Microalgae cultivation in photobioreactors: An overview of light characteristics

General performance, kinetic modification, and key regulating factor recognition of microalgae-based sulfonamide removal

Sulfonamides have been widely detected in water treatment plants. Advanced wastewater treatment for sulfonamide removal based on microalgal cultivation can reduce the ecological risk after discharge, achieve carbon fixation, and simultaneously recover bioresource. However, the general removal performance, key factors and their impacts, degradation kinetics, and potential coupling technologies have not been systematically summarized. To guide the construction and enhance the efficient performance of the purification system, this study summarizes the quantified characteristics of sulfonamide removal based on more than 100 groups of data from the literature. The biodegradation potential of sulfonamides from different subclasses and their toxicity to microalgae were statistically analyzed; therefore, a preferred option for further application was proposed. The mechanisms by which the properties of both sulfonamides and microalgae affect sulfonamide removal were comprehensively summarized. Thereafter, multiple principles for choosing optimal microalgae were proposed from the perspective of engineering applications. Considering the microalgal density and growth status, a modified antibiotic removal kinetic model was proposed with significant physical meaning, thereby resulting in an optimal fit. Based on the mechanism and regulating effect of key factors on sulfonamide removal, sensitive and feasible factors (e.g., water quality regulation, other than initial algal density) and system coupling were screened to guide engineering applications. Finally, we suggested studying the long-term removal performance of antibiotics at environmentally relevant concentrations and toxicity interactions for further research.

A review of emerging membrane-based microalgal-bacterial processes for wastewater treatment: Process configurations, biological and membrane performance, and perspectives

Microalgal-bacterial (MB) consortia create an excellent eco-system for simultaneous COD/BOD and nutrients (N and P) removals in a single step with significant reduction in or complete elimination of aeration and carbonation in the biological wastewater treatment processes. The integration of membrane separation technology with the MB processes has created a new paradigm for research and development. This paper focuses on a comprehensive and critical literature review of recent advances in these emerging processes. Novel membrane process configurations and process conditions affecting the biological performance of these novel systems have been systematically reviewed and discussed. Membrane fouling issues and control of MB biofilm formation and thickness associated with these emerging suspended growth or immobilized biofilm processes are addressed and discussed. The research gaps, challenges, outlooks of these emerging processes are identified and discussed in-depth. The findings from the literature suggest that the membrane-based MB processes are advanced biotechnologies with a significant reduction in energy consumption and process simplification and high process efficiency that are not achievable with current technologies in wastewater treatment. There are endless opportunities for research and development of these novel and emerging membrane-based MB processes.

Reaction of Pyrrolobenzothiazines with Schiff Bases and Carbodiimides: Approach to Angular 6/5/5/5-Tetracyclic Spiroheterocycles

1H-Pyrrole-2,3-diones, fused at [e]-side with a heterocycle, are suitable platforms for the synthesis of various angular polycyclic alkaloid-like spiroheterocycles. Recently discovered sulfur-containing [e]-fused 1H-pyrrole-2,3-diones (aroylpyrrolobenzothiazinetriones) tend to exhibit unusual reactivity. Based on these peculiar representatives of [e]-fused 1H-pyrrole-2,3-diones, we have developed an approach to an unprecedented 6/5/5/5-tetracyclic alkaloid-like spiroheterocyclic system of benzo[d]pyrrolo[3',4':2,3]pyrrolo[2,1-b]thiazole via their reaction with Schiff bases and carbodiimides. The experimental results have been supplemented with DFT computational studies. The synthesized alkaloid-like 6/5/5/5-tetracyclic compounds have been tested for their biotechnological potential as growth stimulants in the green algae Chlorella vulgaris.

Biocompounds from wastewater-grown microalgae: a review of emerging cultivation and harvesting technologies

Microalgae biomass has attracted attention as a feedstock to produce biofuels, biofertilizers, and pigments. However, the high production cost associated with cultivation and separation stages is a challenge for the microalgae biotechnology application on a large scale. A promising approach to overcome the technical-economic limitations of microalgae production is using wastewater as a nutrient and water source for cultivation. This strategy reduces cultivation costs and contributes to valorizing sanitation resources. Therefore, this article presents a comprehensive literature review on the status of microalgae biomass cultivation in wastewater, focusing on production strategies and the accumulation of valuable compounds such as lipids, carbohydrates, proteins, fatty acids, and pigments. This review also covers emerging techniques for harvesting microalgae biomass cultivated in wastewater, discussing the advantages and limitations of the process, as well as pointing out the main research opportunities. The novelty of the study lies in providing a detailed analysis of state-of-the-art and potential advances in the cultivation and harvesting of microalgae, with a special focus on the use of wastewater and implementing innovative strategies to enhance productivity and the accumulation of compounds. In this context, the work aims to guide future research concerning emerging technologies in the field, emphasizing the importance of innovative approaches in cultivating and harvesting microalgae for advancing knowledge and practical applications in this area.

Lighting the way to sustainable development: Physiological response and light control strategy in microalgae-based wastewater treatment under illumination

The Sustainable Development Goals link pollutant control with carbon dioxide reduction. Toward the goal of pollutant and carbon reduction, microalgae-based wastewater treatment (MBWT), which can simultaneously remove pollutants and convert carbon dioxide into biomass with value-added metabolites, has attracted considerable attention. The photosynthetic organism microalgae and the photobioreactor are the functional body and the operational carrier of the MBWT system, respectively; thus, light conditions profoundly influence its performance. Therefore, this review takes the general rules of how light influences the performance of MBWT systems as a starting point to elaborate the light-influenced mechanisms in microalgae and the light control strategies for photobioreactors from the inside out. Wavelength, light intensity and photoperiod solely or interactively affect biomass accumulation, pollutant removal, and value-added metabolite production in MBWT. Physiological processes, including photosynthesis, photooxidative damage, light-regulated gene expression, and nutrient uptake, essentially explain the performance influence of MBWT and are instructive for specific microalgal strain improvement strategies. In addition, light causes unique reactions in MBWT systems as it interacts with components such as photooxidative damage enhancers present in types of wastewater. In order to provide guidance for photobioreactor design and light control in a large-scale MBWT system, wavelength transformation, light transmission, light source distribution, and light-dark cycle should be considered in addition to adjusting the light source characteristics. Finally, based on current research vacancies and challenges, future research orientation should focus on the improvement of microalgae and photobioreactor, as well as the integration of both.

