LCI1, a Chlamydomonas reinhardtii plasma membrane protein, functions in active CO₂ uptake under low CO₂

In response to high CO₂ environmental variability, green algae, such as Chlamydomonas reinhardtii, have evolved multiple physiological states dictated by external CO₂ concentration. Genetic and physiological studies demonstrated that at least three CO₂ physiological states, a high CO₂ (0.5-5% CO₂ ), a low CO₂ (0.03-0.4% CO₂ ) and a very low CO₂ (< 0.02% CO₂ ) state, exist in Chlamydomonas. To acclimate in the low and very low CO₂ states, Chlamydomonas induces a sophisticated strategy known as a CO₂ -concentrating mechanism (CCM) that enables proliferation and survival in these unfavorable CO₂ environments. Active uptake of C_i from the environment is a fundamental aspect in the Chlamydomonas CCM, and consists of CO₂ and HCO₃^- uptake systems that play distinct roles in low and very low CO₂ 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 CO₂ acclimation states and the species of C_i , HCO₃^- or CO₂ , 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 CO₂ uptake in low CO₂ , especially above air-level CO₂ , and that any LCI1 role in very low CO₂ is minimal.