Coral Health and Disease in the Pacific: Vision for Action

What is causing these diseases and where are they coming from?Proving disease causation has been one of the largest challenges in coral diseaseresearch. Because diseases manifest on corals in a limited number ways, corals thatexhibit specific disease signs in the field may be affected by a variety of pathogens thatdiffer spatially or temporally (e.g., the pathogen for WP-II may differ depending on thelocation, affected species, or other factors). To date, causative agents have beenidentified for four of the most common diseases found on Caribbean reefs (BBD, WP-II,ASP, and WPX). Three of these (WP-II, ASP, WPX) were verified through inoculationexperiments with cultured bacterial isolates (through fulfillment of Koch’s postulate),while the cause of BBD was identified using microscopy. In other diseases (YBD, DSD,WBD-II) screening of microbial communities of healthy and diseased tissue (and mucuslayer) using traditional culture methods illustrates a high diversity of microorganisms,along with several bacteria (especially Vibrio spp.) that appear to be more prevalent indiseased samples (Cervino et al., 2001; Gil-Agudelo et al., 2004; Weil, 2006) . Molecularstudies (16S and 18S rRNA/DNA gene sequence amplification of microbialcommunities) have identified complex multi-species microbial communities in corals(including microorganism that may be unculturable) that appear to vary spatially,seasonally, between species, and also between diseased and apparently healthy parts ofthe colony (Rohwer et al., 2002; Pantos et al., 2003; Pantos and Bythell, 2006). In manycases (e.g., WP-II, BBD and WBD) these molecular studies have identified a differentsuite of organisms as potential causative agents than that observed in earlier studies.Black band diseaseThe causative agent of BBD was originally described as the cyanobacteriaOscillatoria submembranaceae and then Phormidium corallyticum based exclusively onfilament morphology, pigmentation and motility determined using light microscopy(Antonius, 1981; Rützler et al., 1983). Other heterotrophic bacteria (Garrett andDucklow, 1975), the sulfide oxidizing bacterium Beggiatoa spp. (Ducklow and Mitchell,1979) and marine fungi (Ramos-Flores, 1983) have also been suggested as the primarypathogen. Richardson and colleagues described BBD as a consortium of microorganismsdominated by a gliding filamentous cyanobacteria (P. corallyticum) that functionstogether with sulfur oxidizing bacteria (Beggiatoa spp.) and sulfur reducing bacteria(Desulfovibrio spp.) to produce anoxia and high levels of sulfide adjacent to the coral,conditions that are lethal to the coral (Carlton and Richardson, 1995; Viehman et al.,2006). More recent work using 16S rRNA gene sequencing identified a complex andvariable assemblage of heterotrophic organisms that includes over 500 species of bacteriaas well as cyanobacteria (Cooney et al. 2002, Frias-Lopez et al. 2002, 2003). Thesemolecular studies identified anomalies in the identification of the cyanobacteria: threeunique taxa of cyanobacteria have been isolated, with differences noted between theCaribbean and IndoPacific. Interestingly, P. corallyticum was not detected in the clonelibraries or evident in the DGGEs (Cooney et al. 2002, Frias-Lopez et al. 2002, 2003).White plagueThe disease pathogen for WP II was identified as a gram negative -proteobacterium (a new species of Sphingomonas later renamed Aurantimonas143