Implications of geometric plasticity for maximizing photosynthesis in branching corals

• Published in: Marine biology 2014-02, Vol.161 (2), p.313-328
• Main Authors: Kaniewska, P., Anthony, K. R. N., Sampayo, E. M., Campbell, P. R., Hoegh-Guldberg, O.
• Format: Article
• Language: English
• Subjects: Acropora humilis; Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Biomedical and Life Sciences; Cnidaria. Ctenaria; Coral reefs; Corals; Frequency distribution; Freshwater & Marine Ecology; Fundamental and applied biological sciences. Psychology; Genotype & phenotype; Invertebrates; Life Sciences; Marine; Marine & Freshwater Sciences; Marine biology; Microbiology; Morphology; Oceanography; Original Paper; Phenotypic plasticity; Photoreception; Photosynthesis; Plasticity; Potential energy; Sea water ecosystems; Sensitivity analysis; Synecology; Underwater light; Zoology
• ISSN: 0025-3162; 1432-1793
• DOI: 10.1007/s00227-013-2336-z

Reef-building corals are an example of plastic photosynthetic organisms that occupy environments of high spatiotemporal variations in incident irradiance. Many phototrophs use a range of photoacclimatory mechanisms to optimize light levels reaching the photosynthetic units within the cells. In this study, we set out to determine whether phenotypic plasticity in branching corals across light habitats optimizes potential light utilization and photosynthesis. In order to do this, we mapped incident light levels across coral surfaces in branching corals and measured the photosynthetic capacity across various within-colony surfaces. Based on the field data and modelled frequency distribution of within-colony surface light levels, our results show that branching corals are substantially self-shaded at both 5 and 18 m, and the modal light level for the within-colony surface is 50 μmol photons m −2  s −1 . Light profiles across different locations showed that the lowest attenuation at both depths was found on the inner surface of the outermost branches, while the most self-shading surface was on the bottom side of these branches. In contrast, vertically extended branches in the central part of the colony showed no differences between the sides of branches. The photosynthetic activity at these coral surfaces confirmed that the outermost branches had the greatest change in sun- and shade-adapted surfaces; the inner surfaces had a 50 % greater relative maximum electron transport rate compared to the outer side of the outermost branches. This was further confirmed by sensitivity analysis, showing that branch position was the most influential parameter in estimating whole-colony relative electron transport rate (rETR). As a whole, shallow colonies have double the photosynthetic capacity compared to deep colonies. In terms of phenotypic plasticity potentially optimizing photosynthetic capacity, we found that at 18 m, the present coral colony morphology increased the whole-colony rETR, while at 5 m, the colony morphology decreased potential light utilization and photosynthetic output. This result of potential energy acquisition being underutilized in shallow, highly lit waters due to the shallow type morphology present may represent a trade-off between optimizing light capture and reducing light damage, as this type morphology can perhaps decrease long-term costs of and effect of photoinhibition. This may be an important strategy as opposed to adopting a type morphology,