Potential local adaptation of corals at acidified and warmed Nikko Bay, Palau

Atsushi Watanabe,Marine Gouezo,Takashi Kawai,Chuki Hongo,Katsunori Kimoto,Haruko Kurihara,Izumi Mimura,Yimnang Golbuu,James Davis Reimer,Asami Tsugi

doi:10.1038/s41598-021-90614-8

Abstract

Ocean warming and acidification caused by increases of atmospheric carbon dioxide are now thought to be major threats to coral reefs on a global scale. Here we evaluated the environmental conditions and benthic community structures in semi-closed Nikko Bay at the inner reef area in Palau, which has high pCO2 and seawater temperature conditions with high zooxanthellate coral coverage. Nikko Bay is a highly sheltered system with organisms showing low connectivity with surrounding environments, making this bay a unique site for evaluating adaptation and acclimatization responses of organisms to warmed and acidified environments. Seawater pCO2/Ωarag showed strong gradation ranging from 380 to 982 µatm (Ωarag: 1.79–3.66), and benthic coverage, including soft corals and turf algae, changed along with Ωarag while hard coral coverage did not change. In contrast to previous studies, net calcification was maintained in Nikko Bay even under very low mean Ωarag (2.44). Reciprocal transplantation of the dominant coral Porites cylindrica showed that the calcification rate of corals from Nikko Bay did not change when transplanted to a reference site, while calcification of reference site corals decreased when transplanted to Nikko Bay. Corals transplanted out of their origin sites also showed the highest interactive respiration (R) and lower gross photosynthesis (Pg) to respiration (Pg:R), indicating higher energy acquirement of corals at their origin site. The results of this study give important insights about the potential local acclimatization and adaptation capacity of corals to different environmental conditions including pCO2 and temperature.

Highlights

Increases in atmospheric carbon dioxide (CO2) simultaneously cause both ocean warming and acidification, which are thought to be the major threats to coral reefs on a global s cale[1,2]
The seawater conditions found in this bay have been suggested to have been maintained at least for the past 150–500 years[12] and long-term selection of coral larvae may have allowed local adaptation or acclimatization responses to environmental conditions found within this bay
Seawater surface pH, Ωarag and temperatures (SST) showed a strong gradient at the entrance into the bay (Fig. 2a, b, e) and the seawater pH range (7.65–8.02) observed within the bay was equivalent to the ocean pH value from present to the value expected by the end of this century (IPCC 2013, RCP 8.5)[29]

Summary

Introduction

Increases in atmospheric carbon dioxide (CO2) simultaneously cause both ocean warming and acidification, which are thought to be the major threats to coral reefs on a global s cale[1,2]. To evaluate the correlation among seawater carbonate chemistry and benthic community structure, six sites (N2-N7) along with the Ωarag gradient (1.28–3.51) inside the bay and one site outside of Nikko Bay (N1) were selected for benthic community observation (Fig. 1, Table S3).

Results

Conclusion