Coral disease diagnostics: what's between a plague and a band?

Abstract

Recently, reports of coral disease have increased significantly across the world's tropical oceans. Despite increasing efforts to understand the changing incidence of coral disease, very few primary pathogens have been identified, and most studies remain dependent on the external appearance of corals for diagnosis. Given this situation, our current understanding of coral disease and the progression and underlying causes thereof is very limited. In the present study, we use structural and microbial studies to differentiate different forms of black band disease: atypical black band disease and typical black band disease. Atypical black band diseased corals were infected with the black band disease microbial consortium yet did not show any of the typical external signs of black band disease based on macroscopic observations. In previous studies, these examples, here referred to as atypical black band disease, would have not been correctly diagnosed. We also differentiate white syndrome from white diseases on the basis of tissue structure and the presence/absence of microbial associates. White diseases are those with dense bacterial communities associated with lesions of symbiont loss and/or extensive necrosis of tissues, while white syndromes are characteristically bacterium free, with evidence for extensive programmed cell death/apoptosis associated with the lesion and the adjacent tissues. The pathology of coral disease as a whole requires further investigation. This study emphasizes the importance of going beyond the external macroscopic signs of coral disease for accurate disease diagnosis.

FIG. 1.

Typical (panels labeled A and a) and atypical (panels labeled B and b) black band disease of Eilat reef corals in the Red Sea. Shown are underwater photographs of infected black band diseased colonies displaying typical (A1 and a1) and atypical (B1 and b1) signs of disease. Tissue necrosis and microbial penetration and disruption of tissue layers in black band diseased colonies is evident by histopathology, using hematoxylin and eosin staining of tissue sections of typical (A2 and a2) and atypical (B2 and b2) diseased colonies, and FISH of typical (A3 and a3) and atypical (B3 and b3) diseased colonies is also shown. Scale bars, 50 μm. Abbreviations: Po, polyp; Ep, epithelium; Ga, gastroderm; Ne, necrosis; Bac, bacteria. FISH and spectral codes: blue, coral tissue; green, Symbiodinium sp.; red, bacteria. Uppercase panel labels demonstrate lower-magnification images; lowercase panel labels denote higher magnification.

FIG. 2.

Diseased Hydnophora sp. (A1) with evident rapid tissue loss and clear lesion borders between the coral tissue and the recently exposed skeleton (a1). Histopathology showed extensive tissue necrosis of cell layers (A2) including epithelium and gastroderm (a2). FISH revealed an extensive microbial population by use of the EUB universal bacterial probe (A3 and a3) associated with tissue necrosis and adjacent tissues. Scale bars, 50 μm. Abbreviations: Ep, epithelium; Ga, gastroderm; Zx, Symbiodinium sp.; Ne, tissue necrosis; Bac, bacteria. FISH and spectral color codes: blue, coral tissue; green, Symbiodinium sp.; red, bacteria. Uppercase panel labels demonstrate lower-magnification images; lowercase panel labels denote higher magnification.

FIG. 3.

FISH identified bacterial communities associated with diseased Hydnophora sp. identified with EUBmix (a) as belonging to δ-proteobacterium (b) and filamentous Cytophaga-Flavobacterium (c) groups. Populations of a Vibrio species were also identified (d). Scale bars, 50 μm. Abbreviations: Ep, epithelium; Ga, gastroderm; Zx, Symbiodinium sp.; Ne, tissue necrosis; Bac, bacteria; GAM, δ-proteobacteria; CF, Cytophaga-Flavobacterium; VB, Vibrio sp. FISH and spectral color codes: blue, coral tissue; green, Symbiodinium sp.; red, bacteria.

FIG. 4.

White disease of Stylophora pistillata displaying tissue loss and partial bleaching in patterns starting at the branch base and progressing rapidly up the branches and through the colony (A1 and a1), with 50% mortality of the colony evident (A1). Hematoxylin and eosin staining shows that tissue structures remain intact at the lesion border (A2 and a2) with some regions of structural loss of the gastroderm adjacent to the lesions (aa2), and FISH using the EUB general bacterial probe identified large populations of bacteria associated with tissues adjacent to the lesion border (A3), a mixed bacterial population dominated by δ-proteobacterial taxa (a3). Scale bars, 50 μm. Abbreviations: Ep, epithelium; Ga, gastroderm; Zx, Symbiodinium sp.; Ne, tissue necrosis; Bac, bacteria; L, lesion; PB, partial bleaching. FISH and spectral color codes: blue, coral tissue; green, Symbiodinium sp.; red, bacteria. Uppercase panel labels demonstrate lower-magnification images; lowercase panel labels denote higher magnification.

FIG. 5.

White syndrome of Acropora sp. (A1) showing a clear lesion boundary between the healthy tissue and the recently exposed skeleton (a1). Investigation of the tissue structure by use of hematoxylin and eosin shows that the tissues appear intact (A2) with no evidence for mass loss of tissue structure or integrity (a2). There is a distinct lack of microbial communities associated with the disease lesion (A3) and no bacterial penetration of colonization of the tissue layers (a3). Scale bars, 50 μm. Abbreviations: Ep, epithelium; Ga, gastroderm; Zx, Symbiodinium sp.; Ne, tissue necrosis; Bac, bacteria; Agg, bacterial aggregates. FISH and spectral color codes: blue, coral tissue; green, Symbiodinium sp.; red, bacteria. Uppercase panel labels demonstrate lower-magnification images; lowercase panel labels denote higher magnification.

FIG. 6.

In situ end labeling of fragmented DNA evident of apoptotic cell death is not detected for atypical (a) or typical (b and c) black band disease or for white diseases of Hydnophora sp. (d, e, and f). Positive staining was evident at the lesion border for the white disease of Stylophora pistillata (g and h) but not for tissues away from the border (i), and a high density of apoptotic cells was associated with white syndrome of Acropora sp. at the lesion border (k and l) but not away from the lesion border (m).

FIG. 7.

Profiling the interaction of holobiont changes and multiple levels of disease signs can be used as a basis for identifying specific criteria for disease diagnosis and to improve understanding of divergent disease processes underlying coral disease progression.