Coral transplantation triggers shift in microbiome and promotion of coral disease associated potential pathogens

Abstract

By cultivating turf algae and aggressively defending their territories, territorial damselfishes in the genus Stegastes play a major role in shaping coral-algal dynamics on coral reefs. The epilithic algal matrix (EAM) inside Stegastes' territories is known to harbor high abundances of potential coral disease pathogens. To determine the impact of territorial grazers on coral microbial assemblages, we established a coral transplant inside and outside of Stegastes' territories. Over the course of one year, the percent mortality of transplanted corals was monitored and coral samples were collected for microbial analysis. As compared to outside damselfish territories, Stegastes were associated with a higher rate of mortality of transplanted corals. However, 16S rDNA sequencing revealed that territorial grazers do not differentially impact the microbial assemblage of corals exposed to the EAM. Regardless of Stegastes presence or absence, coral transplantation resulted in a shift in the coral-associated microbial community and an increase in coral disease associated potential pathogens. Further, transplanted corals that suffer low to high mortality undergo a microbial transition from a microbiome similar to that of healthy corals to that resembling the EAM. These findings demonstrate that coral transplantation significantly impacts coral microbial communities, and transplantation may increase susceptibility to coral disease.

Figure 1. Mortality of transplanted corals in Stegastes’ territories after six months.

(a) Low mortality (0–20%), (b) partial mortality (20–80%), and (c) high mortality (80–99%). Photographs taken by J.M.C.

Figure 2. Percent of transplanted coral fragments (±SE) that suffered mortality after six months of transplantation.

Mortality is categorized as low mortality (0–20%), partial mortality (20–80%), and high mortality (80–99%). Treatments include control plots outside damselfish territories (35 fragments), inside S. apicalis’ territories (34 fragments), and inside S. nigricans’ territories (36 fragments). Bars represent means and standard errors of percentages of fragments in each mortality category across control plots outside damselfish territories and inside different damselfish territories (n = 20 territories in each case). Asterisks indicate significant values (p < 0.05).

Figure 3. Relative abundances of autotrophs, heterotrophs, and potential pathogens in coral fragments.

The relative abundance (±SE) of (a) autotrophs, (b) heterotrophs, and (c) potential pathogens according to damselfish presence (control plots outside damselfish territories, inside S. apicalis’ territories, and inside S. nigricans’ territories) and time after transplantation (baseline coral fragments, transplanted coral fragments after six months, and transplanted coral fragments after one year).

Figure 4. Relative abundances of potential coral disease pathogens in coral fragments.

The relative abundance (±SE) of coral disease associated potential pathogens according to damselfish presence (control plots outside damselfish territories, inside S. apicalis’ territories, and inside S. nigricans’ territories) and time after transplantation (baseline coral fragments, transplanted coral fragments after six months, and transplanted coral fragments after one year).

Figure 5. Principal coordinates analysis (PCoA) showing the percent of variation explained in the microbial community.

Treatments include baseline coral fragments, transplanted corals with low mortality (0–20%), transplanted corals with partial mortality (20–80%), transplanted corals with high mortality (80–99%), and EAM samples (data published in Casey et al.7) outside damselfish territories, inside S. apicalis’ territories, and inside S. nigricans’ territories. The ellipses represent distinct clustering of the baseline corals, transplanted corals, and EAM samples.