FIGURE 8.

A proposed model for dual Ci signaling pathways in Chlorella sp. ArM0029B. In Chlorella sp. ArM0029B, expression of CCM genes is suppressed by high concentration of intracellular Ci mostly derived from the acetate assimilatory pathway. Under this photoheterotrophic growth conditions, bicarbonate sensing and signaling pathway (blue line) are not operating. When high concentration of ${\mathrm{HCO}}_{\mathrm{3}}^{\mathrm{–}}$ is exogenously provided, this bicarbonate pathway becomes active. Constitutive operation of CCM machinery observed in CAC1 mutant implicates that CAC1 suppresses Ci signaling pathway to the CCM genes (red line). Failure of high exogenous CO₂ in CCM activation implicates the presence of membranous CO₂ transporter that allows CO₂ entry fast enough not to be hydrated by external CAHs. Thus, dual signaling pathways for external bicarbonate and internal Ci to the CCM apparatus are probably present in Chlorella sp. ArM0029B. CAH, carbonic anhydrase; Ci, inorganic carbon; LCI, low CO₂ inducible protein.