Experimental assessment and mathematical modelling of the growth of Chlorella vulgaris under photoautotrophic, heterotrophic and mixotrophic conditions

An assessment of the autotrophic/heterotrophic synergism in microalgae under mixotrophic mode and its contribution in high-rate phosphate recovery from wastewater

Dual carbon metabolisms and the synergism contribute to improving nutrient recovery under mixotrophy. However, how synergism influences nutrient recovery has yet to be understood, which is revealed in the current study. Due to dual carbon metabolisms and synergism,the PO4--P recovery rate under mixotrophy reached 0.34 mg L-1 h-1. Due to the internal cycling of respiratory CO2, the mutualistic index (MI) in terms of synergism helped Scenedesmus accumulate 27.49 % more biomass under mixotrophy than sum of the two controls. In contrast, MI contributed 0.26 g L-1 d-1 to the total modeled mixotrophic productivity of 1.15 g L-1 d-1. To total modeled PO4--P recovery, mixotrophic-auto, and mixotrophic-hetero shares were 42 % and 58 %. The synergism under mixotrophy contributed 20 % in total PO4--P recovery. The PO4--P recovery rate under mixotrophywas comparable to other biological P removal methods. These findings emphasize the potential of synergism in improving productivityand promoting resource recovery for sustainable wastewater treatment.

Microalgae create a highway for carbon sequestration in livestock wastewater: Carbon sequestration capacity, sequestration mechanisms, influencing factors, and prospects

Greenhouse gas emissions from the livestock industry are recognized as a major environmental issue. This includes emissions from livestock wastewater. However, the common methods used for carbon sequestration (CS) rarely involve treatment of livestock wastewater, due to an absence of standardized emission points and difficulties in gas collection. To remedy this knowledge gap, this review discusses the sequestration capacity, technical classification, mechanisms, and factors influencing carbon sequestration by microalgae (MCS) in livestock wastewater. First, the carbon emission characteristics of livestock farm are discussed, concluding that, compared with those from enteric fermentation, emissions from waste management are characterized by dispersed emission points, lack of obvious emission patterns, and difficulties in gas collection. Secondly, the use and potential of MCS in livestock wastewater are summarized, with emphasis on the mechanisms involved (both heterotrophic and autotrophic MCS). It was found that development of the heterotrophic microalgal mechanism or combining the use of autotrophic microalgae with bacteria was key to the effective use of MCS for treating livestock wastewater. Finally, physical and chemical factors directly influencing MCS, as well as biological factor (species), were found to determine the potential of MCS. Furthermore, a model for recycling MCS in livestock farms is proposed, providing a novel solution to the achievement of carbon neutrality, resource recycling and ecological environmental protection.

The effect of initial inoculation amount of microalgae on synergistic purification of biogas slurry

In this study, Chlorella and Scenedesmus were inoculated in biogas slurry medium with initial inoculum (OD680) of 0.05, 0.1, 0.2, and 0.3, respectively, and 5% CO2 was continuously injected. The study aimed to examine the carbon sequestration capacity of Chlorella and Scenedesmus, as well as the effectiveness of removing pollutants such as TN, TP, and COD in biogas slurry medium. Additionally, an economic efficiency analysis of energy consumption was conducted. The group with an initial inoculum (OD680) of 0.3 for both types of microalgae exhibited better tolerance to pollutants, entered the logarithmic growth stage earlier, promoted nutrient removal, achieved higher energy efficiency, and reduced carbon emissions compared to the other groups. The highest carbon sequestration rates were 18.03% for Chlorella and 11.05% for Scenedesmus. Furthermore, Chlorella demonstrated corresponding nutrient removal efficiencies of 83.03% for TN, 99.84% for TP, and 90.06% for COD, while Scenedesmus exhibited removal efficiencies of 66.35% for TN, 98.74% for TP, and 77.71% for COD. The highest energy efficiency for pollutants and CO2 removal rates for Chlorella were 49.51 ± 2.20 and 9.91 ± 0.44 USD-1, respectively. In conclusion, the findings demonstrate the feasibility of using microalgae for simultaneous purification of biogas and biogas slurry.

