Dr Wyatt Million was awarded the 2025 Maple-Brown Family Foundation Fellowship. A good understanding of coral tolerance to ocean deoxygenation is required to predict how ocean deoxygenation will change the species composition, coral cover, growth, and success of restoration on future reefs. Wyatt’s research will target this gap in knowledge using multiple ocean deoxygenation experiments that characterize the hypoxia tolerance of Lizard Island. His project update to the Foundation follows.
Problem: Ocean deoxygenation is a pervasive and growing threat to tropical coral reefs – limiting energy-demanding processes (e.g., growth and reproduction), triggering coral bleaching, and causing mortality across reefs. Determining the capacity for resilience to deoxygenation in coral is therefore a priority if we wish to assess and predict coral persistence as oceans lose their oxygen. Importantly, the link between aerobic physiology (i.e., how well a coral can “breathe”) and hypoxia tolerance (i.e., how well a coral can survive without oxygen) is unresolved, highlighting a vital knowledge gap in coral resilience to deoxygenation that if resolved can also provide easily measurable biomarkers, like aerobic thresholds, for coral survival under low oxygen.
Figure 1: Map of sampled colonies.
Objective: Characterize Lizard Island coral responses to two forms of hypoxia:
1) hours-long aerobic hypoxia challenge and 2) multi-day (presumably) lethal levels of deoxygenation. Comparison of the forms of responses will help determine if aerobic thresholds from rapid assays are predictive of lethal thresholds or if the two processes are physiological unlinked despite their shared trigger – oxygen availability.
Approach: 1) I collected fragments of six species of coral to assess hypoxia resilience across a range of taxa. Colonies originated from the near-shore reefs just north of Lizard Island Research Station (Fig. 1). 2) I performed a 10-day deoxygenation experiment exposing fragments of each species to either constant low oxygen conditions (1.3 mg O2 L-1) or ambient oxygen conditions (6.53 mg O2 L-1) generated via a bespoke nitrogen gas control system (Fig. 2A). I measured photochemical efficiency, bleaching, and mortality to determine sublethal and lethal thresholds for each species. 3) I performed incubations for individual fragments within airtight chambers to measure oxygen consumption over a full oxygen gradient (100% to 0% oxygen availability). This rapid assay (~6 hours) enables the quantification of aerobic thresholds that describe the point at which aerobic energy production becomes severely limited.
Figure 2: Average daily oxygen concentration in % air saturation that corals were exposed to within the deoxygenation (cyan) and control (salmon) treatments (A). Average photochemical efficiency, measured as maximum quantum yield (FvFm), for each species throughout the 10 day experiment (B).
Figure 3: Average aerobic thresholds (Pcmax) for each species in % air saturation. Here, lower aerobic thresholds indicate a species is better able to maintain aerobic respiration (and therefore energy production) during deoxygenation.
Outcomes: After 10 days, was no significant effect of deoxygenation exposure on mortality, bleaching, or photochemical efficiency for any species (Fig. 2B). Interestingly, similar levels of deoxygenation have led to mortality in Acroporids and bleaching in Pocilloporids from other regions after just 24 hours. This suggests Lizard Island corals, or more specifically those inhabiting near-shore reefs, possess exceptional tolerance to deoxygenation. Additionally, although there was significant variation in aerobic thresholds among the coral species (Fig. 3), this did not correspond to performance under extreme deoxygenation. When considering the experimental oxygen conditions were at or below the aerobic thresholds for 3 of 6 species, the lack of differentiation between species within the multi-day experiment highlights how little overall hypoxia tolerance depends on the ability to maintain aerobic respiration during deoxygenation.