An intriguing new study by Bachar et al., published in the latest Journal of Experimental Marine Biology and Ecology (JEMBE) shows that carbon derived from different metabolic pathways is used differently by anemones with algal symbionts. It has been well-known for a long time that some anemones, such as the Aiptasia sp. used in their study, form a symbiotic relationship with zooxanthellae (called autotrophy). For a while it was assumed that the anemones persisted by mostly utilizing carbon translocated from its symbionts, but they can also supplement this by heterotrophic feeding on plankton. Bachar and colleagues studied the fate of carbon derived from autotrophy versus the fate of carbon derived from heterotrophy using radiolabeled carbon sources.
To trace the fate of autotrophic carbon the standard is to radiolabel carbon dioxide (14C via sodium carbonate) since this is the carbon substrate of photosynthesis. To trace the fate of heterotrophic carbon they grew brine shrimp (Artemia sp.) fed on algae incubated with the same radiolabeled carbon dioxide source. This what is called a pulse-chase experiment in symbiosis lingo. The idea is that you pulse the anemone with labeled seawater for a certain amount of time, for instance 12 hours in the light and 12 in dark in this study. Then, there is a chase period where you take the anemone out of its radiolabeled environment and let it "digest" the carbon. It is during this time that the products of photosynthesis are making its way from the zooxanthellae into the tissues of the anemone. Then you make an anemone slushy, basically using something resembling a modified coffee bean grinder. Centrifuge the sample, all the algae fall to the bottom while the animal cells lyse into the supernatant. Add a dash of acid to drive out unassimilated inorganic carbon and Voila! You have yourself a delicious Anemone Slushy, a tasty family treat.
What Bachar et al. did was to trace the fate of carbon in the lipid portion of the Anemone Slushy as well as the whole tissue (i.e. animal) portion of the slushy. What they found was very interesting.
"The radioactivity levels of both the lipids and the total tissue of the sea anemones that were fed labelled autotrophic or heterotrophic carbon show that for the autotrophic anemones, the fastest change occurred in the lipid tissue, while for the heterotrophic anemones, it took place in the entire tissue."-Bachar et al. 2007 (see Fig. 2 from their paper below)
This is the first I have seen (and they make that claim in their abstract) that differentiates between the fate of autotrophically and heterotrophically derived carbon in a mixotrophic organism. It suggests that autotrophically derived carbon is convert mostly to lipids, potentially as quick and dirty carbon source for metabolism and respiration, while heterotrophically derived carbon is dispersed throughout the body, possibly for structural purposes (i.e. cell membranes, growth). Autotrophic carbon, derived from carbon dioxide, is short chain molecule so it makes sense that this would be used as an immediate energetic source since it can be easily converted into molecules like pyruvate and acetate which can slide right into the the metabolic cycles. On other hand, heterotrophic carbon is typically longer chain, like fatty acids and sugars, which need to be broken down into smaller parts to be used for metabolism. Hence these might be more appropriate carbon sources for structural components which are typically longer-chain carbon compounds like collagen and phospholipids.
Though this a short study reporting their novel results I am sure they have much greater ambitions in the works so it is worth keeping an eye out for future work from this lab. Symbiosis is fascinating topic. Though scientists have known about algal-cnidarian symbioses for a long time, it has taken decades of work to just figure out what and how. Now Bachar et al. give us where and some further insight in some more how. It is still unclear "why" though and the "how" in an evolutionary sense. Symbiosis is a field still much ripe for exploration. All the studies ever done on host-symbiont phylogenies, carbon translocation, physiological ecology, etc. is just the tip of the iceberg. Autotrophic symbioses are everywhere and occur in many animal phyla from cnidarians to nematodes to molluscs. Some ugly hairy creatures with backbones (a lame evolutionary feature is ask me) of the overexaggerated 5% even form symbiotic relationships with autotrophic organisms!