A model integrating fluid dynamics in photosynthesis and photoinhibition processes

The effect of photosynthesis time scales on microalgae productivity

Microalgae are often seen as a potential biofuel producer. In order to predict achievable productivities in the so called raceway culturing system, the dynamics of photosynthesis has to be taken into account. In particular, the dynamical effect of inhibition by an excess of light (photoinhibition) must be represented. We propose a model considering both photosynthesis and growth dynamics. This model involves three different time scales. We study the response of this model to fluctuating light with different frequencies by slow/fast approximations. Therefore, we identify three different regimes for which a simplified expression for the model can be derived. These expressions give a hint on productivity improvement which can be expected by stimulating photosynthesis with a faster hydrodynamics.

Modeling algal growth in bubble columns under sparging with CO2-enriched air

A theoretical model for predicting biomass growth in semi-continuous mode under sparging with CO(2)-enriched air was developed. The model includes gas-to-liquid mass transfer, algal uptake of carbon dioxide, algal growth kinetics, and light and temperature effects. The model was validated using experimental data on growth of two microalgal species in an internally illuminated photobioreactor: Nannochloropsis salina under gas flow rates of 800 and 1200 mL min(-1) and CO(2) enrichments of 0.5, 1, and 2%; and Scenedesmus sp. at a gas flow rate of 800 mL min(-1) and CO(2) enrichments of 3 and 4%. Temporal algal concentration profiles predicted by the model under semi-continuous mode with harvesting under the different test conditions agreed well with the measured data, with r(2) values ranging from 0.817 to 0.944, p<0.001. As demonstrated, this model can be beneficial in predicting temporal variations in algal concentration and in scheduling harvesting operations under semi-continuous cultivation mode.

Stylized facts in microalgal growth: interpretation in a dynamic energy budget context

A dynamic energy budget (DEB) model for microalgae is proposed. This model deviates from the standard DEB model as it needs more reserves to cope with the variation of assimilation pathways, requiring a different approach to growth based on the synthesizing unit (SU) theory for multiple substrates. It is shown that the model is able to accurately predict experimental data in constant and light-varying conditions with most of the parameter values taken directly from the literature. Also, model simulations are shown to be consistent with stylized facts (SFs) concerning NC ratio. These SFs are reinterpreted and the general conclusion is that all forcing variables (dilution rate, temperature and irradiance) impose changes in the nitrogen or carbon limitation status of the population, and consequently on reserve densities. Model predictions are also evaluated in comparison with SFs on chlorophyll concentration. It is proposed that an extra structure, more dependent on the nitrogen reserve, is required to accurately model chlorophyll dynamics. Finally, SFs concerning extracellular polymeric substances (EPSs) production by benthic diatoms are collected and interpreted and a formulation based on product synthesis and rejection flux is proposed for the EPSs production rate.

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).

Growth of Spirulina platensis enhanced under intermittent illumination

The growth characteristics of microalgae under different light conditions (continuous or intermittent) are essential information for photobioreactor design and operation. In this study, we constructed a thin-layer (10mm) flat plate photobioreactor device with a light/dark (L/D) alternation system to investigate the growth of Spirulina platensis under two different light regimes: (1) continuous illumination in a wide range of light intensities (1.00-77.16 mW cm(-2)); (2) intermittent illumination in medium frequency (0.01-20 Hz). Specific growth rate and light efficiency based on biomass production were determined for each round of experiment. Four regions (light limited region, intermediate region, light saturated region and light inhibition region) were recognized according to the results under continuous illumination. Under intermittent illumination, when L/D frequency increased from 0.01 Hz to 20 Hz, specific growth rate and light efficiency were enhanced. However, the enhancement was different, depending on the applied light intensity and light fraction. The higher the light intensity, the greater the enhancement would be when L/D frequency increased from 0.01 Hz to 20 Hz; and the higher the light intensity, the lower the light fractions is needed to maintain light efficiency as high as that under continuous illumination in light limited region.

Kinetic model of hydrogen generation by Rhodobacter sphaeroides in the presence of NH ions

AIMS

To examine the effects of ammonium ion concentration and illumination intensity on the effectivness of the hydrogen generation process of Rhodobacter sphaeroides.

METHODS AND RESULTS

In all experiments the amount of evolved hydrogen, biomass growth, concentration of ammonium ions, pH and chemical oxygen demand were measured. A nonstructural kinetic model was applied in description of biomass growth, the amount of evolved hydrogen and consumption of organic compounds and ammonium ions. An increase of ammonium ions concentration resulted in a decrease of maximal specific hydrogen potential production. At higher ammonium ion concentrations, no hydrogen evolution was observed. The efficiency of malic acid conversion into hydrogen and the PHB content in the biomass were the highest with lower concentrations of nitrogen compounds.

CONCLUSION

The presence of ammonium ions inhibits hydrogen photogeneration. A good agreement between the experimental data and model simulations were obtained. In all cases, hydrogen evolution started after an exhaustion of ammonium ions and the increase was proportional to the C/N ratio in the medium. The accumulation of PHB competes with the hydrogen evolution process while the conversion of acids into biomass was limited at higher levels of hydrogen generation.

SIGNIFICANCE AND IMPACT OF THE STUDY

Confirmation of the suitability of the selected model for kinetic studies of hydrogen photoevolution.

