Thermal effects of tissue optics in symbiont‐bearing reef‐building corals

Abstract

Reflectance spectroscopy and microscale temperature measurements were used to investigate links between optical and thermal properties of corals. Coral tissue heating showed a species-specific linear correlation to the absorptance of incident irradiance. Heat budgets estimated from absorptance and thermal boundary layer measurements indicated differences in the relative contribution of convection and conduction to heat loss in Porites lobata and Stylophora pistillata, and a higher heat conduction into the skeleton of the thin-tissued branching S. pistillata as compared to the massive thick-tissued P. lobata. Decreasing absorptance associated with bleaching resulted in decreased surface warming of coral tissue. Action spectra of coral tissue heating showed elevated efficiency of heating at wavelengths corresponding to absorption maxima of major zooxanthellae photopigments. Generally, energy-rich radiation (, 500 nm) showed the highest heating efficiency. Speciesspecific relationships between coral tissue heating and absorptance can be strongly affected by differences in the thermal properties of the skeleton and/or tissue arrangement within the skeletal matrix, indicating a yet unresolved potential for coral shape, size, and tissue thickness to affect heat dissipation and especially the conduction of heat into the coral skeleton.

Bio-optical properties and radiative energy budgets in fed and unfed scleractinian corals (Pocillopora sp.) during thermal bleaching

“…S2). Spectral reflectance was in creased for bleached corals, thus reducing the amount of ab sorbed light energy and coral surface warming (En riquez et al 2005, Jimenez et al 2012. We found the same trend of spectral reflectance steadily increasing with thermal stress in corals from both feeding treatments (Fig.…”

“…Furthermore, the absorption of light energy is a major driver of radiative heat generation in coral tissue (Jimenez et al 2012), with the rate of coral heating being directly proportional to the amount of incident light energy (Jimenez et al 2008, Welch & van Gemert 2011. Excess heat from the tissue is dissipated via convection into the surrounding water across a thermal boundary layer (TBL), and via heat conduction down into the coral skeleton (Jimenez et al 2008).…”

In vivoimaging of coral tissue and skeleton with optical coherence tomography

“…tissue thickness and opacity control Symbiodinium light exposure via formation of distinct light gradients (Wangpraseurt et al, 2012a) that are further affected by the presence and distribution of coral host pigments (Lyndby et al, 2016;Salih et al, 2000). The thermal microenvironment of Symbiodinium in hospite is also affected by tissue optical properties (Jimenez et al, 2012;Lyndby et al, 2016).…”

Bio-optical properties and radiative energy budgets in fed and starved scleractinian corals (Pocillopora damicornis) during thermal bleaching

“…However, variable 533 chlorophyll fluorescence imaging data of fed corals did reveal a clear decrease in NPQ after 8 534 days at 30°C (T2) relative to the control corals (T0, Supplementary figure 2). Spectral 535 reflectance was increased for bleached corals, thus reducing the amount of absorbed light 536 energy and coral surface warming (Enríquez et al, 2005;Jimenez et al, 2012). We found the 537 same trend of spectral reflectance steadily increasing with thermal stress in corals from both 538 feeding treatments ( Figure 5), along with a clear negative correlation between coral spectral 539 reflectance and symbiont density (R 2 = 0.99; Supplementary figure 4).…”

“…Enhanced skeletal scattering from bleaching leads to 83 enhanced light absorption by the remaining Symbiodinium cells and can thus induce further 84 light stress, ultimately accelerating the bleaching response; a process known as the optical 85 feedback loop (Enríquez et al, 2005;Wangpraseurt et al, 2017b). 86 Furthermore, the absorption of light energy by symbiont photopigments is a major driver of 87 radiative heat generation in coral tissue (Jimenez et al, 2012), with the rate of coral heating 88 being directly proportional to the amount of incident light energy being absorbed (Jimenez et 89 al., 2008;Welch & van Gemert, 2011). Excess heat from the tissue is dissipated via convection 90 into the surrounding water across a thermal boundary layer (TBL), and via heat conduction 91 down into the coral skeleton (Jimenez et al, 2008).…”