts of its algal food source Vaucheria litoria.  Not only that, but it uses these chloroplasts to go through photosynthesis itself, which it can continue to do 5 months after it last ate V. litoria.  (And this is a slug that only lives for 10 months total.)

This is not as simple as it sounds, however.  You need more than chloroplasts to photosynthesize; you also need genes to encode all the specialized proteins needed to make sunlight into energy!  The big question regarding these slugs was: where did they get these genes?  Scientists working together from Maine, Korea, Iowa and Texas (paper here) compared sections of the nuclear DNA between the slug and its algal food and found identical segments, suggesting that the slug had not evolved these genes on their own, but had acquired them through horizontal gene transfer, or a transfer of DNA from an origin other than one’s own parent.  In this case, they suggest, a segment of the algae’s DNA broke off and joined the slug’s own DNA, an incredibly rare event.  This gene acquisition was so beneficial that it spread through the population, causing E. chlorotica all over the oceans to hoard the chloroplasts of their prey.  And there ya have it, folks: a photosynthetic animal.

Ain’t this a wonderful world we live in?

Learning about the histories of these organisms makes me drool, thinking about the uncategorized protists out there.  What kind of stories do they have to tell?

de Castro, F., Gaedke, U., & Boenigk, J. (2009). Reverse Evolution: Driving Forces Behind the Loss of Acquired Photosynthetic Traits PLoS ONE, 4 (12) DOI: 10.1371/journal.pone.0008465

Rumpho, M., Worful, J., Lee, J., Kannan, K., Tyler, M., Bhattacharya, D., Moustafa, A., & Manhart, J. (2008). Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica Proceedings of the National Academy of Sciences, 105 (46), 17867-17871 DOI: 10.1073/pnas.0804968105