Exploring effects of carbon, nitrogen, and phosphorus on greywater treatment by polyculture microalgae using response surface methodology and machine learning

The microalgae-based wastewater treatment is a promising technique that contribute to achieving sustainable development goals (SDGs), such as SDG-6, "Clean Water and Sanitation". However, it is strongly influenced by the initial composition of wastewater. In this study, the impact of initial organics and nutrient concentration on the removal of total organic carbon (TOC), total carbon (TC), ammonium (NH4+), total nitrogen (TN), and phosphate (PO43-) from greywater using native polyculture microalgae was explored. Response surface methodology was employed along with two machine learning approaches, AdaBoost and XGBoost, to evaluate the interactions among three main factors: TOC, NH4+, and PO43-, and their effects on treatment efficiency. The C/N ratios for achieving maximum TOC and TC removal efficiency of 99.2% and 97.7% were determined to be 10.3, and 65.4-73.6, respectively. Notably, the N/P ratio did not significantly affect their removal. The highest NH4+ removal efficiency, reaching 96.2%, was attained at C/N ratios of 4.3, 24.0, 38.2, and 212.9, coupled with N/P ratios of 0.3, 2.6, and 23.4. Highest TN removal efficiency of 77.2% was achieved at C/N and N/P ratios of 12.2 and 2.0, respectively. Highest PO43- removal of 78.8% was obtained at N/P ratio 12.8. However, C/N ratio did not affect the removal efficiency. Maintaining these specified C/N and N/P ratios in the influent greywater would ensure that the treated greywater meets the required standards for various reuse applications, including flushing, groundwater recharge, and surface water discharge. The integration of RSM with AdaBoost and XGBoost provided accurate predictions of removal efficiencies. For all the models, XGBoost had the highest R2, and lowest MAE and MSE values. The cross validation of RSM models with AdaBoost and XGBoost further reinforced the reliability of these models in predicting treatment outcomes.

Photoheterotroph improved the growth and nutrient levels of Chlorella vulgaris and the related molecular mechanism

Microalgae are rich in fatty acids, proteins, and other nutrients, which have gained the general attention of researchers all over the world. For the development of Chlorella vulgaris in food and feed industry, this study was conducted to investigate the differences in C. vulgaris' growth and nutritional components under different culture conditions (autotrophic, heterotrophic, photoheterotrophic) and the internal factors through cell counting in combination with transcriptome and nutrient analyses. The results showed that, under the photoheterotrophic condition, Chlorella's growth and the contents of lipid and protein were significantly higher than that under the heterotrophic condition, and the moisture content was lower than that under the heterotrophic condition. The saturated fatty acid content under the photoheterotrophic condition was the lowest, while the polyunsaturated fatty acid content was significantly higher than those under the other two conditions. There were 46,583 differentially expressed genes (DEGs), including 33,039 up-regulated DEGs (70.93%) and 13,544 down-regulated DEGs (29.07%), under the photoheterotrophic condition in comparison with the autotrophic condition. The fold change between the two conditions of samples of up-regulated genes was higher than that of the down-regulated genes. The KEGG enrichment showed that the up-regulated DEGs in the photoheterotrophic condition were significantly enriched in 5 pathways, including protein processing in endoplasmic reticulum pathway, photosynthesis pathway, photosynthesis-antenna protein pathway, endocytosis pathway, and phosphonate and phosphinate metabolism pathway. DEGs related to fatty acid metabolic pathways were significantly enriched in the fatty acid biosynthesis pathway and the biosynthesis of unsaturated fatty acid pathway. The qPCR analysis showed that the expression pattern of the selected genes was consistent with that of transcriptome analysis. The results of this study lay a theoretical foundation for the large-scale production of Chlorella and its application in food, feed, and biodiesel. KEY POINTS: • Nutrient levels under photoheterotrophic condition were higher than other conditions. • Six important pathways were discovered that affect changes in nutritional composition. • Explored genes encode important enzymes in the differential metabolic pathways.

