About those ‘solar powered sea slugs’… a new study has given me reason to change their status from super photosynthesizer to plain heterotroph. It seemed too good to be true: sea slugs that can slurp on algae, digesting all but their chloroplasts which are retained in an active state inside the digestive tract conferring a photosynthetic lifestyle to the sea slug hosts. With such an elaborate mechanism for retaining these photosynthetically active chloroplasts, of course it must be to the nutritional advantage of the sea slugs. The early studies on the sea slugs suggested as much. The community of photosynthesis researchers, including myself, was taken with the idea of heterotrophs becoming photosynthetic. Not that we really wanted to convert the human race to autotrophy, but if some organism had figured out a way to enslave chloroplasts, then we should be researching those animals. Was it possible that slugs knew more than humans about the complexities of chloroplast exploitation?
Photosynthesis in plants and algae involves an elaborate exchange of proteins and signals between the chloroplasts and the cell nucleus for proper function. Hundreds of genes are involved and the system acutely senses the needs of the cell and the primary inputs of photosynthesis (light, water, carbon dioxide). Harnessing the chloroplast for the biochemical benefit of a cell is no small feat. Researchers have been looking for answers as to how the slugs might accomplish this. While there have been some instances of an algal gene present in the sea slugs, these are few and far between.
At this point, scientists had to reconsider the hypothesis that the stolen chloroplasts were conferring some kind of autotrophy to the slugs. A recent report in Proceedings of the Royal Society B addresses this assumption. Previous work had established that the presence of kleptoplasts offered an advantage to the slugs during starvation studies, but was it because they were photosynthetically active? Christa and co-authors were able to make more carefully-controlled weight measurements during starvation experiments of different kinds of kleptoplast-containing sea slugs. They compared weight loss of sea slugs starved (1) in the light, (2) in the dark, and (3) in the light in the presence of an inhibitor of photosynthesis. It turns out that there was no difference in the rate of weight loss among these conditions. This means that it doesn’t matter whether the chloroplasts are photosynthetically active or not. Consequently, the slugs are not gaining any autotrophic advantage, but are likely just holding onto them as a food reserve.
The sea slugs and the algal chloroplasts still have an odd relationship that begs for a deeper meaning. The overwhelming majority of the Earth’s biosphere depends on heterotrophs eating photosynthetic organisms, but I can’t think of another example of another heterotroph that saves organelles in specialized structures within its digestive tract (biochemically active or not). It seems like an over-engineered way to stash a midnight snack.
As an omnivore myself, I can’t be too condemning of these sea slugs for feasting on algae. However, given this new information, Elysia chlorotica is the herbivore equivalent of Buffalo Bill* saving parts of its victims as an ornate garment. This revelation definitely gives a creepy vibe to heterotrophy. Are the ‘kleptoplasts’ decorative? Maybe we are on the brink of photosynthetic fashion, remember these guys?
Why chloroplasts and why not some other organelle like mitochondria? Do the sea slugs use some elaborate form of communication based on chlorophyll fluorescence signals? Elysia chlorotica may not need the stolen chloroplasts to be photosynthetically active, but they still need something from them. Researchers have shown that the slugs cannot continue through development past a certain point without ingesting the algae and simultaneously stealing the chloroplasts. For their diet and development to be so tightly linked, there must be some intimate connection between these two species (or species and organelle) that scientists haven’t identified yet. There may be something special about chloroplasts,**but for Elysia chlorotica it isn’t photosynthesis.
This is one strange relationship that doesn’t fit nicely into our typical interspecies interaction categories. It’s not a symbiosis. I would call either organism a parasite. It’s more than an herbivore and its lunch. One thing that is safe to say about it, Elysia chlorotica and its stolen chloroplasts will keep researchers busy for quite some time into the future.
**Of course there’s something special about chloroplasts, haven’t you been reading the blog? But I guess Elysia and I will have to agree to disagree on the importance of photosynthesis.
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