Diversity, nitrogenase activity and salt physiology of Australian cyanobacteria isolated from agricultural and non-agricultural soils

摘要

Cyanobacteria are a tantalising prospect for research into biofertilisers and plant growth promotion. With photolithic autotrophy, broadly distributed capacities for biological N2 fixation and phytohormone production, cyanobacteria have all the necessary requirements for enhancing plant growth in cultivated soils. Research into the use of cyanobacterial inoculants for wheat cultivation, and particularly for salt stress amelioration, has never been assessed in Australia. Therefore, this thesis aimed to explore the potential of Australian cyanobacteria to ameliorate the effects of irrigation salinity on Australian wheat cultivars. This thesis describes the first investigation into the diversity of cyanobacteria from Australian wheat fields and cycad rhizospheres. It also characterises the nitrogenase activity and salt physiology of selected isolates and evaluates their application in ameliorating the salt stress of Australian wheat. The results of this study indicate that there is only a weak relationship between cycad cyanobionts and their associated rhizosphere cyanobacteria. A Cylindrospermum wheat field isolate had the highest nitrogenase activity, being almost double that of Nostoc. Microcoleus vaginatus was also isolated from a wheat field and was found to have nitrogenase activity and to contain cyanobacterial nifH. Analysis of cyanobacterial salt physiology determined that salinity generally lowered O2 evolution and nitrogenase activity of cyanobacteria though the relationships between salinity, O2 evolution and nitrogenase activity were more complicated for the individual isolates studied. The chlorophyll fluorescence parameters failed to explain the observed effects of salinity on O2 evolution. In pot trials, inoculation with cyanobacteria negatively affected the biomass and nutrient concentrations of wheat seedlings at all salinities, though the effects were fewer as salinity increased. The most likely explanation of these results is the sorption of nutrients to cyanobacterial extracellular polymeric substances, making them unavailable for plant uptake. These results indicate that Australian cyanobacteria do not ameliorate salt stress for wheat seedlings but may have a role to play in the stabilisation and improvement of degraded agricultural soils.