Comming to a coral reef near you...!?

A Review of: How feasible is the biological control of coral diseases? Teplitski and Ritchie. 2009. Trends in Ecology and Evolution, 24(7), pp. 378-385.

Increased sea surface temperatures, overfishing and deterioration in coastal water quality are among the factors contributing to the current worldwide coral reef crisis, with recent reviews drawing attention to coral diseases as  a consequence and major cause of coral reef deterioration. As the diseases of corals continue to threaten reefs worldwide, scientists and managers are considering different techniques for mitigation. The first proactive attempt to treat coral disease occurred during the late 1980’s: Black band disease polymicrobial mats were removed from coral surfaces using an underwater aspirator, followed by sealing the disease interface with modeling clay. Although reasonably effective at treating one coral colony at a time, this approach required significant investment of labor and demonstrated the need for novel, logistically feasible approaches for treating coral diseases. This paper evaluates the feasibility of using two forms of biological control: probiotics and phage therapy, as tools for mitigating coral diseases, although much of this research is still in the early stages, and the logistics and mechanism of these approaches are not yet worked out.

Phage therapy is considered to be more promising than antibiotics for treating diseases because phages will multiply as long as their bacterial hosts are present, ensuring that only one administration of the phage is required per treatment. However, as with antibiotics, exposure to phages often selects for resistant bacterial mutants. Most phages have limited host ranges, as they attach only to specific structures on the surfaces of their bacterial hosts and are therefore unable to harm animal cells or most mutualistic bacteria. These properties are an advantage for disrupting reservoirs of specific pathogens and make possible a targeted treatment of bacterial disease. The usefulness of phage therapy will depend on the long-term persistence and effects of phages on the coral-associated microbial communities. Although phage therapy has not yet been tested under field conditions, collective results from a number of investigations suggest that once seeded onto coral fragments, phages persist for extended periods of time and afford at least partial protection against the target pathogen. Before vibriophage therapy is attempted in the field, a better understanding of the role that Vibrios play as coral commensal bacteria is required. Phage-mediated removal of Vibrios from coral-associated microbial communities could have either an overall beneficial, neural or detrimental effect on the reef ecosystem.

An alternative approach for managing opportunistic pathogens of corals could rely on the native coral-associated microbes that have beneficial probiotic properties. Several lines of evidence suggest that, in addition to their own innate immunity mechanisms, marine invertebrates have evolved to rely on mutualistic microbes for defense against pathogens. An array of antibacterial, algicidal, anti-biofouling and cytotoxic compounds have been isolated from marine inverts and their microbial associates, many of which are effective against a broad spectrum of pathogens.

The authors propose three potential opportunities for exploring these properties of native coral-associated microbes in implementing biological control of coral disease: (i) inoculating coral transplants, (ii) using probiotic-related functions as indicators of reef ecosystem recovery, (iii) managing existing reefs to promote robust native microbial communities. However, before these opportunities can be considered as viable strategies, rigorous research is needed to understand the biochemical, physiological and genetic mechanisms that govern the structure and functions of coral-associated microbial communities. Further investigations into how pathogens establish within coral mucus and the associated microbial biofilms, and a better understanding of signal transduction cascades involved in coral immunity, recognition of symbionts and pathogens, and immune memory will help evaluate the feasibility of using immune priming in disease management. Therefore much work remains to be done in the laboratory before various biological control strategies are tested under field conditions.