Diurnally Fluctuating pCO2 Modifies the Physiological Responses of Coral Recruits Under Ocean Acidification.

Lei Jiang,Ya-Juan Guo,Lin Cai,Pei-Yuan Qian,Guo-Wei Zhou,Xiang-Cheng Yuan,Xin-Ming Lei,Ming-Lan Guo,Fang Zhang,Laurence John Mccook,Yu-Yang Zhang,Hui Huang,Jian-Sheng Lian

doi:10.3389/fphys.2018.01952

Abstract

Diurnal pCO2 fluctuations have the potential to modulate the biological impact of ocean acidification (OA) on reef calcifiers, yet little is known about the physiological and biochemical responses of scleractinian corals to fluctuating carbonate chemistry under OA. Here, we exposed newly settled Pocillopora damicornis for 7 days to ambient pCO2, steady and elevated pCO2 (stable OA) and diurnally fluctuating pCO2 under future OA scenario (fluctuating OA). We measured the photo-physiology, growth (lateral growth, budding and calcification), oxidative stress and activities of carbonic anhydrase (CA), Ca-ATPase and Mg-ATPase. Results showed that while OA enhanced the photochemical performance of in hospite symbionts, it also increased catalase activity and lipid peroxidation. Furthermore, both OA treatments altered the activities of host and symbiont CA, suggesting functional changes in the uptake of dissolved inorganic carbon (DIC) for photosynthesis and calcification. Most importantly, only the fluctuating OA treatment resulted in a slight drop in calcification with concurrent up-regulation of Ca-ATPase and Mg-ATPase, implying increased energy expenditure on calcification. Consequently, asexual budding rates decreased by 50% under fluctuating OA. These results suggest that diel pCO2 oscillations could modify the physiological responses and potentially alter the energy budget of coral recruits under future OA, and that fluctuating OA is more energetically expensive for the maintenance of coral recruits than stable OA.

Highlights

Since the Industrial Revolution, about one third of human-emitted CO2 has been absorbed by the ocean, resulting in ocean acidification (OA), a phenomenon characterized by declines in seawater pH, carbonate concentration and saturation state of calcium carbonate (CaCO3) (Sabine et al, 2004)
Current literature provides little information on the effects of seawater acidification on the oxidative stress and antioxidant functioning for scleractinian corals. To address these critical knowledge gaps, the present study investigated the physiological and biochemical responses of the reef coral Pocillopora damicornis to diel pCO2 fluctuations that are characteristic of their natural settings
In the fluctuating OA treatment, corals appeared to compromise asexual reproduction and reactive oxygen species (ROS) detoxification to sustain skeletal growth, indicating potential trade-offs between calcification and other key physiological processes. These findings suggest that diurnal variability in pH/carbonate chemistry is likely to be an overriding factor influencing and determining the early success and recruitment of corals under future OA

Summary

Introduction

Since the Industrial Revolution, about one third of human-emitted CO2 has been absorbed by the ocean, resulting in ocean acidification (OA), a phenomenon characterized by declines in seawater pH, carbonate concentration and saturation state of calcium carbonate (CaCO3) (Sabine et al, 2004). Numerous laboratory experiments have demonstrated negative yet variable effects of OA on coral skeletal growth, with a mean decline in calcification of 15% per unit decrease in aragonite saturation state ( Arg) (Chan and Connolly, 2013) In these empirical studies, reef corals have been exposed to static pH levels consistent with open ocean projections of seawater pH declines of 0.3–0.4 units by the year 2100. Natural variability in carbonate chemistry is characteristic of the shallow coastal reefs (Rivest et al, 2017), and pH and pCO2 could vary by up to 0.7 units and 900 μatm, respectively, over a diel cycle (Santos et al, 2011; Shaw et al, 2012; Chen et al, 2015; Silverman et al, 2015) This environmental variability may greatly confound our current understanding and predictions of OA consequences on marine organisms, especially for those inhabiting high-variance ecosystems (Rivest et al, 2017). Focus is shifting to how natural pCO2 fluctuations will interact with increasing pCO2 levels to affect the future performance of shallow marine organisms

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Conclusion