Distinct Bacterial Communities Associated with the Coral Model Aiptasia in Aposymbiotic and Symbiotic States with Symbiodinium

Abstract

Coral reefs are in decline. The basic functional unit of coral reefs is the coral metaorganism or holobiont consisting of the cnidarian host animal, symbiotic algae of the genus Symbiodinium, and a specific consortium of bacteria (among others), but research is slow due to the difficulty of working with corals. Aiptasia has proven to be a tractable model system to elucidate the intricacies of cnidarian-dinoflagellate symbioses, but characterization of the associated bacterial microbiome is required to provide a complete and integrated understanding of holobiont function. In this work, we characterize and analyze the microbiome of aposymbiotic and symbiotic Aiptasia and show that bacterial associates are distinct in both conditions. We further show that key microbial associates can be cultured without their cnidarian host. Our results suggest that bacteria play an important role in the symbiosis of Aiptasia with Symbiodinium, a finding that underlines the power of the Aiptasia model system where cnidarian hosts can be analyzed in aposymbiotic and symbiotic states. The characterization of the native microbiome and the ability to retrieve culturable isolates contributes to the resources available for the Aiptasia model system. This provides an opportunity to comparatively analyze cnidarian metaorganisms as collective functional holobionts and as separated member species. We hope that this will accelerate research into understanding the intricacies of coral biology, which is urgently needed to develop strategies to mitigate the effects of environmental change.

Laboratory-Cultured Strains of the Sea Anemone Exaiptasia Reveal Distinct Bacterial Communities

“…Symbiotic Exaiptasia of the clonal strain H2 were cultured in clear polycarbonate containers (2 L capacity; Cambro Camwear, USA) filled with autoclaved natural seawater collected from the Red Sea (∼39 PSU salinity) as described in Röthig et al (2016a). Tanks were kept in Intellus Control System Incubators (Model I-22LLVL, Percival Scientific, USA) at 25 • C on a diurnal 12 h light: 12 h dark cycle under whitelight (20-40 µmol photons m −2 s −1 of photosynthetically active radiation).…”

“…Five anemones and one water sample were processed as described in Röthig et al (2016a). Briefly, polyps of ∼1.0 cm size were collected and transferred into 1.5 mL microtubes, and remaining water was carefully removed.…”

“…To allow direct comparisons between the microbiomes of Exaiptasia H2 (this study) and Exaiptasia CC7 (Röthig et al, 2016a), data obtained in this study was processed following the same procedure described by Röthig et al (2016a) for the previously published CC7 data. Sequences were demultiplexed, quality trimmed, preclustered (2 bp difference; Huse et al, 2010), and aligned against the SILVA database (release 119; Pruesse et al, 2007).…”

“…Microbial community relationships can be highly conserved as observed in the cnidarian Hydra sp., where speciesspecific bacterial phylotypes were maintained over long periods of time and across different environments (Fraune and Bosch, 2007). Bacterial associations of Exaiptasia have, however, not been examined until recently (Röthig et al, 2016a), showing that despite its simple body organization, this sea anemone can be associated with diverse bacterial communities. Here, we characterized the bacterial assemblage of symbiotic Exaiptasia H2 (originally isolated from Hawaii, USA) and compared it to the previously published microbiome of Exaiptasia CC7 (originally obtained from North Carolina, USA; Röthig et al, 2016a), which belong to the "global" and "local" lineages, respectively (Grawunder et al, 2015).…”

“…Bacterial associations of Exaiptasia have, however, not been examined until recently (Röthig et al, 2016a), showing that despite its simple body organization, this sea anemone can be associated with diverse bacterial communities. Here, we characterized the bacterial assemblage of symbiotic Exaiptasia H2 (originally isolated from Hawaii, USA) and compared it to the previously published microbiome of Exaiptasia CC7 (originally obtained from North Carolina, USA; Röthig et al, 2016a), which belong to the "global" and "local" lineages, respectively (Grawunder et al, 2015). Knowledge on the bacterial composition of Exaiptasia will be vital for its establishment as a model system for coral research.…”