What physiological acclimation supports increased growth at high CO2 conditions?

Abstract

Chlamydomonas acidophila Negoro is a green algal species abundant in acidic waters (pH 2-3.5), in which inorganic carbon is present only as CO(2). Previous studies have shown that aeration with CO(2) increased its maximum growth rate, suggesting CO(2) limitation under natural conditions. To unravel the underlying physiological mechanisms at high CO(2) conditions that enables increased growth, several physiological characteristics from high- and low-CO(2)-acclimated cells were studied: maximum quantum yield, photosynthetic O(2) evolution (P(max)), affinity constant for CO(2) by photosynthesis (K(0.5,p)), a CO(2)-concentrating mechanism (CCM), cellular Rubisco content and the affinity constant of Rubisco for CO(2) (K(0.5,r)). The results show that at high CO(2) concentrations, C. acidophila had a higher K(0.5,p), P(max), maximum quantum yield, switched off its CCM and had a lower Rubisco content than at low CO(2) conditions. In contrast, the K(0.5,r) was comparable under high and low CO(2) conditions. It is calculated that the higher P(max) can already explain the increased growth rate in a high CO(2) environment. From an ecophysiological point of view, the increased maximum growth rate at high CO(2) will likely not be realised in the field because of other population regulating factors and should be seen as an acclimation to CO(2) and not as proof for a CO(2) limitation.