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Abstract Large amounts of astaxanthin (about 4% DW) can be produced under nitrogen starvation of Haematococcus pluvialis in photobioreactors (PBRs) exposed to high light conditions to induce a light stress. However, in PBR, the large biomass concentration usually achieved leads to strong light attenuation conditions, which makes complex the analysis of this “light stress”. This study aims to elucidate the role of light transfer in astaxanthin cell content and productivity from the microalga Haematococcus pluvialis during nitrogen starvation. Haematococcus pluvialis was cultivated in a flat-panel PBR in a batch mode with sudden nitrogen starvation conditions and an incident photon flux density (PFD) of 250 µmolhν m−2 s−1. Different initial biomass concentrations ( $${C}_{{x}_{0}}$$ C x 0 ) were evaluated, 0.21, 0.52, 1.39 and 2.21 kg m−3. As a result, spectral mass absorption cross-sections of Haematococcus pluvialis were measured at different times during nitrogen starvation, and were used to relate the mean rate of photon absorption (MRPA) to the astaxanthin productivity. A minimum initial MRPA of 7000 ± 500 µmolhν kgx −1 s−1 was found necessary to trigger large accumulation of astaxanthin in Haematococcus pluvialis cells (up to 3.21% DW) during nitrogen starvation conditions. The results also demonstrated the link between the MRPA and the daily astaxanthin productivity of Haematococcus pluvialis cultures, introducing then the MRPA as a physical quantity of interest for a rational optimization of the light culture conditions in PBRs. Graphical Abstract

Large amounts of astaxanthin (about 4% DW) can be produced under nitrogen starvation of Haematococcus pluvialis in photobioreactors (PBRs) exposed to high light conditions to induce a light stress. However, in PBR, the large biomass concentration usually achieved leads to strong light attenuation conditions, which makes complex the analysis of this "light stress". This study aims to elucidate the role of light transfer in astaxanthin cell content and productivity from the microalga Haematococcus pluvialis during nitrogen starvation. Haematococcus pluvialis was cultivated in a flat-panel PBR in a batch mode with sudden nitrogen starvation conditions and an incident photon flux density (PFD) of 250 µmolhν m−2 s−1. Different initial biomass concentrations ( C x 0 ) were evaluated, 0.21, 0.52, 1.39 and 2.21 kg m−3. As a result, spectral mass absorption cross-sections of Haematococcus pluvialis were measured at different times during nitrogen starvation, and were used to relate the mean rate of photon absorption (MRPA) to the astaxanthin productivity. A minimum initial MRPA of 7000 ± 500 µmolhν kgx−1 s−1 was found necessary to trigger large accumulation of astaxanthin in Haematococcus pluvialis cells (up to 3.21% DW) during nitrogen starvation conditions. The results also demonstrated the link between the MRPA and the daily astaxanthin productivity of Haematococcus pluvialis cultures, introducing then the MRPA as a physical quantity of interest for a rational optimization of the light culture conditions in PBRs. [ABSTRACT FROM AUTHOR]

Due to the wide presence of microplastics in water, the interaction between microplastic particles and microalgae cells in medium merits the attention of researchers. Microplastic particles can impact the original transmission of light radiation in water bodies since the refractive index of microplastics is different from that of water bodies. Accordingly, the accumulation of microplastics in water bodies will certainly impact microalgal photosynthesis. Therefore, experimental measurements and theoretical studies characterizing the radiative properties of the interaction between light and microplastic particles are highly significant. The extinction and absorption coefficient/cross-section of polyethylene terephthalate and polypropylene were experimentally measured using transmission and integrating methods in the spectral range of 200–1,100 nm. The absorption cross-section of PET shows remarkable absorption peaks in the vicinity of 326 nm, 700 nm, 711 nm, 767 nm, 823 nm, 913 nm, and 1,046 nm. The absorption cross-section of PP has distinctive absorption peaks near 334 nm, 703 nm, and 1,016 nm. The measured scattering albedo of the microplastic particles is above 0.7, indicating that both microplastics are scattering dominant media. Based on the results of this work, an in-depth understanding of the interaction between microalgal photosynthesis and microplastic particles in the medium will be obtained.

Computational Fluid Dynamics (CFD) have been frequently applied to model the growth conditions in photobioreactors, which are affected in a complex way by multiple, interacting physical processes. We review common photobioreactor types and discuss the processes occurring therein as well as how these processes have been considered in previous CFD models. The analysis reveals that CFD models of photobioreactors do often not consider state-of-the-art modeling approaches. As a comprehensive photobioreactor model consists of several sub-models, we review the most relevant models for the simulation of fluid flows, light propagation, heat and mass transfer and growth kinetics as well as state-of-the-art models for turbulence and interphase forces, revealing their strength and deficiencies. In addition, we review the population balance equation, breakage and coalescence models and discretization methods since the predicted bubble size distribution critically depends on them. This comprehensive overview of the available models provides a unique toolbox for generating CFD models of photobioreactors. Directions future research should take are also discussed, mainly consisting of an extensive experimental validation of the single models for specific photobioreactor geometries, as well as more complete and sophisticated integrated models by virtue of the constant increase of the computational capacity. [ABSTRACT FROM AUTHOR]

Summary: The aim of this study was to investigate the light transfer through sea ice with a focus on bio-optical substances both in fast ice and in the drift ice zones in the northern Baltic Sea. The measurements included snow and ice structure, spectral irradiance and photosynthetically active radiation below the sea ice. We also measured the concentrations of the three main bio-optical substances which are chlorophyll-a, suspended particulate matter, and coloured dissolved organic matter (CDOM). These bio-optical substances were determined for melted ice samples and for the underlying sea water. The present study provides the first spectral light transfer data set for drift ice in the Baltic Sea. We found high CDOM absorption values typical to the Baltic Sea waters also within sea ice. Our results showed that the transmittance through bare ice was lower for the coastal fast ice than for the drift ice sites. Bio-optical substances, in particular CDOM, modified the spectral distribution of light penetrating through the ice cover. Differences in crystal structure and the amount of gas inclusions in the ice caused variation in the light transfer. Snow cover on ice was found to be the dominant factor influencing the light field under ice, confirming previous studies. In conclusion, snow cover dominated the amount of light under the ice, but did not modify its spectral composition. CDOM in the ice absorbs strongly in the short wavelengths. As pure water absorbs most in the long wavelengths, the light transfer through ice was highest in the green (549–585 nm).