Economic analysis of global microalgae biomass energy potential

With the increasing demand for renewable energy, microalgae, as a renewable biomass energy, can fix carbon dioxide and have broad application prospects in alleviating the energy crisis and improving the environment. In this paper, the potential biomass of global microalgae is calculated based on the mathematical growth model of microalgae proposed by predecessors. Based on this, this study further uses Newton's gravity model as the basic model of economic analysis and calculates the economic potential coefficient of microalgae production in various regions of the world by using the data of the world's top 20 cities in terms of urban population and urban GDP in 2020. The study has obtained the current global unused land with the high economic value of large-scale microalgae production areas, such as western North America, northern Africa, and northwest China, etc., which can provide guidance for the future site selection and development of microalgae biomass energy.

A bibliometric analysis of published literature on membrane photobioreactors for wastewater treatment from 2000 to 2022

With the focus on limiting greenhouse gas emissions, microalgae-based technology is a promising approach for wastewater treatment, combining cost-effective operation, nutrient recovery, and assimilation of CO2. In addition, membrane technology supports process intensification and wastewater reclamation. Based on a bibliometric analysis, this paper evaluated the literature on membrane photobioreactors to highlight promising areas for future research. Specifically, efforts should be made on advancing knowledge of interactions between algae and bacteria, analysing different strategies for membrane fouling control and determining the conditions for the most cost-effective operation. The Scopus® database was used to select documents from 2000 to 2022. A set of 126 documents were found. China is the country with the highest number of publications, whereas the most productive researchers belong to the Universitat Politècnica de València (Spain). The analysis of 50 selected articles provides a summary of the main parameters investigated, that focus in increasing the biomass productivity and nutrient removal. In addition, microalgal-bacterial membrane photobioreactor seems to have the greatest commercialisation potential. S-curve fitting confirms that this technology is still in its growth stage.

Effects of Temperature, Light and Salt on the Production of Fucoxanthin from Conticribra weissflogii

Fucoxanthin is a natural active substance derived from diatoms that is beneficial to the growth and immunity of humans and aquatic animals. Temperature, light and salinity are important environmental factors affecting the accumulation of diatom actives; however, their effects on the production of fucoxanthin in C. weissflogii are unclear. In this study, single-factor experiments are designed and followed by an orthogonal experiment to determine the optimal combination of fucoxanthin production conditions in C. weissflogii. The results showed that the optimum conditions for fucoxanthin production were a temperature of 30 °C, a light intensity of 30 umol m-2 s-1 and a salinity of 25. Under these conditions, the cell density, biomass, carotenoid content and fucoxanthin content of C. weissflogii reached 1.97 × 106 cell mL-1, 0.76 g L-1, 2.209 mg L-1 and 1.372 mg g-1, respectively, which were increased to 1.53, 1.71, 2.50 and 1.48 times higher than their initial content. The work sought to give useful information that will lead to an improved understanding of the effective method of cultivation of C. weissflogii for natural fucoxanthin production.

Degradation of Polymethylmethacrylate (PMMA) Bioreactors Used for Algal Cultivation

This paper depicts characteristics of degradation of walls of bioreactors made of polymethylmethacrylate (PMMA) which was used to culture algae. The degradation processes take place stimulated by lighting of external surface and interaction with cultured species on internal surface. Results presented are representative for degradation of a bioreactor tube after the 4-year cultivation of Chlorella sp. Microscopic observations, roughness and transmission tests showed that changes have occurred on the inner surface. The result of use is a decrease in transmission and an increase in roughness. Microscopic observations showed that particles remained after culture, especially in cracks.

Improving biomass and carbohydrate production of microalgae in the rotating cultivation system on natural carriers

Biofilm-based algal technologies have gained popularity due to higher biomass productivity, efficient harvesting, and water-saving over suspended growth systems. A rotating attached system was designed to assess the biofilm-forming capacity of different isolated microalgal strains from the Persian Gulf. Four microalgal strains, including two Chlorella sp., one Picochlorum sp. and one filamentous cyanobacterium Desmonostoc sp. were cultivated on four carriers: jute, cotton, yarn and nylon. The carriers' physicochemical surface characteristics and attachment effects, like contact angle, were investigated. The incorporated biomass and exopolysaccharides (EPS) content in the suspended and biofilm system was calculated and compared. The results showed that the cyanobacterium strain had the biofilm formation capability on both jute and cotton in the attached cultivation system. Under the same culture conditions, the biomass productivity on jute and cotton carriers was significantly higher (4.76 and 3.61 g m^- 2 respectively) than the growth in aqueous suspension (1.19 g m^- 2 d^- 1). The greatest incorporated exopolysaccharides amount was observed on jute (43.62 ± 4.47%) and the lowest amount was obtained from the growth on positive charge yarn (18.62 ± 1.88%). This study showed that in comparison with planktonic growth, the colonization of cyanobacterial cells and subsequent production of extracellular matrix and biofilm formation can lead to increased biomass production.

