A study of the growth and hydrogen production of Cyanothece sp. ATCC 51142

Abstract

Hydrogen (H2) has long been promoted as an ideal fuel, as it permits a completely clean combustion and has great potential to provide clean power needed for transport and electricity generation. The unicellular, nitrogen-fixing cyanobacterium Cyanothece sp. ATCC 51142 is a promising strain with a remarkable capability of producing large quantities of H2. Under anaerobic condition, the cyanobacterium carries out the biological fixation of atmospheric nitrogen (N2) into ammonia (NH3), concurrently producing H2 as by-product. The aim of this thesis was to improve our understanding of the growth and H2 production of Cyanothece sp. ATCC 51142 in order to develop a continuous and practical cyanobacterial H2 production process. In order to achieve effective H2 production, it is prerequisite to grow dense and healthy Cyanothece 51142 cultures. Favourable cyanobacterial growth conditions included a continuous illumination at 207 - 320 μmol m-2 s-1, temperature of 35 °C and nitrogen-replete (addition of nitrate salts) condition. The critical temperature, which induces photoinhibition upon the cyanobacterium, was found at 40 °C. In the case of H2 production, favourable conditions included a continuous illumination at low light intensities of 46 – 92 μmol m-2 s-1, temperature of 30 °C, nitrogen-fixing (sole presence of atmospheric N2) and photoheterotrophic (sole presence of organic glycerol substrate) growth condition. In order to effectively handle incompatible requirements between the cyanobacterial growth and its sequential H2 production, a novel two-stage chemostat photobioreactor (PBR) system was designed and developed, with an aim to improve H2 production yield as well as extend its production duration. The system has been operated non-stop for consecutive 31 days without any losses in its performance and subsequently demonstrated a remarkable improvement in H2 production, with more than 6.4 times higher yield than that of a single-stage batch system. With the continuous mode of operation, a continuous collection of produced biomass from the PBR is also permitted (more than 7.3 times improvement in biomass yield than that of a single-stage batch system). At an industrial scale, this biomass could undergo further downstream processing to generate a multistreamline of high valued by-products such as e.g. vitamins, pharmaceuticals and human nutrition, which can subsequently contribute to a significant improvement in an economic viability of biohydrogen process.