Physiological acclimatization in Hawaiian corals following a 22-month shift in baseline seawater temperature and pH

Abstract

Climate change poses a major threat to coral reefs. We conducted an outdoor 22-month experiment to investigate if coral could not just survive, but also physiologically cope, with chronic ocean warming and acidification conditions expected later this century under the Paris Climate Agreement. We recorded survivorship and measured eleven phenotypic traits to evaluate the holobiont responses of Hawaiian coral: color, Symbiodiniaceae density, calcification, photosynthesis, respiration, total organic carbon flux, carbon budget, biomass, lipids, protein, and maximum Artemia capture rate. Survivorship was lowest in Montipora capitata and only some survivors were able to meet metabolic demand and physiologically cope with future ocean conditions. Most M. capitata survivors bleached through loss of chlorophyll pigments and simultaneously experienced increased respiration rates and negative carbon budgets due to a 236% increase in total organic carbon losses under combined future ocean conditions. Porites compressa and Porites lobata had the highest survivorship and coped well under future ocean conditions with positive calcification and increased biomass, maintenance of lipids, and the capacity to exceed their metabolic demand through photosynthesis and heterotrophy. Thus, our findings show that significant biological diversity within resilient corals like Porites, and some genotypes of sensitive species, will persist this century provided atmospheric carbon dioxide levels are controlled. Since Porites corals are ubiquitous throughout the world's oceans and often major reef builders, the persistence of this resilient genus provides hope for future reef ecosystem function globally.

Figure 1

(a) Map of four coral collection sites around the island of Oʻahu, Hawaiʻi (black text), experimental location at the Hawaiʻi Institute of Marine Biology (HIMB, blue text), and photos of the study species (b) M. capitata, (c) P. compressa, and (d) P. lobata. Photographs courtesy of Keoki Stender.

Figure 2

Representative photos of the mesocosms after twenty-two months of exposure to: (a) control, (b) ocean acidification, (c) ocean warming, and (d) future ocean treatment conditions.

Figure 3

Weekly mean ± 1 SD mid-day (12:00) (a) salinity, (b) temperature, (c) pH, and (d) pCO₂ in each experimental treatment: control (green), ocean acidification (blue), ocean warming (orange) and combined future ocean (red) throughout the 22-month experimental period. Dates provided as MM/DD/YYYY.

Figure 4

Survivorship of corals following 22 months exposure under control (CO, green), ocean acidification (OA, blue), ocean warming (OW, orange), or combined future ocean (FO, red) conditions for (a) M. capitata across sites, (b) P. compressa across site, (c) P. lobata across sites. Survivorship of the individual genets of each species collected from (d–e) Moku o Loʻe, (f–h) Sampan, (i–k) Waimānalo, and (l–n) Haleʻiwa.

Figure 5

Nonparametric multidimensional scaling plots (NMDS) of coral physiological profiles for (a) Montipora capitata, (b) Porites compressa, and (c) Porites lobata. Data colors correspond to treatments: controls (CO, green circles), ocean acidification (OA, blue diamonds), ocean warming (OW, orange squares), or combined future ocean (FO, red triangles) treatments. Summary of pairwise PERMANOVA tests between CO and each of the treatments is shown in the bottom left of each panel. Additional statistical details in Table S7.

Figure 6

Mean (±1SE) of physiological traits in (a-k) Montipora capitata, (l-v) Porites compressa, and (w-ag) Porites lobata following 22-months exposure to control (CO, green), ocean acidification (OA, blue), ocean warming (OW, orange), or combined future ocean (FO, red) experimental treatment conditions. Uppercase letters above bars indicate the results of post-hoc tests when ANOVA was significant, whereby averages sharing letters did not significantly differ from each other. Sample sizes for each variable are shown within each bar in the top row and is the same for all panels below. Statistical details in Table S8. TOC = Total Organic Carbon Flux. CTAR = Contribution of the Total acquired fixed carbon relative to Animal Respiration.