Modulation of the symbionts light environment in hospite in scleractinian corals

Abstract

The upregulation of animal chromoproteins (CPs) during thermal stress produces “colorful” bleached corals that facilitate coral recovery after bleaching. In situ measurements indicate that animal CPs present in coral tissues reduce the elevated internal light environment of the remaining symbionts in bleached or low-pigmented stressed corals. However, there is still a lack of understanding regarding the extent to which animal CPs contribute to modifying the internal light environment of the symbionts in hospite. In this study, we evaluate the effect of three animal CPs on the optical properties of the coral tissue and their internal light environment using a numerical model. The model allows estimations of the absorbance spectra of corals as a function of changes in symbiont and animal pigmentation, as well as descriptions of the light environment in hospite of the symbionts. These descriptions were derived from the quantification of the contribution of each pigment component to light absorption, together with the contribution of the coral skeleton’s reflectance. Simulations indicate that animal CPs upregulation modifies the spectral distribution and the intensity of the internal light field. Animal CPs can reduce up to 11% of the light intensity in hospite when present individually, and up to 24% when present in combination. Such reduction may play a critical role in preventing the full development of the bleached phenotype when irradiance rises to excessive levels at low coral pigmentation, facilitating coral recovery and symbiont tissue re-colonization after bleaching. Accordingly, coral’s CPs components need to also be considered when selecting coral species for future restoration efforts.

The role of host pigments in coral photobiology

“…This is of high relevance for CPs found in thintissued corals, where in the absence of host pigments the symbionts experience a light-enhancing environment in hospite, meaning light inside the tissue is higher than outside thanks to the scattering properties of tissue and skeleton (Enrıquez et al, 2005;Bollati et al, 2022;Wangpraseurt et al, 2017a). The presence of CPs can change this environment to become more light attenuating within the pigment-specific absorption spectrum (Bollati et al, 2022;Galindo-Martıńez et al, 2022), potentially resulting in a 50% reduction in chlorophyll excitation as estimated from in vitro studies (Smith et al, 2013). Besides absorption, coral host pigments can also dissipate excess light energy via elastic scattering (Lyndby et al, 2016;Wangpraseurt et al, 2017b), and FRET between different host pigments might also serve as a photoprotective mechanism via energy conversion (Gilmore et al, 2003;Salih et al, 2004).…”

“…This approach appears particularly promising to understand some of the tradeoffs of host pigment production, which are likely to be a major selection driver for color polymorphism (Gittins et al, 2014;Quick et al, 2018). For example, the trade-off between light modulation (Bollati et al, 2022;Galindo-Martıńez et al, 2022) and heat budgets (Lyndby et al, 2019) is likely critical in determining recovery potential and eventually survival of corals during bleaching events (Bollati et al, 2020).…”

Fitted Fv/Fm temperature response curves: applying lessons from plant ecophysiology to acute thermal stress experiments in coral holobionts.

“…Non-invasive complementary techniques to the maximum quantum yield of PSII, such as absorption measurements, are crucial for assessing the physiological condition of corals under heat-stress that results in coral bleaching. For instance, corals exhibit photoprotective mechanisms like tissue retraction, the production of fluorescent proteins, and endolithic algae in the skeleton, which are not detected by Fv/Fm measurements alone and change with increasing heat-stress (Salih et al 2000;Brown et al 2002;Dove 2004;Dizon et al 2021;Satoh et al 2021;Bollati et al 2022;Galindo-Martínez et al 2022b, 2022a. Absorptance (A), estimated from reflectance (R) measurements, describes the relative amount of solar energy/incident light that can potentially be used in photosynthesis for organic carbon fixation.…”