Irradiance penetration distribution and flashing light frequency simultaneously affected with microalgal cell absorption and CO2 bubble scattering in a raceway pond

In order to investigate light penetration and flashing light frequency for microalgal cell-CO₂ bubble culture system in a raceway pond, user-defined function for CO₂ mass transfer and bubble scattering models coupled with discrete ordinates radiation model were adopted to clarify simultaneous effects of microalgal cell absorption and CO₂ bubble scattering. Light intensity along the microalgal suspension depth attenuated more rapidly with increased biomass concentration, decreased bubble generation diameter, increased CO₂ gas content and incident light intensity. Ratio of light zone decreased from 81.13 % to 20.00 % when biomass concentration increased from 0 to 0.4 g/L because of light absorption and shading effects of microalgae. When bubble generation diameter increased from 0.1 to 1.6 mm, ratio of light zone increased from 37.95 % to 42.64 %, while microalgal flashing light cycle first decreased to a valley of 1.81 s at 0.8 mm and then increased. Local light intensity in the upper layers was more enhanced due to lots of CO₂ bubbles gathering and reflecting more light with decreased bubble diameter and increased gas content. Light attenuated more rapidly in microalgal suspension with decreased bubble generation diameter and increased CO₂ gas content because of increased bubble diffraction coefficient and contact area. When initial CO₂ volume fraction increased from 0.02 to 0.2, flashing light frequency of microalgal cells decreased from 0.55 to 0.29 Hz and light zone time ratio φ decreased from 36.90 % to 18.40 %. At a biomass concentration of 0.1 g/L and a bubble flow rate of 0.1 m/s, the maximum light penetration and microalgal growth rate was achieved when bubble diameter, incident light intensity and gas content were optimally at 0.8 mm, 200 W/m² and 0.02, respectively. This work provides data support and theoretical guidance for photobioreactor design and optimization of light energy utilization.

Effect of different mixed light-emitting diode light wavelengths on CO2 absorption from biogas and nutrient removal from biogas slurry by microalgae and fungi induced using strigolactone and endophytic bacteria

In this study, biogas and biogas slurry were simultaneously treated using two symbiotic systems: Chlorella vulgaris-Ganoderma lucidum-S395-2 (endophytic bacteria) and Scenedesmus obliquus-G. lucidum-S395-2. The influence of different mixed illumination (red and blue) intensity ratios on the algal symbionts' extracellular carbonic anhydrase activities was investigated, as well as the rates of microalgal growth and photosynthesis. The treatment performance was simultaneously assessed in terms of the efficiency of organic matter or nutrient removal and the level of CO2 absorption. The results indicated that red-blue light combinations with an intensity ratio of 5:5 were optimal. When comparing the performance of the two symbiotic systems, the C. vulgaris-G. lucidum-S395-2 symbiont co-culture system achieved significantly improved photosynthetic rates, biomass growth, and treatment effects. Under the optimal treatment conditions, the organic matter and nutrient removal rates were 81.06% ± 7.06% for chemical oxygen demand, 82.32% ± 7.18% for total nitrogen, and 82.98% ± 7.26% for total phosphorus. In addition, the rate of CO2 removal from biogas was 63.38% ± 5.35%. PRACTITIONER POINTS: The red and blue light intensity ratio of 5:5 showed the best removal performance. C. vulgaris-G. lucidum-S395-2 system obtained the best photosynthetic performance. The carbonic anhydrase activity had positive effects on CO2 removal performance.

Boosting β-carotene and storage materials productivities by two-stage mixed and monochromatic exposure stresses on Dunaliella salina

Growth and product formation of Dunaliella salina, a potent β-carotene source, were investigated under single and two-stage monochromic and mixed illuminations using two LEDs, each emitting red (R), blue (B), or white (W) light. Targeting cell growth in single-stage, WW, RR, and BB, as well as RB illumination, were compared and mixed RB illumination was found most supportive showing the highest cell growth of 1.81 ± 0.008 g/L. Subsequently, new two-stage illuminations (RB-BB and RB-RR) were designed to investigate growth and bio-product formation using RB illumination similarly in the 1st stage followed by separate BB and RR illuminations within the 2nd stage. RB-BB strategy resulted in enhanced productivities of lipid (7.6 mg/L/day), starch (20 mg/L/day), and β-carotene (0.4 mg/L/day) which were respectively higher by 80, 70, and 81% compared to single-stage control (WW). RB-RR strategy stimulated cell growth while it resulted in decreased productivities of products (other than chlorophyll). The highest biomass level of 2.2 g/L and nitrate removal of 80% were obtained in RB-RR while RB-BB resulted in the lowest values of 1.2 g/L and 48%, respectively. Appropriate selection of illuminations in two-stage strategies, therefore, functions to enhance the productivity of important metabolites or cell growth which can have generic applications in other microalgae.NOVELTY STATEMENTAlthough the effects of a variety of stressful conditions on microalgae product lines have been investigated so far, the effects of two-stage mixed and monochromatic exposure as a light management strategy have not yet been considered. This strategy was inspired by the fact that cell mass alongside the cell content of a product contributes to product productivity. Accordingly, the growth of Dunaliella salina was first examined under single-stage mixed and monochromatic exposure where mixed red-blue light led to the highest biomass formation. Shifting from mixed to different monochromatic exposures was then examined as a stress factor to stimulate product formation. Higher cell factories obtained under mixed exposure in the 1st stage escalated product productivities within the 2nd stage when exposed to monochromatic light.

