Pollock, F. Joseph, Wada, Naoisha, Torda, Gergely, Willis, Bette L., and Bourne, David G. (2017) White syndrome-affected corals have a distinct microbiome at disease lesion fronts. Applied and Environmental Microbiology, 83 (2). e02799-16. pp. 1-15.

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Abstract

Tissue loss coral diseases, collectively known as white syndromes (WSs), induce significant mortality on reefs throughout the Indo-Pacific, yet definitive confirmation of WS etiologies remains elusive. In this study, we integrate ecological disease monitoring, bacterial community profiling, in situ visualization of microbe/host interactions, and cellular responses of the host coral through an 18-month repeated sampling regime. We assert that the observed pathogenesis of WS lesions on acroporid corals at Lizard Island (Great Barrier Reef) is not the result of apoptosis or infection by Vibrio bacteria, ciliates, fungi, cyanobacteria or helminthes. Histological analyses detected helminths, ciliates, fungi and cyanobacteria in less than 25% of WS samples, and helminthes and fungi were also observed in 12% of visually healthy samples. The abundance of Vibrio-affiliated sequences (assessed using 16S rRNA amplicon sequencing) did not differ significantly between health states and never exceeded 3.3% of reads in any individual sample. In situ visualization detected Vibrio bacteria only in summer WS lesion samples and revealed no signs of these bacteria in winter disease samples (or any healthy tissue samples), despite continued disease progression year-round. However, a 4-fold increase in Rhodobacteraceae-affiliated bacterial sequences at WS lesion fronts suggests this group of bacteria could play a role in WS pathogenesis and/or serve as a diagnostic criterion for disease differentiation. While the causative agent(s) underlying WSs remain elusive, the microbial and cellular processes identified in this study will help to identify and differentiate visually similar, but potentially distinct WS etiologies.

Importance: Over the past decade, a virulent group of coral diseases known as white syndromes have impacted coral reefs throughout Indian and Pacific Oceans. This manuscript provides the first detailed case study of white syndromes to combine disease ecology, high throughput microbial community profiling and cellular-scale host-microbe visualization over seasonal time scales. We provide novel insights into the etiology of this devastating disease and reveal new diagnostic criteria that could be used to differentiate visually similar, but etiologically distinct forms of white syndrome.

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