I’ve mentioned before that studying photosynthesis is important because scientists would ultimately like to harness that power to do useful work for ourselves- for both increased food production and biofuels for the rest of our energy demands. Well, I am sorry to report that we have lost the photosynthesis-race… to a slug. A sea slug called Elysia chlorotica.
This animal can effectively steal photosynthesis from the algae (aka pond scum) it eats. You’re probably thinking, “Yeah, that’s what I did today when I ate a salad for lunch.” Well, you’d be wrong. This sea slug is able to suck the chloroplasts right out of its favorite meal (Vaucheria litorea, a filamentous algae) like a straw and retain them within its cells instead of digesting them. This allows the slugs to become photosynthetic. I’ll let that sink in a minute while you ponder how effective your digestive system really was with that salad you ate earlier. While you do that, check out this cool youtube video of a juvenile slug dining on algae.
You don’t really think that scientists were just going to observe that phenomenon and say, “Hmm, well that’s interesting to the point of impossible” and just leave it at that. No, they had to dig deeper to find out how this really happens.
I’m sure you read yesterday’s post on chloroplasts and are well-versed in the current state of our knowledge of them. Go ahead and click the link; I’ll wait until you get back.
Caught up? Great! Let’s continue…
I mentioned that within plant (and algal) cells, chloroplasts cannot stand alone. There is an intricate messaging system between the chloroplast and the nucleus that keeps photosynthesis going. If you want to put a number on how dependent a chloroplast is on the nucleus of a cell, it would be 90%. The nuclear genome of photosynthetic organisms provides 90% of the protein components in the chloroplast. So how can the sea slug possibly have useful chloroplasts?
Your first thought may be, “Those chloroplasts are just green decoration. They aren’t really useful.” Scientists thought of that too. As it turns out, the ingested chloroplasts of Elysia chlorotica are functional enough to provide the sugars necessary to sustain the slugs for up to 10 months without another food source. Its. Entire. Life. Span.That’s pretty amazing, especially given the fact that the photosynthetic apparatus within the chloroplasts of normal photosynthetic organisms requires upkeep on the minutes timescale.
Again I ask- how can these chloroplasts be functional? Is there something different about these algal chloroplasts that make them more independent from a nucleus? Or is there something special about Elysia chlorotica cells that make this feat possible? To address these questions, scientists decided to sequence the genomes of both the algal chloroplast and the slug’s nucleus. Enough is already known from other species about what genes are universally required for photosynthesis so that scientists can simply look for these in each of the sequenced genomes. They found that the algal chloroplast is nothing special; it still lacks many of the genes required for photosynthesis typical of other chloroplast genomes and is therefor utterly dependent on a nuclear genome with these missing genes. Surprisingly, researchers found these photosynthetic genes in the nuclear genome of the sea slug. So it seems that Elysia chlorotica is not only good at stealing chloroplasts, but also the genes it needs to maintain them. Both of these things are not occurring in the same step- Elysia chlorotica cells already have these genes waiting in their nuclei. At some point in the history of this relationship, the sea slug managed to incorporate these useful genes as part of their own genetic makeup.
Having photosynthetic genes in the nuclear genome of the sea slug definitely answers some questions about this strange lifestyle, but the mystery is far from solved. These stolen chloroplasts do not divide within the Elysia chorotica cells nor are they inherited by progeny slugs. Thus, individual sea slugs must acquire all of their chloroplasts from the algae they eat. There must be an elaborate system within the digestive tract of Elysia chlorotica to recognize chloroplasts, retain them, and ingest them into their cells while discarding all other algal cell contents.
We are still far away from understanding this process enough to steal photosynthesis for ourselves or other animals we care about.* If sea slug can manage it there is hope for us, but I don’t think humans will be dropping a trophic level any time soon. After all, I still like the taste of my salad (among many, many other things) very much.
*Wouldn’t it be nice to have a photosynthetic fish or dog? You wouldn’t have to feed them as often, only give them access to sunlight. It would make vacationing away from your pets much easier.
**Here is another youtube video talking about Elysia chorotica. Note there is a typo on one slide. Plasmid should be plastid (a general term for chloroplast or related organelle). Plasmid is a completely unrelated entity in molecular biology.
*** The scientist that knows most about the secrets of Elysia chlorotica endosymbiosis is Dr. Mary Rumpho Kennedy at the Univeristy of Maine. Check out this research spotlight.