Insertional Mutants of Chlamydomonas reinhardtii That Require Elevated CO2 for Survival

Abstract

Aquatic photosynthetic organisms live in quite variable conditions of CO 2 availability. To survive in limiting CO 2 conditions, Chlamydomonas reinhardtii and other microalgae show adaptive changes, such as induction of a CO 2 -concentrating mechanism, changes in cell organization, increased photorespiratory enzyme activity, induction of periplasmic carbonic anhydrase and specific polypeptides (mitochondrial carbonic anhydrases and putative chloroplast carrier proteins), and transient down-regulation in the synthesis of Rubisco. The signal for acclimation to limiting CO 2 in C. reinhardtii is unidentified, and it is not known how they sense a change of CO 2 level. The limiting CO 2 signals must be transduced into the changes in gene expression observed during acclimation, so mutational analyses should be helpful for investigating the signal transduction pathway for low CO 2 acclimation. Eight independently isolated mutants of C. reinhardtii that require high CO 2 for photoautotrophic growth were tested by complementation group analysis. These mutants are likely to be defective in some aspects of the acclimation to low CO 2 because they differ from wild type in their growth and in the expression patterns of five low CO 2 -inducible genes (Cah1, Mca1, Mca2, Ccp1, and Ccp2). Two of the new mutants formed a single complementation group along with the previously described mutant cia-5, which appears to be defective in the signal transduction pathway for low CO 2 acclimation. The other mutations represent six additional, independent complementation groups.Acclimation to changed environmental conditions is a key to survival for all organisms. In response to perceived environmental signals, organisms may exhibit specific adaptive changes, such as changes in the expression of key genes to survive specific environmental changes. Because CO 2 can vary substantially in aquatic habitats and represents the major substrate for photosynthetic CO 2 fixation via the enzyme Rubisco, CO 2 concentration is an important environmental signal in aquatic photosynthetic organisms including cyanobacteria and Chlamydomonas reinhardtii.Unlike terrestrial higher plants, aquatic photosynthetic organisms can face difficulties in acquiring CO 2 . Because the CO 2 diffusion rate in water is much slower than that in air (Badger and Spalding, 2000), the CO 2 supply to Rubisco in these aquatic photosynthetic organisms can become limited. C. reinhardtii and other aquatic photosynthetic organisms have a genetic program to allow them to acclimate to low CO 2 . This acclimation includes induction of a CO 2 -concentrating mechanism (CCM) that allows the cells to acquire CO 2 efficiently by increasing the CO 2 concentration around Rubisco under limiting CO 2 conditions (Badger et al., 1980; for review, see Spalding, 1998; Kaplan and Reinhold, 1999).Along with the induction of the CCM, C. reinhardtii shows adaptive changes to limiting CO 2 conditions, such as changes in cell organization (Geraghty and Spalding, 1996), increased photorespiratory enzyme ac...

Systems Analysis of the Response of Photosynthesis, Metabolism, and Growth to an Increase in Irradiance in the Photosynthetic Model OrganismChlamydomonas reinhardtii

“…CCP2 (low-CO 2 -inducible chloroplast envelope protein 2) and LCI1 (low-CO 2 -inducible membrane protein 1) were also strongly induced by increased light. As these genes are induced by low CO 2 (Burow et al, 1996;Van et al, 2001;Pollock et al, 2004), they have been proposed to be part of the inorganic carbon concentrating mechanism. High light may induce LCI1 because the resulting increase in the rate of photosynthesis leads to a temporary decrease in the internal CO 2 concentration, which induces genes involved in the carbon concentrating mechanism (McGinn et al, 2003;Spijkerman, 2011).…”

An inorganic carbon transport system responsible for acclimation specific to air levels of CO 2 in Chlamydomonas reinhardtii

“…Although initially identified as a probable Ci transport mutant, a recent study reported that the expression profiles of several CO 2 responsive genes in pmp1 differ from those in wild type and suggested that the Pmp1 gene product might regulate the expression of Ci transporter genes (14). Another recent observation regarding pmp1 is its unusual, air-dieing phenotype (15,16): it grows well in either high (5%) or very low (Ͻ200 ppm) CO 2 , but dies in low (air-level) CO 2 (350-450 ppm). This conspicuous phenotype distinguishes pmp1 from most other high CO 2 -requiring mutants and indicates the existence of multiple Ci transport systems in C. reinhardtii corresponding to multiple, CO 2 level-dependent acclimation states.…”

Hydrogen Photoproduction Is Attenuated by Disruption of an Isoamylase Gene in Chlamydomonas reinhardtii

“…Screening DNA insertional mutagenesis libraries in C. reinhardtii has become a popular strategy for identifying important genes involved in specific cellular pathways and processes (Debuchy et al, 1989;Rochaix, 1995;Tam and Lefebvre, 1995;Niyogi et al, 1997;Adam and Loppes, 1998;Davies et al, 1999;Moseley et al, 2000; Van et al, 2001;Dame et al, 2002;Polle et al, 2003). Our screening efforts resulted in the identification of several mutants that were compromised in their ability to photoproduce H 2 .…”