Kathryn Knight

Organisms that have struck up symbiotic relationships have found a wonderful quid pro quo lifestyle solution. For example, animal hosts provide essential nutrients and a safe refuge to their algal lodgers, while the algae provide a plentiful supply of energy harnessed by photosynthesis. ‘Most earlier studies had pointed to glycerol as being the primary photosynthetic metabolite [energy source] transferred from dinoflagellate algae to their cnidarian animal hosts’, explains John Pringle from Stanford University. However, other studies had suggested that symbionts may supply their hosts with alternative materials, such as photosynthetic glucose. Pringle, a yeast geneticist by trade, says that he was attracted to the problem because of his fascination with coral reefs. However, he recalls that when he began working on symbiosis he had no preconceptions about which materials the symbionts might deliver to their hosts. ‘When you change fields you look at everything with a sceptical eye’, he remembers. Teaming up with graduate student Matthew Burriesci, Pringle wanted to tackle the question of material transfer from a fresh – and softer – perspective. He decided to find out which materials the symbiotic anemone Aiptasia receives from its algal occupants (p. 3467).

According to Pringle, symbiotic algae are remarkably robust, surviving separation from their host, and in the past, many researchers had measured the materials released by the isolated algae. Instead, Burriesci and Pringle supplied the intact anemone with heavy carbon dioxide – where the 12C atoms were replaced with 13C atoms – and allowed the algae, secure inside their symbiotic hosts, to photosynthesise for a day before separating the two. Then Burriesci and Theodore Raab analysed which materials in the host tissue had acquired the heavy carbon marker by isolating individual compounds with gas chromatography and identifying them by mass spectrometry.

‘Glycerol does eventually appear in the host's body along with succinate, fumarate, various amino acids and other compounds’, recalls Pringle, adding that most host materials become labelled eventually if left for long enough. However, in order to find out which metabolic compound was being delivered directly to the host by its residents, Burriesci and Pringle realised that they would have to separate the symbiotic partners more quickly.

Burriesci went off and designed and built a bespoke filter holder; ‘He did all the machining himself. I didn't know about it until after it was done,’ recalls Pringle, adding, ‘Then Matt could take an anemone sitting in a tank that had been provided with 13CO2, disrupt it, separate the mixture into an algal fraction and an animal cytoplasm fraction and then freeze them in liquid nitrogen, all within about two minutes’. Under the right conditions, this would allow the team to capture the first carbon compounds that the algae delivered to its anemone host after exposure to the heavy carbon dioxide.

Switching the lights on for a few hours – to make sure that the algae were happily photosynthesising – before supplying the anemone with the heavy carbon dioxide, Burriesci swiftly separated the algae from the anemone 2–10 min later. Then he began analysing the animal tissue for any evidence of transfer of the heavy carbon from the photosynthesising algae to the anemone.

‘The results were, to my eye, remarkably clear cut’, recalls Pringle. Mass spectral analysis showed that the only compound in the anemone tissue carrying the 13C signature was glucose. Instead of delivering glycerol to its host, the algae were supplying it with glucose. However, Pringle is quick to point out that this does not necessarily mean that all symbiotic algae provide glucose to their hosts, adding that they could deliver glycerol or other compounds under different circumstances.