Anaerobic Acclimation in Chlamydomonas reinhardtii

Abstract

Both prokaryotic and eukaryotic photosynthetic microbes experience conditions of anoxia, especially during the night when photosynthetic activity ceases. In Chlamydomonas reinhardtii, dark anoxia is characterized by the activation of an extensive set of fermentation pathways that act in concert to provide cellular energy, while limiting the accumulation of potentially toxic fermentative products. Metabolite analyses, quantitative PCR, and high density Chlamydomonas DNA microarrays were used to monitor changes in metabolite accumulation and gene expression during acclimation of the cells to anoxia. Elevated levels of transcripts encoding proteins associated with the production of H 2 , organic acids, and ethanol were observed in congruence with the accumulation of fermentation products. The levels of over 500 transcripts increased significantly during acclimation of the cells to anoxic conditions. Among these were transcripts encoding transcription/translation regulators, prolyl hydroxylases, hybrid cluster proteins, proteases, transhydrogenase, catalase, and several putative proteins of unknown function. Overall, this study uses metabolite, genomic, and transcriptome data to provide genome-wide insights into the regulation of the complex metabolic networks utilized by Chlamydomonas under the anaerobic conditions associated with H 2 production.

The Interplay of Proton, Electron, and Metabolite Supply for Photosynthetic H2 Production in Chlamydomonas reinhardtii

“…3A). It has been reported previously that metabolite levels of fermentative products and transcript levels of genes of fermentative pathways increase during anaerobic cultivation in the dark (21). Recent metabolome studies have dem- (38).…”

“…When the culture is subjected to sulfur limitation but still aerobic, starch, fatty acid, and lipid synthesis are strongly induced. In contrast when anaerobic conditions are established, it seems that pyruvate is mainly used for amino acid synthesis and fermentative pathways to control the cellular redox balance (21,24,25,46) (Figs. 2 and 4 -6).…”

“…Pyruvate can also be fermented to formate (catalyzed by the pyruvate formate lyase), converted to acetyl-CoA (catalyzed by the pyruvate ferredoxin oxidoreductase), or metabolized to acetate or ethanol (catalyzed by the acetate kinase), or the acetaldehyde dehydrogenase/alcohol dehydrogenase (ADH) (21,24) (Fig. 11).…”

“…Most recently, more attention was paid to systems biology approaches (21-25) to obtain a more detailed picture of the processes that are interconnected with hydrogenase activity (21,24,26,27). The aim of all of these systematic approaches was to understand the regulatory mechanisms of substrate supply to hydrogenase and to identify bottlenecks of the H 2 production process as a precondition for subsequent targeted metabolic engineering approaches.…”

Biochemical and Kinetic Characterization of the Eukaryotic Phosphotransacetylase Class IIa Enzyme from Phytophthora ramorum

“…Proteomic studies have suggested that the Pat1-Ack2 pathway is localized to mitochondria (note that phosphotransacetylase is designated Pat in Chlamydomonas), and the Pat2-Ack1 pathway is localized to chloroplasts (13,17). Acetate has been found to be one of the major fermentative products excreted by Chlamydomonas during growth under dark, anoxic conditions, and ACK1, ACK2, PAT1, and PAT2 transcript levels are increased, in agreement with a role for the Pat-Ack pathway in acetate production (18). The ack1 and pat2 mutants were the most vulnerable to anoxia, and strains could not be recovered after a 24-h exposure to anoxia (19).…”