Contemporary reliance on bicarbonate acquisition predicts increased growth of seagrass Amphibolis antarctica in a high-CO(2) world

Rising atmospheric CO(2) is increasing the availability of dissolved CO(2) in the ocean relative to HCO(3)(−). Currently, many marine primary producers use HCO(3)(−) for photosynthesis, but this is energetically costly. Increasing passive CO(2) uptake relative to HCO(3)(−) pathways could provide energy savings, leading to increased productivity and growth of marine plants. Inorganic carbon-uptake mechanisms in the seagrass Amphibolis antarctica were determined using the carbonic anhydrase inhibitor acetazolamide (AZ) and the buffer tris(hydroxymethyl)aminomethane (TRIS). Amphibolis antarctica seedlings were also maintained in current and forecasted CO(2) concentrations to measure their physiology and growth. Photosynthesis of A. antarctica was significantly reduced by AZ and TRIS, indicating utilization of HCO(3)(−)-uptake mechanisms. When acclimated plants were switched between CO(2) treatments, the photosynthetic rate was dependent on measurement conditions but not growth conditions, indicating a dynamic response to changes in dissolved CO(2) concentration, rather than lasting effects of acclimation. At forecast CO(2) concentrations, seedlings had a greater maximum electron transport rate (1.4-fold), photosynthesis (2.1-fold), below-ground biomass (1.7-fold) and increase in leaf number (2-fold) relative to plants in the current CO(2) concentration. The greater increase in photosynthesis (measured as O(2) production) compared with the electron transport rate at forecasted CO(2) concentration suggests that photosynthetic efficiency increased, possibly due to a decrease in photorespiration. Thus, it appears that the photosynthesis and growth of seagrasses reliant on energetically costly HCO(3)(−) acquisition, such as A. antarctica, might increase at forecasted CO(2) concentrations. Greater growth might enhance the future prosperity and rehabilitation of these important habitat-forming plants, which have experienced declines of global significance.