Microalgae are considered one of the most efficient and environmentally friendly ways for carbon dioxide fixation. The bubbles play an important role in analyzing the radiation transfer in photobioreactors during microalgae growth. Herein, Chlorella sp. and Scenedesmus obliquus were cultured in the airlift flat plate photobioreactor and evaluated for the temporal evolution of radiation characteristics. A one-dimensional model of bubbles on time-dependent radiation transfer in a photobioreactor was proposed, and it was well verified with the experimental result. The results indicated that with the increase of bubble volume fraction or the decrease of bubble radius, the local irradiance increased at the illuminated surface of the microalgal culture and was attenuated more rapidly along with the radiation transfer. The average specific growth rate of microalgae decreases as bubble volume fraction increases or bubble radius decreases. The volume fraction of 0.003 and a radius of 3.5 mm are the optimal operating conditions in this study for microalgae growth and carbon dioxide fixation. The presented analysis would facilitate the design and optimization of the optical and aeration configurations of photobioreactors for carbon dioxide fixation.

Light interaction with plant tissue varies significantly in different components with the structural, chemical, and optical characteristics in most agro products at the microscale. In light-tissue interaction, the tissues can generally be treated as being primarily composed of absorption and scattering particles, and thus the light propagation through tissues can be simplified as mainly involving the process of photon interactions with the absorption and scattering particles. When entering the tissue, the light can be absorbed and/or scattered, represented by the absorption coefficient (μa) and reduced scattering coefficient (μ's), respectively. Quantification of optical properties (i.e., μa and μ's) can greatly contribute to clarifying the measured data, optimizing optical devices, and finally improving the quality and safety assessment of agro-products. Alternatively, spatial-frequency domain imaging (SFDI) has widely been used to measure the optical properties, and then to evaluate the quality/safety of agro products last decades, showing the wide-field and noncontact imaging, depth- and resolution-varying, as well as signal enhancement. SFDI can also be used to reconstruct the three-dimensional distribution of optical features related to tissue physicochemical properties in the field of nondestructive detection. This study first overviewed the origins and development of SFDI in the field of agricultural engineering, and then introduced the main working principles of SFDI, including system components, light propagation model, data measurement and processing, and inverse algorithms for optical property estimation. Specifically, the SFDI was first applied to the nondestructive detection of bruising on Golden Delicious apples in 2007, indicating a better performance to distinguish the bruised apple from the sound one. The SFDI system is mainly composed of a light source, a digital projector, a CCD camera, a wavelength selective device, and a sample stage, the former three of which are the core components to directly determine the quality of structured illumination and collected images, as well as the testing efficiency. Calibration is also required for the SFDI system with the standard samples before evaluation. The specific procedure is followed. The images of target samples are first captured by the SFDI system. The light uniformity correction, image demodulation, system response calibration, and surface profile correction are then conducted to obtain the diffuse reflectance images for the quality and safety evaluation directly, or for the optical property estimation coupled with inverse algorithms. After that, the application status of SFDI was summarized in the field of agricultural engineering, including the measurement of optical property and quality/safety assessment of several thin-skinned fruits, such as apple, pear, kiwifruit, cucumber, and peach. The challenges and future perspectives of the SFDI technique were also presented eventually. Nevertheless, the current SFDI technique is derived mostly from the diffusion approximation, thereby hindering the application easy to introduce large measurement errors. There are great challenges when measuring the optical property of two- and multi-layered agro products. It is also lacking a standardized optical system for accurate estimation of the optical property. The SFDI presents better performance in the depth-varying detection, but the penetration depth is a bit shallow limited to the millimeter level. Moreover, the demand for portable handheld devices of the SFDI technique is ever increasing in recent years. This review can provide a critical overview of the development of the SFDI technique for better understanding in the field of agricultural engineering. [ABSTRACT FROM AUTHOR]

The industrial production of astaxanthin from Haematococcus pluvialis is mainly operated following a two-stage cultivation strategy in photobioreactors (PBRs) operating in batch mode. This study provides a strategy for optimizing continuous astaxanthin production in a one-stage approach, by optimizing both astaxanthin accumulation and biomass productivity of H. pluvialis under nitrogen-limited condition. To achieve the good balance in culture conditions to maintain growth under nitrogen-limited conditions, while triggering significant astaxanthin accumulation in reddish vegetative cells, the role of light absorption, as represented by the mean rate of photon absorption (MRPA), and nitrate concentration was especially investigated. For that purpose, H. pluvialis cultures were grown in a flat-panel PBR with constant dilution rate D of 0.015 h−1, different photon flux densities PFDs ranging from 75 to 750 μmol hν ·m−2·s−1 and different nitrogen concentration levels in the feeding medium [NO 3 −] of 1, 3 and 8.8 mM. A parabolic relationship between MRPA and astaxanthin production rate was obtained. This indicated that astaxanthin synthesis was limited by the MRPA, opening further optimization by adjusting incident light intensity, nitrogen concentration in the feeding medium or culture dilution rate. After optimization of operating conditions, we demonstrated that a large quantity of astaxanthin can be produced in continuous mode, following then a one-stage strategy which may be advantageous for industrial use. A maximum astaxanthin productivity of 1.27 ± 0.03 g·m−2·d−1 with an astaxanthin accumulation of 3.9 ± 0.2 % DW was reached for the culture featuring MRPA of 9000 μmol hν ·kg x −1·s−1. The process was also proved reversible, allowing to tailor the biomass composition and physiological state (from green to red cells enriched in astaxanthin) by adjusting operating conditions, opening perspectives from an optimized coupling with downstream processing steps. • The continuous culture of H. pluvialis enriched in astaxanthin was optimized. • MRPA was found relevant to control the light-stress in N-limitation. • A minimum chlorophyll content was found necessary to sustain sufficient growth. • An astaxanthin productivity of 1.27 g·m−2·d−1 with 3.9%DW accumulation was obtained. • A reversibility of cell response was demonstrated after almost one year of culture. [ABSTRACT FROM AUTHOR]

