Co-culture of the aerial microalga Coelastrella rubescens KGU-HN001 and the nitrogen fixing-bacterium Rhodobacter sphaeroides under aerial-phase conditions

The co-culture of an aerial microalga and a nitrogen-fixing bacterium was investigated under aerial-phase conditions, carbon (C) and nitrogen (N) were exchanged between the alga and the bacterium, where the microalga contributed organic C while the bacterium contributed inorganic N. The aerial microalga Coelastrella rubescens KGU-HN001, previously isolated from the surface of steel signs, and the N-fixing bacterium Rhodobacter sphaeroides were cultured as biofilms under aerial-phase conditions using a medium without C and N sources. During co-culture under aerial-phase (i.e., aerobic) conditions, the cell number, total organic C, and total N content in cells increased. For 7 days of co-culture, the number of algal cells per filter increased by 1.8 times when the biofilms were arranged in a bilayer, compared with mono-cultured algal cells. These results indicate that C fixation by the microalga and N fixation by the bacterium occurred under aerial-phase conditions, and both microorganisms were able to grow by supplying each other with essential nutrients.