Changes in carbon uptake mechanisms in two green marine algae by reduced seawater pH

Abstract

Acidification of the oceans, as a consequence of anthropogenic CO2 emissions has reduced the pH at the surface of oceans by 0.1 unit compared to pre-industrial values. The reduction of seawater pH changes the relative proportion of the inorganic carbon (Ci) species which could potentially affect the modes of Ci assimilation by marine microalgae. In this study the effects of changes in external pH on the modes of Ci uptake over the pH range 5.0 to 7.5 were determined mass spectrometrically in Stichococcus minor Naegeli and S. cylindricus Butcher et Umbauk. Both species were found to tolerate a broad range of pH from pH 5.0 to 9.5 but the optimum for growth of both species was 8.2. Both species were also found to grow over a range of salinities and are best described as brackish water species rather than marine species, since they grow over wide ranges of salinity and pH. Neither species expresses external carbonic anhydrase (CA) activity. In both species, cells grown at pH 5.0, where the bulk of dissolved inorganic carbon (DIC) is in the form of CO2, active HCO3− and CO2 uptake were absent and cells appear to take up CO2 by diffusion. However, active HCO3− uptake was present in cells of both species grown at pH 6.0, 7.0 and 7.5 but active CO2 uptake was not detectable. Cells of both species, when grown at pH 8.2, display both active CO2 uptake and active HCO3− uptake.