The CO 2 -concentrating mechanism in the physiological context: lowering the CO 2 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.

A Metabolomic Approach to Study Major Metabolite Changes during Acclimation to Limiting CO2 in Chlamydomonas reinhardtii

“…The same argument may be used to explain the increase in free bases (A,U, and G) and guanosine/uridine by RNA degradation. There are very significant interactions between CO 2 availability and nitrogen availability on the expression of the CCM in C. reinhardtii (Giordano et al, 2003), and interactions between carbon and nitrogen metabolism with changing CO 2 supply are to be expected (Thyssen et al, 2001). There seems to be no information on the effects of growth at different CO 2 concentrations on the carbon-nitrogen ratio of C. reinhardtii, and data for other algae show that an increased carbon-nitrogen ratio in microalgal cells grown at high CO 2 does occur but is by no means universal (Beardall et al, 2005b;Finkel et al, 2010).…”

“…Rearrangement of starch from granules to the pyrenoid sheath is characteristic for the early part of the low-CO 2 acclimation response (Thyssen et al, 2001). The absolute level of starch synthesis is lower in low-CO 2 cells, although it represents a larger fraction of the total carbon fixed.…”

“…A decline in starch content is also detectable within 30 min after transfer to low CO 2 (Kuchitsu et al, 1988). This decline in starch is followed, after about 2 h, by a net synthesis of starch (Thyssen et al, 2001) that is mainly deposited around the pyrenoid, rather than as the starch grains distributed in the stroma normally found in high-CO 2 -grown cells. When the CCM is fully induced, the alga can concentrate C i inside the cell/chloroplast against a free-energy gradient.…”

Circadian Clock Regulation of Starch Metabolism Establishes GBSSI as a Major Contributor to Amylopectin Synthesis in Chlamydomonas reinhardtii

“…The importance of circadian clock regulation of starch metabolism in vascular plants is discussed. It has been previously noted that photoautotrophic cultures of C. reinhardtii grown either in low or high CO 2 displayed a diurnal rhythm of starch content that did not correlate with lights off and on (Klein, 1987;Thyssen et al, 2001). Because the minimal starch content is reached at the middle of the light phase, we investigated the possibility that starch metabolism as a whole (as reflected by starch content) could be under tight circadian clock control in the monocellular alga C. reinhardtii.…”

Microalgae as a Feedstock for Biofuel Precursors and Value-Added Products: Green Fuels and Golden Opportunities

“…Similarly, the activities of carbonic anhydrase and nitrate reductase enzymes in C. pyrenoidosa and Chlamydomonas reinhardtii exhibited different responses to elevation of dissolved CO2 content (Xia and Gao 2005). Moreover, other studies demonstrated that increasing CO2 content from ambient to 2-5% improves biomass growth but has either negative (up to 2-3 fold reduction) or no effect on starch accumulation when applied without the cell arrest (Thyssen et al 2001;Izumo et al 2007;Li et al 2013). Lowering CO2 concentration can induce carbon dioxide concentrating mechanisms (CCM) that may enhance CO2 supply to ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), and thus improve photosynthesis efficiency and carbon storage (Raven 2010).…”