Final Scientific/Technical Report

Abstract

The potential for developing commercially viable microbial H 2 -production systems as a renewable source of biofuel has been limited by the need for an anaerobic environment to enable photobiological H 2 -production in capable bacterial and algal species. In this project, we have shown that the cyanobacterium Cyanothece sp. ATCC 51142 has the capacity for highly efficient H 2 -production under natural aerobic conditions. The marine cyanobacterium Cyanothece sp. ATCC 51142 has a diurnal metabolic cycle; photosynthesis and carbon fixation occur during daylight hours and then at night, high rates of respiration create a suboxic intracellular environment that enables O 2 -sensitive processes to occur, including N2-fixation and H 2 -production. We developed a two-stage approach to monitor H 2 -production by Cyanothece 51142. In the first stage, we grew the bacteria aerobically in an alternating 12 hour light/dark cycle. A second ‘incubation’ stage was then carried out in which we took cells from the end of a 12 hour light growth period and incubated them in air-tight vials for a further 12 hours under continuous illumination. Analysis of the headspace in the vial revealed high specific rates of H 2 -production (>150 μmol of H 2 per mg chlorophyll per hour) during this incubation period. Furthermore, the rate of H 2 -production could be enhanced by growing the cells in the presence of high levels of CO 2 or glycerol. We also confirmed that H 2 -production was mediated by the nitrogenase system found in these Cyanothece cells. Interestingly, in the absence of molecular N 2 , nitrogenase systems channel all available electrons towards H 2 -production. Accordingly, when we incubated glycerol-supplemented Cyanothece 51142 cells in the absence of N 2 , the rate of H 2 -production increased up to 467 μmol of H 2 per mg chlorophyll per hour, which is an order of magnitude greater than those rates previously observed in other wild type H 2 producing model photosynthetic microorganisms under anaerobic conditions. As glycerol and CO 2 are both abundantly available as industrial waste products, the fact that they substantially enhance aerobic H 2 -production suggests that Cyanothece 51142 is a potentially viable system for producing biohydrogen as a renewable fuel source. During this project, we have conducted a detailed systems level analysis of this interesting cyanobacterial system, and have created a rich knowledgebase for the use of Cyanothece cells as efficient biohydrogen producers in large scale.