The key to coral reefs’ immense productivity and biodiversity is the close and long-term interaction (symbiosis) between corals and photosynthetic algae, which live inside the coral cells and transfer essential nutrients to their hosts. Rising seawater temperature can cause this symbiosis to break down, a phenomenon known as coral ‘bleaching’. The number of bleaching incidents around the world are increasing and have been linked to global climate change. At present the molecular mechanisms that underpin the intimate partnership between coral and algae are not well understood, largely because corals are not suitable for molecular analysis in the laboratory. The EU-funded project ESYMBIOSIS has further developed a novel model system using Aiptasia, a marine sea anemone, to uncover fundamental aspects of coral symbiosis. A novel experimental system Aiptasia lives in a stable symbiosis with the same types of algae as corals. However, unlike corals, Aiptasia can be easily kept as asexually-reproducing clonal lines under laboratory conditions. Furthermore, sexual reproduction can be induced, allowing unlimited access to non-symbiotic larvae that readily take up symbionts from the environment. “We characterised distinct anemone lines and symbiont strains used in the laboratory using molecular phylogeny (the evolutionary history of a species) and zoology,” says project coordinator Dr Annika Guse. Expanding on studies by a collaborating lab to use blue light emitting diodes to simulate a full moon, the researchers then developed an efficient protocol for inducing the sexual reproduction of anemones to produce anemone larvae on a regular basis. Such larvae, which acquire symbionts from the environment, are used to study symbiosis establishment using modern molecular tools. “We described larval development and symbiont uptake at the cellular level and developed tools to analyse gene expression and localise proteins. In addition, we identified key players of symbiosis, and performed comparative experiments with corals collected in the field,” explains Dr Guse. Scientists sequenced the Aiptasia genome and established various other molecular, cell biological and biochemical techniques including metabolomics, lipidomics and transcriptomic approaches. Dr Guse claims: “Our efforts have paved the way to use Aiptasia larvae as a novel experimental system for coral symbiosis research and resulted in various peer-reviewed publications.” How corals survive ESYMBIOSIS now seeks to answer important questions like whether there are specific cells that acquire the symbionts. It is also investigating recognition mechanisms for symbiont uptake, how the symbionts avoid being destroyed by their host, and how key nutrients are transferred between the partners. “We are investigating how evolutionary conserved lipid transporters that are involved in (chole)sterol transfer from symbiont to host work – a prerequisite for coral survival because they have lost the ability to synthesise (chole)sterols themselves,” reveals Dr Guse. As in medical research, understanding the healthy state of coral symbiosis can provide a basis for determining what happens during disease or coral bleaching incidents. “Our research lays the groundwork for a more targeted investigation of coral bleaching and for developing strategies for coral reef protection,” comments Dr Guse. “It also raises awareness of coral reef ecosystems and the challenges they face due to pollution from human activities,” she concludes.
ESYMBIOSIS, coral, symbiosis, Aiptasia, algae