Re-evaluation of the emergence of reef coral diseases based on the importance of ciliate infections

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Global CO2 emissions are produced mainly by western economies in the temperate zones, however the impacts of climate change are mainly being seen at the climatic extremes of the poles and tropical zones. While the poles are scarcely populated, coral reefs play a vital role in directly supporting at least 500 million people worldwide, despite only representing 0.1% of the world's ocean area. The Coral Triangle in south-east Asia alone includes over 100 million people who are almost entirely dependent on coastal resources. Coral diseases have contributed significantly to global declines in coral reefs (some scientists put the figure at about 40% loss over the last 40 years), leaving few options for coastal peoples of developing countries. Many scientists have linked the emergence of coral diseases to climate change that affects the overall health and disease resistance of the host as well as promoting the activity of some pathogens. However, our understanding of coral diseases is driven mainly on the assumption that most are caused by bacteria. Following a general lack of success in identifying causal agents using traditional culture-based approaches, our group began using modern culture-independent methods based on analysing bacterial DNA in environmental samples over ten years ago. While this work has been successful in advancing understanding of the microbial ecology of several common coral diseases, there have been few breakthroughs in determining the causal agents of disease. More recently, working on a NERC-funded project to investigate temperature stress effects on coral susceptibility to disease, we have discovered that several of the most important types of coral disease are associated with mass infections by protist pathogens, as well as bacteria. The protists (ciliates similar to Paramecium) act as pathogens kill the coral by ingesting the tissues. Ongoing work will address the relative changes in ciliate and bacterial pathogen populations during the disease process, but there is no doubt that the ciliates are important agents in disease transmission and pathology and may be the primary pathogens. We have also shown that these diseases are highly temperature-dependent, which may explain the global increase in disease prevalence in the last 20-30 years. The proposed study therefore addresses the ciliate diseases specifically and will test whether they are acting as primary pathogens (causal agents) of the disease or secondary, opportunistic pathogens invading the tissues after another primary (possibly bacterial) pathogen. To do this we will apply traditional Koch's postulates, isolating the potential pathogens in culture and innoculating healthy corals in controlled incubations. We will also survey a number of locations worldwide to determine whether diseases with very similar signs are also associated with ciliates. Some of these diseases have caused serious ecological impacts, for example one (White Band Disease) has elimnated elkhorn coral as the dominant coral species in the whole Caribbean region. Since the diseases are highly temperature-dependent, we will conduct experiments to allow us to more accurately model the impacts of future climate change scenarios on coral mortality. The experiments will distinguish the effects of temperature on increased pathogen activity and changes in host coral susceptibility. We will further investigate the changes in susceptibility to determine the likely mechanisms by which corals resist ciliate infections under healthy conditions. Together, these studies will allow a mechanistic understanding of how temperature affects the disease process, so we can model the effects of future climate change, rather than just model past history. The final synthesis of the research will allow us to fundamentally re-evaluate the emergence of coral diseases in the last 20-30 years as well as predict future changes and propose potential management solutions.