Algae Taking Over the World, at a Snail's Pace

A marine slug is now doing what algae have been doing for millions of years, using the sun's energy to turn carbon dioxide into sugar - i.e. photosynthesis!

There are plenty of examples of what we call symbiosis, where algal cells live within the body of some animal (e.g. a coral) but in this case it seems the chloroplasts - the bits that do the trapping of light and the follow-up chemical reactions - are not only living happily inside the slug cells but the slug's genes are (apparently) producing raw materials for them.

I'm indebted to Darren Brown's tweets for alerting me to this story, and you can read the full posting at Wired Science.

The slug is called Elysia chlorotica. Other slugs also carry around algal chloroplasts but this seems to be the only species that holds them in working order for its entire life, about a year.

Excitingly for me, the algae it eats is called Vaucheria. This is the algae I spend four years studying for my PhD. There are about 20 or so species in Australia and they grow just about everywhere - on garden soil, in lakes and streams, in saltmarsh and sometimes, intertidally. Although pretty dull in the field (a green felty mat) they are quite pretty under the microscope.

But back to the green slug... When young it eats lots of my once favourite alga, extracting the chloroplasts from the long tube that is Vaucheria, and then doesn't have to eat again for the rest of its life! As they say in the media release, then 'all it has to do is sunbathe'.

The really exciting, and slightly controversial, bit is that chloroplasts need a supply of chlorophyll and other chemicals. In the algal cell the nucleus generates these compounds for the chloroplasts. The question here is whether the chloroplasts in the slug keep running on stored compounds or have to generate their own.

Sidney Pierce from the University of South Florida, and his colleagues found some genes in the slug's own DNA that seem to be related to photosynthesis. These were presumably 'lifted' from the algal partner. (As an aside, this a Genetically Modified Organism - GMO - being created 'naturally'. Of course genetic modification goes on all the time but between animals and plants it is perhaps less common.)

Using radioactive markers, Pierce and friends showed that the sunbaking slug produced chorophyll from these genes and his results have just been published the results in a journal called Symbiosis.

Apparently there was some scepticism at the meeting where the results were presented, with concerns about possible algal contamination in the experiements. Still, the possiblitiy of genes between animals and plants being mixed was 'intriguing' and 'could certainly complicate tracing out evolutionary history'.

Images: At the top, a picture of the slug from the Wired Science page, and at the end, a close up of one of 'my' Vaucherias.