The coral symbiont CandidatusAquarickettsia is variably abundant in threatened Caribbean acroporids and transmitted horizontally

The symbiont “CandidatusAquarickettsia rohweri” infects a diversity of aquatic hosts. In the threatened Caribbean coral, Acropora cervicornis, Aquarickettsiaproliferates in response to increased nutrient exposure, resulting in suppressed growth and increased disease susceptibility and mortality of coral. This study evaluated the extent, as well as the ecology and evolution of Aquarickettsiainfecting threatened corals, Ac. cervicornis, and Ac. palmataand their hybrid (“Ac. prolifera”). Aquarickettsiawas found in all acroporids, with coral host and geographic location impacting the infection magnitude. Phylogenomic and genome-wide single-nucleotide variant analysis of Aquarickettsiafound phylogenetic clustering by geographic region, not by coral taxon. Analysis of Aquarickettsiafixation indices suggests multiple sequential infections of the same coral colony are unlikely. Furthermore, relative to other Rickettsiales species, Aquarickettsiais undergoing positive selection, with Florida populations experiencing greater positive selection relative to other Caribbean locations. This may be due in part to Aquarickettsiaproliferating in response to greater nutrient stress in Florida, as indicated by greater in situ replication rates in these corals. Aquarickettsiawas not found to significantly codiversify with either the coral animal or the coral’s algal symbiont (Symbiodinium“fitti”). Quantitative PCR analysis showed that gametes, larvae, recruits, and juveniles from susceptible, captive-reared coral genets were not infected with Aquarickettsia. Thus, horizontal transmission of Aquarickettsiavia coral mucocytes or an unidentified host is more likely. The prevalence of Aquarickettsiain Ac. cervicornisand its high abundance in the Florida coral population suggests that coral disease mitigation efforts focus on preventing early infection via horizontal transmission.