Influence of Different Light-Emitting Diode Colors on Growth and Phycobiliprotein Generation of Arthrospira platensis

Light-emitting diodes (LED) can be utilized as tailorable artificial light sources for the cultivation of cyanobacteria such as Arthrospira platensis (AP). To study the influence of different LED light colors on phototrophic growth and biomass composition, AP was cultured in closed bioreactors and exposed to red, green, blue, or white LED lights. The illumination with red LED light resulted in the highest cell growth and highest cell densities compared to all other light sources (order of cell densities: red > white > green > blue LED light). In contrast, the highest phycocyanin concentrations were found when AP was cultured under blue LED light (e.g., order of concentrations: blue > white > red > green LED light). LED-blue light stimulated the accumulation of nitrogen compounds in the form of phycobiliproteins at the expense of cell growth. The results of the study revealed that exposure to different LED light colors can improve the quality and quantity of the biomass gained in AP cultures.

Modeling and Simulation of Photobioreactors with Computational Fluid Dynamics—A Comprehensive Review

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.

Effect of light intensity and wavelength on nitrogen and phosphate removal from municipal wastewater by microalgae under semi-batch cultivation

Domestic, agricultural and industrial water activities lead to organic and inorganic pollution of the environment. Biotreatment of municipal wastewater with the potential production of biomass is a valuable feature of microalgae. In this study we evaluated the effects of wavelength and light intensity on phosphate and ammonium removal on the one hand, and biomass and protein production on the other hand by Spirulina platensis in municipal wastewater treatment under semi batch cultivation. S. platensis was inoculated at 40% in artificial wastewater open pond system. Red, blue and purple light with 3800, 4800 and 5800 lux light intensity under 12 h light and 12 h darkness were investigated. Cultivation was conducted in semi-batch conditions; after four days cultivation, one third of the culture was replaced with fresh medium. The highest biomass and protein concentrations were observed under blue light at 5800 lux light intensity, 5.45 and 3 g/l respectively cumulatively; while the highest amount of phosphate and ammonium removal were about 145 and 218 mg/l under purple light at 5800 lux intensity, respectively. The amounts of biomass and protein produced, as well as phosphate and ammonium removed, are therefore impacted by wavelength, light intensity, results show that light intensity and wavelength can be customized to reach on the one hand the highest biomass and protein production, and on the other hand to maximize the removal of phosphorous and ammonium.

Modeling and Optimizing the Effect of Light Color, Sodium Chloride and Glucose Concentration on Biomass Production and the Quality of Arthrospira platensis Using Response Surface Methodology (RSM)

Arthrospira platensis (Spirulina) biomass is a valuable source of sustainable proteins, and the basis for new food and feed products. State-of-the-art production of Spirulina biomass in open pond systems only allows limited control of essential process parameters, such as light color, salinity control, or mixotrophic growth, due to the high risk of contaminations. Closed photobioreactors offer a highly controllable system to optimize all process parameters affecting Spirulina biomass production (quantity) and biomass composition (quality). However, a comprehensive analysis of the impact of light color, salinity effects, and mixotrophic growth modes of Spirulina biomass production has not been performed yet. In this study, Response Surface Methodology (RSM) was employed to develop statistical models, and define optimal mixotrophic process conditions yielding maximum quantitative biomass productivity and high-quality biomass composition related to cellular protein and phycocyanin content. The individual and interaction effects of 0, 5, 15, and 30 g/L of sodium chloride (S), and 0, 1.5, 2, and 2.5 g/L of glucose (G) in three costume-made LED panels (L) where the dominant color was white (W), red (R), and yellow (Y) were investigated in a full factorial design. Spirulina was cultivated in 200 mL cell culture flasks in different treatments, and data were collected at the end of the log growth phase. The lack-of-fit test showed that the cubic model was the most suitable to predict the biomass concentration and protein content, and the two-factor interaction (2FI) was preferred to predict the cellular phycocyanin content (p > 0.05). The reduced models were produced by excluding insignificant terms (p > 0.05). The experimental validation of the RSM optimization showed that the highest biomass concentration (1.09, 1.08, and 0.85 g/L), with improved phycocyanin content of 82.27, 59.47, 107 mg/g, and protein content of 46.18, 39.76, 53.16%, was obtained under the process parameter configuration WL4.28S2.5G, RL10.63S1.33G, and YL1.00S0.88G, respectively.

Semi-continuous immobilized cultivation of Porphyridium cruentum for sulfated polysaccharides production

In this study, semi-continuous immobilized cultivation of Porphyridium cruentum through calcium alginate beads was performed for sulfated polysaccharides (SPs) production. The cell biomass and daily SPs productivity in the calcium alginate bead immobilized culture were increased by up to 79 ± 3.4% and 45.6 ± 3.2%, compared to those in the control, respectively. Furthermore, simultaneous application of immobilization and blue wavelength illumination further increased the phycobiliproteins content by 260 ± 9%, compared to those in the control. Similarly, nutrient deficiencies in combination with immobilization increased daily SPs productivity by about twice that of the control. The chemical composition and biological activity of the extracellular polymeric substances produced through immobilization were similar to those of the control. This study suggests the potential application of calcium alginate beads-based immobilization for continuous and high-efficiency SPs production using P. cruentum.

Influence of wavelength photoperiods and N/P ratio on wastewater treatment with microalgae-bacteria

This research investigates the effect of mixing wavelength light photoperiods (12 h blue, 8 h blue: 4 h green, 4 h blue: 8 h green, and 12 h green) and N/P ratios (1.3 to 8.3) on the growth microalgae-bacteria systems, organic matter, and nutrient removals. The highest microalgae-bacteria growth performance (μ = 0.2 d^-1, 481.1 ± 15.3 mg DW L^-1) was observed when a 8 h blue: 4 h green mixed wavelength and a low N/P ratio were used. For both N/P ratios, biomass productivity was favored when using the blue light dominated at longer time periods. Mechanisms for nitrogen removal by assimilation depend on the N/P ratio, achieving assimilation between 49 and 65% at a low N/P ratio. High nitrogen removal (>50%) showed a strong relation with alkalinity culture conditions (pH > 8.5). The mixing of wavelength photoperiods seems to be a promising strategy to achieve high biomass productivity and nutrient removal. However, for optimal conditions, N/P ratios in the wastewater should be considered.

