Impacts of coral bleaching on pH and oxygen gradients across the coral concentration boundary layer: a microsensor study
Reef-building corals are surrounded by complex microenvironments (i.e. concentration boundary layers) that partially isolate them from the ambient seawater. Although the presence of such concentration boundary layers (CBLs) could potentially play a role in mitigating the negative impacts of climate change stressors, their role is poorly understood. Furthermore, it is largely unknown how heat stress-induced bleaching affects O2 and pH dynamics across the CBLs of coral, particularly in branching species. We experimentally exposed the common coral species Acropora aspera to heat stress for 13 d and conducted a range of physiological and daytime microsensor measurements to determine the effects of bleaching on O2 and pH gradients across the CBL. Heat stress equivalent to 24 degree heating days (3.4 degree heating weeks) resulted in visible bleaching and significant declines in photochemical efficiency, photosynthesis rates and photosynthesis to respiration (P/R) ratios, whereas dark respiration and calcification rates were not impacted. As a consequence, bleached A. aspera had significantly lower (− 13%) surface O2 concentrations during the day than healthy corals, with concentrations being lower than that of the ambient seawater, thus resulting in O2 uptake from the seawater. Furthermore, we show here that Acropora, and potentially branching corals in general, have among the lowest surface pH elevation of all corals studied to date (0.041 units), which could contribute to their higher sensitivity to ocean acidification. Additionally, bleached A. aspera no longer elevated their surface pH above ambient seawater values and, therefore, had essentially no [H+] CBL. These findings demonstrate that heat stress-induced bleaching has negative effects on pH elevation and [H+] CBL thickness, which may increase the overall susceptibility of coral to the combined impacts of ocean acidification and warming.
KeywordsAcropora aspera Metabolism Calcification Diffusive oxygen flux Concentration gradients Heat stress
Funding for this study was provided by the Australian Research Council (ARC) Centre of Excellence for Coral Reef Studies, the Western Australian Marine Science Institution (WAMSI), an ARC Laureate Fellowship awarded to MM and an ARC DECRA Award (DE160100668) awarded to SC.
Compliance with ethical standards
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
- Cai W-J, Ma Y, Hopkinson BM, Grottoli AG, Warner ME, Ding Q, Hu X, Yuan X, Schoepf V, Xu H, Han C, Melman TF, Hoadley KD, Pettay DT, Matsui Y, Baumann JH, Levas S, Ying Y, Wang Y (2016) Microelectrode characterization of coral daytime interior pH and carbonate chemistry. Nat Comm 7:11144CrossRefGoogle Scholar
- Chan NCS, Wangpraseurt D, Kühl M, Connolly S (2016) Flow and coral morphology control coral surface pH: Implications for the effects of ocean acidification. Frontiers in Marine Science 3: https://doi.org/10.3389/fmars.2016.00010
- Dickson AG, Sabine CL, Christian JR (2007) Guide to Best Practices for Ocean CO2 Measurements. PICES Special Publication 3:191 ppGoogle Scholar
- Hoegh-Guldberg O, Mumby PJ, Hooten AJ, Steneck R, Greenfield P, Gomez E, Harvell CD, Sale PF, Edwards AJ, Caldeira K, Knowlton N, Eakin CM, Iglesias-Prieto R, Muthiga N, Bradbury RH, Dubi A, Hatziolos ME (2007) Coral reefs under rapid climate change and ocean acidification. Science 318:1737–1742CrossRefGoogle Scholar
- Hughes TP, Anderson KD, Connolly SR, Heron SF, Kerry JT, Lough JM, Baird AH, Baum JK, Berumen ML, Bridge T, Claar DC, Eakin CAM, Gilmour JP, Graham NAJ, Harrison H, Hobbs JPA, Hoey AS, Hoogenboom MO, Lowe RJ, McCulloch M, Pandolfi JM, Pratchett MS, Schoepf V, Torda G, Wilson SK (2018) Spatial and temporal patterns of mass bleaching of corals in the Anthropocene. Science 359:80–83CrossRefGoogle Scholar
- Hughes TP, Kerry JT, Álvarez-Noriega M, Álvarez-Romero JG, Anderson KD, Baird AH, Babcock RC, Beger M, Bellwood DR, Berkelmans R, Bridge TC, Butler IR, Byrne M, Cantin NE, Comeau S, Connolly SR, Cumming GS, Dalton SJ, Diaz-Pulido G, Eakin CM, Figueira WF, Gilmour JP, Harrison HB, Heron SF, Hoey AS, Hobbs J-PA, Hoogenboom MO, Kennedy EV, C-y Kuo, Lough JM, Lowe RJ, Liu G, McCulloch MT, Malcolm HA, McWilliam MJ, Pandolfi JM, Pears RJ, Pratchett MS, Schoepf V, Simpson T, Skirving WJ, Sommer B, Torda G, Wachenfeld DR, Willis BL, Wilson SK (2017) Global warming and recurrent mass bleaching of corals. Nature 543:373–377CrossRefGoogle Scholar
- IPCC (2013) Climate Change 2013: The physical science basis. Summary for Policy Makers., http://www.ipcc.ch website
- Jokiel PL, Maragos JE, Franzisket L (1978) Coral growth: buoyant weight technique. In: Stoddart DR, Johannes RE (eds) Coral Reefs: Resesarch Methods. UNESCO, Paris, pp 529–541Google Scholar
- Jorgensen BB, Revsbech NP (1985) Diffusive boundary layers and the oxygen uptake of sediments and detritus. Limnol Oceanogr 30:112–122Google Scholar
- Legendre P, Legendre L (1998) Numerical Ecology. ElsevierGoogle Scholar
- NOAA (2016) NOAA Coral Reef Watch Climatology version 2 NOAA Coral Reef Watch: MethodologyGoogle Scholar
- Wangpraseurt D, Larkum AWD, Ralph PJ, Kühl M (2012) Light gradients and optical microniches in coral tissues. Front Microbiol 3: https://doi.org/10.3389/fmicb.2012.00316