Elevated Inorganic Carbon Concentrating Mechanism Confers Tolerance to High Light in an Arctic Chlorella sp. ArM0029B

Abstract

Microalgae and higher plants employ an inorganic carbon (Ci) concentrating mechanism (CCM) to increase CO₂ availability to Rubisco. Operation of the CCM should enhance the activity of the Calvin cycle, which could act as an electron sink for electrons generated by photosynthesis, and lower the redox status of photosynthetic electron transport chains. In this study, a hypothesis that microalgal cells with fully operating CCM are less likely to be photodamaged was tested by comparing a <i>Chlorella</i> mutant with its wild type (WT). The mutant acquired by screening gamma-ray-induced mutant libraries of <i>Chlorella</i> sp. ArM0029B exhibited constitutively active CCM (CAC) even in the presence of additional Ci sources under mixotrophic growth conditions. In comparison to the WT alga, the mutant named to constitutively active CCM1 (CAC1) showed more transcript levels for genes coding proteins related to CCM such as Ci transporters and carbonic anhydrases (CA), and greater levels of intracellular Ci content and CA activity regardless of whether growth is limited by light or not. Under photoinhibitory conditions, CAC1 mutant showed faster growth than WT cells with more PSII reaction center core component D1 protein (encoded by <i>psbA</i>), higher photochemical efficiency as estimated by the chlorophyll fluorescence parameter (Fv/Fm), and fewer reactive oxygen species (ROS). Interestingly, high light (HL)-induced increase in ROS contents in WT cells was significantly inhibited by bicarbonate supplementation. It is concluded that constitutive operation of CCM endows <i>Chlorella</i> cells with resistance to HL partly by reducing the endogenous generation of ROS. These results will provide useful information on the interaction between CCM expression, ROS production, and photodamage in <i>Chlorella</i> and related microalgae.