High-CO2 Response Mechanisms in Microalgae

Abstract

The concentrations of atmospheric CO2 and aquatic inorganic carbon have decreased over geologic time with minor fluctuations. In contrast, O2 concentration has increased through the actions of photosynthetic organisms. Therefore, photosynthetic organisms must adapt to such dramatic environmental change. Aquatic photosynthetic microorganisms, namely eukaryotic microalgae, cyanobacteria, and non-oxygen-evolving photosynthetic bacteria, have developed the ability to utilize CO2 efficiently for photosynthesis because CO2 is a enzyme ribulose-1,5-bisphosphate substrate for the primary CO2-fixing carboxylase/oxygenase (Rubisco) and its related metabolic pathways such as the Calvin– Benson cycle (C3 cycle). As the Rubisco carboxylase reaction is suppressed by elevated O2 concentrations via competition with CO2, photosynthetic organisms have developed special mechanisms for acclimating and adapting to changes in both CO2 and O2 concentrations. Examples of such mechanisms are the microalgal CO2-concentrating mechanisms (CCM), the facilitation of “indirect CO2 supply” with the aid of carbonic anhydrase and dissolved inorganic carbon (DIC)-transporters (see Section 3), and C4- photoysnthesis (for review, see Giordano et al., 2005; Raven, 2010). Many reports on low- CO2-acclimation/adaptation mechanisms have been published, particularly in relation to certain cyanobacteria and unicellular eukaryotes. However, knowledge of high-CO2- acclimation/adaptation mechanisms is very limited. We recently identified an acceptable high-CO2-inducible extracellular marker protein, H43/Fea1 (Hanawa et al., 2007) and a cis-element involved in high-CO2-inducible gene expression in the unicellular green alga Chlamydomonas reinhardtii (Baba et al., 2011a). We also identified other high-CO2-inducible proteins in the same alga using proteomic analysis (Baba et al. 2011b). In this chapter, we briefly introduce low-CO2-inducible phenomena and mechanisms as background and then review recent progress in elucidating the molecular mechanisms of the high-CO2 response in microalgae.