A novel mechanistic modelling approach for microbial selection dynamics: Towards improved design and control of raceway reactors for purple bacteria

Purple phototrophic bacteria (PPB) show an underexplored potential for resource recovery from wastewater. Raceway reactors offer a more affordable full-scale solution on wastewater and enable useful additional aerobic processes. Current mathematical models of PPB systems provide useful mechanistic insights, but do not represent the full metabolic versatility of PPB and thus require further advancement to simulate the process for technology development and control. In this study, a new modelling approach for PPB that integrates the photoheterotrophic, and both anaerobic and aerobic chemoheterotrophic metabolic pathways through an empirical parallel metabolic growth constant was proposed. It aimed the modelling of microbial selection dynamics in competition with aerobic and anaerobic microbial community under different operational scenarios. A sensitivity analysis was carried out to identify the most influential parameters within the model and calibrate them based on experimental data. Process perturbation scenarios were simulated, which showed a good performance of the model.

Mixotrophic culture of Chaetoceros sp. and the synergistic carbon and energy metabolism

This research studied the metabolic mechanism of the mixotrophic Chaetoceros sp. The results showed this alga had the highest cell density and growth rate of 47.72 × 105 cells mL-1 and 0.41 d-1, respectively, with a maximum dry weight of 2.90 g/L, when compared to photoautotrophic and photoheterotrophic modes. Compared to photoheterotrophy, transcriptomics results showed the Rubisco, PGK, and GAPDH related genes were separately up-regulated by 1.03, 2.36, and 1.36 times in CBB cycle in mixotrophic mode, suggesting intermediate metabolites of EMP and PPP can enter the chloroplast via transporter proteins, or membrane permeation, and feedback inhibition regulates the reduction of multiple reactions in CBB cycle. Chaetoceros sp. achieves high biomass by utilizing ATP and carbon structures from EMP and PPP pathways, and the addition of NaHCO3 leads to an up-regulation of CBB cycle for the mixotrophic alga, resulting in higher biomass compared to the photoheterotrophic mode.

A Coculture of Photoautotrophs and Hydrolytic Heterotrophs Enables Efficient Upcycling of Starch from Wastewater toward Biomass-Derived Products: Synergistic Interactions Impacting Metabolism of the Consortium

Even with particular interest in sustainable development, due to the limited types of bioavailable carbon sources that could support heterotrophic/mixotrophic growth, microalgae-derived products still suffer from inconsistent yield and high costs. This study demonstrates a successful cocultivation of the photoautotroph Chlorella vulgaris with a hydrolytic-enzyme-abundant heterotroph, Saccharomycopsis fibuligera, enabling efficient starch upcycling from water/wastewater toward enhancing microalgae-dominant biomass and lipid production. The enzymatic activities of S. fibuligera contributed to the hydrolysis of starch into glucose, generating a 7-fold higher biomass through mixotrophic/heterotrophic growth of C. vulgaris. Further, scanning transmission electron microscopy (STEM) and quantitative analysis suggested a significantly induced accumulation of lipids in C. vulgaris. Results of meta-transcriptomics revealed the critical regulatory role of illumination in interaction shifting. Gene expression for glycolysis and lipid biosynthesis of C. vulgaris were highly activated during dark periods. Meanwhile, during illumination periods, genes coding for glucoamylase and the sulfur-related activities in S. fibuligera were significantly upregulated, leading to induced starch hydrolysis and potential increased competition for sulfur utilization, respectively. This study indicates that hydrolytic organisms could collaborate to make starch bioavailable for nonhydrolytic microalgae, thus broadening the substrate spectrum and making starch a novel biotechnological feedstock for microalgae-derived products, e.g., biofuels or single-cell protein.

The growth and lipid accumulation of Scenedesmus quadricauda under nitrogen starvation stress during xylose mixotrophic/heterotrophic cultivation

