Coral thermal resilience and gene expression after acclimatization to semi-diurnal cooling

Abstract

The sensitivity of reef corals to thermal bleaching varies across space and time. Fluctuating thermal regimes were identified as a major mitigating factor in coral bleaching severity. However, corals acclimatized to fluctuating temperatures were found to respond in different ways; some developed increased resilience after exposure to fluctuating temperatures and others reduced resilience. The molecular processes behind this phenomenon require more investigation, studies exploring acclimatization are needed across a range of coral species and thermal regimes. Hong Kong presents a somewhat unique environment for corals due to wide seasonal temperature ranges (~15°C – >32 °C), turbid waters, and pronounced daily temperature fluctuations during the hottest months. The extent to which acclimatization to this extreme thermal regime may mitigate or exacerbate coral bleaching responses remains unknown. In this study, Acropora digitifera collected in Hong Kong were acclimatized to twice-daily cooling (28 °C to 24 °C) for 30 days and then exposed to heat stress for 18 days (reaching 33.5 °C and 4.1 °C-days). Their physiological health (photosynthetic efficiency, Symbiodinium density, and chlorophyll a content) and gene expression were then compared to corals acclimatized to a constant (28 °C) temperature. It was found that acclimatization to semi-diurnal cooling did not result in any significant differences in coral physiology and gene expression during heat stress and bleaching for Acropora digitifera corals from Hong Kong, suggesting that acclimatization to pulses of low temperature may have no effect in coral thermal resilience. Instead, significant gene expression and photosynthetic efficiency differences were found between the coral colonies, with colonies sustaining less photosystem damage displaying a muted gene expression response to thermal stress. This finding helps to elucidate the complicated relationship between acclimatization and breakdown of the coral-algae symbiosis during thermal stress, which should enhance thermal bleaching predictions and targeted mitigation efforts.

Date of Award	2023
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Alex WYATT (Supervisor)