Studies On The Regulation Of Nitrogen Fixation In Cyanobacteria With Specal Reference To The Effects Of Oxygen PDF Download

"The concern about the exhaustion of fossil fuels and the harmful effects of their emissions to the environment is guiding efforts to find new energy alternatives. One of the most promising energy resources is hydrogen (H2). Cyanobacteria are among the most important candidates for photobiological H2 production. Some filamentous cyanobacteria have evolved to differentiate specialized cells called heterocysts where nitrogen fixation and H2 production take place. The complete understanding of the process of heterocyst differentiation is of great importance for photobiological H2 production. Prior to this study, there were no quantitative culture-level models that describe the effects of cellular activities and cultivation conditions on the heterocyst differentiation. Such a model was developed in this thesis, incorporating photosynthetic growth, heterocyst differentiation, self-shading effect on light penetration, and nitrogen fixation. This first-generation model was found to describe well the experimental results and it was able to predict different culture properties. Since the productivity and economics of all photosynthsis-related processes are influenced by the light utilization efficiency, the effect of red vs. white light, and their intensities, on cell growth and heterocyst differentiation was also investigated. Red light was found not only to stimulate the growth of the studied microorganisms but also to give higher heterocyst frequencies. Gas vesicles were another interesting structure of some cyanobacteria studied in this thesis. Gas vesicles provide cyanobacteria with a mechanism of buoyancy regulation. Purified gas vesicle suspensions had been shown to improve oxygenation in oxygen limited systems. In order to improve the cell-collection efficacy of cyanobacteria with gas vesicles, three different flocculants were tested and compaired, i.e. polyethyleneimine (PEI), chitosan and MPE-50 (a commerical product from Ondeo Nalco Center, exact formulation unknown). Even though, they showed similar flocculation efficacy at low cell concentrations, chitosan flocculation ability was greatly increased when the pH of the cell suspension was adjusted to lower values. Chitosan was proved to the useful for cyanobacterial cell collection using a concentration of 50 mg/L and adjusting pH of the cell sample to 5-7. The sensitivity of chitosan to changes in pH may be useful also in the later step of recovery and purification of gas vesicles after cell lysis."--Abstract