Effects of Monochromatic Illumination with LEDs Lights on the Growth and Photosynthetic Performance of Auxenochlorella protothecoides in Photo- and Mixotrophic Conditions

This study examined the effects of monochromatic illumination (blue, red, green and yellow) employing light-emitting diodes (LEDs), trophic conditions (photoautotrophic and mixotrophic), and nitrogen availability (high and low peptone concentration) on the growth and biochemical composition of Auxenochlorella protothecoides. The results revealed that mixotrophic conditions did not favor A. protothecoides, giving lower growth rates compared to heterotrophy (dark conditions). However, mixotrophy gave significantly higher growth rates compared to photoautotrophy. The best light wavelengths for mixotrophic cultivation were that of white and red. In all cases investigated in this study, high peptone concentration (4 g/L) resulted in decreased growth rates. Regarding the biochemical composition of A. protothecoides, the strongest effect, irrespective of trophic conditions, was caused by nitrogen availability (peptone concentration). Specifically, at nitrogen replete conditions (4 g/L peptone), biomass was rich in proteins (32-67%), whereas under deplete conditions (0.5 g/L peptone), A. protothecoides accumulated mainly carbohydrates (up to 56%). Mixotrophic conditions generally favored higher carbohydrate content, whereas photoautotrophic conditions favored higher protein content. The different illumination spectra did not have any clear effect on the biochemical composition (metabolites content), except that, in all trophic conditions, the use of the green spectrum resulted in higher chlorophyll b content. Chlorophyll a fluorescence studies revealed that the trophic conditions and the high peptone concentrations impacted the photosystem II (PSII) performance, and also affected plastoquinone re-oxidation kinetics and the heterogeneity of the PSII reaction centers.

Digestate as Sustainable Nutrient Source for Microalgae—Challenges and Prospects

The interest in microalgae products has been increasing, and therefore the cultivation industry is growing steadily. To reduce the environmental impact and production costs arising from nutrients, research needs to find alternatives to the currently used artificial nutrients. Microalgae cultivation in anaerobic effluents (more specifically, digestate) represents a promising strategy for increasing sustainability and obtaining valuable products. However, digestate must be processed prior to its use as nutrient source. Depending on its composition, different methods are suitable for removing solids (e.g., centrifugation) and adjusting nutrient concentrations and ratios (e.g., dilution, ammonia stripping). Moreover, the resulting cultivation medium must be light-permeable. Various studies show that growth rates comparable to those in artificial media can be achieved when proper digestate treatment is used. The necessary steps for obtaining a suitable cultivation medium also depend on the microalgae species to be cultivated. Concerning the application of the biomass, legal aspects and impurities originating from digestate must be considered. Furthermore, microalgae species and their application fields are essential criteria when selecting downstream processing methods (harvest, disintegration, dehydration, product purification). Microalgae grown on digestate can be used to produce various products (e.g., bioenergy, animal feed, bioplastics, and biofertilizers). This review gives insight into the origin and composition of digestate, processing options to meet requirements for microalgae cultivation and challenges regarding downstream processing and products.

Hydrothermal liquefaction of Nostoc ellipsosporum biomass grown in municipal wastewater under optimized conditions for bio-oil production

Microalgae offer numerous potential applications, however the industrial scale-up of algal technology still remains a challenge due to high production cost. Optimization of growth conditions and integration with waste streams can improve the economic viability of microalgal production systems. This study investigated on the optimal growth conditions of microalgae Nostoc ellipsosporum cultivated in municipal wastewater with the objective of achieving maximum biomass production, nutrient removal efficiency and bio-oil yield. The effect of light intensity, photoperiod, wavelength, aeration and growth media composition were studied. Different formulations of municipal wastewater blended with Fog's nutrient were used as growth medium. Optimization of growth conditions and acclimatization to wastewater enhanced the biomass yield of Nostoc ellipsosporum from 1.42 to 2.9 g L^-1, achieving 87.59% of nitrogen removal and 88.31% of phosphate removal from wastewater. Furthermore, hydrothermal liquefaction of biomass produced bio-oil yield of 24.62% at 300 °C.

The Effect of Chemical Sulfide Oxidation on the Oxygenic Activity of an Alkaliphilic Microalgae Consortium Deployed for Biogas Upgrading

The oxygenic photosynthetic activity (OPA) of an alkaliphilic microalgae consortium was evaluated at different concentrations of dissolved sulfide under room temperature and well-defined conditions of irradiance and pH in a tubular closed photobioreactor. The kinetic assays showed that it was optimal at a sulfide concentration of 3.2 mg/L under an external photosynthetically active radiation of 50 and 120 μE/m2 s together with a pH of 8.5 and 9.2. In contrast, the oxygenic photosynthetic activity was insignificant at 15 μE/m2 s with a pH of 7.3, both in the absence and presence of sulfide. Consecutive pulse additions of dissolved sulfide evidenced that the accumulation rate of dissolved oxygen was decreased by the spontaneous chemical oxidation of sulfide with dissolved oxygen in alkaline culture media, mainly at high sulfide levels. At 3.2 mg/L of sulfide, the oxygenic photosynthetic activity was improved by around 60% compared to the treatment without sulfide at external irradiances of 120 μE/m2 s, 30 °C, and pH of 8.5 and 9.2. Additionally, an even higher OPA enhancement (around 85%) was observed in the same previous conditions but using 16 mg/L of sulfide. Thiosulfate was the major end-product of sulfide by oxic chemical reaction, both in biotic and abiotic assays with yields of 0.80 and 0.68, respectively.

