title: Niche specialization of reef-building corals in the mesophotic zone: metabolic trade-offs between divergent Symbiodinium types.
abstract: The photobiology of two reef corals and the distribution of associated symbiont types were investigated over a depth gradient of 0-60 m at Scott Reef, Western Australia. Pachyseris speciosa hosted mainly the same Symbiodinium C type similar to C3 irrespective of sampling depth. By contrast, Seriatopora hystrix hosted predominantly Symbiodinium type D1a or D1a-like at shallow depths while those in deeper water were dominated by a Symbiodinium C type closely related to C1. The photosynthesis/respiration (P/R) ratio increased consistently with depth at the two sampling times (November 2008 and April 2009) for P. speciosa and in November 2008 only for S. hystrix, suggesting a reduction in metabolic energy expended for every unit of energy obtained from photosynthesis. However, in April 2009, shallow colonies of S. hystrix exhibited decreased P/R ratios down to depths of approximately 23 m, below which the ratio increased towards the maximum depth sampled. This pattern was mirrored by changes in tissue biomass determined as total protein content. The depth of change in the direction of the P/R ratio correlated with a shift from Symbiodinium D to C-dominated colonies. We conclude that while photobiological flexibility is vital for persistence in contrasting light regimes, a shift in Symbiodinium type may also confer a functional advantage albeit at a metabolic cost with increased depth.
authors.affiliation: Australian Institute of Marine Science, UWA Oceans Institute (M096), 35 Stirling Highway, Crawley, Western Australia 6009, Australia. t.cooper@aims.gov.au
authors.forename: Timothy F
authors.initials: TF
authors.lastname: Cooper
authors.forename: Karin E
authors.initials: KE
authors.lastname: Ulstrup
authors.forename: Sana S
authors.initials: SS
authors.lastname: Dandan
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Heyward
authors.forename: Michael
authors.initials: M
authors.lastname: Kühl
authors.forename: Andrew
authors.initials: A
authors.lastname: Muirhead
authors.forename: Rebecca A
authors.initials: RA
authors.lastname: O'Leary
authors.forename: Bibi E F
authors.initials: BE
authors.lastname: Ziersen
authors.forename: Madeleine J H
authors.initials: MJ
authors.lastname: Van Oppen
mesh_terms: D000818:Animals; D037421:Anthozoa / Q000378:metabolism*; D044822:Biodiversity; D003001:Cloning, Molecular; D057894:Coral Reefs; D004141:Dinoflagellida / Q000378:metabolism*; D017753:Ecosystem; D008969:Molecular Sequence Data; D010788:Photosynthesis; D017422:Sequence Analysis, DNA; D013045:Species Specificity; D013559:Symbiosis; D014904:Western Australia