Proteomic Analysis of High-CO2-Inducible Extracellular Proteins in the Unicellular Green Alga, Chlamydomonas reinhardtii

The unicellular green alga Chlamydomonas reinhardtii can acclimate to a wide range of CO2 concentrations through the regulation of a CO2-concentrating mechanism (CCM). By proteomic analysis, here we identified the proteins which were specifically accumulated under high-CO2 conditions in a cell wall-less strain of C. reinhardtii which release their extracellular matrix into the medium. When the CO2 concentration was elevated from the ambient air level to 3% during culture, the algal growth rate increased 1.5-fold and the composition of extracellular proteins, but not intracellular soluble and insoluble proteins, clearly changed. Proteomic analysis data showed that the levels of 22 of 129 extracellular proteins increased for 1 and 3 d and such multiple high-CO2-inducible proteins include gametogenesis-related proteins and hydroxyproline-rich glycoproteins. However, we could not prove the induction of gametogenesis under high-CO2 conditions, suggesting that the inductive signal might be incomplete, not strong enough or that only high-CO2 conditions might be not sufficient for the cell stage to proceed to the formation of sexually active gametes. However, these gametogenesis-related proteins and/or hydroxyproline-rich glycoproteins may have novel roles outside the cell under high-CO2 conditions.