Direct effects of pH and inorganic carbon on macroalgal photosynthesis and growth

The maximum final pHs reached during extended photosynthesis were 8.83 for the red algae Delesseria sanguinea and 10.19 for the brown algae Fucus vesiculosus, as long as the concentrations of exceeded 1 mmol l−1, suggesting the existence of an upper pH limit for macroalgal photosynthesis. In experiments initiated with low concentrations of inorganic carbon, the final pH decreased with reduced initial pH, indicating that inorganic carbon became limiting. Effects of high O2 concentrations could be ruled out as a contributing factor in pH drift experiments initiated at or above pH 7.5. The growth rates of both species decreased when pH exceeded 8.5. However, although the growth of D. sanguinea stopped at pH 9.1–9.3, depending on acclimation, F. vesiculosus could maintain its growth up to a pH of 9.8 in these experiments. Additional experiments carried out on F. vesiculosus at two pH levels, 8 and 9.3, and a range of dissolved inorganic carbon concentrations revealed that the growth of F. vesiculosus was reduced at pH 9.3 compared with pH 8, irrespectively of inorganic carbon concentration. The results suggest that high pH may be an important, but so far overlooked, factor in regulating coastal primary production.