• The optimal amounts of exogenous COD and CAP for attached microalgae were determined. • Proteomic analysis revealed COD (60 mg/L) promoted both algal auto- and heterotrophic growth. • CAP (5–10 mg/L) promoted autotrophic metabolism of surface cells in biofilm. • COD and CAP inhibited light transport to deep layer, but both favored N utilization. • Attached cultures of Chlorella had stable resourcing capacity within 24 days. During the wastewater treatment and resource recovery process by attached microalgae, the chemical oxygen demand (COD) can cause biotic contamination in algal culture systems, which can be mitigated by adding an appropriate dosage of antibiotics. The transport of COD and additive antibiotic (chloramphenicol, CAP) in algal biofilms and their influence on algal physiology were studied. The results showed that COD (60 mg/L) affected key metabolic pathways, such as photosystem II and oxidative phosphorylation, improved biofilm autotrophic and heterotrophic metabolic intensities, increased nutrient demand, and promoted biomass accumulation by 55.9 %, which was the most suitable COD concentration for attached microalgae. CAP (5–10 mg/L) effectively stimulated photosynthetic pigment accumulation and nutrient utilization in pelagic microalgal cells. In conclusion, controlling the COD concentration (approximately 60 mg/L) in the medium and adding the appropriate CAP concentration (5–10 mg/L) are conducive to improving attached microalgal biomass production and resource recovery potential from wastewater. [ABSTRACT FROM AUTHOR]

Computational Fluid Dynamics (CFD) have been frequently applied to model the growth conditions in photobioreactors, which are affected in a complex way by multiple, interacting physical processes. We review common photobioreactor types and discuss the processes occurring therein as well as how these processes have been considered in previous CFD models. The analysis reveals that CFD models of photobioreactors do often not consider state-of-the-art modeling approaches. As a comprehensive photobioreactor model consists of several sub-models, we review the most relevant models for the simulation of fluid flows, light propagation, heat and mass transfer and growth kinetics as well as state-of-the-art models for turbulence and interphase forces, revealing their strength and deficiencies. In addition, we review the population balance equation, breakage and coalescence models and discretization methods since the predicted bubble size distribution critically depends on them. This comprehensive overview of the available models provides a unique toolbox for generating CFD models of photobioreactors. Directions future research should take are also discussed, mainly consisting of an extensive experimental validation of the single models for specific photobioreactor geometries, as well as more complete and sophisticated integrated models by virtue of the constant increase of the computational capacity.

To gain more understanding of lake ice melting process, field research was carried out in an arctic tundra lake, Kilpisjärvi (surface area 37.1 km2, maximum depth 57 m) in the melting periods of 2013 and 2014. The heat budget of the ice cover was dominated by the radiation balance; turbulent heat fluxes were large in 2013 due to warm air advection but small in 2014. Transmittance of solar radiation through ice was 0.25 in 2013 and 0.10 in 2014, snow-ice was absent in 2013 but in 2014 accounted for 50% of the ice cover. The melting rate was 4.4 cm d-1 in 2013, 1.9 cm d-1 in 2014. The portions of surface, bottom and internal melting were, respectively, 2.9, 1.0 and 0.5 cm d-1 in 2013 and 0.8, 1.0 and 0.1 cm d-1 in 2014. Internal melting was realized in increase of ice porosity. In 2013 a rapid ice breakage event completed the ice breakup in short time when ice porosity had reached 40-50%. A lake ice melting model should include the thickness and porosity of ice, with porosity connected to an ice strength criterion.

To gain more understanding of lake ice melting process, field research was carried out in an arctic tundra lake, Kilpisjärvi (surface area 37.1 km², maximum depth 57 m) in the melting periods of 2013 and 2014. The heat budget of the ice cover was dominated by the radiation balance; turbulent heat fluxes were large in 2013 due to warm air advection but small in 2014. Transmittance of solar radiation through ice was 0.25 in 2013 and 0.10 in 2014, snow-ice was absent in 2013 but in 2014 accounted for 50% of the ice cover. The melting rate was 4.4 cm d-1 in 2013, 1.9 cm d-1 in 2014. The portions of surface, bottom and internal melting were, respectively, 2.9, 1.0 and 0.5 cm d-1 in 2013 and 0.8, 1.0 and 0.1 cm d-1 in 2014. internal melting was realized in increase of ice porosity. in 2013 a rapid ice breakage event completed the ice breakup in short time when ice porosity had reached 40-50%. A lake ice melting model should include the thickness and porosity of ice, with porosity connected to an ice strength criterion. [ABSTRACT FROM AUTHOR]

This study investigates the effect of condensed water droplets on the areal biomass productivity of outdoor culture systems with a free surface, protected by a transparent window or cover to prevent contamination and to control the growth conditions. Under solar radiation, evaporation from the culture causes droplets to condense on the interior surface of the cover. To quantify the effect of droplets on the system’s performance, the bidirectional transmittance of a droplet-covered window was predicted using the Monte Carlo ray-tracing method. It was combined with a growth kinetics model of Chlorella vulgaris to predict the temporal evolution of the biomass concentration on 21 June and 23 September in Los Angeles, CA. A droplet contact angle of 30∘ or 90∘ and a surface area coverage of 50% or 90% were considered. Light scattering by the condensed droplets changed the direction of the incident sunlight while reducing the amount of light reaching the culture by up to 37%. The combined effect decreased the daily areal biomass productivity with increasing droplet contact angle and surface area coverage by as much as 18%. Furthermore, the areal biomass productivity of the system was found to scale with the ratio X0/a of the initial biomass concentration X0 and the specific illuminated area a, as previously established for different photobioreactor geometries, but even in the presence of droplets. Finally, for a given day of the year, the optical thickness of the culture that yielded the maximum productivity was independent of the window condition. Thus, the design and operation of such a system should focus on maintaining a small droplet contact angle and surface area coverage and an optimum optical thickness to maximize productivity.

