Their microscopic examination of these coral heads revealed the presence in significant numbers of the floating bacterium Beggiatoa, which they believed caused the disease. Predatory bacteria attracted to the released mucus apparently penetrated the stressed coral tissue and participated in the destruction of the cnidarium colonies. Our initial work (RUTZLER & SANTAVY, 1983) on the morphology and fine structure of the putative cyanophyte pathogen led to taxonomic reevaluation and comparison of Belizean and Bermuda populations of this organism, named Phormidium corallyticum RurzLER & SANTAVY.
Additionally, we consider the ecological requirements of the blackband cyanophyte and assess its current and potential impact on the coral reef community. Black band disease was sampled underwater by digging and scraping off affected parts of the coral and piping (with a swabbing syringe) the loosened material into scalable plastic bags. In the laboratory, the progress of the disease was documented by measurements from the point of vaccination and by close-up photography.
Our initial work (RUTZ-LER & SANTAVY, 1983) on the morphology and fine structure of a putative cyanophyte pathogen led to a taxonomic re-evaluation and comparison of the Belize and Bermuda populations of these organisms, called Phormidium corallyticwn. One to four visits were made at intervals of 3 to 20 days to measure and photograph the progression of the disease (plotted in Table 2; .. a specimen of Montastrea I coral is depicted in Fig. 2, Diploria in Fig. la, b ). The greatest distance traveled by the disease on one coral was 160 mm in 41 days of observation in April-May, 400 mm during the duration of the infection (recorded in December 1982).
One to four visits were made at 3–20 day intervals to measure and photograph disease progression (plotted in Table 2; coral sample Montastrea I is shown in Fig. 2, Diploria in Fig. 1a, B). Plankton flows over reef areas affected by black band disease have so far contained no floating blue-green algae other than tufts of the planktonic Trichodesmium species. In most cases this marks the beginning of the disease process, which manifests itself within four to six hours (Figures 3,4).
Phormidium material placed on a 150/im mesh net (bolted cloth) supported 5 mm above a healthy coral surface (Diploria) moves to the lower surface of the net and forms a mat there, but is unable to penetrate coral tissue. Although mechanical dissociation of the large Phormidium corallyticum is quite easy (see Materials and Methods), we did not obtain any axenic cultures for observation. Phormidium material placed on a 150 jum mesh net (bolted cloth) supported 5 mm above a healthy coral surface (Diploria) moves to the lower surface of the net and forms a mat there, but is unable to penetrate coral tissue.
No intact coral cells can be seen in contact with or in the immediate vicinity of the Phormidium filaments. The dense web of several layers of filaments remains on top of the mash. In the older parts of the carpet (away from the living coral), these sulfur bacteria grow over the Phormidium carpet, where they form a whitish zone (Fig. 4c).
In the oldest parts of the mat (away from the living corals), these sulfur bacteria grow on the Phormidium mat, where they form a white area (Fig. 4 c).
Laboratory experiments
To confirm the nutritional value of coral for Phormidium growth, we added autoclaved and untreated Montastrea tissue (10 each) to small clusters of the organism on agar plates. Growth was determined by measuring the longest perpendicular diameters of the Phormidium colonies; plain agar served as control. Cultures in plain seawater, in or on any of the enriched media tested, or over decaying coral debris could only be maintained at reduced light levels; the best growth of cultured Phormidium away from live coral was recorded on coral waste at a light level of 6 % of full sunlight (30 g cal - cm2 • day').
As shown in Table 3, only inoculations involving Phormidium corallyticum were positive and induced the previously described disease stages. Some bands contained numerous specimens of the nematode Araeolaimus sp. and a few bacteria, including Beggiatoa (Fig. 15), but not Migeltalk. After four days under low water flow conditions in the aquarium, Beggiatoa growth appeared along some wound edges, but the cyanophytes neither spread nor grew.
Not one of the twenty-seven gorgonian band inoculations took, whereas 100% of the Phormidium corallyticum control infections were successful. Coils dominated by Schizothrix mexicana initially spread within the wound area and filaments concentrated peripherally, but the organism did not become established in any of the coral species. Responses of scleractinian and gorgonian hosts to inoculations of foreign but not disease-associated organisms were tested.
Active, unstressed corals and gorgonians were able to free themselves from foreign objects with the help of mucus and ciliary action and polyp movements (Fig. 13). This may be why at least one earlier report did not mention cyanobacteria as part of black line disease (GARRETT & DUCKLOW, 1975). Most easily infected Atlantic corals belong to genera that do not occur in the Indo-Pacific, and Atlantic species of common Indo-Pacific genera, such as Acropora and Porites, are resistant to the disease (Table 1).
The rate of spread of the infection, once established on a coral, is remarkably rapid compared to the growth of competitors for space, such as sponges. The fact that polyphosphate bodies (Fig. 10a) are also larger and more common near the active edge of the disease than away from it - as in the Beggiatoa zone - indicates a build-up of energy storage (SKUFTIG,. Very infected parts of the reefs along Bermuda, is also far from possible stress factors produced by these islands.
Inoculation experiments have shown that access to subectodermal coral tissue is all that is required for Phormidium filaments to cause histolysis, which is the beginning of the disease process. Moreover, toxic substances extracted from some cyanophytes cause general cellular damage and severe injury to parenchymal cells of the liver of injected rats (ASHWORTH & MASON, 1946).
