Effect of light-path length in outdoor flat plate reactors on output rate of cell mass and of EPA in Nannochloropsis sp.

The effect of light-path length (i.e. reactor width or thickness) of flat plate glass reactors on outdoor production of eicosapentaenoic acid (EPA) and cell mass of Nannochloropsis sp. was tested, using a range of light-paths from 1.3 to 17.0 cm. Volumetric productivity of cell mass and optimal, as well as maximal cell density which represents the highest sustainable cell density under the experimental conditions, decreased with increase in light-path. Daily areal output rate (g dry weight m⁻² day⁻¹) increased with increased light-path, in contrast with results obtained in similar reactors with Spirulina cultures, in which areal output rates increased when the light-path was reduced. Maximal areal productivity of Nannochloropsis sp. (12.8 and 22.4 g ash-free dry weight per day per m² of irradiated reactor surfaces, in winter and summer, respectively), reflecting maximal efficiency in light utilization, was obtained with the long light-paths, i.e. 10.4 and 17.0 cm. Increasing the light-path from 1.3 to 17.0 cm resulted in an increase in areal EPA productivity, from 66.7 to 278.2 mg m⁻² day⁻¹ in winter and from 232.1 to 515.7 mg m⁻² day⁻¹ in summer. This enhancement in areal productivity of EPA stems from increased productivity of cell mass which was associated with the increase in light-path. We concluded that the optimal light-path, which must be defined for each algal species, represents an important parameter which determines optimal culture density (i.e. resulting in the highest output rate of cell mass per irradiated reactor surface), as well as productivity of cell mass and cell products. Under our conditions the optimal light-path for culturing Nannochloropsis in vertical reactors was ca 10 cm.