The relationship between performance and working conditions in photobioreactor hydrogen production systems illuminated by a variable intensity light source has been described quantitatively using relational expressions. First, based on the finite volume method and the Michaelis-Menten model, the hydrogen production process of a photobioreactor (PBR) system was simulated numerically. Then, the performance of the PBR system was evaluated considering the hydrogen production rate, dimensionless hydrogen production rate, hydrogen production thrust coefficient and conversion efficiency of light energy to hydrogen energy rate as performance parameters, and the relationship between these parameters and working conditions was studied. Finally, the improved quantum-behaved particle swarm optimization (IQPSO) and surface fitting technique based on the curve fitting method were used to obtain relational expressions about the performance and working conditions of the PBR. All of the results show that the method can obtain accurately relational expressions for the performance optimization and forecasts of the PBR system.

Microalgae are seen as the most promising flexible feedstock, being considered the crop of the future. They grow fast, just needing sunlight, carbon dioxide and minerals. They contain high value ingredients, such as proteins, carbohydrates, lipids, nucleic acids and others (carotenoids and polymers). Thus, they can be produced for a wide range of markets, including human and animal nutrition, cosmetics, pharmaceuticals and biofuels. However, the production cost is still high, limiting their commercial applications to high-valued compounds. The reduction of these costs can be obtained with efficient bioreactor designs, which are able to achieve high areal biomass productivities. In this context, Computational Fluid Dynamics (CFD) may play an important role in the optimization of bioreactor design, analysing the interaction of hydrodynamics, light supply, heat and mass transfer and biological kinetics. This study addresses the recent advances in CFD modelling of both open pond and closed bioreactors. [ABSTRACT FROM AUTHOR]

In this study, the light transfer procedure and photobiological hydrogen production of microalgae in the photobioreactors (PBRs) at different locations of China are investigated thoroughly. Firstly, the solar irradiation projected on the ground of four different regions (Harbin, Beijing, Shanghai and Sanya) at the same moment and that of Harbin at four different moments are calculated by the SMART model. Based on the finite volume method and the photobiological hydrogen production dynamic model, the effects of different irradiance on the photobiological hydrogen production rate in the PBRs are analyzed. In addition, the effect of microalgae cell concentration and the scattering and absorbing of gas bubbles on the hydrogen production have also been studied. All the results elucidate these two factors play a significant role in the light transfer and hydrogen production rate in the PBRs. Meanwhile, the light intensity and microalgae cell density both have the saturation inhibition effect on the hydrogen production process. The distribution of the highly efficient hydrogen production region in the reactor varies with different bubbles concentration. [ABSTRACT FROM AUTHOR]

This study aims to develop a methodology to predict the absorption cross-section of colonial microalgae such as those of the Volvocaceae family including Eudorina , Pleodorina , and Volvox consisting of an ordered assembly of large and optically soft absorbing cells embedded within a non-absorbing spherical extracellular matrix (ECM). The absorption cross-section of spherical colonies, such as Eudorina , containing 16, 32, and 64 equidistant photosynthetic cells distributed on the surface of a sphere within a concentric spherical ECM was predicted by the superposition T-matrix method for ECM size parameters as large as 500 and by the Monte Carlo ray-tracing (MCRT) method for ECM size parameters as large as 900. The predicted absorption cross-sections given by the two methods were in excellent agreement despite the fact that the conditions for geometric optics were not rigorously satisfied. The absorption cross-section of the microalgae colonies considered was found to increase with increasing cell radius, absorption index or cell pigment concentration, and/or number of cells. Shading among cells decreased the mass absorption cross-section and was increasingly important for colonies with strongly absorbing cells, large cell radius, and/or large number of cells. These results demonstrate that accounting for shading effects is necessary to accurately predict the absorption cross-section of microalgae colonies. Furthermore, the study demonstrated that the MCRT method is an accurate and efficient method for modeling light absorption by an ensemble of many large, ordered, and optically soft particles. Finally, the impact of colony formation as well as pigment and biomass concentrations on the local (LRPA) and mean (MRPA) rates of photon absorption within a microalgae culture was also assessed. At low biomass concentrations, the LRPA and MRPA decreased in the presence of colonies. This effect was more pronounced at higher pigment concentrations. • The absorption cross-section of colonial Volvocaceae is predicted. • Colonies consist of large, equidistant, optically soft cells in a clear spherical ECM. • T-matrix and ray tracing methods are compared over a wide range of size parameters. • Ray tracing method is accurate while requiring less computing resources. • Mutual shading among cells reduced light absorption, even at small cell volume fraction. [ABSTRACT FROM AUTHOR]

Due to the wide presence of microplastics in water, the interaction between microplastic particles and microalgae cells in medium merits the attention of researchers. Microplastic particles can impact the original transmission of light radiation in water bodies since the refractive index of microplastics is different from that of water bodies. Accordingly, the accumulation of microplastics in water bodies will certainly impact microalgal photosynthesis. Therefore, experimental measurements and theoretical studies characterizing the radiative properties of the interaction between light and microplastic particles are highly significant. The extinction and absorption coefficient/cross-section of polyethylene terephthalate and polypropylene were experimentally measured using transmission and integrating methods in the spectral range of 200-1,100 nm. The absorption cross-section of PET shows remarkable absorption peaks in the vicinity of 326 nm, 700 nm, 711 nm, 767 nm, 823 nm, 913 nm, and 1,046 nm. The absorption cross-section of PP has distinctive absorption peaks near 334 nm, 703 nm, and 1,016 nm. The measured scattering albedo of the microplastic particles is above 0.7, indicating that both microplastics are scattering dominant media. Based on the results of this work, an in-depth understanding of the interaction between microalgal photosynthesis and microplastic particles in the medium will be obtained., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Yang and Ma.)

