LCI1, a <i>Chlamydomonas reinhardtii</i> plasma membrane protein, functions in active CO<sub>2</sub> uptake under low CO<sub>2</sub>
Alfredo Kono, Martin H. Spalding
Abstract
Summary In response to high CO 2 environmental variability, green algae, such as Chlamydomonas reinhardtii , have evolved multiple physiological states dictated by external CO 2 concentration. Genetic and physiological studies demonstrated that at least three CO 2 physiological states, a high CO 2 (0.5–5% CO 2 ), a low CO 2 (0.03–0.4% CO 2 ) and a very low CO 2 (< 0.02% CO 2 ) state, exist in Chlamydomonas . To acclimate in the low and very low CO 2 states, Chlamydomonas induces a sophisticated strategy known as a CO 2 ‐concentrating mechanism (CCM) that enables proliferation and survival in these unfavorable CO 2 environments. Active uptake of C i from the environment is a fundamental aspect in the Chlamydomonas CCM, and consists of CO 2 and HCO 3 – uptake systems that play distinct roles in low and very low CO 2 acclimation states. LCI1, a putative plasma membrane C i transporter, has been linked through conditional overexpression to active C i uptake. However, both the role of LCI1 in various CO 2 acclimation states and the species of C i , HCO 3 – or CO 2 , that LCI1 transports remain obscure. Here we report the impact of an LCI1 loss‐of‐function mutant on growth and photosynthesis in different genetic backgrounds at multiple pH values. These studies show that LCI1 appears to be associated with active CO 2 uptake in low CO 2 , especially above air‐level CO 2 , and that any LCI1 role in very low CO 2 is minimal.