Yui Sato

Dynamics of seasonal outbreaks of black band disease in an assemblage of Montipora species at Pelorus Island (Great Barrier Reef, Australia)

Recurring summer outbreaks of black band disease (BBD) on an inshore reef in the central Great Barrier Reef (GBR) constitute the first recorded BBD epizootic in the region. In a 2.7 year study of 485 colonies of Montipora species, BBD affected up to 10 per cent of colonies in the assemblage. Mean maximum abundance of BBD reached 16±6 colonies per 100 m2 (n=3 quadrats, each 100 m2) in summer, and decreased to 0–1 colony per 100 m2 in winter. On average, BBD lesions caused 40 per cent tissue loss and 5 per cent of infections led to whole colony mortality. BBD reappearance on previously infected colonies and continuous tissue loss after the BBD signs had disappeared suggest that the disease impacts are of longer duration than indicated by the presence of characteristic signs. Rates of new infections and linear progression of lesions were both positively correlated with seasonal fluctuations in sea water temperatures and light, suggesting that seasonal increases in these environmental parameters promote virulence of the disease. Overall, the impacts of BBD are greater than previously reported on the GBR and likely to escalate with ocean warming.

Successional changes in bacterial communities during the development of black band disease on the reef coral, Montipora hispida.

Black band disease (BBD) consists of a mat-forming microbial consortium that migrates across coral colonies causing rapid tissue loss. Although BBD-associated microbial communities have been well characterized, little is known regarding how these complex bacterial consortia develop. This study analyzed successional changes in microbial communities leading to the development of BBD. Long-term monitoring of tagged corals throughout outbreaks of BBD in the central Great Barrier Reef documented cyanobacterium-infected lesions, herein termed cyanobacterial patch(es) (CP), which were macroscopically distinct from BBD and preceded the onset of BBD in 19% of the cases. Dominant cyanobacteria within CP lesions were morphologically distinct from ones dominating BBD lesions. Clone libraries and terminal restriction fragment length polymorphism analysis confirmed shifts within cyanobacterial assemblages, from Blennothrix sp.-affiliated sequences dominating CP lesions, to Oscillatoria sp.-affiliated sequences, similar to those retrieved from other BBD samples worldwide, dominating BBD lesions. Bacterial 16S ribosomal RNA clone libraries also showed shifts in bacterial ribotypes during transitions from CP to BBD, with Alphaproteobacteria-affiliated sequences dominant in CP libraries, whereas gammaproteobacterial and cyanobacterial ribotypes were more abundant in BBD clone libraries. Sequences affiliated with organisms identified in sulfur cycling were commonly retrieved from lesions showing characteristic field signs of BBD. As high sulfide concentrations have been implicated in BBD-mediated coral tissue degradation, proliferation of a microbial community actively involved in sulfur cycling potentially contributes to the higher progression rates found for BBD compared with CP lesions. Results show how microbial colonization of indistinct lesions may facilitate a common coral disease with proven ecological effects on coral populations.

Changes in sulfate-reducing bacterial populations during the onset of black band disease

Factors that facilitate the onset of black band disease (BBD) of corals remain elusive, though anoxic conditions under the complex microbial mat and production of sulfide are implicated in necrosis of underlying coral tissues. This study investigated the diversity and quantitative shifts of sulfate-reducing bacterial (SRB) populations during the onset of BBD using real-time PCR (RT-PCR) and cloning approaches targeting the dissimilatory (bi)sulfite reductase (dsrA) gene. A quantitativePCR (qPCR) assay targeting the 16S rRNA gene also provided an estimate of total bacteria, and allowed the relative percentage of SRB within the lesions to be determined. Three Montipora sp. coral colonies identified with lesions previously termed cyanobacterial patches (CPs) (comprising microbial communities unlike those of BBD lesions), were tagged and followed through time as CP developed into BBD. The dsrA-targeted qPCR detected few copies of the gene in the CP samples (<65 per ng DNA), though copy numbers increased in BBD lesions (>2500 per ng DNA). SRB in CP samples were less than 1% of the bacterial population, though represented up to 7.5% of the BBD population. Clone libraries also demonstrated a shift in the dominant dsrA sequences as lesions shifted from CP into BBD. Results from this study confirm that SRB increase during the onset of BBD, likely increasing sulfide concentrations at the base of the microbial mat and facilitating the pathogenesis of BBD.

Biogeochemical conditions determine virulence of black band disease in corals

The microenvironmental dynamics of the microbial mat of black band disease (BBD) and its less virulent precursor, cyanobacterial patch (CP), were extensively profiled using microsensors under different light intensities with respect to O2, pH and H2S. BBD mats exhibited vertical stratification into an upper phototrophic and lower anoxic and sulphidic zone. At the progression front of BBD lesions, high sulphide levels up to 4977 μM were measured in darkness along with lower than ambient levels of pH (7.43±0.20). At the base of the coral–BBD microbial mat, conditions were hypoxic or anoxic depending on light intensity exposure. In contrast, CP mats did not exhibit strong microchemical stratification with mostly supersaturated oxygen conditions throughout the mats at all light intensities and with levels of pH generally higher than in BBD. Two of three replicate CP mats were devoid of sulphide, while the third replicate showed only low levels of sulphide (up to 42 μM) present in darkness and at intermediate light levels. The level of oxygenation and sulphide correlated well with lesion migration rates, that is virulence of the mats, which were greater in BBD than in CP. The results suggest that biogeochemical microgradients of BBD shaped by the complex microbial community, rather than a defined pathogen, are the major trigger for high virulence and the associated derived coral mortality of this disease.

