Biomass production and N2-fixation by Azolla filiculoides in outdoor mass culture

Investigations carried out at our Research Centre since 1978 have shown that symbiosis Azolla-Anabaena is an efficient biolgical system for converting inorganic carbon and elementar nitrogen into organic compounds at the expense of solar energy. This paper deals with recent progress in the development of a culture technique suitable for growing Azolla spp. either on a mineral solution without combined nitrogen or on sugar-refinery waste water. It has been shown that the maintenance of an appropriate planting density in the culture (about 50 g of dry weight pe sq.metre) is very important for attaining high yields. Under the climatic conditions of Florence, Azolla filiculoides gave the highest yield of biomass and combined nitrogen among the species tested in outdoor culture of mineral solution (two-fifths strenght Hoagland's type solution). From a culture of A. filiculoides operated on a semicontinuos regime in a 100 sq.metres pond for 180 days (May-October) a mean yield of 11 g (dry weight) m-2 day-1. The maximum growth rate corresponded to a doubling time of 2.5 days. During the period July-September the mass culture of Azolla was carried out on sugar refinery waste water, previously treated with photoanaerobic bacteria. This effluent was characterized by a low nitrogen content, a relatively high amount of K,Mg,Ca,P and Fe and by a BOD5 of 200-300 ppm. On this waste water the yield were comparable to those attained on synthetic medium under the same climatic conditions. The nitrogen content of the biomass was about 5% of dry-weight. Hence the amount of nitrogen fixed amounted to 5 Kg ha-1 day-1.