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.