Molecular mechanisms behind the adjustment of phototrophic light-harvesting and mixotrophic utilization of cellulosic carbon sources in Chlamydomonas reinhardtii

Plants, green algae and cyanobacteria perform photosynthetic conversion of sunlight into chemical energy in a permanently changing natural environment, where the efficient utilization of light and inorganic carbon represent the most critical factors. Photosynthetic organisms have developed different acclimation strategies to adapt changing light conditions and insufficient carbon source supply in order to survive and to assure optimal growth and protection. This thesis provides further insights into the molecular mechanisms of the acclimation response of the green algae Chlamydomonas reinhardtii. An important acclimation mechanism to altering light conditions involves the post-transcriptional regulation of the nuclear-encoded photosystem II (PSII)-associated light-harvesting complex (LHCII) genes via cytosolic translational control mediated through the RNA-binding protein NAB1. In the active state, NAB1 represses the cytosolic translation of LHCII mRNAs by sequestration into translational silent messenger ribonucleoprotein complexes (mRNPs). The overexpression of NAB1 decreases LHCII protein amount whereas NAB1 knockout leads to an increased level of LHCII proteins. Consequently, NAB1 is part of a control system regulating the size and composition of the LHCII complex at the posttranscriptional level. The repressor activity of this specific factor is controlled by two posttranslational modifications: i) by methylation of arginines in the glycine-arginine rich (GAR) motif of the protein, ii) by the thiol status of two C-terminal cysteines. This work provides evidence that arginine methylation represents a slowly reacting modulator, which is required for the maintenance of the repressor activity of NAB1 and is responsive to the availability of light. At the same time, cysteine modification is regarded as the fine-tuning mechanism that dynamically responds to changes in the cytosolic redox-state. Moreover, the observations suggest that the regulation via arginine methylation operates independently from cysteine-based redox control, with its extent strongly depending on the growth conditions. The high methylation state is found under photoautotrophic, and the low methylation state under heterotrophic growth conditions. Photosynthetic performance is also dependent on inorganic carbon (Ci) supply, because the light-harvesting capacity and the utilization of captured energy have to be balanced. The insufficient Ci-availability can be compensated by diverse organic carbon sources, since some phototrophs can assimilate acetate, glucose or other sugars for mixothrophic growth. The green alga C. reinhardtii was so far reported to grow on acetate, but not on hexoses. Intriguingly, in silico analysis of the genome of C. reinhardtii revealed that it contains genes encoding glycoside hydrolases of different families, known to be involved in cellulose- and hemicellulose degradation, even though the cell wall of this alga does not contain cellulose and is solely composed of hydroxyproline-rich glycoproteins. This work characterizes the capability of C. reinhardtii for cellulose degradation as seen by the digestion of cellulosic materials such as carboxymethyl cellulose, Avicel and filter paper. Furthermore, the results of the present work indicate an assimilation of the breakdown products, in particular cellobiose. Cellulose degradation into cellodextrins (cellobiose-cellopentaose) was shown to be performed by extracellular cellulases CrCel9B and CrCel9C that belong to glycoside hydrolase family 9 (GHF9/subgroup E2). These enzymes display homology to cellulases from Metazoa and phylogenetic analyses suggested that they originated from an ancient eukaryotic ancestor. Furthermore, a positive effect on specific growth rates was observable under different growth conditions (high, low and very-low CO2 as well as acetate) after media supplementation with cellulosic material. In conclusion, this work provides advanced insights into the molecular regulation mechanisms of light-acclimation and utilization of an abundant organic carbon source in Chlamydomonas reinhardtii. These new findings could help to achieve higher biomass productivity by improved photosynthetic conversion efficiency and additionally, the use of abundant organic carbon sources as an integral part of new photoheterotrophic cultivation concepts.