Abstract
Since the Industrial Revolution, the partial pressure of carbon dioxide (pCO2) has been increasing and global ocean surface waters have absorbed 30% of the anthropogenic CO2 released into the atmosphere. An increase in pCO2 in surface ocean waters causes an increase in bicarbonate ions (HCO3 − ) and protons (H + ) and a decrease in carbonate ions (CO3 2− ), thereby decreasing the pH and the saturation state of the seawater with respect to CO3 2− . These changes in ocean chemistry (termed ocean acidification) are expected to have negative impacts on marine calcifying organisms. Because calcifying marine primary producers are important to the carbon cycle and rocky shore habitat structure and stability, investigating how they will respond to future oceanic pCO2 levels is a relevant and important topic of research. Due to a recent strong increase in the number of studies investigating the responses of calcifying marine macroalgae to elevated pCO2, this review aims to present the state of knowledge on the response of calcifying macroalgae to ocean acidification alone and in combination with global and local stressors. We discuss the physiological responses of calcifying macroalgae to elevated pCO2 within the contexts biogeo - graphy, taxonomy, and calcification mechanisms. Generally, coralline algae that deposit high-Mg calcite are most susceptible to high pCO2, and polar species are particularly at risk. However, some dolomite-depositing species may be able to acclimate to high pCO2. Calcifiers generally show sensitivity to overgrowth and outcompetition by noncalcifying algae when grown under elevated CO2 conditions, and this trend could be amplified under conditions of high inorganic nutrients. However, it still remains unknown whether or not calcifiers will be able to adapt to their rapidly changing environments. We discuss the lack of research on this topic, and provide some suggestions for how this knowledge gap can be filled by future research.
Highlights
The purpose of this review is to discuss the stateof-the-art of research that has been conducted on calcifying macroalgae within the context of ocean acidification
We conclude that many calcifying macroalgae are sensitive to ocean acidification, but that variations in their responses are due to calcification mechanisms, skeletal mineralogy, carbon uptake mechanisms and local environmental conditions
These changes often result in communities dominated by noncalcifiers, with the exception of communities where Padina spp. are common, in which case they become prominent under elevated CO2 conditions despite decreases in skeletal inorganic carbon
Summary
The purpose of this review is to discuss the stateof-the-art of research that has been conducted on calcifying macroalgae within the context of ocean acidification. We first provide an introduction to the chemistry of ocean acidification, how it relates to calcification in the marine environment, and how the link between calcification and photosynthesis is an important factor influencing the response of calcifying macroalgae to elevated pCO2. Meta-analysis of studies (from 1987 to 2013) showing the frequency of calcification responses (no response, negative, positive, acclimation, or parabolic) reported for calcifying macroalgae exposed to elevated CO2, according to the type of carbonate mineral deposited in the skeleton and the authors suggest that these changes could make L. glaciale more susceptible to biological and physical erosion. The mechanism and location of calcification probably play a role in determining their sensitivity to low pH, but recent studies have suggested that increased dissolution rates, rather than decreased calcification rates, are the real threat to calcifying organisms under ocean acidification conditions (Ries 2011b, Rodolfo-Metalpa et al 2011, Roleda et al 2012). Higher CO2 concentrations benefit the noncalcifying alga by increasing the diffusion gradient outside the cell relative to inside, while the calcifier
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