White syndrome in Acropora muricata: nonspecific bacterial infection and ciliate histophagy

Selective antibiotic treatment of white syndrome (WS)-affected corals (Acropora muricata) from Fiji was used to identify 3 potential bacterial pathogens of the disease. Interestingly, the suite of bacterial associates of the disease was different to that recently identified using identical primer sets for WS on the GBR and in the Solomon Islands. In addition to the three bacterial pathogenic candidates and as previously shown for WS and more recently for white band disease (WBD) in the Caribbean, all samples of the disease were specifically associated with the histophagous ciliate Philaster lucinda. From the pattern of disease progression and histopathology in relation to the selective elimination of microbial groups, we conclude that these 'white' diseases are a result of a nonspecific bacterial infection and a 'secondary' infection by the P. lucinda ciliate. Although we have not observed the initiation of infection, a nonspecific, multispecies bacterial infection appears to be a corequirement for WS lesion progression and we hypothesize that the bacterial infection occurs initially, weakening the defences of the host to predation by the ciliates. Such ciliate histophagy gives rise to the characteristic white band of denuded coral skeleton that gives these diseases their names. The characteristics of the microbial communities of WBD and WS appear identical, and since the bacterial associates of WS vary geographically (and/or temporally), there appears to be no logical distinction between WS in the Indo-Pacific and WBD in the Caribbean.