Growth, photosynthetic properties and Rubisco activities and amounts of marine macroalgae grown under current and elevated seawater CO₂ concentrations

Growth rates, photosynthetic responses and the activity, amount and CO₂ affinity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) were determined for common marine macroalgae grown in seawater (containing 14.5 ± 2.1 μm CO₂) or CO₂- enriched seawater (averaging 52.8 ± 19.2 μm CO₂). The algae were grown in 40 L fiber- glass tanks (outdoor) for 4-15 weeks and in a field experimental setup for 5 days. Growth rates of the species studied (representing the three major divisions, i.e. Chlorophyta, Rhodophyta and Phaeophyta) were generally not significantly affected by the increased CO₂ concentrations in the seawater medium. Rubisco characteristics of algae cultivated in CO₂-enriched seawater were similar to those of algae grown in nonenriched seawater. The lack of response of photosynthetic traits in these aquatic plants is likely to be because of the presence of CO₂ concentrating mechanisms (CCMs) which rely on HCO₃^- utilization, the inorganic carbon (Ci) form that dominates the total Ci pool available in seawater. Significant changes on the productivity of these particular marine algae species would not be anticipated when facing future increasing atmospheric CO₂ levels.