Abstract
Beneficial effects of CO2 on photosynthetic organisms will be a key driver of ecosystem change under ocean acidification. Predicting the responses of macroalgal species to ocean acidification is complex, but we demonstrate that the response of assemblages to elevated CO2 are correlated with inorganic carbon physiology. We assessed abundance patterns and a proxy for CO2:HCO3− use (δ13C values) of macroalgae along a gradient of CO2 at a volcanic seep, and examined how shifts in species abundance at other Mediterranean seeps are related to macroalgal inorganic carbon physiology. Five macroalgal species capable of using both HCO3− and CO2 had greater CO2 use as concentrations increased. These species (and one unable to use HCO3−) increased in abundance with elevated CO2 whereas obligate calcifying species, and non-calcareous macroalgae whose CO2 use did not increase consistently with concentration, declined in abundance. Physiological groupings provide a mechanistic understanding that will aid us in determining which species will benefit from ocean acidification and why.
Highlights
Benefit fleshy macroalgae, as dissolved inorganic carbon (DIC) is not always limiting for growth or photosynthesis[25]
Dictyota dichotoma was the species whose abundance increased the most between the pH 8.04 and 7.69 locations (17-fold increase). This was followed by Caulerpa racemosa (6 fold increase), Padina pavonica (5-fold increase), Sargassum muticum (3-fold increase), and Dictyopteris polypodioides (2-fold increase)
Crustose coralline algae (CCA) and 8 individual species of macroalgae decreased in abundance between the pH 8.04 and 7.69 locations
Summary
Benefit fleshy macroalgae, as DIC is not always limiting for growth or photosynthesis[25]. Hepburn et al.[18] suggested a template for predicting the responses of the future abundance of macroalgae to elevated CO2 based on their inorganic carbon physiology: calcareous species will decline, non-CCM species will increase, and CCM species will either be unaffected or may increase. To date these predictions regarding fleshy macroalgae remain untested in the field, and the groups used by Hepburn et al may even be too broad. These include group (4) the obligate calcifiers (calcareous reds and greens), and (5) non-obligate calcifiers (calcareous browns, Ochrophyta; Padina spp.) which would be grouped based on the characteristics of their CCM)
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