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.