Modulation of Light-Enhancement to Symbiotic Algae by Light-Scattering in Corals and Evolutionary Trends in Bleaching

Abstract

Calcium carbonate skeletons of scleractinian corals amplify light availability to their algal symbionts by diffuse scattering, optimizing photosynthetic energy acquisition. However, the mechanism of scattering and its role in coral evolution and dissolution of algal symbioses during “bleaching” events are largely unknown. Here we show that differences in skeletal fractal architecture at nano/micro-lengthscales within 96 coral taxa result in an 8-fold variation in light-scattering and considerably alter the algal light environment. We identified a continuum of properties that fall between two extremes: (1) corals with low skeletal fractality that are efficient at transporting and redistributing light throughout the colony with low scatter but are at higher risk of bleaching and (2) corals with high skeletal fractality that are inefficient at transporting and redistributing light with high scatter and are at lower risk of bleaching. While levels of excess light derived from the coral skeleton is similar in both groups, the low-scatter corals have a higher rate of light-amplification increase when symbiont concentration is reduced during bleaching, thus creating a positive feedback-loop between symbiont concentration and light-amplification that exposes the remaining symbionts to increasingly higher light intensities. By placing our findings in an evolutionary framework, in conjunction with a novel empirical index of coral bleaching susceptibility, we find significant correlations between bleaching susceptibility and light-scattering despite rich homoplasy in both characters; suggesting that the cost of enhancing light-amplification to the algae is revealed in decreased resilience of the partnership to stress.

Effects of seawater pCO2 and temperature on calcification and productivity in the coral genus Porites spp.: an exploration of potential interaction mechanisms

“…Calcification rates, normalised to coral surface area, did not vary significantly between replicates, but both gross photosynthesis and respiration rates did vary significantly (p B 0.05 two-tailed t test). Light variations over the tank footprint were small (\ 50 lmol photons m -2 s -1 ), but photosynthesis variations between replicates may reflect individual topography or modulation of light scattering (Marcelino et al 2013). We maintained each colony in the same tank position for the duration of each experiment ensuring that measurements of the same colony in different temperatures are directly comparable.…”

Physiological integration of coral colonies is correlated with bleaching resistance

“…For example, skeletal light scattering can increase light availability to in hospite Symbiodinium (Enríquez et al 2005, Stambler & Dubinsky 2005, Terán et al 2010 and is modulated by calcium carbonate microstructure (Marcelino et al 2013, Swain et al 2016b in concert with optical properties of coral tissues (Kühl et al 1995, Wangpraseurt et al 2012. Coloniality may be intertwined with the relationship between internal light enhancement and bleaching response as a contributing factor to observed species-specific differential bleaching resistance, or it may directly influence the internal light environment itself (En ríquez et al 2017) and thereby contribute to differ ential bleaching.…”

The New Systematics of Scleractinia: Integrating Molecular and Morphological Evidence

“…Therefore, these genes are more informative for reconstructing deep coral phylogenies. Other than the 16S rDNA that established widespread subordinal non-monophyly Palumbi 1996, 1997;Le GoffVitry et al 2004), 12S rDNA, cytochrome b and cytochrome c oxidase subunit I (COI) were purposed for corals relatively early (Medina et al 1999;van Oppen et al 1999;Fukami et al 2000) and have been used for inferring large scleractinian trees effectively (Chen et al 2002;Fukami et al 2004bFukami et al , 2008Barbeitos et al 2010;Kitahara et al 2010bKitahara et al , 2013Stolarski et al 2011;Arrigoni et al 2012Arrigoni et al , 2014cHuang 2012;Huang andRoy 2013, 2015;Marcelino et al 2013;Curnick et al 2015; Fig. 4.1).…”