The Great Barrier Reef Susceptibility to Diseases

It was only around three years ago when a misleading and inaccurate “Great Barrier Reef Obituary” went viral, pronouncing the Earth’s largest living structure as dead. Fortunately, this turned out not to be the case, though the damage and severe bleaching that the reef has taken in just the past three years since that article was posted alone doesn’t leave its condition faring all that much better.

And with all of these extreme bleaching events occurring and causing the condition of the Great Barrier reef to seemingly to worsen by the minute, it’s no surprise that researchers from all around the world have begun to take a closer look into the different factors that may impact coral colonies’ susceptibility to different coral diseases, including the lesser known but arguably more deadly white syndrome (WS)- a collective term for coral tissue loss diseases, which from 2008 to 2013 killed up to 96 percent of the Acropora plate coral living in the reefs of Christmas Island, an Australian external territory, and has affected coral from around the world.

One such group of researchers viewed reports of localized increased disease prevalence at offshore reef platforms on the Great Barrier Reef as an opportunity to perform an eight-month case study to investigate the naturally occurring relationship between environmental stress, coral-associated microbial communities, coral immune function and disease onset. The researchers created a baseline understanding of coral host-microbiota interactions during environmental stress that end in disease by combining a variety of different techniques including ecological disease monitoring, water quality assessment, protein-based immune function characterization and microbial community profiling using 16S rRNA gene amplicon pyrosequencing. The study consisted of comparing coral immune function and coral-associated microbial communities both between healthy corals adjacent to reef platforms versus healthy corals at a nearby control site, and among corals before, during and after initial signs of coral disease WSs.

The case study centered around a control site and 2 tourist platforms moored about 5m from the reef crest at Hardy Reef, a mid-shelf reef about 75 km offshore of the Whitsunday Island group in the central region of the Great Barrier Reef Marine Park. Only one of the platforms, which could typically see about 400 visitors per day, was actually being used at the time of the study, as the other smaller platform’s use had stopped about a year before the study began. Weather was monitored daily and beginning in January, five replicate water samples were collected at each sampling location and time point, with sub-samples being analyzed for dissolved inorganic and organic nutrients. Eight similarly-sized visually healthy colonies of the coral Acropora millepora within each of the three locations monitored were tagged, with care being taken to select colonies separated by about 5m to minimize the risk of confoundment from possible vector transmission of infectious disease agents. Tagged colonies were photographed before and after sample collection and their health states visually assessed each month, with samples for bacterial community profiling and immunological analyses being collected at four time points: November, January, February and June. At each time point, one branch was cut and sampled from the middle of each tagged colony; if a colony displayed signs of disease, an apparently healthy portion of a branch was collected approximately 1 cm from the disease lesion boundary.

Genomic DNA was extracted from crushed coral fragments, purified, and then sent off for PCR amplification of the small-subunit rRNA 16S gene. Sequence reads were processed using the QIIME pipeline and operational taxonomic units were identified. Total potential phenoloxidase (tpPO) activity was measured as a proxy for innate immune function; total tissue protein content was determined using the DC Protein Assay. Different data analyses were conducted including an assessment of bacterial community (alpha) diversity and beta diversity (between sample diversity). Differences in water quality variables among sampling locations and months, and interactions between these factors were also determined.

Interestingly, the researchers found that community stability among healthy samples didn’t significantly differ between platform and control sites, suggesting that alpha diversity rather than beta diversity underlines detected dissimilarity between locations. TpPO activity levels of healthy corals also varied significantly between locations, but not among months, with a significant interaction between location and month (in both November and June, mean tpPO activity of healthy corals near reef platforms was about 50% lower than mean activity of control site corals). Overall, 31% of tagged colonies adjacent to platforms developed signs of white syndrome (WS) over the course of the study, while all control colonies on a platform-free reef remained visually healthy. Two key changes in bacterial community structure were also noted in apparently healthy corals that’d develop WS disease signs- a significant reduction in bacterial diversity associated with individual corals and a significant increase in among-colony bacterial community heterogeneity in corals that developed WS, months before the first visual signs of disease. Corals adjacent to platforms also experienced significant reductions in coral immune function and the corals at platform sites that remained visually healthy throughout the study had reduced bacterial diversity compared to healthy colonies at the platform-free site.

So why is this information important? Tourism on the Great Barrier Reef (GBR) represents a $6.4 billion per year commercial industry in Australia, meaning there’s strong motivation for tourism operators to protect reefs associated with platforms. The identification of environmental drivers in this study will be critical to effectively managing these offshore reef platforms.

Moreover, the fate of the coral host seems to be linked to the diversity and associated microbial composition. These results support that a loss of microbial diversity may impact coral holobiont resilience, and when taken together, suggest that an overall disruption in microbial community structure, not infection by a single pathogenic species, could contribute to the development of disease. They also highlight the important role that diversity is likely to play in stabilizing microbial communities that govern coral health and imply that activities associated with nearness to reef platforms impact coral immunocompetence and associated bacterial community, which affects the corals’ susceptibility to disease.