Emerging coral diseases: a temperature‐driven process?

Abstract

In the last few decades there has been a surge in research focusing on coral disease. While climate change, specifically rising sea surface temperature, has been proposed as a major and growing driver of the emergence of marine diseases, to date a solid connection between disease epizootics and elevated sea surface temperature has not been established. However, a wealth of data now exists, compiled from many different perspectives, that may support such a connection. In this work we provide a comprehensive review targeting one coral disease, black band disease, that spans the infection process, pathobiology, and epizootiology, and links specific mechanisms of the disease process to increasing temperatures. This temperature‐driven pattern of infection can be expanded to include similar processes associated with other temperature‐related coral diseases. The conclusions presented here are based upon the results of many studies using a diverse suite of approaches that have been synthesized to argue that the emergence and continuing spread of black band disease is linked to warming sea surface temperatures. In summary, as global ocean temperatures increase seasonally and over decades, the environment shifts to become more favorable for the growth of potentially pathogenic microorganisms endemic to the immediate environment of the reef. The increase in the relative number of potential pathogens in the microbial community produces microenvironments conducive to the growth of other potential pathogens, leading to infection by a polymicrobial consortium. This consortium is easily perturbed by a (seasonal) temperature decrease, but remains associated with the coral host and can be reactivated with a subsequent seasonal increase in temperature, resulting in a cycle of temperature‐dependent disease emergence.

Changes in the bacterial community associated with black band disease in a Red Sea coral, Favia sp., in relation to disease phases

“…The changes in disease signs correlate with both season and water temperature changes (Rützler et al 1983, Kuta & Richardson 1996, Zvuloni et al 2009, Miller & Richardson 2015. In order to clarify the dynamics of this disease, we assessed and compared the bacterial communities found during the 3 stages of the disease, highly active black band stage (collected during the summer when water temperature reached an average of 28°C) the waning stage (fall when water temperature average decreased to 25°C) and the non-active stage (winter when water temp average was 21°C).…”

“…In the Red Sea, as in other areas, the signs of BBD have a seasonal appearance: increasing in the summer with rising water temperatures and disappearing in winter with the decreasing water temperatures (Zvuloni et al 2009, Miller & Richardson 2015. Often the disease signs reappear on the same coral colony the following year (Carlton & Richardson 1995, Zvuloni et al 2009).…”

“…To date, a primary pathogen has not been characterized for this disease, and it is thought to be caused by a complex and diverse microbial consortium with regional specificity (Carlton & Richardson 1995, Richardson 2004, Barneah et al 2007, Arotsker et al 2009, Aeby et al 2015. Environmental parameters, such as water depth and clarity, as well as water temperature, have been correlated with this disease's prevalence and dispersal (Miller & Richardson 2015), though the roles of these parameters are as yet unclear (Rützler et al 1983, Kuta & Richardson 2002, Zvuloni et al 2009, Krediet et al 2013). For example, Sato et al (2011) found that high light intensity (440 µmol m −2 s −1 ) significantly enhanced BBD progression in the coral Montipora hispida, while high temperatures were not found to statistically significantly affect disease progression.…”

“…Differences in strains (Sekar et al 2009, Sato et al 2011 found in corals from different geographic regions may also be attributed to species specificity or to tolerance of certain cyanobacterial strains to water temperature fluctuations (5 to 10°C fluctuations in Eilat, the Red Sea). The persistence of cyanobacteria in affected corals throughout the year, even when signs are not visible during low temperatures of the winter/spring, provides further evidence for their importance in reinitiation of the disease when environmental and physiological conditions are right (Miller & Richardson 2015). Indeed in a study of the BBD in the Caribbean Casamatta et al (2012) experimentally showed that the closely related BBD-associated cyanobacterial strain Roseofilum reptotaenium infected the corals.…”

“…Aeby et al (2015) showed that the isolate from BBD-affected Montipora capitata in the Hawaiian archipelago was 99% similar to P. coralii (BgP10_4S T ) from the Red Sea. In the Caribbean a cyanobacterium Roseofilum reptotaenium was isolated from BBD-affected Diploria stri gosa and Siderastrea siderea corals; this strain was shown to synthesize microcystin-LR and experiments showed that the strain is capable of initiating an infection on healthy coral that resembles in situ BBD (Casamatta et al 2012).In the Red Sea, as in other areas, the signs of BBD have a seasonal appearance: increasing in the summer with rising water temperatures and disappearing in winter with the decreasing water temperatures (Zvuloni et al 2009, Miller & Richardson 2015. Often the disease signs reappear on the same coral colony the following year (Carlton & Richardson 1995, Zvuloni et al 2009).…”