It's quite easy working out what we consume in a day – measuring what goes in is straightforward. But it's a different matter when your favourite food source is packed away inside you. Keeping track of the metabolic trade between symbionts seems virtually impossible. Lynne Whitehead explains that the nutritional traffic in an intact symbioses had only been successfully traced in one symbiotic partnership, and that most metabolite identification was done on symbionts that had been separated. But Whitehead realised that she might be able to untangle a symbioses' complex food web if she could make the donor symbiont dormant, and then feed the host a simulated cocktail of the symbiont's products to see if it satisfied the recipient's metabolic requirements (p.3149). Unsure whether her unconventional approach would reveal anything about the symbioses'nutritional traffic, she decided to `just try, things might work'.
First Whitehead needed a symbiotic tenant that she could shut down as simply as throwing a switch; a photosynthetic tenant. Whitehead could control the symbiont's donations to its host simply by switching off the lights. Knowing that the small green sea anemone, Anemonia viridis, harboured the photosynthetic dinoflagellate, Symbiodinium, Whitehead left her lab in York, and travelled across the UK to a cold beach in North Wales. Up to her knees in the icy water, Whitehead collected the small green anemones from their tide pool homes, ready to see whether the animals could thrive on a nutrient cocktail when it's dinoflagellate lodger was dormant.
Back in the lab, Whithead provided the intact symbioses with radiolabelled bicarbonate and allowed the dinoflagellates to photosynthesise for a few minutes before separating the symbioses' metabolic products with ion exchange resin and thin layer chromatography. But the amount of each radioactive metabolite produced by the anemone was vanishingly small; Whitehead had a tantalising 6-week wait while she exposed the chromatography plate to film before she could see the metabolite pattern produced by the micro-organism and its host.
But would the symbioses produce the same distribution of metabolites when the dinoflagellates were on standby? Instead of dimming the lights, Whitehead used dichlorophenyl dimethyl urea to inhibit the dinflagellates' photoreactive cascade, then she exposed the anemones to the radiolabeled sugars and organic acids that she suspected were released by the micro-organisms. After another nailbiting wait, Whitehead compared the metabolite patterns produced by the inhibited symbioses with the photosynthesising symbioses. They were essentially identical! The anemones that had been supplied with glucose, and fumarate or succinate, had produced a realistic metabolite pattern.
Knowing that several microbial symbionts seemed to produce high levels of glycerol when separated from their host, Whitehead also tried supplementing the half active symbioses with glycerol. But instead of producing a natural pattern of metabolites, she realised that these anemones didn't consume the sugar. She suspects that glycerol release from the microbes could turn out to be an artefact of separation, rather than an essential nutrient for some symbioses. Amazed that the long-shot had worked, Whitehead is optimistic that it might be possible to begin identifying who's supplying who, and with what,by shutting off other microbe's energy supplies in intact symbioses. But for the time being, she's sticking with the little green anemone, and its tidal tastes.