The CO₂-concentrating mechanism in the physiological context: lowering the CO₂ supply diminishes culture growth and economises starch utilisation in Chlamydomonas reinhardtii

Abstract.

In a synchronously grown Chlamydomonas reinhardtii (Chlorophyceae) culture the CO₂-concentrating mechanism (CCM) was induced by lowering the CO₂ level from 4% to 0.036% CO₂ (culture HL). The effects of the reduced carbon supply on starch levels were studied over a period of up to 100 h and compared with control cultures kept either at 4% CO₂ (culture H) or continuously at ambient air (0.036% CO₂, culture L). Lowering the CO₂ supply reduced culture growth as estimated by chlorophyll, protein and cell density. The starch level continued to show diurnal variations with an initially reduced rate of starch synthesis at reduced or abolished culture growth. Subsequently, starch maxima and minima increased. After 4 days the resulting pattern for culture HL was similar to that of culture L, which possessed higher minima but identical maxima to culture H. The intracellular starch localisation was examined on electron micrographs. Cell extracts were assayed for ADP-glucose pyrophosphorylase (EC 2.7.7.27) and starch phosphorylase (EC 2.4.1.1) activities. Over the assayed period of 2 days, there was a good correlation between the observed changes in the starch levels and the measured enzyme activities. The rate of CO₂-dependent oxygen evolution of culture HL declined from 100% to 60% of the control over the day. This indicates that the diminished or abolished growth and the impairment of starch accumulation upon CO₂ depletion are not simply consequences of the lowered level of the substrate CO₂. The diminished growth and the peculiar starch accumulation pattern with higher positions of the starch minima in low-CO₂ cells are interpreted as economised starch utilisation as long-term aspects of induction of the CCM.