n a recent study, published in PLOS ONE Journal, the influence of light intensity on the growth and lipid productivity of Nannochloropsis salina was investigated in a flat-bed photobioreactor designed to minimize cells self-shading. The influence of various light intensities was studied with both continuous illumination and alternation of light and dark cycles at various frequencies, which mimic illumination variations in a photobioreactor due to mixing.
Results show that Nannochloropsis can efficiently exploit even very intense light, provided that dark cycles occur to allow for re-oxidation of the electron transporters of the photosynthetic apparatus. If alternation of light and dark is not optimal, algae undergo radiation damage and photosynthetic productivity is greatly reduced. The results demonstrate that, in a photobioreactor for the cultivation of algae, optimizing mixing is essential in order to ensure that the algae exploit light energy efficiently.
When cells are exposed to illumination, one component of the photosynthetic apparatus, photosystem II (PSII), is continually damaged and must be continually repaired by re-synthesis of damaged components. Photosynthetic organisms exposed to saturating light can also reduce oxidative damage by thermal dissipation of excess energy. Both the repair of damaged photosystems and the dissipation of energy reduce the overall efficiency of light use and should be minimized, if higher productivity is to be achieved.
Algae in photobioreactors are inevitably exposed to variable incident light due to diurnal and seasonal differences in irradiation. Nannochloropsis species have been shown to be capable of growing in a large range of illumination intensities, acclimating to changing conditions by optimizing the composition of their photosynthetic machinery to irradiation. Observed responses to different light intensities include modulation of pigment composition and concentrations of enzymes involved in carbon fixation.
It should also be noted that algal cultures in photobioreactors have high optical density, which causes highly inhomogeneous light distribution. As a consequence, surface-exposed cells absorb most of the light, leaving only a residual part of the radiation for the cells underneath, which are thus limited in their growth.
Instead, external layers are easily exposed to excess light and they must thermally dissipate up to 80% of their photons in order to avoid radiation damage. This greatly reduces their light use efficiency.
Following this idea, it has been shown that the overall efficiency of photobioreactors increases when the light path is diminished, reducing the inhomogeneity of light distribution. Unfortunately, very short light paths are difficult to be implemented in large-scale plants, due to practical and economic reasons.
Another factor to be considered is that cells in photobioreactors are rapidly mixed and move abruptly from darkness to full sunlight. Mixing cycles vary greatly according to cultivation system, and mixing rate on a millisecond time-scale can be achieved in closed tubular reactors or optical fiber-based photobioreactors, thanks to turbulent eddies. Conversely, in raceway ponds, laminar flows often affect efficient mixing.
Alternation of light/dark periods has been suggested to be beneficial to photosynthetic efficiency. In some cases, the possibility of achieving light integration has been shown, meaning that fluctuating light can be exploited with the same efficiency as continuous light of equal average intensity. However, the experiments reported in the literature focused on very different ranges of flash frequencies, from 5 Hz to 1 kHz, making a complete comparison of results difficult.
Experiments with photobioreactors also suggest that mixing rates affect photosynthetic productivity and, in particular, that the latter increases with the frequency of light/dark alternation. However, this conclusion has not always been confirmed, and other reports show that higher mixing rates do not improve photosynthetic efficiency, clearly indicating that deeper understanding of the influence of light fluctuations on photosynthetic productivity is needed.
Authors of this study were: Eleonora Sforza, Dipartimento di Ingegneria Industriale DII, Università di Padova, Padova, Italy; Diana Simionato, Dipartimento di Biologia, Università di Padova, Padova, Italy; Giorgio Mario Giacometti, Dipartimento di Biologia, Università di Padova, Padova, Italy; Alberto Bertucco, Dipartimento di Ingegneria Industriale DII, Università di Padova, Padova, Italy; Tomas Morosinotto