Growth, photosynthesis, and gene expression in <i>Chlamydomonas</i> over a range of CO₂ concentrations and CO₂/O₂ ratios: CO₂ regulates multiple acclimation states

Abstract

Growth, photosynthesis, and induction of two low CO 2 -inducible genes of Chlamydomonas reinhardtii Dangeard strain CC125 were quantified in a range of physiologically relevant CO 2 and O 2 concentrations (5%–0.005% CO 2 and 20% or 2% O 2 ) using airlift bioreactors to facilitate the simultaneous measurement of both growth and in situ photosynthetic rates. Within these CO 2 concentration ranges, O 2 concentrations (20% vs. 2%) had no discernable effect on growth, photosynthetic rate, or induction of the periplasmic carbonic anhydrase (Cah1) and glycolate dehydrogenase (Gdh) genes in wild-type C. reinhardtii. These results failed to support the hypothesis that the CO 2 /O 2 ratio plays any role in signaling for the up-regulation of limiting CO 2 -induced genes and (or) of the CO 2 -concentrating mechanism (CCM). The mRNA abundance of the Cah1 and Gdh genes appeared to be regulated in concert, suggesting co-regulation by the same signaling pathway, which, because of a lack of an O 2 effect, seems unlikely to involve photorespiration or a photorespiratory metabolite. Instead, it appeared that the CO 2 concentration alone was responsible for regulation of limiting CO 2 acclimation responses. Based on growth, photosynthesis, and gene expression characteristics, three distinct CO 2 -regulated physiological states were recognized within the studied parameters, a high CO 2 (5%–0.5%) state, a low CO 2 (0.4%–0.03%) state, and a very low CO 2 (0.01%–0.005%) state. Induction of Cah1 expression and Gdh up-regulation occurred at a CO 2 concentration between 0.5% and 0.4% CO 2 , delineating the high from the low CO 2 states. Photosynthetic characteristics also were distinct in the three CO 2 -regulated physiological states, e.g., the estimated K 0.5 (CO 2 ) of the high CO 2 , low CO 2 , and very low CO 2 states were 72, 10, and 0.9 µmol·L –1 CO 2 , respectively. In addition to a greater photosynthetic CO 2 affinity, the very low CO 2 state could be distinguished from the low CO 2 state by an increased cell-doubling time and a smaller cell size.Key words: algae, Chlamydomonas, CO 2 , gene expression, induction, photorespiration, photosynthesis.