In order to conquer the block of high cost and low yields which limit to realize the commercialization of microalgal biodiesel, the mixotrophic and heterotrophic cultivation of Scenedesmus quadricauda FACHB-1297 fed on xylose was separately studied employing six forms of media: phosphorus sufficient, phosphorus restricted, and phosphorus starvation were combined with nitrogen sufficient and nitrogen starvation conditions. The maximum lipid content (about 41% of dry weight) was obtained on the 5th day (heterotrophic cultivation) and 8th day (mixotrophic cultivation) under the nitrogen starved and phosphorus sufficient (N0&P) conditions, which was about twofold in comparison to the final lipid content on the sufficient nitrogen condition (control). Under mixotrophic and heterotrophic modes, the highest lipid production was achieved in the N0&P trial, with the value of 274.96 mg/L and 193.77 mg/L, respectively. Xylose utilization rate of 30-96% under heterotrophic modes was apparently higher than that of 20-50% in mixotrophic modes. In contrast, phosphorus uptake rate of 100% under mixotrophic cultivation was significantly more than that of 60-90% in heterotrophic cultivation. Furthermore, under the condition of heterotrophic cultivation using xylose as a carbon source, the phosphorus had a positive impact on microalgae cell synthesis and the lipid content enhanced with the augmentation in phosphorus concentrations. We suggested that sufficient phosphorus should be supplied for obtaining higher microalgal lipid production in the lack of nitrogen under xylose heterotrophic/mixotrophic condition. This was a highly effective way to obtain efficient microalgae lipid production.

Unexpected growth promotion of Chlorella sacchrarophila triggered by herbicides DCMU

The ecotoxicological effects of herbicide contamination on the autotrophic growth of microalgae in aquatic environments have been major concerns. However, little is known about the influence of herbicides on the mixotrophic growth of microalgae. This study investigated the ecotoxicological effect of 3-(3,4-dichlorophenyl)-1,1-dimethyl-urea (DCMU) on the mixotrophic growth of Chlorella sacchrarophila FACHB 4. Results showed that C. sacchrarophila in mixotrophy was more resistant to DCMU than in photoautotrophy. Moreover, a low content of DCMU (20-80 μg·L^-1) promoted the mixotrophic growth of C. sacchrarophila, and the promotion effect was obviously enhanced with the increase in light intensity. The chlorophyll content and glucose absorption rate of C. sacchrarophila were found to increase after incubation with DCMU for 24 h. Transcriptome analyses revealed that the mechanism of DCMU to promote the mixotrophic growth of C. sacchrarophila was probably through accelerating glucose uptake and utilization, which was accomplished by reducing photodamage and increasing the chlorophyll content of C. sacchrarophila. This study not only revealed an unexpected bloom of mixotrophic microalgae triggered by herbicides, but it also shed new light on an effective and low-cost strategy to improve the microalgae productivity for utilization.

A reliable multi-nutrient model for the rapid production of high-density microalgal biomass over a broad spectrum of mixotrophic conditions

The superior microalgal biomass productivities obtained under mixotrophic conditions have been widely demonstrated. However, to attain the full potential of the method, optimal conditions for biomass production and resource utilization need to be determined and successfully exploited throughout the process operation. Detailed kinetic mathematical models have often proved most efficient tools for predicting process behavior and governing its overall operation. This paper presents an extensive study for obtaining a highly reliable model for mixotrophic production of microalgae covering a wide set and range of nutritional conditions (10-fold the concentration range of Bold's Basal Medium) and biomass yields up to 6.68 g.L^-1 after only 6 days. The final reduced model includes a total of five state variables and nine parameters: model calibration resulted in very small 95% confidence intervals and relative errors below 5% for all parameters. Model validation showed high reliability with R² correlation values between 0.77 and 0.99.

A universal dynamical metabolic model representing mixotrophic growth of Chlorella sp. on wastes

An emerging idea is to couple wastewater treatment and biofuel production using microalgae to achieve higher productivities and lower costs. This paper proposes a metabolic modeling of Chlorella sp. growing on fermentation wastes (blend of acetate, butyrate and other acids) in mixotrophic conditions, accounting also for the possible inhibitory substrates. This model extends previous works by modifying the metabolic network to include the consumption of glycerol and glucose by Chlorella sp., with the goal to test the addition of these substrates in order to overcome butyrate inhibition. The metabolic model was built using the DRUM framework and consists of 188 reactions and 173 metabolites. After a calibration phase, the model was successfully challenged with data from 122 experiments collected from scientific literature in autotrophic, heterotrophic and mixotrophic conditions. The optimal feeding strategy estimated with the model reduces the time to consume the volatile fatty acids from 16 days to 2 days. The high prediction capability of this model opens new routes for enhancing design and operation in waste valorization using microalgae.

