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.

Figure 1.

(a) Map of Scott Reef on the Northwest Shelf of Australia where a small remotely operated vehicle (ROV) and video benthic grab were used for sampling of reef-building corals over a large (0–60 m) depth gradient. (b) Sampling sites for genetic and physiological measurements (circles) at South Scott Reef.

Figure 2.

(a) Schematic of the measuring sequence of net photosynthesis (P_n) rate and post-illumination respiration rate (R_L) at eight incremental light levels (shown in italics, µmol photons m⁻² s⁻¹). Downward arrows indicate light off following each irradiance treatment. Steady state levels of oxygen were obtained within 15 min of each light or dark treatment. Asterisk (*) denotes initial measure of dark-adapted respiration (R_D). (b) Diagram of the calculation of R_L in the dark and P_n in the light. Rate of gross photosynthesis (P_g) for each light level was calculated as the sum of P_n and R_L measured for that light level.

Figure 3.

Box and whisker plots of the influence of depth on the physiological variables of P. speciosa at South Scott Reef. (a) Symbiont genotype; (b) P_max; (c) E_k; (d) α; (e) dark respiration; (f) P/R ratio; (g) symbiont density; (h) chlorophyll a/symbiont; and (i) total protein. For the lines in a box and whisker plot: error bars are the 95% confidence interval, the bottom and top of the box are the 25th and 75th percentiles, the line inside the box is the 50th percentile (median), and any outliers are shown as open circles.

Figure 4.

Box and whisker plots of the influence of depth on the physiological variables of S. hystrix at South Scott Reef. (a) Symbiont genotype; (b) P_max; (c) E_k; (d) α; (e) dark respiration; (f) P/R ratio; (g) symbiont density; (h) chlorophyll a/symbiont; and (i) total protein.

Figure 5.

The influence of depth on the ratio between gross photosynthesis and dark respiration of S. hystrix for each symbiont type ((a) Symbiodinium C and (b) Symbiodinium D). Symbols: filled circles, November 2008; open circles, April 2009.