Carbon-concentrating mechanisms in acidophilic algae

Seven phylogenetically diverse strains of microalgae were isolated from a naturally acidic lake and river system in Patagonia. Half-saturation constants for CO2 at atmospheric levels of oxygen varied between 2 and 12.7 mu mol L-1. CO2 uptake was oxygen sensitive in the spheroidal form of Watanabea sp. (Trebouxiophyceae), Palmellopsis sp. (Chlorophyceae) and an unidentified, Prymnesiophyte but insensitive in the ellipsoidal form of Watanabea sp. and ill Euglena mutabilis (Euglenophyta). The kinetic evidence suggests that a carbon-concentrating mechanism (CCM) is absent in Palmellopsis sp. and Gloeochrysis sp. (Chrysophyta) and the spheroidal form of Watanabea sp. but present in the ellipsoidal form of Watanabea sp. The low K-1/2, mu mol L-1 in the unidentified Prymnesiophyte suggests, that a CCM may be present, but the kinetic responses were oxygen sensitive; although, K-1/2 was not affected by oxygen. Cat-boil uptake in E mutabilis was not oxygen sensitive. but the K-1/2 was 6.4 mu mol L-1, which makes the diagnosis of the presence of a CCM equivocal. The results for Watanabea sp. appear to be the first where different forms of the same species possess or lack a CCM when grown under identical conditions. Rates of growth of Watanabea sp., largely comprising the spheroidal form, were between 86 and 95%, of maximal rates at air equilibrium concentrations of CO2, depending on the oxygen concentration. The high affinity of these species for CO2 even in the apparent absence of a CCM, suggests that when an acid lake is close to atmospheric equilibrium, rates of photosynthesis and growth are unlikely to be limited by CO2.