CARBON‐USE STRATEGIES IN MACROALGAE: DIFFERENTIAL RESPONSES TO LOWERED PH AND IMPLICATIONS FOR OCEAN ACIDIFICATION¹

Abstract

Ocean acidification (OA) is a reduction in oceanic pH due to increased absorption of anthropogenically produced CO2 . This change alters the seawater concentrations of inorganic carbon species that are utilized by macroalgae for photosynthesis and calcification: CO2 and HCO3 (-) increase; CO3 (2-) decreases. Two common methods of experimentally reducing seawater pH differentially alter other aspects of carbonate chemistry: the addition of CO2 gas mimics changes predicted due to OA, while the addition of HCl results in a comparatively lower [HCO3 (-) ]. We measured the short-term photosynthetic responses of five macroalgal species with various carbon-use strategies in one of three seawater pH treatments: pH 7.5 lowered by bubbling CO2 gas, pH 7.5 lowered by HCl, and ambient pH 7.9. There was no difference in photosynthetic rates between the CO2 , HCl, or pH 7.9 treatments for any of the species examined. However, the ability of macroalgae to raise the pH of the surrounding seawater through carbon uptake was greatest in the pH 7.5 treatments. Modeling of pH change due to carbon assimilation indicated that macroalgal species that could utilize HCO3 (-) increased their use of CO2 in the pH 7.5 treatments compared to pH 7.9 treatments. Species only capable of using CO2 did so exclusively in all treatments. Although CO2 is not likely to be limiting for photosynthesis for the macroalgal species examined, the diffusive uptake of CO2 is less energetically expensive than active HCO3 (-) uptake, and so HCO3 (-) -using macroalgae may benefit in future seawater with elevated CO2 .

The regulation of coralline algal physiology, an in situ study of <i>Corallina officinalis</i> (Corallinales, Rhodophyta)

“…winter. Maintenance of net production over July and September daytime tidal emersion, despite decreases in rock pool pCO 2 of 84 and 39 %, respectively, highlight the ability of C. officinalis to effectively utilize both CO 2 and HCO − 3 as substrates for photosynthesis, as previously noted (Cornwall et al, 2012). This allows access to the relatively high HCO − 3 concentrations in seawater when CO 2 diffusion is limiting (Koch et al, 2013).…”

The other ocean acidification problem: CO 2 as a resource among competitors for ecosystem dominance

“…Indeed, the ability of some algae with CCMs to benefit from enriched CO 2 lends insights into the potential mechanisms for CO 2 effects. Species with CCMs can shift away from HCO À 3 towards aqueous CO 2 when CO 2 levels are high [25]. Thus, aqueous CO 2 may facilitate CCM C-acquisition or make it less energetically costly, and this capacity varies among species [26].…”

Divergent responses in growth and nutritional quality of coastal macroalgae to the combination of increased pCO2 and nutrients

“…These studies used concentrations of pCO 2 expected in the next 50-100 years, whereas our study focused on the more extreme pCO 2 projection for the year 2100. However, the response of macroalgae is still highly species-specific, and neutral or negative impacts of ocean acidification on growth rate have been observed in non-calcifying macroalgae (Cornwall et al, 2012;Gutow et al, 2014;Israel and Hophy, 2002;Mercado and Gordillo, 2011). Divergent responses of macroalgae to acidification are likely due to the differences in CCM effectiveness, potentially giving certain species more independence from the environment, or CCMs are optimized for higher pH conditions and their activity is sensitive to pH (Axelsson et al, 2000;Moulin et al, 2011).…”