PNNL

Abstract

We analyzed the metabolic rhythms and differential gene transcription in the unicellular, diazotrophic cyanobacterium Cyanothece sp. ATCC51142 under N2-fixing conditions with 12h light-12h dark cycles followed by 36 h continuous light. Cultures were grown in a 6-L bioreactor that was specially designed for photosynthetic microorganisms and that permitted continuous monitoring of parameters such as pH and dissolved oxygen. Our main objective was to determine the strategies used by these cells to perform N2 fixation under normal day-night conditions, as well as under greater stress caused by continuous light. Our results strongly suggested that the level of N2 fixation is dependent upon respiration for energy production and for removal of intracellular O2. We determined that N2 fixation cycled in continuous light, but that the N2 fixation peak was lower and that glycogen degradation and respiration were also lower under these conditions. We also demonstrated that nifH (the gene encoding the Fe protein) and nifB and nifX were strongly induced in the continuous light; this is consistent with the mode of operation of these proteins relative to the MoFe protein and suggested that any regulation of N2 fixation was at a posttranscriptional level. Also, many soluble electron carriers (e.g., ferredoxins), as well as redox carriers (e.g., thioredoxin and glutathione) were strongly induced during N2 fixation in continuous light. We suggest that these carriers were required to generate enhanced cyclic electron transport and phosphorylation for energy production and to maintain appropriate redox levels in the presence of enhanced O2, respectively.