Azolla-Anabaena symbionts and microbial mat as nitrogen-fixing biocatalysts for bioregenerative space life support

This study was conducted to characterize the responses of selected nitrogen-fixing biocatalysts to various environmental parameters that are likely to be encountered in a scaled-up nitrogen-fixing bioregenerative unit for use in a biologically based life support system (BLSS). The results showed that while both Azolla filiculoides and Azolla nilotica thrived on standard nitrogen-free liquid media, only Azolla filiculoides maintained its growth rate in the presence of 50% by volume of aerobic-bioreactor potato effluent (ABPE) in its liquid medium. The growth rate and Anabaena cavity population of Azolla filiculoides also remained undiminished in the presence of as much as 10 mM of NO3- in its liquid medium. The tolerance of Azolla filiculoides for NO3- however, lay somewhere between 10 and 20 mM of NO3-. Azolla filiculoides failed to grow at 5 mM or greater concentrations of NH4+. The growth rate of the Oscillatoria-dominated microbial mat was significantly elevated by increased silage loading density (16 g/L) as well as by adding 5 mM of NO3- into its liquid medium. Neither the growth rate of Azolla filiculoides nor that of the microbial mat was affected by three levels of photosynthetic photon flux (PPF) (520, 265, and 125 micromoles m-2 s-1) under high-pressure sodium or cool fluorescent lamps.