Adjusted light and dark cycles can optimize photosynthetic efficiency in algae growing in photobioreactors

Abstract

Biofuels from algae are highly interesting as renewable energy sources to replace, at least partially, fossil fuels, but great research efforts are still needed to optimize growth parameters to develop competitive large-scale cultivation systems. One factor with a seminal influence on productivity is light availability. Light energy fully supports algal growth, but it leads to oxidative stress if illumination is in excess. In this work, 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. Our 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.

Figure 1. Nannochloropsis salina growth under different light intensities in a flat-bed photobioreactor.

A) Growth kinetics of algae exposed to differing light intensities from 5 to 1000 µE m⁻² s⁻¹. Data with 5, 50, 120, 150, 250, 350 and 1000 µE m⁻² s⁻¹ shown in light blue, black, red, green, dark blue, pink and yellow, respectively. B) Growth parameters determined from curves in A, specific growth rate (black squares) and cellular concentration after 8 days of growth (red circles). C) Cellular concentrations reported in B normalized to light intensity: this may be used as approximate estimate of biomass production, as no significant deviation of cell size or DW/cell ratio was observed.

Figure 2. Algal growth kinetics under pulsed light.

A–B) Nannochloropsis growth curves in pulsed light of differing intensity and frequency, 1200 µE m⁻² s⁻¹ (10, 5, 1 Hz, respectively in red, blue and green, A) and 350 µE m⁻² s⁻¹ (10 and 30 Hz in red and blue, B). Kinetics with 120 µE m⁻² s⁻¹ continuous light reported for comparison (black). C) Growth rate (columns) and cellular concentration after 8 days of growth (red squares) extrapolated from curves in A–B. Values with 120 and 1000 µE m⁻² s⁻¹ constant illumination from Figure 1 reported for comparison. D) Cell concentration in C reported normalized to integrated light intensity. 1200-10, 5, 1 Hz and 350-10, 30 Hz reported in dark blue, pink, light blue, red and green respectively.

Figure 3. Dependence of photosynthetic efficiency (Fv/Fm) on illumination conditions.

Fv/Fm values at end of exponential phase, compared between cells grown under continuous illumination of differing intensity (black squares) and pulsed light of differing intensity and frequency, 1200 µE m⁻² s⁻¹ (10, 5, 1 Hz, red, green and blue circles), 350 µE m⁻² s⁻¹ (10 and 30 Hz, pink and light blue diamonds). Cells at 5 µE m⁻² s⁻¹ were too dilute to provide reliable results.

Figure 4. Acclimation response in cells grown in continous vs. pulsed light.

Cells grown under differing light intensities, either continuous or pulsed, compared with their Chl content per cell (black) and Chl/Car ratio (red), parameters indicating activation of acclimation response to pulsed light conditions.

Figure 5. Evaluation of lipid productivity.

Dependence of lipid production on illumination intensity. Productivity values with constant light (black squares) compared with the ones with light flashes of 350 and 1200 µE m⁻² s⁻¹ at various frequencies.