Sea slugs photosynthesise

We all know we should eat our greens, but for photosynthetic sea slugs, this takes on a whole other meaning. These small marine creatures feed on algae, and steal the photosynthetic chloroplasts in the algae as they digest, incorporating the chloroplasts into specialized pockets of their digestive system where they can be kept, still photosynthesising, for up to a year in some species. Vesa Havurinne from the University of Turku, has recently found that the sea slugs induce protective changes in the chloroplasts they steal and that this allows the chloroplasts to keep photosynthesising outside of the algae. Eva Higginbotham heard how…

Vesa - The term sea slug - it refers to a large group of animals. They are gastropods just like your regular garden snails and slugs. And they can look quite alien to be frank. But when you're talking about photosynthetic sea slugs, you are referring to a very specific group of slugs and they are called sap-sucking sea slugs, or sacoglossans.

Eva - And you've actually sent me a couple of pictures of some sea slugs. So I'm just looking at them here and they're quite cute!

Vesa - Well, if you're into that sort of thing, but yeah!

Eva - And so they've sort of got a white, or sort of opaque-ish, white-looking body with some parts of them that are green. Where does the green color come from?

Vesa - Yeah, the green color comes from the chloroplasts that they steal from the algae they feed on.

Eva - So what makes the sea slugs photosynthetic?

Vesa - Well, they steal the chloroplasts, which are the organelles inside plant cells or algae cells that utilise light energy to convert it to sugars by fixing CO2 from the atmosphere or the surroundings. So basically these slugs, they are taking that organelle from the algae that they eat. And then they take those foreign organelles chloroplasts inside their own animal cells. And then they basically carry out photosynthesis, just like the algae.

Eva - Why do they do that?

Vesa - Well, usually when people are answering this question, they are stating the obvious - like that the slugs would get a energetic benefit, but it's still debated.

Eva - So what were you trying to find out in this study?

Vesa - We were trying to figure out if the slugs induce some protective changes to the chloroplasts they steal from the algae. The thing is that when the chloroplasts are inside the slug cells, they are actually quite isolated. They are, for example, cut off from most of these repair mechanisms that are available to them in their normal environment, meaning the algae cell. So because the photosynthetic machinery of the chloroplasts is actually constantly being damaged by light, even though light is required for photosynthesis basically. So we were trying to figure out if they somehow reduce the damage to the chloroplast in the first place, so that they wouldn't have to fix so much damage.

Eva - And what did you find?

Vesa - We compared photosynthesis in the algae versus the slugs. And we found out that the slugs do actually induce photoprotective changes to the chloroplast. The first thing that we noticed was that the slugs actually maintain the photosynthetic machinery in a so-called 'emptier state', electron wise, than the algae. And this allows for more fluent conversion of light energy into sugars and reduces the risk of this excess energy going into unwanted directions, for example, to oxygen, which can create reactive oxygen species and damage the chloroplasts and the slug for that matter. The second thing that we found was that the slugs actually have a more efficient way of preventing excess light energy actually reaching the photosynthetic machinery than the algae. And the third point was that if all these other measures fail, the slugs can still utilise the same safe energy sinks that the algae actually use. So when they are exposed to a very strong light, for example, they can still deal with that excess energy safely without producing reactive oxygen species,

Eva - Are photosynthetic sea slugs the only sort of animals that eat plants and steal their chloroplasts to use in photosynthesis in this way, or is this a mechanism that other species have too?

Vesa - For a long time it was thought that these sap-sucking sea slugs were the only animals capable of this, but there was a recent paper where they showed that actually there are these marine flatworms that are able to do the same trick. And it would be interesting to get my hands on those as well, to see if there are some common characteristics within these two different animal groups. And if so, if there are some general rules for these like stealing chloroplasts and using them for your own gains.