The spectral absorption cross-section of microalgae is an essential parameter in modeling the microalgae metabolism and growth kinetics as well as in estimating the productivity and efficiency of photobioreactors. This paper presents a simple experimental procedure for retrieving the average spectral absorption cross-section of concentrated microalgal suspensions. The method combines experimental measurements of the normal–hemispherical transmittance and reflectance of the suspensions in conventional cuvettes with an inverse method and analytical expressions obtained from the modified two-flux approximation accounting for absorption and multiple scattering. The method was validated with direct measurements of the scattering phase function and of the absorption and scattering cross-sections of freshwater microalgae Chlorella vulgaris . It was able to retrieve the absorption cross-section with acceptable accuracy. The latter was then used to estimate successfully the fluence rate using a simplified light transfer model, the local rate of photon absorption, and the biomass productivity in flat plate PBRs. [ABSTRACT FROM AUTHOR]

This paper aims to experimentally and directly validate a recent theoretical method for predicting the radiation characteristics of photosynthetic microorganisms. Such predictions would facilitate light transfer analysis in photobioreactors (PBRs) to control their operation and to maximize their production of biofuel and other high-value products. The state of the art experimental method can be applied to microorganisms of any shape and inherently accounts for their non-spherical and heterogeneous nature. On the other hand, the theoretical method treats the microorganisms as polydisperse homogeneous spheres with some effective optical properties. The absorption index is expressed as the weighted sum of the pigment mass absorption cross-sections and the refractive index is estimated based on the subtractive Kramers–Kronig relationship given an anchor refractive index and wavelength. Here, particular attention was paid to green microalgae Chlamydomonas reinhardtii grown under nitrogen-replete and nitrogen-limited conditions and to Chlorella vulgaris grown under nitrogen-replete conditions. First, relatively good agreement was found between the two methods for determining the mass absorption and scattering cross-sections and the asymmetry factor of both nitrogen-replete and nitrogen-limited C. reinhardtii with the proper anchor point. However, the homogeneous sphere approximation significantly overestimated the absorption cross-section of C. vulgaris cells. The latter were instead modeled as polydisperse coated spheres consisting of an absorbing core containing pigments and a non-absorbing but strongly refracting wall made of sporopollenin. The coated sphere approximation gave good predictions of the experimentally measured integral radiation characteristics of C. vulgaris . In both cases, the homogeneous and coated sphere approximations predicted resonance in the scattering phase function that were not observed experimentally. However, these approximations were sufficiently accurate to predict the fluence rate and local rate of photon absorption in PBRs. [ABSTRACT FROM AUTHOR]

This study provides insight into the key variables that drive sun-induced chlorophyll fluorescence (SIF) emanating from vegetation canopies, based on a global sensitivity analysis (GSA) of the Soil-Canopy Observation of Photosynthesis and Energy (SCOPE) balance model. An updated version of the SCOPE model was used here (v1.53) which contains novel leaf physiological modules for determination of the steady state fluorescence yield: a photosynthesis model coupled with (a) submodels having empirically derived relationships, identified as TB12 for unstressed and TB12-D for drought conditions and (b) a mechanistic (MD12) submodel based on theoretical relationships. By inspecting Sobol's total order (main effect and all the interactions) sensitivity index ( S Ti ) rankings, the influential and non-influential variables were determined. Two experiments were conducted for the different leaf physiology modules in SCOPE considering (1) only vegetation variables, and (2) all SCOPE variables, i.e., including micrometeorological, aerodynamic and geometry variables. Considering TB12-D S Ti results using only vegetation input variables, the canopy-leaving broadband (641–800 nm) SIF variability was determined mainly by leaf optical properties and canopy structural variables. The most important variables were (with decreasing importance) leaf chlorophyll content (Cab), leaf inclination (LIDFa) and leaf area index (LAI). These three variables alone determined 77.9% of the SIF variability. V cmo , the variable related to photosynthetic capacity, determined 11.4% of overall SIF variability, and its importance declined considerably when moving from the first emission peak (SIF red ; with maximal relevance of 17.9% at 676 nm) to the second emission peak (SIF NIR ; e.g., 9.6% at 740 nm). Stronger relationships with V cmo were obtained when retrieving the full broadband SIF flux and calculating total fluorescence yield (F yield , determined as the integral of the hemispherical broadband SIF flux divided by the total absorbed PAR), of which 35% of the variability was influenced by V cmo . Using the TB12 submodel, the major drivers of SIF flux were similar to TB12-D except that V cmo accounted for very little (< 2%) variability. The MD12 submodel identified the components of long-term PSII photoprotection and photodamage as the dominant factors for SIF variability: these two variables alone accounted for 51.4% of the variability of SIF flux and 61% of F yield , whereas V cmo explained only 9.7% and 10.9% of variability in SIF flux and F yield , respectively. Analysis of the relative importance of all SCOPE variables revealed that in addition to the key vegetation variables, micrometeorological variables were important in driving SIF variability, especially incoming shortwave radiation (Rin) and to a lesser extent air temperature (Ta), atmospheric vapor pressure (ea) and atmospheric CO 2 concentration (Ca). Their impact further reduced the relative importance of V cmo . The GSA experiments led to the following conclusions: (1) explicit knowledge of key variables driving the SIF flux is essential in order to achieve unbiased SIF interpretation related to photosynthetic activity at local and global scales; (2) information related to photosynthetic activity is found more in the first emission peak (SIF red ) than in the second peak (SIF NIR ), and more in the full broadband SIF emission, which allows calculation of F yield , than in individual wavebands. [ABSTRACT FROM AUTHOR]

Sun-induced chlorophyll fluorescence (SIF) at leaf level is emitted in both upward and downward directions in the red and far-red part of the spectrum (650–850 nm) when a leaf is illuminated from the upper leaf surface. Hence, total SIF is represented by the sum of the upward and downward emission components. Nevertheless, the downward component of leaf SIF is often not considered despite that downward fluorescence yield (↓FY) can amount up to 40% of the total fluorescence yield (FY tot ). Downward SIF is mainly emitted in the far-red, since this part of fluoresced light is highly scattered within leaves, unlike red Chl fluorescence, which is mostly reabsorbed. While total FY can be quite different among distinct species, the relative partitioning between upward and downward fluorescence shows more similarities among different leaf types, especially in the far-red. It is shown that bidirectional SIF emission properties in the far-red can be attributed to the scattering properties of the leaf, whereby an equifacial leaf follows a different trend compared to bifacial leaves. This was done by comparing SIF data with simultaneously measured reflectance, transmittance and fluorescence data by means of the FluoWat leaf clip coupled with an ASD hyperspectral spectroradiometer. These results could further improve Chl fluorescence modeling at leaf level, and help to advance the interpretation of SIF at the canopy level. [ABSTRACT FROM AUTHOR]