Multiscale numerical workflow describing microalgae motion and light pattern incidence towards population growth in a photobioreactor

In this article, a numerical workflow describing the microalgal growth inside of a photobioreactor is proposed. CFD is used to compute reactor internal hydrodynamics taking into account marine impeller rotation and sparged bubbles motion. Lagrangian approach is used to track microalgae motion inside of the culture vessel. The illumination across the reactor is obtained using the classical Beer-Lambert's law. The combination of light field and cell motion allows to reconstruct the light history of each microalgae. These histories are then supplied to Han's model which predicts individual growth rate and experienced photodamages. Once computed, several thousands of trajectories are agglomerated at the population level yielding the photobioreactor performances. After having ensured properties convergence, this procedure is applied to a large range of optical density (0 to 4.0), i.e. cell concentration, and incident light intensities (0 to 2000 μmolPhoton/m2/s). From this exploration, it is possible to determine the photobioreactor response surfaces in terms of growth rate and photodamages. These are latter used to propose an optimal lighting strategy for biomass production - reducing photobioreactor operation time by 16% compared to classical two-step procedure - and assist light induced stress with the aim of triggering secondary metabolites production.

Light-Emitting Diode Power Conversion Capability and CO2 Fixation Rate of Microalgae Biofilm Cultured Under Different Light Spectra

Microalgae biofilm-based culture has attracted much interest due to its high harvest efficiency and low energy requirements. Using light-emitting diodes (LEDs) as light source for microalgae culture has been considered as a promising choice to enhance the economic feasibility of microalgae-based commodities. In this work, the LED power conversion capability and CO2 fixation rate of microalgae biofilms (Chlorella ellipsoidea and Chlorella pyrenoidosa) cultured under different light spectra (white, blue, green and red) were studied. The results indicated that the power-to-biomass conversion capabilities of these two microalgae biofilms cultured under blue and white LEDs were much higher than those under green and red LEDs (C. ellipsoidea: 32%–33% higher, C. pyrenoidosa: 34%–46% higher), and their power-to-lipid conversion capabilities cultured under blue LEDs were 61%–66% higher than those under green LEDs. The CO2 fixation rates of these two biofilms cultured under blue LEDs were 13% and 31% higher, respectively, than those under green LEDs. The results of this study have important implications for selecting the optimal energy-efficient LEDs using in microalgae biofilm-based culture systems.

Autotrophic and Heterotrophic Growth Conditions Modify Biomolecole Production in the Microalga Galdieria sulphuraria (Cyanidiophyceae, Rhodophyta)

Algae have multiple similarities with fungi, with both belonging to the Thallophyte, a polyphyletic group of non-mobile organisms grouped together on the basis of similar characteristics, but not sharing a common ancestor. The main difference between algae and fungi is noted in their metabolism. In fact, although algae have chlorophyll-bearing thalloids and are autotrophic organisms, fungi lack chlorophyll and are heterotrophic, not able to synthesize their own nutrients. However, our studies have shown that the extremophilic microalga Galderia sulphuraria (GS) can also grow very well in heterotrophic conditions like fungi. This study was carried out using several approaches such as scanning electron microscope (SEM), gas chromatography/mass spectrometry (GC/MS), and infrared spectrophotometry (ATR-FTIR). Results showed that the GS, strain ACUF 064, cultured in autotrophic (AGS) and heterotrophic (HGS) conditions, produced different biomolecules. In particular, when grown in HGS, the algae (i) was 30% larger, with an increase in carbon mass that was 20% greater than AGS; (ii) produced higher quantities of stearic acid, oleic acid, monounsaturated fatty acids (MUFAs), and ergosterol; (iii) produced lower quantities of fatty acid methyl esters (FAMEs) such as methyl palmytate, and methyl linoleate, saturated fatty acids (SFAs), and poyliunsaturated fatty acids (PUFAs). ATR-FTIR and principal component analysis (PCA) statistical analysis confirmed that the macromolecular content of HGS was significantly different from AGS. The ability to produce different macromolecules by changing the trophic conditions may represent an interesting strategy to induce microalgae to produce different biomolecules that can find applications in several fields such as food, feed, nutraceutical, or energy production.

The effect of medium and light wavelength towards Stichococcus bacillaris fatty acid production and composition

Introduction of novel species will highlight technical feasibility of microalgae-based biofuels for commercial applications. This paper reports the effect of culture medium and light wavelength on biomass and fatty acid production of S. bacillaris which holds some advantages as short life cycle, easy cultivation, high lipid content, diversity of fatty acids and stability under harsh environmental conditions. The results displayed that, soil extract (SE) greatly enhance growth rate of cultures. Maximum biomass and lipid productivity were achieved in TAP medium as 81 mg/L·day, 19.44 mg/L·day; respectively. Light wavelength didn't significantly change growth kinetics but played a critical role on chlorophyll-a accumulation. C14:0, C16:0 and C18:0 fatty acids were abundant which are suitable for biodiesel conversion. Interestingly, blue and red light increased longer chain fatty acids content. These results indicated that; S. bacillaris holds potential for further development of biodiesel production and feasibility of algal biodiesel for fundamental and applied sciences.

Influence of Photoperiods on Microalgae Biofilm: Photosynthetic Performance, Biomass Yield, and Cellular Composition

Microalgae have immense potential as biological sources to produce biofuels and high-value biomolecules. Biofilm-based microalgae cultivation has attracted much interest recently because of its high biomass productivity, reduced water use, and low cost of harvesting. This study aimed to understand the effect of photoperiod on three microalgae biofilms, including Nannochloris oculata, Chlorella sp., and Chlorella pyrenoidosa. The examined photoperiods were 3:3 s, 5:5 s, 30:30 min, 12:12 h (light-period-to-dark-period ratio), and continuous lighting. By determining the maximum quantum yield and relative electron transport rate of photosystem II, we found that photoperiods on the seconds scale improved photosynthetic performance of microalgae biofilm. Biomass yield and lipid content of these three microalgae cultured under the photoperiod with the seconds scale increased by 11%–24% and 7%–22%, respectively, compared with those cultured under continuous lighting. In addition, the photoperiods of 3:3 s, 5:5 s, 30:30 min, and 12:12 h were beneficial for protein synthesis. These results have important implications in establishing suitable light regimes for microalgae biofilm-based cultivation systems.

