Outplanted Acropora cervicornis microbiome
A pervasive association exists between the bacterium Aquarickettsia rohweri and Caribbean Acropora, as A. rohweri dominates the microbiomes of field-collected samples of this coral species. In particular, this bacterial species is highly abundant in genotypes of A. cervicornis susceptible to white band disease with reduced abundance in disease-resistant genotypes. A. rohweri, a member of the order Rickettsiales, is hypothesized to be an obligate symbiont dependent on the coral holobiont for nutrition and energy. Many other closely-related parasites within Rickettsiales are transmitted vertically, and A. rohweri is unlikely to persist in a free-living stage due to its reduced metabolic capabilities. This bacterial parasite was therefore expected to be transmitted vertically from one acroporid generation to the next. However, phylogenomic analyses of Acropora spp. and A. rohweri did not reveal the co-evolutionary characteristics expected of a vertically transmitted symbiont. These characteristics could be obscured, however, by horizontal transmission between hosts. The identification of A. rohweri in evolutionarily distant aquatic hosts ranging from ctenophores to sponges also strongly supports horizontal transmission of this species. To better understand the transmission dynamics of Aquarickettsia, populations of this bacteria were quantified in early life stages of A. cervicornis produced and raised in a land-based nursery setting at Mote Marine Laboratory in the Florida Keys. These corals were produced via controlled two-parent crosses involving six different genotypes across three annual spawning events. We found that Aquarickettsia was absent from these captive-raised individuals though present in parental genotypes maintained in Mote Marine Laboratorys in situ coral nursery. In March 2021, offspring were transferred to the same in situ nursery or outplanted to reef plots either near to or far from adult A. cervicornis to determine if proximity to other colonies affected parasite acquisition. Corals were sampled one week, one month, and two months post-transplantation to assess timing of Aquarickettsia infection and to examine shifts in the coral microbiome overall due to transplantation location. As tens of thousands of A. cervicornis are outplanted onto degraded Florida reefs each year, we aim to characterize routes of acquisition of this dominant parasite of Acropora to inform improved restoration practices.