We propose to simulate the four-level stimulated Raman adiabatic passage in a four-waveguide array. We find that an Abelian geometric phase factor emerges in this adiabatic passage, and the transfer of light between the waveguides can solely depend on the geometric phase. The robustness of geometric phase may bring additional benefits for high-precision light manipulation in some all-optical devices. [ABSTRACT FROM AUTHOR]

The rainy season affects the development of maize in Liaoning Province in China. Continuous, rainy weather and scant sunlight result in poor pollination, bald tips, and in an abnormally high, barren stalk. Field studies were conducted at the kernel formation stage (3-11 d after silking). Paired, near-isogenic lines of nonbarren stalk (Shennong 98B) and barren stalk (Shennong 98A) were exposed to 38, 60, and 75% shading to investigate changes in photosynthesis and chlorophyll (Chl) fluorescence characteristics under different light intensities. Net photosynthetic rate ( P), leaf maximum photochemical efficiency of PSII (F/F), photochemical quenching of Chl fluorescence (q), and actual photochemical efficiency of PSII (Φ) of Shennong 98B were always higher than those of Shennong 98A under natural light, contrary to nonphotochemical quenching (NPQ). F/F, Φ, and q increased, while P and electron transport rate (ETR) decreased after shading, and this was aggravated with increasing shade intensity. P, q, Φ, and ETR were lower than the values in natural light condition after seven days of shading. NPQ, F/F, Φ, q, and ETR recovered, when shading was removed. The P of two inbred lines returned soon to the control levels after 38% shade. Under shade and natural light conditions, the P and Chl fluorescence characteristics of Shennong 98A were both lower than those of Shennong 98B. We suggest that a poor adaptability to low light is an important physiological reason for inducing barren stalk in low light-sensitive maize. [ABSTRACT FROM AUTHOR]

This study experimentally demonstrated a feed-forward inversion control scheme for maintaining an optimum incident irradiance on photobioreactors (PBRs) during batch cultivation. A data-based model-free optimization using quadratic fit was utilized to rapidly estimate the optimum average fluence rate set point value that rendered maximum microalgae growth rate. Then, the feed-forward inversion control scheme adjusted the incident irradiance with respect to the in-process measured mass concentration to maintain the optimum average fluence rate inside the PBR. Optimization of growth conditions with respect to light is of prime importance for increasing biomass and lipid productivity in microalgae cultivation. The present approach can rapidly identify the optimum average fluence rate for any given species, reduce the lag time, and increase the growth rate and productivity of microalgae. This was illustrated with Nannochloropsis oculata batch grown in a flat-plate PBR illuminated from both sides. [ABSTRACT FROM AUTHOR]

Abstract: This study provides design and operational guidelines for achieving maximum biomass productivity in outdoor photobioreactors (PBRs). Detailed simulations of coupled light transfer and growth kinetics of microalgae were performed for open ponds, vertical flat-plate, and tubular PBRs operated in batch mode and exposed to time-dependent collimated and diffuse solar irradiance. The temporal evolution of microalgae concentration was predicted by accounting for light saturation, photoinhibition, and respiration. Three-dimensional spectral light transfer simulations of collimated and diffuse solar radiation in the PBRs were performed at different times of the day. The green microalgae Chlamydomonas reinhardtii was used for illustration purposes. The study demonstrated that the daily productivity per unit of illuminated surface area for PBRs operated in batch mode was identical and depended uniquely on the ratio where X 0 is the initial microalgae concentration and a is the illuminated surface area per unit volume of PBR. A maximum daily productivity of about 0.045kg/m2/day was achieved for . Remarkably, similar results were obtained with experimental data and other simulation results based on different models reported in the literature, for different microorganisms and PBRs operated in continuous mode. The PBR optical thickness, represented by , constitutes a convenient parameter for designing (via a) and operating (via X 0) these PBRs to achieve their maximum performance. [Copyright &y& Elsevier]

Highlights: [•] Marine microalgae N. oculata were grown under white or red LEDs with different irradiances. [•] Their absorption and scattering cross-sections were measured in the PAR region. [•] Cross-sections and retrieved refraction index did not depend on incident spectra and irradiance. [•] Absorption cross-section and absorption index decreased under nutrient limited conditions. [ABSTRACT FROM AUTHOR]

Abstract: This article is a critical review of the state-of-the-art of computational fluid dynamics (CFD) used to investigate bioreactors that produce biomethane and biohydrogen by means of biochemical conversion technology. First, the issue on rheology of biomaterials is addressed. Second, the CFD development in six major bioreactors is documented. The bioreactors studied are: (1) anaerobic lagoon, (2) plug-flow digester, (3) complete-mix digester, (4) anaerobic biohydrogen fermenter, (5) anaerobic biofilm reactor, and (6) photobioreactor. Third, the CFD simulation strategy that includes pre-processing and solver as well as post-processing is summarized. Fourth, opportunities and challenges for CFD applications in bioenergy are presented. Finally, specific comments and suggestions are made on: rheology, heat transfer, mass transfer, light transfer, turbulence, and multiphase flow. The primary goal of CFD research in biorenewable energy to develop a comprehensive model that integrates physical and biological processes is highlighted. [Copyright &y& Elsevier]

The results from a winter field study in 2009-2010 in Lake Vanajanselkä are presented. This lake is shallow, eutrophic, and annually ice-covered on average for 5 months. The ice sheet was 41-55 cm thick at the annual maximum, consisting of snow-ice and congelation ice. The e-folding depth of light intensity was 50-100 cm for congelation ice and 5-10 cm for snow. The water body had a 4-m thick upper mixed layer and lower continuously stratified layer. Fall cooling process was crucial to determine the temperature of the lower layer at freeze-up, anything within 0-4 °C. Oxygen concentration decreased in winter, especially close to the bottom sediments, and carbon dioxide concentration increased due to respiration activity. Phytoplankton production and biomass level were low or very low and, therefore, heterotrophic and mixotrophic species were abundant. Oxygen depletion in the hypolimnium had several chemical and ecological consequences, such as release of phosphorus from the bottom sediments. In spring, just before the ice-out, photosynthesis was at a high level beneath the ice due to improved light conditions and started to elevate the oxygen concentration in the topmost water layer. [ABSTRACT FROM AUTHOR]

