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Symbiotic associations of the deepest recorded photosynthetic scleractinian coral (172 m depth)


The symbiosis between scleractinian corals and photosynthetic algae from the family Symbiodiniaceae underpins the health and productivity of tropical coral reef ecosystems. While this photosymbiotic association has been extensively studied in shallow waters (<30 m depth), we do not know how deeper corals, inhabiting large and vastly underexplored mesophotic coral ecosystems, modulate their symbiotic associations to grow in environments that receive less than 1% of surface irradiance. Here we report on the deepest photosymbiotic scleractinian corals collected to date (172 m depth), and use amplicon sequencing to identify the associated symbiotic communities. The corals, identified as Leptoseris hawaiiensis, were confirmed to host Symbiodiniaceae, predominantly of the genus Cladocopium, a single species of endolithic algae from the genus Ostreobium, and diverse communities of prokaryotes. Our results expand the reported depth range of photosynthetic scleractinian corals (0–172 m depth), and provide new insights on their symbiotic associations at the lower depth extremes of tropical coral reefs.

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Fig. 1: Sampling location of the deepest photosymbiotic scleractinian coral recorded to date.
Fig. 2: Microbial communities harbored by the three deep colonies.


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We sincerely thank the divers of the Under The Pole (UTP) Team: GB, JL, GL who performed the extreme CCR deep dive to 172 m depth to collect the specimens for this study. In addition, we are grateful to all UTP III Expedition crew who greatly contributed to the success of the one-year-long mesophotic expedition. We also thank West N for assistance with the BIO2MAR platform. We acknowledge del Campo J for constructive discussions on Ostreobium phylogeny and sharing his updated 16S-sequences database. We are also grateful for constructive reviews from Hume B and one anonymous referee. This research was funded by the ANR DEEPHOPE (ANRAAPG 2017 #168722), the Délégation à la Recherche DEEPCORAL, the CNRS DEEPREEF and the IFRECOR. MM was supported by NSF OCE 1442206. The technical dives for coral sampling were funded through the Under The Pole Expedition III.

Under The Pole Consortium

G. Bardout9, E. Périé-Bardout9, E. Marivint9, G. Lagarrigue9, J. Leblond9, F. Gazzola9, S. Pujolle9, N. Mollon9, A. Mittau9, J. Fauchet9, N. Paulme9, R. Pete9, K. Peyrusse9, A. Ferucci9, A. Magnan9, M. Horlaville9, C. Breton9, M. Gouin9, T. Markocic9, I. Jubert9, P. Herrmann9.

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All authors conceived and designed this study; LH and HR coordinated the study; HR, GP-R, MP, UTP III consortium and LH collected field data; MP identified coral species; HR carried out analysis and graphics and drafted the manuscript; HR, PG, MM, JBR, PB, and LH wrote the manuscript. All authors commented on the manuscript and gave their approval to submit.

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Correspondence to Héloïse Rouzé.

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Members of the Under The Pole Consortium are listed below Acknowledgements.

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Rouzé, H., Galand, P.E., Medina, M. et al. Symbiotic associations of the deepest recorded photosynthetic scleractinian coral (172 m depth). ISME J (2021).

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