Gram staining reveals diverse bacterial associations in coral cell-associated microbial aggregates in the Pacific Ocean

Cell-associated microbial aggregates (CAMAs) (also referred to as coral-associated microbial aggregates) have been observed in 24 coral species from the Pacific Ocean, and studies indicate most contain gram-negative bacilli from the genus <i>Endozoicomonas.</i> Here, we used histology with Gram staining to evaluate the morphology and distribution of CAMAs in six species of scleractinian corals from Hawaii and Palmyra. Within CAMAs, we observed the coexistence of bacteria with differing morphologies and Gram-staining properties both within and among coral species. <i>Pocillopora</i> and <i>Acropora</i> had mostly gram-negative bacilli, whereas gram-negative cocci dominated in <i>Porites</i>. <i>Acropora</i> had the highest abundance of gram-positive CAMAs. The anatomical distribution of CAMAs varied by coral species. CAMAs dominated in the tentacles of <i>Pocillopora meandrina</i>, <i>Pocillopora grandis</i>, and <i>Porites evermanni,</i> were mostly in the coenenchyme of <i>Acropora cytherea</i>, and were found equally between tentacles and coenenchyme in <i>Porites compressa</i> and <i>Porites lobata</i>. Tissue-layer distribution also varied, with CAMAs mainly in the epidermis of <i>Pocillopora</i> but in the gastrodermis of <i>Porites</i> and <i>Acropora</i>. The diversity of bacteria in CAMAs and their anatomic distribution in Pacific corals may be more complex than previously understood. This indicates other bacterial species, in addition to <i>Endozoicomonas,</i> are colonizing CAMAs in corals from the Pacific Ocean.