Abstract: Solar radiation is the energy source driving the metabolic activity of microorganisms able to photobiologically fixate carbon dioxide and convert solar energy into biofuels. Thus, careful radiation transfer analysis must be conducted in order to design and operate efficient photobioreactors. This review paper first introduces light harvesting mechanisms used by microorganisms as well as photosynthesis and photobiological fuel production. It then provides a thorough and critical review of both experimental and modeling efforts focusing on radiation transfer in microalgae suspension. Experimental methods to determine the radiation characteristics of microalgae are presented. Methods for solving the radiation transfer equation in photobioreactors with or without bubbles are also discussed. Sample measurements and numerical solutions are provided. Finally, novel strategies for achieving optimum light delivery and maximizing sunlight utilization in photobioreactors are discussed including genetic engineering of microorganisms with truncated chlorophyll antenna. [Copyright &y& Elsevier]

The production of biofuels from microalgae requires efficient photobioreactors in order to meet the tight constraints of energy efficiency and economic profitability. Current cultivation systems are designed for high-value products rather than for mass production of cheap energy carriers. Future bioreactors will imply innovative solutions in terms of energy efficiency, light and gas transfer or attainable biomass concentration to lower the energy demand and cut down production costs. A new generation of highly developed reactor designs demonstrates the enormous potential of photobioreactors. However, a net energy production with microalgae remains challenging. Therefore, it is essential to review all aspects and production steps for optimization potential. This includes a custom process design according to production organism, desired product and production site. Moreover, the potential of microalgae to synthesize valuable products additionally to the energetic use can be integrated into a production concept as well as waste streams for carbon supply or temperature control. [ABSTRACT FROM AUTHOR]

Abstract: The objective of this study is to experimentally measure the radiation characteristics of hydrogen producing microorganisms. Special attention is paid to the filamentous cyanobacteria Anabaena variabilis ATCC 29413-U and the unicellular purple bacteria Rhodobacter sphaeroides ATCC 49419 two of the widely studied photobiological hydrogen producers. The extinction and absorption coefficients are measured in the spectral range from 300 to 1300nm using a spectrophotometer with and without an integrating sphere. Moreover, a nephelometer has been constructed to measure the scattering phase function of the microorganisms at 632.8nm. The data are used to recover the mass specific absorption, scattering, and extinction cross-sections, the single scattering albedo, and the scattering phase function of the microorganisms. The scattering phase functions of both microorganisms were peaked strongly in the forward direction as expected from their size parameter and shape. The results reported in this study can be used with the radiative transport equation (RTE) to accurately predict and optimize light transport in photobioreactors for photobiological hydrogen production. Finally, the results show that absorption cross-sections of A. variabilis and R. sphaeroides have peaks that do not overlap but rather enlarge the spectral width of the absorption cross-section of a potential symbiotic culture promising more efficient utilization of solar radiation from light transfer point of view. [Copyright &y& Elsevier]

Abstract: This paper presents a parametric study simulating light transfer in a photobioreactor containing gas bubbles and filamentous cyanobacteria Anabaena variabilis suspended in water. To the best of our knowledge, this paper presents for the first time a model for such system: (i) using a consistent set of radiation characteristics of the medium derived from experimental data and from Mie theory; (ii) accounting for anisotropic scattering by both the bubbles and the filamentous microorganisms; (iii) considering the spectral dependency of radiation characteristics in the spectral range from 400 to 700nm using a box model, and (iv) evaluating light transfer in a photobioreactor containing genetically engineered microorganisms with reduced pigment content. The steady-state one-dimensional radiation transfer equation is solved using the modified method of characteristics and a quadrature with 24 directions per hemisphere adapted to forward scattering media. The parameters investigated include the bacteria concentration, the bubble radius, and the void fraction, as well as the approximate scattering phase function. It was established that the strongly forward scattering by the bubbles must be accounted for and the truncated phase function (TPF) is recommended. In the absence of bubbles, ignoring in-scattering by the bacteria leads to errors as high as 20%. On the other hand, accounting for in-scattering with isotropic phase function gives acceptable results. Moreover, genetically reducing the pigment content of the microorganisms by an order of magnitude increases the significance of forward scattering of light by the microorganisms. This in turn, increases the penetration depth and can be accounted for by either the Henyey–Greenstein or the TPF approximations. Finally, the model presented can also be applied to (i) other types of microorganisms such as unicellular green algae or photosynthetic bacteria, (ii) different photobioreactor processes such as food product or pharmaceutical production, or (iii) photochemical reactors. [Copyright &y& Elsevier]

In this first of two papers (hereinafter called Paper I), we present a universal approach for simply realizing monolithic photonic integration based on asymmetric twin-waveguide (ATG) technology. The concepts and important developments leading to ATG integration technology will be reviewed. The ATG structure consists of active and/or passive devices formed in separate, vertically displaced waveguides. Light is transferred between the waveguides via very low loss, lateral, adiabatic tapered mode transformers, allowing different optical functions to be realized in the different waveguides. The design of the adiabatic tapered mode transformer uses an algorithm based on perturbation theory. We show that the same designs can also be deduced from coupled local mode theory. Using the perturbation algorithm to design the taper coupler in an ATG based high bandwidth photodiode, a transfer efficiency of greater than 90% from the fiber waveguide to the coupling waveguide is achieved while the taper length can be reduced by 35% compared to conventional two-section linear taper couplers. The taper design algorithm is further optimized to make the adiabatic taper couplers tolerant to variations in incident light polarization, operation wavelength, and dimensional control during fabrication. Finally, we propose and design a taper that adiabatically couples light from the fundamental mode to the first-order mode. Such a taper coupler is useful in an integrated semiconductor optical amplifier/p-i-n detector circuit. [ABSTRACT FROM PUBLISHER]

