Differential susceptibility of reef-building corals to deoxygenation reveals remarkable hypoxia tolerance

Maggie D Johnson,Emily N Nixon,Valerie J Paul,Sara D Swaminathan,Andrew H Altieri

doi:10.1038/s41598-021-01078-9

Abstract

Ocean deoxygenation threatens the persistence of coastal ecosystems worldwide. Despite an increasing awareness that coastal deoxygenation impacts tropical habitats, there remains a paucity of empirical data on the effects of oxygen limitation on reef-building corals. To address this knowledge gap, we conducted laboratory experiments with ecologically important Caribbean corals Acropora cervicornis and Orbicella faveolata. We tested the effects of continuous exposure to conditions ranging from extreme deoxygenation to normoxia (~ 1.0 to 6.25 mg L−1 dissolved oxygen) on coral bleaching, photophysiology, and survival. Coral species demonstrated markedly different temporal resistance to deoxygenation, and within a species there were minimal genotype-specific treatment effects. Acropora cervicornis suffered tissue loss and mortality within a day of exposure to severe deoxygenation (~ 1.0 mg L−1), whereas O. faveolata remained unaffected after 11 days of continuous exposure to 1.0 mg L−1. Intermediate deoxygenation treatments (~ 2.25 mg L−1, ~ 4.25 mg L−1) elicited minimal responses in both species, indicating a low oxygen threshold for coral mortality and coral resilience to oxygen concentrations that are lethal for other marine organisms. These findings demonstrate the potential for variability in species-specific hypoxia thresholds, which has important implications for our ability to predict how coral reefs may be affected as ocean deoxygenation intensifies. With deoxygenation emerging as a critical threat to tropical habitats, there is an urgent need to incorporate deoxygenation into coral reef research, management, and action plans to facilitate better stewardship of coral reefs in an era of rapid environmental change.

Highlights

Ocean deoxygenation threatens the persistence of coastal ecosystems worldwide
Tissue loss was first apparent in A. cervicornis on Day 2 in the severe deoxygenation treatment, and was significantly higher in only the 1.0 mg L−1 treatment relative to all others on Days 4 and 5 (Tukey’s post hoc, P < 0.05 each day) (Fig. 2b)
No tissue loss or partial mortality were observed in A. cervicornis in the 4.25 and 6.25 mg L −1 dissolved oxygen treatments (Fig. 2b)

Summary

Introduction

Ocean deoxygenation threatens the persistence of coastal ecosystems worldwide. Despite an increasing awareness that coastal deoxygenation impacts tropical habitats, there remains a paucity of empirical data on the effects of oxygen limitation on reef-building corals. Intermediate deoxygenation treatments (~ 2.25 mg L−1, ~ 4.25 mg L−1) elicited minimal responses in both species, indicating a low oxygen threshold for coral mortality and coral resilience to oxygen concentrations that are lethal for other marine organisms These findings demonstrate the potential for variability in species-specific hypoxia thresholds, which has important implications for our ability to predict how coral reefs may be affected as ocean deoxygenation intensifies. 13% of all coral reef habitat is at elevated risk to deoxygenation events, and that number is likely underestimated by an order of m agnitude[13] Such events can cause mass mortality and dramatically impact reef ecosystems and the goods and services they provide to coastal c ommunities[14]. Identifying species and genotypes with differential susceptibilities to common environmental stressors is an essential step towards understanding and better predicting how reef habitats will shift in response to escalating local and global threats, and can be used to inform strategies to protect, manage, and restore important reef h abitat[37,38]

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Conclusion