Simultaneous oxygen production and nitrogenase activity of an ammonia-excreting mutant of the cyanobacterium Anabaena variabilis in a co-culture with wheat

A mutant strain of Anabaena variabilis, strain SA-1 that supported growth of wheat plants in a hydroponic co-culture in nitrogen (N) free medium also produced enough oxygen (O2) to support root respiration. The steady-state concentration of net O2 in the coculture was dependent on incident light intensity. At an incident photosynthetic photoflux (PPF) of 1000 micromoles photons.m-2.s-1, net O2 evolution by the co-culture in the root zone reached a maximum value of about 220 micromoles O2 evolved.h-1.mg chlorophyll (Chl)-1. The O2 concentration in the rhizosphere of the co-culture stayed above the ambient air level. O2 uptake in the dark by strain SA-1-supplemented wheat roots washed free of cyanobacterium was higher than the root respiration of nitrate-grown plants. Nitrate-grown plants required aeration for maximum growth while the wheat-cyanobacterial co-culture can be cultured without aeration. These results show that O2 produced by strain SA-1 can be used to supply the O2 needs for root respiration of wheat. Respiration reduced net O2 evolution by the mutant SA-1, decreasing the partial pressure of O2 at the sites of cyanobacterial attachment to the roots. This led to an increase in the specific activity of nitrogenase of the co-culture at the high light intensities used to support wheat growth. This activity of about 30 micromoles ethylene produced.mg Chl-1.h-1 was threefold higher than the activities obtained with the free-living strain SA-1 assayed at the same light intensity. In the co-culture, ammonia produced by the mutant strain SA-1 was not detectable. The NH4+ produced by strain SA-1 was used by the wheat plants and, under these conditions, the total N content of the plants reached as high as 85% of the total N content of nitrate-grown plants. In the co-culture system the metabolism of wheat and the cyanobacterium complemented each other, leading to higher plant growth in N-free medium.