This study investigates the effect of condensed water droplets on the areal biomass productivity of outdoor culture systems with a free surface, protected by a transparent window or cover to prevent contamination and to control the growth conditions. Under solar radiation, evaporation from the culture causes droplets to condense on the interior surface of the cover. To quantify the effect of droplets on the system's performance, the bidirectional transmittance of a droplet-covered window was predicted using the Monte Carlo ray-tracing method. It was combined with a growth kinetics model of Chlorella vulgaris to predict the temporal evolution of the biomass concentration on 21 June and 23 September in Los Angeles, CA. A droplet contact angle of 30 ∘ or 90 ∘ and a surface area coverage of 50% or 90% were considered. Light scattering by the condensed droplets changed the direction of the incident sunlight while reducing the amount of light reaching the culture by up to 37%. The combined effect decreased the daily areal biomass productivity with increasing droplet contact angle and surface area coverage by as much as 18%. Furthermore, the areal biomass productivity of the system was found to scale with the ratio X 0 / a of the initial biomass concentration X 0 and the specific illuminated area a, as previously established for different photobioreactor geometries, but even in the presence of droplets. Finally, for a given day of the year, the optical thickness of the culture that yielded the maximum productivity was independent of the window condition. Thus, the design and operation of such a system should focus on maintaining a small droplet contact angle and surface area coverage and an optimum optical thickness to maximize productivity. [ABSTRACT FROM AUTHOR]

This study elucidates the effect of colony formation on light absorption by Botryococcus braunii microalgae cells. The spectral average mass absorption cross-section of suspensions of B. braunii cultures with free-floating cells or colonies was measured experimentally across the photosynthetically active radiation region. The average mass absorption cross-section was found to decrease significantly across the spectrum in the presence of colonies. This observation could be attributed to (i) reduced pigment concentrations due to nutrient limitations, (ii) mutual shading of the aggregated cells, and/or (iii) the presence of the colonies' extracellular matrix. The Monte Carlo ray tracing method was used to elucidate the contribution of each of these phenomena on the mass absorption cross-section of cells in colonies. Colonies were modeled either as fractal aggregates of monodisperse cells or as an ensemble of monodisperse cells regularly arranged at the periphery of a hollow sphere embedded in a spherical extracellular matrix or as a volume and average projected area equivalent coated sphere. The change in pigment concentrations due to nutrient limitation was found to be the most important factor. In addition, the mass absorption cross-section of cells in colonies was found to decrease due to mutual shading among cells. This effect was stronger with increasing number of cells in the colony and increasing cell absorption index. The effect of extracellular matrix on the mass absorption cross-section was found to be negligible. Finally, good agreement was found between the equivalent coated sphere approximation and the colonies modeled as fractal aggregates comprised of monodisperse cells. This study also established that the Monte Carlo ray tracing method can be used for a variety of microalgae species and colony configurations, whose absorption cross-section are not readily calculated by standard methods due to their complex geometry, inhomogeneous nature, and large size compared to the wavelength of the incident light. • Cell mutual shading decreases light absorption by cells in colonies. • The effect of colony extracellular matrix on light absorption is negligible. • Ray tracing can be used to simulate light absorption by optically soft cells. • Equivalent coated sphere approximation can be used to model absorption by colonies. [ABSTRACT FROM AUTHOR]

• The adiabatic light transfer and split couplers are designed in a three waveguide system. • The performance of dressed-state couplers is analyzed. • The feasibility of the present design is verified by the beam propagation method. We theoretically perform a dressed state approach to investigate a fast adiabatic light transfer and split in the three-waveguide coupled system. We compare the performance of the waveguide couplers based on the dressed-state method with the stimulated Raman adiabatic passage (STIRAP) couplers. We find that the dressed-state couplers can achieve a higher fidelity at a shorter device length and can be more robust against various uncertainties than the original STIRAP couplers. [ABSTRACT FROM AUTHOR]

The relationship between performance and working conditions in photobioreactor hydrogen production systems illuminated by a variable intensity light source has been described quantitatively using relational expressions. First, based on the finite volume method and the Michaelis-Menten model, the hydrogen production process of a photobioreactor (PBR) system was simulated numerically. Then, the performance of the PBR system was evaluated considering the hydrogen production rate, dimensionless hydrogen production rate, hydrogen production thrust coefficient and conversion efficiency of light energy to hydrogen energy rate as performance parameters, and the relationship between these parameters and working conditions was studied. Finally, the improved quantum-behaved particle swarm optimization (IQPSO) and surface fitting technique based on the curve fitting method were used to obtain relational expressions about the performance and working conditions of the PBR. All of the results show that the method can obtain accurately relational expressions for the performance optimization and forecasts of the PBR system. [ABSTRACT FROM AUTHOR]

Abstract This study aims to elucidate the role of light absorption rate and nitrate concentration on triacylglycerol (TAG) productivity and cellular TAG accumulation in nitrogen limited Parachlorella kessleri cultures grown in chemostat mode. In batch operated nitrogen starved cultures the cells accumulate large amounts of TAGs at the expense of biomass growth leading to severe decrease in cell division rate prompting eventual cell death. In PBRs operated in continuous mode cell multiplication can persist in nitrogen limited conditions with simultaneous TAG accumulation. P. kessleri cultures were grown in a flat-plate PBR with constant dilution rate of 0.01 1/h and feed-medium nitrate concentration ranging from 1 to 16 mM. The steady-state biomass, pigment, carbohydrate, protein, total lipid, and TAG concentrations as well as the spectral absorption and scattering cross-sections of the microalgae were measured for each culture. These were used to estimate the specific mean rate of photon absorption (MRPA) in the PBR. Maximum areal TAG productivity of 2.6 g/m2⋅day was obtained for the culture grown with nitrate concentration equal to 3.65 mM and a mean rate of photon absorption equal to 17,700 μmol hν /kg X A ⋅ s. TAG productivity and cellular TAG accumulation varied with the nitrate concentration in the feed medium and MRPA. Finally, our results show that TAG productivity can be further increased by optimizing the mean rate of photon absorption through adjusting culture dilution rate and feed-medium nitrate concentration. Highlights • TAG and carbohydrate rich Parachlorella kessleri cultures were grown in chemostat mode • TAG productivity depended on photon absorption rate and nitrate concentration • Rate of photon absorption must be expressed per kg of active biomass • Cells preferred production of TAG or carbohydrates depending on medium nitrate concentration [ABSTRACT FROM AUTHOR]