Nutrient removal in an algal membrane photobioreactor: effects of wastewater composition and light/dark cycle

Graesiella emersonii was cultivated in an osmotic membrane photobioreactor (OMPBR) for nutrients removal from synthetic wastewater in continuous mode. At 1.5 days of hydraulic retention time and under continuous illumination, the microalgae removed nitrogen (N) completely at influent NH₄⁺-N concentrations of 4-16 mg/L, with removal rates of 3.03-12.1 mg/L-day. Phosphorus (P) removal in the OMPBR was through biological assimilation as well as membrane rejection, but PO₄^3--P assimilation by microalgae could be improved at higher NH₄⁺-N concentrations. Microalgae biomass composition was affected by N/P ratio in wastewater, and a higher N/P ratio resulted in higher P accumulation in the biomass. The OMPBR accumulated about 0.35 g/L biomass after 12 days of operation under continuous illumination. However, OMPBR operation under 12 h light/12 h dark cycle lowered biomass productivity by 60%, which resulted in 20% decrease in NH₄⁺-N removal and nearly threefold increase in PO₄^3--P accumulation in the OMPBR. Prolonged dark phase also affected carbohydrate accumulation in biomass, although its effects on lipid and protein accumulation were negligible. The microalgae also exhibited high tendency to aggregate and settle, which could be attributed to reduction in cell surface charge and enrichment of soluble algal products in the OMPBR. Due to a relatively shorter operating period, membrane biofouling and salt accumulation did not influence the permeate flux significantly. These results improve the understanding of the effects of N/P ratio and light/dark cycle on biomass accumulation and nutrients removal in the OMPBR.

Effects of incident light intensity and light path length on cell growth and oil accumulation in Botryococcus braunii (Chlorophyta)

Botryococcus braunii was cultured in different light path length under different incident light intensity to investigate the effect of light on alga growth as well as hydrocarbon and fatty acid accumulation. Results indicated that longer light path length required higher incident light intensity in order to meet the light requirement of algal growth and hydrocarbon accumulation during the course of cultivation. However, hydrocarbon profile was only affected by the incident light intensity and not influenced by the light path length. High incident light intensity enhanced the accumulation of hydrocarbons with longer carbon chains. Besides, the fatty acid content and profiles were significantly influenced by both incident light intensity and light path. Higher fatty acid content and higher percentage of C18 and monounsaturated fatty acid components were achieved at the higher incident light intensity and lower light path length. Taken together, these results are benefit to improve its biomass and oil productivity through the optimization of light and photobioreactor design.

Microalgae and cyanobacteria modeling in water resource recovery facilities: A critical review

Microalgal and cyanobacterial resource recovery systems could significantly advance nutrient recovery from wastewater by achieving effluent nitrogen (N) and phosphorus (P) levels below the current limit of technology. The successful implementation of phytoplankton, however, requires the formulation of process models that balance fidelity and simplicity to accurately simulate dynamic performance in response to environmental conditions. This work synthesizes the range of model structures that have been leveraged for algae and cyanobacteria modeling and core model features that are required to enable reliable process modeling in the context of water resource recovery facilities. Results from an extensive literature review of over 300 published phytoplankton models are presented, with particular attention to similarities with and differences from existing strategies to model chemotrophic wastewater treatment processes (e.g., via the Activated Sludge Models, ASMs). Building on published process models, the core requirements of a model structure for algal and cyanobacterial processes are presented, including detailed recommendations for the prediction of growth (under phototrophic, heterotrophic, and mixotrophic conditions), nutrient uptake, carbon uptake and storage, and respiration.

Potential of Microalgae Carotenoids for Industrial Application

Microalgae cultivation, when compared to the growth of higher plants, presents many advantages such as faster growth, higher biomass productivity, and smaller land area requirement for cultivation. For this reason, microalgae are an alternative platform for carotenoid production when compared to the traditional sources. Currently, commercial microalgae production is not well developed but, fortunately, there are several studies aiming to make the large-scale production feasible by, for example, employing different cultivation systems. This review focuses on the main carotenoids from microalgae, comparing them to the traditional sources, as well as a critical analysis about different microalgae cultivation regimes that are currently available and applicable for carotenoid accumulation. Throughout this review paper, we present relevant information about the main commercial microalgae carotenoid producers; the comparison between carotenoid content from food, vegetables, fruits, and microalgae; and the great importance and impact of these molecule applications, such as in food (nutraceuticals and functional foods), cosmetics and pharmaceutical industries, feed (colorants and additives), and healthcare area. Lastly, the different operating systems applied to these photosynthetic cultivations are critically discussed, and conclusions and perspectives are made concerning the best operating system for acquiring high cell densities and, consequently, high carotenoid accumulation.

Complex mathematical analysis of photobioreactor system

Modeling as a tool solves extremely difficult tasks in life sciences. Recently, schemes of culturing of microalgae have received special attention because of its unique features and possible uses in many industrial applications for renewable energy production and high value products isolation. The goal of this review is to present the use of system analysis theory applied to microalgae culturing modeling and process development. The review mainly focuses on the modeling of the key steps of autotrophic growth under the integral biorefinery concept of the microalgae biomass. The system approach follows systematically a procedure showing the difficulties by modeling of sub-systems. The development of microalgae kinetics and computational fluid dynamics (CFD) studies were analyzed in details as sub-systems in advanced design of photobioreactor (PBR). This review logically follows the trends of the modeling procedure and clarifies how this approach may save time and money during the research efforts. The result of this work is a successful development of a complex PBR mathematical analysis in the frame of the integral biorefinery concept.

