Fixing N2 into cyanophycin: continuous cultivation of Nostoc sp. PCC 7120.

Two diazotrophic cyanobacteria (Anabaena cylindrica PCC 7122 and Nostoc sp. PCC 7120) were cultivated to produce cyanophycin, a nitrogen reserve compound, under nitrogen fixing conditions. In preliminary continuous experiments, Nostoc sp. was shown to be more efficient, accumulating a higher amount of cyanophycin and showing a greater capability to fix atmospheric nitrogen in the biomass (67 mgN d-1 of fixed nitrogen per liter of culture). The operating conditions were then optimized to maximize the cyanophycin productivity: the effect of incident light intensity, residence time and nitrogen availability were investigated. Nitrogen availability and/or pH played a major role with respect to biomass production, whereas phosphorus limitation was the main variable to maximize cyanophycin accumulation. In this way, it was possible to achieve a stable and continuous production of cyanophycin (CGP) under diazotrophic conditions, obtaining a maximum cyanophycin productivity of 15 mgCGP L-1 d-1. KEY POINTS: • Diazotrophic cyanobacteria produce stable amount of cyanophycin in continuous PBR. • Nostoc sp. proved to be more efficient in producing cyanophycin than Anabaena sp. • P deprivation is the major variable to increase cyanophycin productivity in continuous.

Mixotrophy in Synechocystis sp. for the treatment of wastewater with high nutrient content: effect of CO2 and light.

Synechocystis sp. has remarkable mixotrophic capabilities, as well as an efficient exploitation of nitrogen and phosphorus, that may be applied in wastewater treatment based on cyanobacteria. To better investigate the exploitation of algal mixotrophy in bioremediation, this species was used in axenic respirometric tests to ascertain the effect of high light and non limiting CO2 supply on the overall regulation of mixotrophy, resulting in an inhibition of the exploitation of organic carbon. The same species was then cultured in real, unsterilized effluent obtained from the acidogenic fermentation of sludge, which contains a high concentration of nutrients (approximately 600, 90 and 6000 mg L-1 of N, P and COD, respectively) and it is often inhibiting for many microalgal species. On the contrary, Synechocystis sp., showed a remarkable growth and a removal up to 96% of phosphorus, 66% of nitrogen and of 68% of COD in such a complex waste stream.