Ocean acidification causes bleaching and productivity loss in coral reef builders

Abstract

Ocean acidification represents a key threat to coral reefs by reducing the calcification rate of framework builders. In addition, acidification is likely to affect the relationship between corals and their symbiotic dinoflagellates and the productivity of this association. However, little is known about how acidification impacts on the physiology of reef builders and how acidification interacts with warming. Here, we report on an 8-week study that compared bleaching, productivity, and calcification responses of crustose coralline algae (CCA) and branching (Acropora) and massive (Porites) coral species in response to acidification and warming. Using a 30-tank experimental system, we manipulated CO(2) levels to simulate doubling and three- to fourfold increases [Intergovernmental Panel on Climate Change (IPCC) projection categories IV and VI] relative to present-day levels under cool and warm scenarios. Results indicated that high CO(2) is a bleaching agent for corals and CCA under high irradiance, acting synergistically with warming to lower thermal bleaching thresholds. We propose that CO(2) induces bleaching via its impact on photoprotective mechanisms of the photosystems. Overall, acidification impacted more strongly on bleaching and productivity than on calcification. Interestingly, the intermediate, warm CO(2) scenario led to a 30% increase in productivity in Acropora, whereas high CO(2) lead to zero productivity in both corals. CCA were most sensitive to acidification, with high CO(2) leading to negative productivity and high rates of net dissolution. Our findings suggest that sensitive reef-building species such as CCA may be pushed beyond their thresholds for growth and survival within the next few decades whereas corals will show delayed and mixed responses.

Fig. 1.

Effects of experimental ocean acidification (CO₂ level) and warming on three key performance variables of three major coral reef builders: (A–C) crustose coralline algae (CCA, Porolithon onkodes), (D–F) branching Acropora (A. intermedia), and (G–I) massive Porites (P. lobata). Gray and black bars show low- and high-temperature treatments, respectively. Data are means ± SEM of n = 15–25 specimens for each combination of CO₂ and temperature. Levels of CO₂ represented the present-day control condition (380 ppm atmospheric CO₂) and projected scenarios for high categories IV (520–700 ppm) and VI (1000–1300 ppm) by the IPCC. See Table 1 for summary of values for water-chemistry parameters and Table S1 for results of analyses. Scale bar, 5 cm.