Performance of the mixed LED light quality on the growth and energy efficiency of Arthrospira platensis

The effect of mixed light quality with red, blue, and green LED lamps on the growth of Arthrospira platensis was studied, so as to lay the theoretical and technical basis for establishing a photo-bioreactor lighting system for application in space. Meanwhile, indexes, like morphology, growth rate, photosynthetic pigment compositions, energy efficiency, and main nutritional components, were measured respectively. The results showed that the blue light combined with red light could decrease the tightness of filament, and the effect of green light was opposite. The combination of blue light or green light with red light induced the filaments to get shorter in length. The 8R2B treatment could promote the growth of Arthrospira platensis significantly, and its dry weight reached 1.36 g L^-1, which was 25.93% higher than the control. What's more, 8R2B treatment had the highest contents of carbohydrate and lipid, while 8R2G was rich in protein. 8R0.5G1.5B had the highest efficiency of biomass production, which was 161.53 mg L^-1 kW^-1 h^-1. Therefore, the combination of red and blue light is more conducive to the growth of Arthrospira platensis, and a higher biomass production and energy utilization efficiency can be achieved simultaneously under the mixed light quality with the ratio of 8R0.5G1.5B.

Boosting Nannochloropsis oculata growth and lipid accumulation in a lab-scale open raceway pond characterized by improved light distributions employing built-in planar waveguide modules

Aiming at alleviating the adverse effect of poor light penetrability on microalgae growth, planar waveguide modules functioned as diluting and redistributing the intense incident light within microalgae culture more homogeneously were introduced into a lab-scale open raceway pond (ORP) for Nannochloropsis oculata cultivation. As compared to the conventional ORP, the illumination surface area to volume ratio and effective illuminated volume percentage in the proposed ORP were respectively improved by 5.53 times and 19.68-172.72%. Consequently, the superior light distribution characteristics in the proposed ORP contributed to 193.33% and 443.71% increase in biomass concentration and lipid yield relative to those obtained in conventional ORP, respectively. Subsequently, the maximum biomass concentration (2.31 g L^-1) and lipid yield (1258.65 mg L^-1) was obtained when the interval between adjacent planar waveguide modules was 18 mm. The biodiesel produced in PWM-ORPs showed better properties than conventional ORP due to higher MUFA and C18:1 components proportions.

Designing a CO2 supply strategy for microalgal biodiesel production under diurnal light in a cylindrical-membrane photobioreactor

A cylindrical membrane photobioreactor with high CO₂ mass transfer coefficient was designed and installed under customized unidirectional lighting. Combinatorial effect of light and CO₂ on the growth of Chlorella sp. FC2 IITG was studied and an optimal CO₂ supply without pH control strategy was developed under diurnal light similar to sunlight (17-2000-17 µE m^-2 s^-1). Unprecedentedly, broad range of saturated light levels (700-1500 µE m^-2 s^-1), reversible photoinhibition, no pH control requirement and dark-phase growth were noticed altogether in the strain. Under diurnal light, final biomass titer of 5.79 g L^-1 and overall biomass productivity of 1.29 g L^-1 day^-1 were observed. The results were similar to optimal light (1130 µE m^-2 s^-1) and CO₂ (2%) conditions. Subsequently, a highest FAME productivity of 265 mg L^-1 day^-1 was observed in last two days of lipid induction phase.

Enhanced production of astaxanthin by Chromochloris zofingiensis in a microplate-based culture system under high light irradiation

The green microalga Chromochloris zofingiensis is a promising producer of natural astaxanthin. In the present study, C. zofingiensis was first cultivated in shake flasks under low light irradiation and then subjected to continuous high light irradiation, which effectively promoted astaxanthin production. In addition, a microplate-based culture system in concert with high light irradiation from blue light and white light above 150μmolm^-2s^-1 was constructed and applied to improve astaxanthin production. Blue light exerted more positive influences on astaxanthin accumulation, but when the light intensity was increased to 300μmolm^-2s^-1, astaxanthin biosynthesis was substantially inhibited. Conversely, in a nitrogen-deprived culture under white light, the highest astaxanthin content for C. zofingiensis, 7.1mg/g, was obtained. The highest astaxanthin yield achieved was 38.9mg/L in a culture with 0.1g/L nitrate under the same culture conditions. This study demonstrates that C. zofingiensis has great potential for natural astaxanthin production.

Light intensity as major factor to maximize biomass and lipid productivity of Ettlia sp. in CO2-controlled photoautotrophic chemostat

The optimal culture conditions are critical factors for high microalgal biomass and lipid productivity. To optimize the photoautotrophic culture conditions, combination of the pH (regulated by CO₂ supply), dilution rate, and light intensity was systematically investigated for Ettlia sp. YC001 cultivation in a chemostat during 143days. The biomass productivity increased with the increase in dilution rate and light intensity, but decreased with increasing pH. The average lipid content was 19.8% and statistically non-variable among the tested conditions. The highest biomass and lipid productivities were 1.48gL^-1d^-1 and 291.4mgL^-1d^-1 with a pH of 6.5, dilution rate of 0.78d^-1, and light intensity of 1500μmolphotonsm^-2s^-1. With a sufficient supply of CO₂ and nutrients, the light intensity was the main determinant of the photosynthetic rate. Therefore, the surface-to-volume ratio of a photobioreactor should enable efficient light distribution to enhance microalgal growth.