Ocean Acidification Differentially Affects the Photosynthesis of Key New England Macrophytes

While the influence of anthropogenic CO₂ emissions on seawater chemistry is detrimental to calcification among CaCO₃ reliant organisms such as commercially important shellfish species, non-calcareous macrophytes like seagrasses and seaweeds can experience enhanced growth under elevated pCO₂ conditions and may be a substantial, if ephemeral, CO₂ sink. Most marine macrophytes rely on enzyme conversion of HCO₃^- to supply the inorganic carbon necessary for photosynthesis; the ability to down-regulate this energetically expensive carbon acquisition under high pCO₂ conditions could determine future species success. We exposed four commercially and ecologically relevant New England macrophytes (Saccharina latissima, Fucus vesiculosus, Ulva lactuca, and Zostera marina) to pre-industrial (280 uatm), present (400 and 520 utam – recorded in Casco Bay) and future (640, 880 and 1120 uatm – as predicted by the IPCC) pCO₂ levels in 1.5 hr long respirometry assays after 72 hrs acclimation. CO₂ consumption, photosynthetic quotient (Q = CO₂ consumed:O₂ evolved), and change in carbonate saturation state (Ω_calcite) were calculated for each sample using differences in initial and final carbonate chemistry and dissolved oxygen concentrations. All species experienced increases in rate of CO₂ uptake and Q under elevated pCO₂ treatments, but response level differed across species. Saccharina latissima had the greatest relative effect on all parameters measured, consuming 4 times more carbon at high pCO₂ levels than the lowest performing species. While all macrophytes were able to raise Ω_calcite, the magnitude of change decreased at higher pCO₂ levels, suggesting a limitation to the degree to which photosynthesis can locally raise calcification potential for sensitive native or farmed populations of shellfish in the future. The varied responses observed across species have implications for future community structures and for phytoremediation efforts.