Abstract
Coral animals harbor diverse microorganisms in their tissues, including archaea, bacteria, viruses, and zooxanthellae. The extent to which coral-bacterial associations are specific and the mechanisms for their maintenance across generations in the environment are unknown. The high diversity of bacteria in adult coral colonies has made it challenging to identify species-specific patterns. Localization of bacteria in gametes and larvae of corals presents an opportunity for determining when bacterial-coral associations are initiated and whether they are dynamic throughout early development. This study focuses on the early onset of bacterial associations in the mass spawning corals Montastraea annularis, M. franksi, M. faveolata, Acropora palmata, A. cervicornis, Diploria strigosa, and A. humilis. The presence of bacteria and timing of bacterial colonization was evaluated in gametes, swimming planulae, and newly settled polyps by fluorescence in situ hybridization (FISH) using general eubacterial probes and laser-scanning confocal microscopy. The coral species investigated in this study do not appear to transmit bacteria via their gametes, and bacteria are not detectable in or on the corals until after settlement and metamorphosis. This study suggests that mass-spawning corals do not acquire, or are not colonized by, detectable numbers of bacteria until after larval settlement and development of the juvenile polyp. This timing lays the groundwork for developing and testing new hypotheses regarding general regulatory mechanisms that control bacterial colonization and infection of corals, and how interactions among bacteria and juvenile polyps influence the structure of bacterial assemblages in corals.
Highlights
Coral holobionts are dynamic assemblages consisting of the animal host, symbiotic dinoflagellates, bacteria, archaea, fungi, and viruses [1]
EUB338 fluorescence in situ hybridization (FISH) results from M. faveolata early development, a time series of stages spanning from newly released eggs to 24 h post-settlement (Figure 3a–f), show that bacteria were not detectable in the eggs or in M. faveolata planulae through 120 h post-release
In the spawning corals examined in this study, throughout the 5-day swimming larval period, bacteria did not appear to be taken up by planulae, nor were they detectable on the planula surface
Summary
Coral holobionts are dynamic assemblages consisting of the animal host, symbiotic dinoflagellates (zooxanthellae), bacteria, archaea, fungi, and viruses [1]. There is still little known about what controls coral-bacterial interactions and whether true symbioses, or long-term species-specific associations between corals and bacteria, exist. Pandemic outbreaks of diverse bacterial diseases in corals across the globe, among other factors including the demise of herbivorous fish and sea urchins, have facilitated the overgrowth and dominance of macroalgae on reefs and dramatically shifted the ecology of reef habitats [5,6]. Though identification of pathogens is critical for management and prevention of coral diseases, it is important to define and establish a baseline for bacterial diversity associated with healthy corals. A thorough understanding of the dynamic coralassociated bacterial communities, how they establish interactions with coral animals, and the roles they play in coral health is critical for effective reef management
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