Photosynthesis in Flashing Light

Effects of photosynthetic photon flux density, frequency, duty ratio, and their interactions on net photosynthetic rate of cos lettuce leaves under pulsed light: explanation based on photosynthetic-intermediate pool dynamics

Square-wave pulsed light is characterized by three parameters, namely average photosynthetic photon flux density (PPFD), pulsed-light frequency, and duty ratio (the ratio of light-period duration to that of the light-dark cycle). In addition, the light-period PPFD is determined by the averaged PPFD and duty ratio. We investigated the effects of these parameters and their interactions on net photosynthetic rate (P n) of cos lettuce leaves for every combination of parameters. Averaged PPFD values were 0-500 µmol m-2 s-1. Frequency values were 0.1-1000 Hz. White LED arrays were used as the light source. Every parameter affected P n and interactions between parameters were observed for all combinations. The P n under pulsed light was lower than that measured under continuous light of the same averaged PPFD, and this difference was enhanced with decreasing frequency and increasing light-period PPFD. A mechanistic model was constructed to estimate the amount of stored photosynthetic intermediates over time under pulsed light. The results indicated that all effects of parameters and their interactions on P n were explainable by consideration of the dynamics of accumulation and consumption of photosynthetic intermediates.

Scenedesmus dimorphus biofilm: Photoefficiency and biomass production under intermittent lighting

This study investigated the effect of intermittent lighting on the growth performances of a Scenedesmus dimorphus biofilm. Flashing light effect (FLE) is common in traditional suspended cultures of microalgae; yet, publications about this phenomenon, in the context of biofilm cultivation, are scarce. In this work we demonstrate that, thanks to intermittent illumination, it is possible for attached cultivations to fulfill FLE, improve photoefficiency and productivity as well as providing protection from photoinhibition using much lower flashing light frequencies than those usually required with suspended cultures. Medium frequency intermittent lighting (0.1 Hz) guaranteed excellent light integration resulting in 9.13 g m⁻² d⁻¹ biomass productivity, which was 8.9% higher than with continuous lighting. Results showed that a light fraction value of 0.5 always improved photoefficiency values as related to continuous light with a 118.8% maximum increase.

Optimization of microalgal production in a shallow outdoor flume

The marine diatom Cyclotella cryptica was grown over a period of 13 months in a 48-m(2) shallow outdoor flume. The use of foil arrays at intervals of 1.2 m to effect systematic vertical mixing in the flume was found to significantly enhance microalgal production (p = 0.006). Average photosynthetic efficiencies (based on visible irradiance) with and without the foil arrays in place were 9.6 +/- 0.8 and 7.5 +/- 0.5% (+/-95% confidence intervals), respectively. A cost-benefit analysis indicated that the foil arrays were cost-effective if the value of the algae exceeded about $2.28 kg(1) of ash-free dry weight (AFDW). Parallel experiments performed in four 9.2-m(2) flumes showed that production was maximized when the cells were grown on a 2-day batch cycle between harvests rather than on a 1- or 3-day batch cycle. The optimum initial concentration (immediately after harvesting) of the algae was negatively correlated with the time interval between harvests and ranged from approximately 39 g AFDW/m(3) on a 3-day cycle to 213 g AFDW/m(3) on a 1-day cycle. The increase in production resulting from growth on a 2-day rather than a 1-day batch cycle was about 19% and was statistically significant at p = 0.0003. Growth of C. cryptica over a total period of 122 days during the 13-month study in the 48-m(2) flume under near-optimal conditions (2-day batch cycle, initial concentration 155 g AFDW/m(3)) resulted in an average production rate (+/-95% confidence interval) of 29.7 +/- 2.7 g AFDW/m(2) d.

Analysis of light regime in continuous light distributions in photobioreactors

Maximum photobioreactor (PBR) efficiency is a must in applications such as the obtention of microalgae-derived fuels. Improving PBR performance requires a better understanding of the "light regime", the varying irradiance that microalgal cells moving in a dense culture are exposed to. We propose a definition of light regime that can be used consistently to describe the continuously varying light patterns in PBRs as well as in light/dark cycles. Equivalent continuous and light/dark regimes have been experimentally compared and the results show that continuous variations are not well represented by light/dark cycles, as had been widely accepted. It has been shown that a correct light regime allows obtaining photosynthetic rates higher than the corresponding to continuous light, the so-called "flashing light effect" and that this is possible in commercial PBRs. A correct PBR operation could result in photosynthetic efficiency close to the optimum eight quanta per O(2).

Photosynthesis in modulated light: quantitative dependence of photosynthetic enhancement on flashing rate

In order to predict the potential benefit associated with mixing devices designed to introduce periodic light modulations in dense cultures of microalgae, it is necessary to develop a quantitative understanding of the relationship between the frequency of the modulations and the resulting photosynthetic efficiency enhancement. To explore this relationship, the photosynthetic rate of cells of Phaeodactylum tricornutum from a dense steady state culture was determined as a function of modulation frequency, intensity of light received, and the proportion of the total cycle period during which the cells were illuminated. At high flash frequencies, the photosynthetic rate was determined by the average intensity received by the cells (full light intensity integration), while at low frequencies the cells responded to the instantaneous intensity (no light intensity integration). Full integration was approached asymptotically with increasing flash frequency. The frequency response could be described by a rectangular hyperbola, and the parameters of this hyperbola were nearly independent of the illumination intensity and the flash proportion. The saturation constant of the hyperbola, at which the response is one-half of the maximum, was 0.67 Hz.

The 1932 experiments

The 1932 papers of Emerson and Arnold were great ones, both conceptually and experimentally. The results were so far out of synch with the thinking of the times that it took many years for them to become a cozy part of the dogma of photosynthesis. I reflect on this twenty-four year segment of history.