Inorganic carbon physiology underpins macroalgal responses to elevated CO2

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.

Responses of macroalgae to CO2 enrichment cannot be inferred solely from their inorganic carbon uptake strategy

“…The results from this study and prior studies (Table ) highlight that—even though carbon use strategy will play a role—the responses of macroalgae to CO 2 enrichment cannot be inferred solely from the carbon uptake strategy as has been implicated in field studies (Cornwall et al, ; Diaz‐Pulido et al, ; Hepburn et al, ). We suggest that the key to predicting the response(s) of macroalgae to a future high CO 2 ocean is to understand which species have growth rates that are limited by DIC availability under current CO 2 levels, and which species are saturated.…”

“…Extensive field surveys have been conducted to study CCM and non‐CCM species in temperate (Cornwall et al, ; Hepburn et al, ) and tropical systems (Diaz‐Pulido, Cornwall, Gartrell, Hurd, & Tran, ), and along a gradient of CO 2 /pH in a volcanic vent in Italy (Cornwall et al, ). These surveys indicate that the responses of macroalgae to CO 2 enrichment will depend on their mechanism(s) of inorganic carbon uptake, that is, their carbon uptake strategies (Hepburn et al, ).…”

“…For CCM species, the predominance of the bicarbonate ion in seawater (90% under current pH) may mean that their growth is not limited by dissolved inorganic carbon (DIC) supply and there will be no future CO 2 enrichment effect. However, field surveys have illustrated that within CCM species we can further distinguish between (a) those that are able to use the 200% additional CO 2 predicted for the future and (b) those that cannot (Cornwall et al, ; Hepburn et al, ; Kübler & Dudgeon, ): (a) Species able to use additional CO 2 either exhibit a low affinity for DIC (carbon‐limited CCM species) or have CCMs that can be downregulated (i.e., decreased CCM activity); (b) CCM species that are considered unresponsive to future CO 2 concentrations have either a high affinity for CO 2 or CCMs that cannot be downregulated under elevated CO 2 conditions (Cornwall et al, ). In contrast, growth and photosynthesis of non‐CCM species are thought to be limited under current CO 2 concentrations and they are predicted to benefit from the future 200% increase in dissolved CO 2 (Cornwall et al, ; Hepburn et al, ; Kübler & Dudgeon, ).…”

Coralline algal skeletal mineralogy affects grazer impacts

“…Evidence from other systems suggests increased dissolution of grazing mollusk shells (Hall-Spencer et al, 2008;Rodolfo-Metalpa et al, 2011), effects on radula, and changes in feeding and metabolic rates (Leung, Russell, & Connell, 2017;Russell et al, 2013), all of which will affect grazing pressure through effects on grazer abundance, age-structure, and feeding rates. Acidification effects on noncalcified macroalgae are difficult to generalize across species and environments, but are likely to affect coralline-algal grazers as noncalcified macroalgae compete with coralline algae directly and indirectly through grazer-mediated apparent competition (Celis-Plá et al, 2015;Cornwall et al, 2011Cornwall et al, , 2017Hepburn et al, 2011;Nunes et al, 2015). Overall, there is mounting evidence that trophic control in marine systems is changing and may potentially play a stabilizing role in the resonance of climate change responses across marine ecosystems (Falkenberg et al, 2013;Ghedini et al, 2015;Kroeker, Kordas, & Harley, 2017;McCoy & Pfister, 2014;McCoy et al, 2016).…”

Effects of light intensity on the photosynthetic responses ofSargassum fusiformeseedlings to future CO2rising

“…(Gao, Aruga, Asada, & Kiyohara, ) were not significantly impacted by high CO 2 levels. Therefore, macroalgae with various photosynthetic affinities for DIC may respond differentially to elevated CO 2 levels (Cornwall et al, ). In addition, environmental changes in the coastal waters and human activities within coastal ecosystems can interact with ocean acidification (Beardall, Sobrino, & Stojkovic, ).…”