value at 40 8C. The most pronounced response to
temperature was recorded in the transition from 40 to
35 8C, with a decrease in productivity of about 4 g (dry
biomass) m2 per day (Table 1).
Cell density represents another key factor governing
culture productivity, since it critically affects the availability
of light to the cells (mutual shading). For a given
irradiance, there is an optimum value of cell
population*/the optimal cell density*/which yields
the higher output rate, and therefore, the highest
photosynthetic efficiency [15]. Fig. 1 shows how the
value for optimal cell density is determined, by analyzing
the effect of cell density on biomass productivity.
This parameter varies with irradiance, and it should be
established at least once every season. In the particular
case of Fig. 1 (autumn), the optimal value for cell