Photosynthetic usable energy explains vertical patterns of biodiversity in zooxanthellate corals

Abstract

The biodiversity in coral reef ecosystems is distributed heterogeneously across spatial and temporal scales, being commonly influenced by biogeographic factors, habitat area and disturbance frequency. A potential association between gradients of usable energy and biodiversity patterns has received little empirical support in these ecosystems. Here, we analyzed the productivity and biodiversity variation over depth gradients in symbiotic coral communities, whose members rely on the energy translocated by photosynthetic algal symbionts (zooxanthellae). Using a mechanistic model we explored the association between the depth-dependent variation in photosynthetic usable energy to corals and gradients of species diversity, comparing reefs with contrasting water clarity and biodiversity patterns across global hotspots of marine biodiversity. The productivity-biodiversity model explained between 64 and 95% of the depth-related variation in coral species richness, indicating that much of the variation in species richness with depth is driven by changes in the fractional contribution of photosynthetically fixed energy by the zooxanthellae. These results suggest a fundamental role of solar energy availability and photosynthetic production in explaining global-scale patterns of coral biodiversity and community structure along depth gradients. Accordingly, the maintenance of water optical quality in coral reefs is fundamental to protect coral biodiversity and prevent reef degradation.

Effects of surface geometry on light exposure, photoacclimation and photosynthetic energy acquisition in zooxanthellate corals

“…The change in PSII half-time (PSII t 1/2 ) as a function of light exposure was calculated with the power function: PSII t 1/2 = ME Δ , where M represents the maximum theoretical value of PSII t 1/2 , E represents the estimated light exposure at each inclination setting, and Δ denotes the rate of change of PSII t 1/2 with light availability. The values employed in this analysis for M and Δ were 28.10 and -0.50, respectively, derived from experiments conducted with the Caribbean coral Porites astreoides [ 27 ]. Despite our study being centered on a different coral species, we chose these values due the notable similar trends observed in the rates of PSII photodamage with light exposure in experiments involving several coral species [ 27 , 28 ].…”

“…The values employed in this analysis for M and Δ were 28.10 and -0.50, respectively, derived from experiments conducted with the Caribbean coral Porites astreoides [ 27 ]. Despite our study being centered on a different coral species, we chose these values due the notable similar trends observed in the rates of PSII photodamage with light exposure in experiments involving several coral species [ 27 , 28 ]. The consistency of this pattern highlights the paramount influence of light intensity on the rates of photodamage and consequent costs of PSII repair, which appear to be highly consistent among coral species and in hospite zooxanthella types.…”

“…The costs of repair from photodamage ( C a ) were determined as percentage of the total amount of photosynthetically fixed energy ( P g ) over a diurnal cycle (12 hours of daylight), through the relation: C a = (12 / PSII t 1/2 ) R , where R represents the relative energy cost of protein turnover for the reassembly of PSII reaction centers ( R = 15% as in López-Londoño, Gómez-Campo (27)). The PUES at each inclination setting was calculated by subtracting the estimated costs of repair from the photosynthetic output of the zooxanthellae ( PUES = P g — C a ) [ 27 ].…”

“…In areas from coral colonies exposed to high-light intensities, the reduction in the translocation of photosynthetic usable energy can occur due to increased costs of PSII repair in the photosynthetic apparatus of the zooxanthellae. Conversely, in areas exposed to low-light intensities, such reductions may arise as a consequence of diminished photosynthetic activity and photosynthetically fixed energy [ 27 ]. Crucially, a uniform pattern in the rates of PSII photodamage as a function of light exposure has been observed across experiments involving multiple coral species subjected to varying intensity and quality of light [ 27 , 28 ].…”

“…Conversely, in areas exposed to low-light intensities, such reductions may arise as a consequence of diminished photosynthetic activity and photosynthetically fixed energy [ 27 ]. Crucially, a uniform pattern in the rates of PSII photodamage as a function of light exposure has been observed across experiments involving multiple coral species subjected to varying intensity and quality of light [ 27 , 28 ]. The consistency of these trends emphasizes the pivotal role of light intensity in determining the rates of PSII photodamage.…”