One more thing about those solar-powered sea slugs (aka heterotroph in autotroph’s clothing)…

OK, well really this post is about the algae they eat. I’ve dedicated enough space to an animal on this blog. Regardless of whether or not (and it’s decidedly not) the slugs need the chloroplasts to be photosynthetically active, the kleptoplasts are remarkably stable. At least as far as the light reactions go based on long term chlorophyll fluorescence measurements, they are capable of photosynthetic electron transfer after months of residing within the digestive tract of the slug.

Genome Biol Evol 20132013  5(12) 2540-8, Fig. 1._

How remarkable is that? Remember last time I mentioned that it took hundreds of genes coming from the nuclear genome of a photosynthetic eukaryote to keep their chloroplasts functional? It’s no small feat.

Remember how I told you that photosynthesis researchers had to be fast and cold (and often dark) when it comes to biochemical preparations of chloroplasts and thylakoid membranes? I’ll tell you isolated chloroplasts kept on ice in the dark won’t remain that active for a week much less at ambient ocean temperatures.

I’m sure the slugs still have some yet-to-be-discovered secrets of chloroplast maintenance, but the first one is knowing where to steal their chloroplasts. Acetabularia acetabulum is the food of choice of the chloroplast-thieving sacoglossan slug Elysia timida. Researchers sequenced the chloroplast genome of Acetabularia acetabulum and identified clues as to the longevity of the stolen chloroplasts- the ftsH and tufA genes.

Here’s why that’s important: As far as stability goes, the weak link in the photosynthetic electron transfer chain is the Photosystem II (PSII) enzyme. As part of its normal activity, the core D1 subunit is irreversibly damaged. The damaged protein must be removed and replaced with a newly synthesized version. In photosynthetic organisms, an elaborate PSII damage-repair cycle keeps the light reactions functional. One of the auxiliary factors required for this process is FtsH, a protease involved in the removal of the damaged D1 protein. Normally, the ftsH gene is encoded within the nuclear genome of photosynthetic eukaryotes. Translation elongation factor Tu (encoded by tufA) is essential for the translation of chloroplast proteins. Since the PSII damage-repair cycle and the longevity of chloroplasts require a significant amount of new protein synthesis, the translation elongation factor Tu would be limiting over a period of months if the chloroplasts couldn’t make this protein on their own.

In the case of Acetabularia acetabulum, ftsH and tufA are encoded within the chloroplast genome. Similar trends were found in aquatic algae that were food sources for the sacoglossan slugs. This means the chloroplasts can make this critical factor for themselves, giving them a higher level of autonomy for maintaining their photosynthetic machinery. Elysia timida wouldn’t be so lucky if it tried stealing chloroplasts from the alga Chlamydomonas. Likewise for any plant chloroplast.

Genome Biol Evol 20132013  5(12) 2540-8, Fig. 3._

Additional experiments are necessary to directly evaluate the effect these chloroplast genes have on the long term retention of active kleptoplasts in the sea slugs. I’m sure there’s more to the story of how the stolen chloroplasts are maintained, but the ability to independently make these two essential proteins is a good start.

Johnna

Reference (open access)

http://gbe.oxfordjournals.org/content/5/12/2540.full

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4 thoughts on “One more thing about those solar-powered sea slugs (aka heterotroph in autotroph’s clothing)…

  1. galicolagfb

    That’s very interesting!
    Do you know if anyone tried to feed the slugs with other algae that don’t encode these genes in their chloroplasts and A) see if the slugs actually eat it and B) if they can retain kleptoplasts from these?

    Also, did anyone try to maintain chloroplasts from Acetabularia acetabulum in vitro and see if this makes a difference?

    Reply
    1. johnnaroose Post author

      I don’t think researchers have disentangled these related questions yet. Do the slugs have to feed on a particular algae to obtain some metabolite critical for development or can it come from one of several algae? (only a hypothesis at the moment and mystery metabolites are unknown) Will they eat and can they retain the chloroplasts from other algae? (Not sure if the slugs are very picky eaters) The in vitro question about Acetabularia acetabulum is an interesting question because it would tease apart how much of the long term retention is due to inherent properties of the chloroplasts vs. special adaptations of the sea slugs’ digestive tracts. I will ask the Gould lab if they are working on it since he sent me the links to this recent paper.

      Reply
    2. johnnaroose Post author

      OK, Sven Gould has been gracious enough to answer some of these questions…

      In the laboratory, will the sea slugs eat any kind of algae you feed them? I know there is some specificity, but how specific is it? If they at least need one type of food source, can you supplement with another strain by offering two types of algae (say, their favorite and something like Chlamydomonas)? If they will eat other algae, do they retain those chloroplasts?

      No. Although different slugs feed on different algae they have pretty much specialised in a few. Some can feed on only ONE species, and those are also often those that can survive starvation the longest. If you offer them something those slugs something else, they simply starve. Their “oral apparatus“ has specialised and can tap only a specific type of algae.

      How stable are the Acetabularia acetabulum chloroplasts in vitro (under say, biochemist ice bucket conditions and/or buffer and temperature conditions similar to the slug’s digestive tract)? There was a question about how much of the long term retention is coming from the slug side vs. the chloroplast side.

      Not as stable as in the slugs, but yes, this is a good point. It has been shown that those plastids survive and remain active for about 2 weeks outside the algae in an appropriate buffer (or even eggs they were injected into), so much longer than their land plant “relatives“. So the slugs contribute the right environment for sure, but the kleptoplasts already bring along the crucial tools to take care of themselves.

      Reply
      1. galicolagfb

        Thank you and Prof. Gould for the elaborate answers.
        Its really interesting.

        I wonder if these chloroplasts would be good candidates for the development of biochemical photo-voltaic cells (I remember a lecture by an Israeli researcher that is trying to develop photosynthetic batteries with in vitro PS II).

        I wonder what is the evolutionary advantage for the algae.

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