Microalgal blooms in the skeletons of bleached corals during the 2020 bleaching event on Heron Island, Australia

Abstract

Climate change is increasing the frequency of marine heatwaves around the world, causing widespread degradation of coral reefs. Endolithic microalgae inhabiting the coral skeleton have been highlighted as potentially important mediators of the consequences of heatwaves on coral reefs. These microalgae often bloom during heat stress due to greater light availability, theoretically delaying coral starvation by providing photoassimilates. However, these microalgae also dissolve coral skeletons at an accelerated rate during marine heatwaves, affecting the structural complexity of the reef. Despite their ecological role, no studies have examined endolithic algal blooms during a natural bleaching event. We quantified blooms of endolithic microalgae in the skeletons of lagoon corals bleaching on Heron Island in the austral summer of 2020. At the peak of heat stress, 20-30% of bleached corals across 9 genera at 3 sites had blooms. They were predominantly seen in branching Acropora spp. (37.8, 65.7 and 66.7% at three sites), which are primary reef builders at Heron Island. At the end of the bleaching event, the overall prevalence varied between 5 and 42%, and nearly all blooms were observed in acroporids. The relative high frequency of these blooms highlights the ongoing need to understand the role of these microbes during coral bleaching events.

The impact of rising temperatures on the prevalence of coral diseases and its predictability: A global meta‐analysis

“…Such an increase in variability has never been formerly reported before, yet this can be explained by varying responses by different coral species, as with the rise in disease-free corals observed. In addition, coral reefs are complex habitats, and there is substantial variation in bleaching and heat stress responses that occur within and between reefs during bleaching events (Ainsworth et al, 2021;Fordyce et al, 2021;Page et al, 2019). A rise in disease prevalence variation with increasing average summer SST indicates that it will be more difficult to predict disease prevalence as average F I G U R E 4 Three oceans' predicted non-zero values (mu) of disease prevalence (community-level) per fixed variable.…”

The impact of rising temperatures on the prevalence of coral diseases and its predictability: a global meta-analysis

“…Such an increase in variability has never been formerly reported before, yet this can be explained by varying responses by different coral species, as with the rise in disease-free corals observed. In addition, coral reefs are complex habitats, and there is substantial variation in bleaching and heat stress responses that occur within and between reefs during bleaching events (Ainsworth et al, 2021;Fordyce et al, 2021;Page et al, 2019). A rise in disease prevalence variation with increasing average summer SST indicates that it will be more difficult to predict disease prevalence as average summer SST rises.…”