Enhanced Algal Photosynthetic Photon Efficiency by Pulsed Light

Abstract

We present experimental results demonstrating that, relative to continuous illumination, an increase of a factor of 3-10 in the photon efficiency of algal photosynthesis is attainable via the judicious application of pulsed light for light intensities of practical interest (e.g., average-to-peak solar irradiance). We also propose a simple model that can account for all the measurements. The model (1) reflects the essential rate-limiting elements in bioproductivity, (2) incorporates the impact of photon arrival-time statistics, and (3) accounts for how the enhancement in photon efficiency depends on the timescales of light pulsing and photon flux density. The key is avoiding "clogging" of the photosynthetic pathway by properly timing the light-dark cycles experienced by algal cells. We show how this can be realized with pulsed light sources, or by producing pulsed-light effects from continuous illumination via turbulent mixing in dense algal cultures in thin photobioreactors.

Keywords: biochemical analyses; biochemical mechanism; plant biochemistry.

Graphical abstract

Figure 1

Key Trends in Algal Photobioreactor Performance (A) Biomass production rate per unit time and per unit area, P, as a function of instantaneous photon flux density. Under continuous light, the instantaneous and time-averaged photon flux densities are identical. For pulsed light, however, I_{instantaneous} refers to its value only during T_pulse, which is why P can be noticeably greater under pulsed light. (B) The corresponding variation of P/I (biomass production per photon) with instantaneous photon flux density.

Figure 2

Measured Normalized Specific Growth Rate As a function of: (A) T_dark for pulsed light with I = 1,000 μE/(m²-s) at T_pulse = 5, 10, and 15 ms and (B) I for both continuous irradiation and pulsed light. Vertical bars indicate ±1 standard deviation about the average. Each data point comes from 20 replications for experiments with continuous light and 8 replications for experiments with pulsed light.

Figure 3

Schematic of the Rate-Limiting Step in Algal Bioproductivity as Implemented in Our Model

Figure 4

Comparison of Model Predictions for a P-I Curve against Our Measurements for Continuous Illumination Vertical bars indicate ±1 standard deviation about the average for the measured data, with each point coming from 20 replications.

Figure 5

Calculated η_ph versus Pulse Duration At two values of photon flux density (from Equation 11). A = 1 nm², N_Pool = 7, and τ_del = 10 ms.

Figure 6

Measured and Calculated η_ph η_ph versus (A) T_pulse at I = 1,000 μE/(m²-s) (at the optimal T_dark) and N_pool = 7, τ_del = 10 ms, A = 1 nm² for model calculations; (B) I for T_pulse = 10 ms, T_dark = 290 ms; and (C) I for T_pulse = 150 ms, T_dark = 250 ms. Vertical bars indicate ±1 standard deviation about the average for the experimental data, with each measured point coming from 8 replications.

Figure 7

Comparison of Simple Average Model Predictions for η_ph against Data with Pulse Times ≥ 10 ms Calculated values show the sensitivity to the assumed τ_del. (A) For two pulse durations at a relatively low I = 350 μE/(m²-s). (B) For three pulse durations at an intermediate I = 1,200 μE/(m²-s). The vertical bars of ±1 standard deviation about the average of each measured data point were taken from the original reference Simionato et al. (2013) with no further elaboration therein.

Figure 8

Comparisons of Model Predictions Against Data for η_ph For (A) I = 200, 500, and 1,000 μE/(m²-s), at T_pulse = 10 ms and T_dark = 290 ms; (B) T_pulse = 5, 10, and 15 ms, at I = 1,000 μE/(m²-s). The vertical bars of ±1 standard deviation about the average (1) come from 8 replications for each measured point and (2) correspond to the inherent standard deviations associated with photon arrival statistics for the model (computed) results, as elaborated in the text.

Figure 9

Comparisons of Model Predictions against Data for η_ph For (A) (T_pulse, T_dark) = (11 ms, 22 ms), and (T_pulse, T_dark) = (33.33 ms, 66.67 ms), at a relatively low value of I = 350 μE/(m²-s); (B) (T_pulseT_dark) = (10 ms, 20 ms), (T_pulse, T_dark) = (20 ms, 180 ms), and (T_pulse, T_dark) = (100 ms, 900 ms) at I = 1,200 μE/(m²-s). The vertical bars of ±1 standard deviation about the average (1) were taken from Simionato et al. (2013) with no further elaboration therein and (2) correspond to the inherent standard deviations associated with photon arrival statistics for the model (computed) results, as elaborated in the text.