The effect of temperature and aquaria conditions on bacterial communities inhabiting healthy tissues of Diploria strigosa

The occurrence of coral diseases associated with bacterial infection increases dramatically with increasing sea surface temperatures. However, the mechanism by which elevated seawater temperature actually induces or causes coral disease is poorly understood. One possibility is that increasing seawater temperature causes a change in the community structure of bacteria inhabiting healthy coral tissues, To assess the effect of increasing seawater temperature on the bacterial communities inhabiting the brain coral D. strigosa, colonies were collected directly from the reef were compared to colonies maintained in both heated and control flow-through aquaria. Heated aquaria were maintained at 30.5o C (2.5o C above ambient) and coral colonies were left in the aquaria for up to two weeks prior to sampling. Bacterial assemblages were compared statistically through the use of Terminal-Restriction Fragment Length Polymorphisms (T-RFLP) of 16S rRNA genes. The bacterial communities inhabiting coral colonies maintained in the unheated flow-through aquaria were statistically different from colonies sampled directly from the reef. However, colonies maintained in the heated aquaria were statistically similar to those sampled from the unheated aquaria. K e y w o r d s Bacteria, Coral disease, T-RFLP, Temperature Introduction Recent reports have highlighted the impact of coral diseases on reefs worldwide (Green and Bruckner 2000), and suggest that both the intensity and number of different diseases may be on the rise (Richardson 1998; Harvell et al. 1999). The correlation of disease occurrence to numerous environmental factors, including temperature (Gil-Agudelo and Garzon-Ferreira 2001;Kuta and Richardson 2002; Richardson et al. 1998; Rosenberg and Ben-Haim 2002), depth (Kuta and Richardson 2002), sedimentation (Bruckner et al. 1997; Taylor 1983) nutrients (Antonius 1988; Kim and Harvell 2000; Taylor 1983) and pollution, (Antonius 1988; Bruckner et al. 1997) hints at the complex interactions between disease and the environment. Of these factors, one of the most striking correlations to disease occurrence is temperature (Kuta and Richardson 2002, Rosenberg and Ben-Haim 2002). In the case of bleaching by Vibrio shiloi increased temperature triggers the expression of virulence factors (Rosenberg and BenHaim 2002). At high seawater temperatures, V. shiloi produces an adhesin that allows it to adhere to and then penetrate coral surfaces. Once inside the coral tissue V. shiloi toxins inhibit and lyse symbiotic zooxanthellae. In other coral diseases such as plague, dark spot disease and black band disease, the correlation to temperature is less absolute, and the mechanism by which elevated temperatures affect disease incidence is poorly understood. In these cases, the relationship between temperature and disease may not be entirely tied to the pathogen(s), but instead may involve the temperature response of a whole microbial community. One of the best defense mechanisms to the growth of pathogenic microbes is the maintenance of a healthy microbial community. These microorganisms can inhibit pathogens through interspecific competition and secretion of antibiotic substances (Klaus et al. 2005; Rohwer et al. 2002, Pantos et al. 2003) It is well established that coral tissues harbor diverse bacterial communities, primarily associated with the coral surface microlayer or coelenteron (Bythell et al. 2002; Frias-Lopez et al. 2002; Gast et al. 1998; Rohwer et al. 2001). However, the affect of seawater temperature on these communities remains poorly understood. To further understand the ecological process link between coral disease and temperature, additional information is needed on how the healthy bacterial community associated with coral tissue responds to varying temperature conditions. The goal of the present study was to assess the effect of temperature on the bacterial communities of healthy colonies of Diploria strigosa. The bacterial community associated with naturally occurring colonies as well as those maintained in heated and unheated experimental aquaria were characterized through the use of terminal restriction fragment length polymorphisms (T-RFLP) of 16S rRNA genes. The variation in bacterial communities was analyzed quantitatively using a combination of statistical procedures, including multidimensional scaling (MDS), one-way analysis of similarity (ANOSIM), and