Model-supported optimization of phototrophic growth in a stirred-tank photobioreactor

A fuzzy-logic light attenuation model was developed and validated for a stirred-tank photobioreactor. Based on this model, local light intensities were used to calculate local specific growth rates of the cyanobacteria Synechococcus PCC 7942. The light regime for maximization of biomass space-time yield in a batch process was estimated using a genetic algorithm taking into account the integral average of the individual growth rates.

Analyzing and modeling of photobioreactors by combining first principles of physiology and hydrodynamics

Mixing in photobioreactors is known to enhance biomass productivity considerably, and flow dynamics play a significant role in the reactor's performance, as they determine the mixing and the cells' movement. In this work we focus on analyzing the effects of mixing and flow dynamics on the photobioreactor performance. Based on hydrodynamic findings from the CARPT(Computer Automated Radioactive Particle Tracking) technique, a possible mechanism for the interaction between the mixing and the physiology of photosynthesis is presented, and the effects of flow dynamics on light availability and light intensity fluctuation are discussed and quantitatively characterized. Furthermore, a dynamic modeling approach is developed for photobioreactor performance evaluation, which integrates first principles of photosynthesis, hydrodynamics, and irradiance distribution within the reactor. The results demonstrate the reliability and the possible applicability of this approach to commercially interesting microalgae/cyanobacteria culture systems.

Modeling stability of photoheterotrophic continuous cultures in photobioreactors

Continuous cultures of the purple non-sulfur bacterium Rhodospirillum rubrum were grown in a cylindrical photobioreactor in photoheterotrophic conditions, using acetate as carbon source. A new kinetic and stoichiometric knowledge model was developed, and its ability to simulate experimental results obtained under varying incident light fluxes and residence times is discussed. The model accurately predicts the stable, unstable, or oscillating behavior observed for the reactor productivity. In particular, the values of residence time corresponding to a subcritical bifurcation with a typical hysteresis effect are calculated and analyzed. The robustness of the proposed model allows the engineering operating domain of the photobioreactor function to be set and offers a promising tool for the design and control of such photoheterotrophic processes.

Optimisation of cultivation parameters in photobioreactors for microalgae cultivation using the A-stat technique

Light availability inside the reactor is often the bottleneck in microalgal cultivation and for this reason much attention is being given to light limited growth kinetics of microalgae, aiming at the increase of productivity in photobioreactors. Steady-state culture characteristics are commonly used for productivity optimisation and for cell physiology studies in continuous cultures, and are normally achieved using chemostat cultivations. In the present study, we investigated the applicability of a new and dynamic cultivation method called acceleration-stat (A-stat) to microalgae cultivations where light is the limiting substrate. In the A-stat, the dilution rate is increased at a constant rate. This acceleration rate should be a compromise between a short cultivation time, in order to make it a fast process, and the metabolic adaptation rate of the microorganism to changes in the environment. Simulations of the A-stat were done with different acceleration rates to have an indication of the best rate to use. An A-stat was performed in a pilot plant bubble column (65 l) with Dunaliella tertiolecta as a model organism, and results showed that a pseudo steady state was maintained throughout the experiment. From this work, it was concluded that the A-stat can be used as a fast and accurate tool to determine kinetic parameters and to optimise any specific type of photobioreactor.

Benefits and limitations of modeling for optimization of Porphyridium cruentum cultures in an annular photobioreactor

A deterministic Markov process was developed to estimate the efficiency of a fully controlled photobioreactor by calculating the growth of the Rhodophyte Porphyridium cruentum. It was assumed that microalgal growth, in strictly controlled and non-nutrient-limited conditions, is a function of the amount of light energy received. The light sources delivered a continuous 206 microE m(-2) s(-1) average photon flux density to cultures reaching concentrations of 3 g l(-1) in batch and 0.7 g l(-1) in chemostat. The concentration time-courses calculated compared satisfactorily with measured results for both batch and continuous cultures. The quality of simulation in these two cases validated the hypotheses made, especially for the linearity of light bioconversion, and allowed the model to be used for further exploration of the photobioreactor-operating domain. Distribution of the specific growth rate as a function of time and radial position was compared for the two simulated cases. The use of an initial batch of several days prior to the dilution of a continuous culture proved theoretically beneficial for overall production. The influence of the dilution rate and of light-path length on surface productivity and concentration at steady state were demonstrated.

Simulation of algae growth in a bench-scale bubble column reactor

The growth of the marine red microalga Porphyridium sp. in a bubble-column photobioreactor was simulated. The proposed model constitutes a dynamic integration of the kinetics of photosynthesis and photoinhibition with the fluid dynamics of the bubble column, including the effects of shear stress on the kinetics of growth. The kinetic data used in the model were obtained in independent experiments run in a thin-film photobioreactor with defined light/dark cycles. The maintenance term was modified to take into account the effects of liquid flow in the bioreactor on the growth rate. A hybrid method proposed for the approximate solution of the equations gave an appreciable reduction of the calculation time. Extrapolations of the model indicated the possibility of predicting the optimal diameter for an assembly of bubble column photobioreactors. Satisfactory fit was found with the experimental results of biomass growth in a 13-liter bubble column.