Outdoor cultivation ofArthrospira platensis during autumn and winter in temperate climates

Arthrospira (Spirulina) platensis M2 was grown outdoors in 50-mm diameter tubular reactors under the climatic conditions of central Italy (Florence) from September to December 1995 and in March 1996. Except for September, the cultures temperature was regulated. Mean productivities of 0.83, 0.44 and 0.61 g dry wt L^–1 d^–1 were achieved in autumn (September–October), winter (November–December) and March, respectively. In autumn and winter, the photosynthetic efficiency of the cultures and the degree of correlation between productivity and solar irradiance were significantly greater than in summer. The effect of cell density and aeration rate on productivity was evaluated in September. The productivity of cultures operated at high supra-optimal population density was about 30% less at high aeration rate (1.0 LL^–1 min^–1), and 50% less at standard aeration rate (0.17 LL^–1 min^–1), than that of control cultures kept at optimal population density and standard aeration rate. The reduction of productivity in high-density cultures was due to lower daylight output rates and higher night biomass losses (the latter were particularly relevant under standard aeration conditions). The main factor limiting productivity in closed reactors during autumn was the night temperature. Heating the cultures during daylight hours on sunny days did not cause any significant increase of the yields, since under sunlight the unheated cultures also reached the optimal temperature for growth early in the morning. On cloudy days, the day-time temperature of the unheated cultures remained well below the optimum, however this had only a limited effect on productivity since algal growth was mainly light-limited.