Black band disease microbial community variation on corals in three regions of the wider Caribbean

Black band disease (BBD) is a pathogenic consortium of microorganisms that primarily affects massive framework-building scleractinian corals on reefs worldwide. There has been considerable debate concerning the microbial community composition of BBD. The aim of this study was to utilize microbial profiling to assess overall patterns of variation in the BBD bacterial community with respect to geographic location, host coral species, time, and nutrient regime. Length heterogeneity polymerase chain reaction (LH-PCR) was employed to differentiate BBD communities based on the natural variation in the sequence lengths within hypervariable domains of the 16S rRNA gene. Analysis of LH-PCR profiles of 97 BBD samples using multivariate ordination methods and analysis of similarity revealed significant clustering with respect to geographic region when comparing BBD sampled from reefs near Lee Stocking Island in the Bahamas' Exuma Chain, the Northern Florida Keys (NFK), and St. John in the US Virgin Islands. There was much variability in BBD community composition on a regional basis, between sites in the NFK, and in terms of coral host species. The observed differences among BBD microbial community profiles were driven primarily by variation in relative abundance of 313-316-bp amplicons, which correspond to cyanobacteria and alpha-proteobacteria. The results obtained in this study support previous reports of intrinsic variability and complexity of the BBD microbial community but also suggest that this variability has biogeographic patterns.

The presence of the cyanobacterial toxin microcystin in black band disease of corals

Black band disease (BBD) is a migrating, cyanobacterial dominated, sulfide-rich microbial mat that moves across coral colonies lysing coral tissue. While it is known that BBD sulfate-reducing bacteria contribute to BBD pathogenicity by production of sulfide, additional mechanisms of toxicity may be involved. Using HPLC/MS, the cyanotoxin microcystin was detected in 22 field samples of BBD collected from five coral species on nine reefs of the wider Caribbean (Florida Keys and Bahamas). Two cyanobacterial cultures isolated from BBD, Geitlerinema and Leptolyngbya sp. contained microcystin based on HPLC/MS, with toxic activity confirmed using the protein phosphatase inhibition assay. The gene mcyA from the microcystin synthesis complex was detected in two field samples and from both BBD cyanobacterial cultures. Microcystin was not detected in six BBD samples from a different area of the Caribbean (St Croix, USVI) and the Philippines, suggesting regional specificity for BBD microcystin. This is the first report of the presence of microcystin in a coral disease.

Microbial communities in the surface mucopolysaccharide layer and the black band microbial mat of black band-diseased Siderastrea siderea

Microbial community profiles and species composition associated with two black band-diseased colonies of the coral Siderastrea siderea were studied by 16S rRNA-targeted gene cloning, sequencing, and amplicon-length heterogeneity PCR (LH-PCR). Bacterial communities associated with the surface mucopolysaccharide layer (SML) of apparently healthy tissues of the infected colonies, together with samples of the black band disease (BBD) infections, were analyzed using the same techniques for comparison. Gene sequences, ranging from 424 to 1,537 bp, were retrieved from all positive clones (n = 43 to 48) in each of the four clone libraries generated and used for comparative sequence analysis. In addition to LH-PCR community profiling, all of the clone sequences were aligned with LH-PCR primer sequences, and the theoretical lengths of the amplicons were determined. Results revealed that the community profiles were significantly different between BBD and SML samples. The SML samples were dominated by gamma-proteobacteria (53 to 64%), followed by beta-proteobacteria (18 to 21%) and alpha-proteobacteria (5 to 11%). In contrast, both BBD clone libraries were dominated by alpha-proteobacteria (58 to 87%), followed by verrucomicrobia (2 to 10%) and 0 to 6% each of delta-proteobacteria, bacteroidetes, firmicutes, and cyanobacteria. Alphaproteobacterial sequence types related to the bacteria associated with toxin-producing dinoflagellates were observed in BBD clone libraries but were not found in the SML libraries. Similarly, sequences affiliated with the family Desulfobacteraceae and toxin-producing cyanobacteria, both believed to be involved in BBD pathogenesis, were found only in BBD libraries. These data provide evidence for an association of numerous toxin-producing heterotrophic microorganisms with BBD of corals.

Ecological physiology of a coral pathogen and the coral reef environment

Laboratory studies on the ecological physiology of a coral pathogen were carried out to investigate growth potential in terms of environmental factors that may control coral diseases on reefs. The disease chosen for this study, white plague type II, is considered to be one of the major diseases of Caribbean scleractinian corals, affecting a wide range of coral hosts and causing rapid and widespread tissue loss. It is caused by a single pathogen, the bacterium Aurantimonas coralicida. A series of laboratory experiments using a pure culture of the pathogen was carried out to examine the roles of temperature, pH, and O(2) concentration on growth rate. Results revealed optimal growth between 30 and 35 degrees Celsius, and between pH values of 6 and 8. There was a distinctive synergistic relationship between pH and temperature. Increasing temperature from 25 to 35 degrees Celsius expanded the range of pH tolerance from a minimum of 6.0 down to 5.0. O(2) concentration directly affected growth rate, which increased with increasing O(2). The combined effects of increasing O(2) and increasing temperature resulted in a synergistic effect of more rapid growth. These laboratory results are discussed in terms of the coral host and the range of the environmental factors that occur on coral reefs. We conclude that changing environmental conditions in the reef environment, in particular observed increases in water temperature, may be promoting coral diseases by allowing coral pathogens to expand their ecological niches. In the case of the white plague type II pathogen, elevated temperature would allow A. coralicida to colonize the low pH environment of the coral surface mucopolysaccharide layer as an initial stage of infection. The synergistic effect between temperature and oxygen concentration appeared to be less environmentally relevant for this coral pathogen.

Development and field application of a molecular probe for the primary pathogen of the coral disease white plague type II

One of the current problems in the field of coral disease research is that of tracking coral pathogens in the natural environment. A promising method to do this is by use of pathogen-specific molecular probes. However, this approach has been little used to date. We constructed, and validated in the laboratory, a fluorochrome-labeled molecular probe specific to Aurantimonas coralicida, the bacterial pathogen of the Caribbean coral disease white plague type II (WPIl). We then used the probe to test field samples of diseased coral tissue for the presence of this pathogen. Probe design was based on a unique subset (25 nucleotides) of the complete l6S rRNA gene sequence derived from a pure culture of the pathogen. The pathogen-specific probe was labeled with the fluorochrome GreenStar* FITC (fluorescein isothiocyanate, GeneDetect Ltd, New Zealand). As a control, we used the universal eubacterial probe EUB 338, labeled with a different fluorochrome (TRITC, tetra-methylrhodamine isothiocyanate). Both probes were applied to laboratory samples of pure cultures of bacteria, and field samples collected from the surface of the disease line of corals exhibiting signs of white plague (types I and II), healthy controls, and corals with an uncharacterized disease ("patchy necrosis"). All samples were analyzed using fluorescence in situ hybridization (FISH). We have determined that the probe is specific to our laboratory culture of the coral pathogen, and does not react with other bacterial species (the eubacterial probe does). The WPII pathogen was detected in association with diseased coral samples collected from coral colonies on reefs of the Bahamas (n= 9 samples) exhibiting signs of both WPI and WPII. Diseased (and healthy) tissue samples (n- 4) from corals exhibiting signs of "patchy necrosis" were also assayed. In this case the results were negative, indicating that the same pathogen is not involved in the two diseases. Incorporation and use of pathogen-specific probes can significantly expand our knowledge of the etiology of coral diseases.