Cyanotoxins are not implicated in the etiology of coral black band disease outbreaks on Pelorus Island, Great Barrier Reef

Cyanobacterial toxins (i.e. microcystins) produced within the microbial mat of coral black band disease (BBD) have been implicated in disease pathogenicity. This study investigated the presence of toxins within BBD lesions and other cyanobacterial patch (CP) lesions, which, in some instances ( approximately 19%), facilitated the onset of BBD, from an outbreak site at Pelorus Island on the inshore, central Great Barrier Reef (GBR). Cyanobacterial species that dominated the biomass of CP and BBD lesions were cultivated and identified, based on morphology and 16S rRNA gene sequences, as Blennothrix- and Oscillatoria-affiliated species, respectively, and identical to cyanobacterial sequences retrieved from previous molecular studies from this site. The presence of the cyanotoxins microcystin, cylindrospermopsin, saxitoxin, nodularin and anatoxin and their respective gene operons in field samples of CP and BBD lesions and their respective culture isolations was tested using genetic (PCR-based screenings), chemical (HPLC-UV, FTICR-MS and LC/MS(n)) and biochemical (PP2A) methods. Cyanotoxins and cyanotoxin synthetase genes were not detected in any of the samples. Cyanobacterial species dominant within CP and BBD lesions were phylogenetically distinct from species previously shown to produce cyanotoxins and isolated from BBD lesions. The results from this study demonstrate that cyanobacterial toxins appear to play no role in the pathogenicity of CP and BBD at this site on the GBR.

Sulfur-oxidizing bacterial populations within cyanobacterial dominated coral disease lesions.

This study investigated the diversity and quantitative shifts of sulfur-oxidizing bacteria (SOB) during the onset of black band disease (BBD) in corals using quantitative PCR (qPCR) and cloning approaches targeting the soxB gene, involved in sulfur oxidation. Four Montipora sp. coral colonies identified with lesions previously termed cyanobacterial patches (CP) (comprising microbial communities different from those of BBD lesions), was monitored in situ as CP developed into BBD. The overall abundance of SOB in both CP and BBD lesions were very low and near the detection limit of the qPCR assay, although consistently indicated that SOB populations decreased as the lesions transitioned from CP to BBD. Phylogenetic assessment of retrieved soxB genes showed that SOB in both CP and BBD lesions were dominated by one sequence type, representing > 70% of all soxB gene sequences and affiliated with members of the Rhodobacteraceae within the α-Proteobacteria. This study represents the first assessment targeting SOB within BBD lesions and clearly shows that SOB are not highly diverse or abundant in this complex microbial mat. The lack of oxidation of reduced sulfur compounds by SOB likely aids the accumulation of high levels of sulfide at the base of the BBD mat, a compound contributing to the pathogenicity of BBD lesions.

Unraveling the microbial processes of black band disease in corals through integrated genomics

Coral disease outbreaks contribute to the ongoing degradation of reef ecosystems, however, microbial mechanisms underlying the onset and progression of most coral diseases are poorly understood. Black band disease (BBD) manifests as a cyanobacterial-dominated microbial mat that destroys coral tissues as it rapidly spreads over coral colonies. To elucidate BBD pathogenesis, we apply a comparative metagenomic and metatranscriptomic approach to identify taxonomic and functional changes within microbial lesions during in-situ development of BBD from a comparatively benign stage termed cyanobacterial patches. Results suggest that photosynthetic CO2-fixation in Cyanobacteria substantially enhances productivity of organic matter within the lesion during disease development. Photosynthates appear to subsequently promote sulfide-production by Deltaproteobacteria, facilitating the major virulence factor of BBD. Interestingly, our metagenome-enabled transcriptomic analysis reveals that BBD-associated cyanobacteria have a putative mechanism that enables them to adapt to higher levels of hydrogen sulfide within lesions, underpinning the pivotal roles of the dominant cyanobacterium within the polymicrobial lesions during the onset of BBD. The current study presents sequence-based evidence derived from whole microbial communities that unravel the mechanism of development and progression of BBD.

Decadal erosion of coral assemblages by multiple disturbances in the Palm Islands, central Great Barrier Reef

Increases in the frequency of perturbations that drive coral community structure, such as severe thermal anomalies and high intensity storms, highlight the need to understand how coral communities recover following multiple disturbances. We describe the dynamics of cover and assemblage composition of corals on exposed inshore reefs in the Palm Islands, central Great Barrier Reef, over 19 years encapsulating major disturbance events such as the severe bleaching event in 1998 and Cyclone Yasi in 2011, along with other minor storm and heat stress events. Over this time, 47.8% of hard coral cover was lost, with a concomitant shift in coral assemblage composition due to taxon-specific rates of mortality during the disturbances, and asymmetric recovery in the aftermath thereof. High recruitment rates of some broadcast-spawning corals, particularly corymbose Acropora spp., even in the absence of adult colonies, indicate that a strong external larval supply replenished the stocks. Conversely, the time required for recovery of slow-growing coral morphologies and life histories was longer than the recurrence times of major disturbances. With interludes between bleaching and cyclones predicted to decrease, the probability of another severe disturbance event before coral cover and assemblage composition approximates historical levels suggests that reefs will continue to erode.