A mechanistic model of coral bleaching due to temperature-mediated light-driven reactive oxygen build-up in zooxanthellae.

Abstract

Mass coral bleaching has emerged in the 21st century as the greatest threat to the health of the world's reefs. A sophisticated process understanding of bleaching at the polyp scale has now been achieved through laboratory and field studies, but this knowledge is yet to be applied in mechanistic models of shelf-scale reef systems. In this study we develop a mechanistic model of the coral-symbiont relationship that considers temperature-mediated build-up of reactive oxygen species due to excess light, leading to zooxanthellae expulsion. The model explicitly represents the coral host biomass, as well as zooxanthellae biomass, intracellular pigment concentration, nutrient status, and the dynamics of reaction centres and the xanthophyll cycle. Photophysiological processes represented include photoadaptation, xanthophyll cycle dynamics, and reaction centre state transitions. The mechanistic model of the coral-symbiont relationship is incorporated into a $\sim$1 km resolution coupled hydrodynamic - biogeochemical model that encompasses the entire $\sim$2000 km length of the Great Barrier Reef. A simulation of the 2016 bleaching event shows the model is able to capture the broadscale features of the observed bleaching, but fails to capture bleaching on offshore reefs due to the model's grid being unable to resolve the bathymetry of shallow platforms surrounded by deep water. To further analyse the model behaviour, a $\sim$200 m resolution nested simulation of Davies Reef (18$^{\circ}$49'S, 147$^{\circ}$38'E) is undertaken. We use this nested model to demonstrate the depth gradient in zooxanthellae response to thermal stress. Finally, we discuss the uncertainties in the bleaching model, which lie primarily in quantifying the link between reactive oxygen build-up and the expulsion process. Through the mechanistic representation of environmental forcing, this model of coral bleaching applied in realistic environmental conditions has the potential to generate more detailed predictions than the presently-available satellite-based coral bleaching metrics, and can be used to evaluate proposed management strategies.

Optimising cool-water injections to reduce thermal stress on coral reefs of the Great Barrier Reef

“…The eReefs project includes semi-automated model generation that allows high-resolution models to be nested within the 1 km regional hindcast (RECOM—RElocatable Coastal Ocean Model, [ 5 , 19 ]). We created reef scale models around ∼50 reefs, 19 of which are shown in this paper.…”

“…The main drivers of coral bleaching are thermal and light stress [ 3 ]. The mechanism by which temperature and light drives bleaching is through the build-up of reactive oxygen stress inside the coral host due to inactivation of the zooxanthallae photosystem at anomalously-high summertime temperatures [ 4 , 5 ]. As a result of the direct dependence of bleaching on environmental temperature and light levels, researchers have advocated interventions to reduce light reaching corals and/or to cool the water surrounding corals [ 6 – 8 ].…”

Budidaya Karang Hias Polip Besar Pada Kedalaman Yang Berbeda Di Alam Dan Sistem Resirkulasi

“…Apabila karang mengalami stres karang akan mengalami perubahan konsentrasi pigmen warna karang, pemenuhan kebutuhan nutrien dan bahkan zooxanthellea akan keluar dari tubuh karang yang sebelumnya sebagai simbionnya (Baird et al, 2018).…”

Modeling Photoprotection at Global Scale: The Relative Role of Nonphotosynthetic Pigments, Physiological State, and Species Composition

“…Polimene et al (2012) proposed a model of DD and DT production through conversion of other pigments and de novo synthesis and hence predict the long-term photoprotective response in Bacilliarophyta and Haptophyta. In both cases, the focus on a specific pigment type or mechanism of action permitted the authors to gain insight into particular situations, such as coastal areas dominated by diatoms (Polimene et al, 2014) or corals reefs of the Great Barrier Reef (Baird et al, 2018). In both cases, the focus on a specific pigment type or mechanism of action permitted the authors to gain insight into particular situations, such as coastal areas dominated by diatoms (Polimene et al, 2014) or corals reefs of the Great Barrier Reef (Baird et al, 2018).…”

“…Gustafsson et al (2014) proposed a model that describes xanthophyll synthesis and cycling in zooxanthella and hence predicts the short-term oxidative stress that leads to coral bleaching. In both cases, the focus on a specific pigment type or mechanism of action permitted the authors to gain insight into particular situations, such as coastal areas dominated by diatoms (Polimene et al, 2014) or corals reefs of the Great Barrier Reef (Baird et al, 2018). Pigment-or mechanism-dependent models are challenging to extrapolate to diverse phytoplankton communities.…”