Photosynthetically produced sucrose by immobilized Synechocystis sp. PCC 6803 drives biotransformation in E. coli

BACKGROUND

Whole-cell biotransformation is a promising emerging technology for the production of chemicals. When using heterotrophic organisms such as E. coli and yeast as biocatalysts, the dependence on organic carbon source impairs the sustainability and economic viability of the process. As a promising alternative, photosynthetic cyanobacteria with low nutrient requirements and versatile metabolism, could offer a sustainable platform for the heterologous production of organic compounds directly from sunlight and CO₂. This strategy has been applied for the photoautotrophic production of sucrose by a genetically engineered cyanobacterium, Synechocystis sp. PCC 6803 strain S02. As the key concept in the current work, this can be further used to generate organic carbon compounds for different heterotrophic applications, including for the whole-cell biotransformation by yeast and bacteria.

RESULTS

Entrapment of Synechocystis S02 cells in Ca²⁺-cross-linked alginate hydrogel beads improves the specific sucrose productivity by 86% compared to suspension cultures during 7 days of cultivation under salt stress. The process was further prolonged by periodically changing the medium in the vials for up to 17 days of efficient production, giving the final sucrose yield slightly above 3000 mg l^-1. We successfully demonstrated that the medium enriched with photosynthetically produced sucrose by immobilized Synechocystis S02 cells supports the biotransformation of cyclohexanone to ε-caprolactone by the E. coli WΔcscR Inv:Parvi strain engineered to (i) utilize low concentrations of sucrose and (ii) perform biotransformation of cyclohexanone to ε-caprolactone.

CONCLUSION

We conclude that cell entrapment in Ca²⁺-alginate beads is an effective method to prolong sucrose production by the engineered cyanobacteria, while allowing efficient separation of the cells from the medium. This advantage opens up novel possibilities to create advanced autotroph-heterotroph coupled cultivation systems for solar-driven production of chemicals via biotransformation, as demonstrated in this work by utilizing the photosynthetically produced sucrose to drive the conversion of cyclohexanone to ε-caprolactone by engineered E. coli.

Applying mixotrophy strategy to enhance biomass production and nutrient recovery of Chlorella pyrenoidosa from biogas slurry: An assessment of the mixotrophic synergistic effect

Using biogas slurry to cultivate microalgae can simultaneously obtain microalgal biomass and allow nutrient recovery. Mixotrophic microalgae are widely recognized for their high biomass accumulation and low light dependence, making it possible to overcome the drawbacks of photoautotrophy. In this study, three complete metabolic modes of photoautotrophy, heterotrophy, mixotrophy and two incomplete metabolic modes with the addition of diuron and rotenone were applied to investigate Chlorella pyrenoidosa growth in biogas slurry. The results showed that the mixotrophic group obtained 1.15 g/L biomass, 30 % starch content, 99.40 % ammonium removal and 81.69 % total phosphorus removal, which were highly promoted compared to the others. The decline in chlorophyll, the simultaneous downregulation of Rubisco and citrate synthase and the increase in the actual quantum yield of PSII under mixotrophy revealed a synergistic effect: the complementation of photophosphorylation and oxidative phosphorylation greatly contributed to maximizing energy metabolism efficiency and minimizing energy dissipation loss.

Focus on the role of synthetic phytohormone for mixotrophic growth and lipid accumulation by Chlorella pyrenoidosa

Synthetic phytohormone (SP) is regarded as an attractive candidate for microalgae cultivation due to its potential for high-value microalgae biomass production. Herein, α-naphthylacetic acid (NAA), indomethacin (IN) and 2,4-dichlorophenoxyacetic acid (2,4-D) were used for the mixotrophic cultivation of Chlorella pyrenoidosa with mariculture wastewater (MW) acidogenic fermentation effluent. The growth and lipid accumulation of Chlorella pyrenoidosa added with SP were enhanced, given their high bioavailability of the nutrients. Among these three SPs, IN was optimal for Chlorella pyrenoidosa growth, with the maximum optical density of 1.81. NAA exhibited the best performance for lipid production and the proportion of lipid reached 50.24%. Furthermore, the energy of Chlorella pyrenoidosa cultured with SP preferentially allocated to lipogenesis. To understand the mechanism of lipid accumulation in Chlorella pyrenoidosa in response to SP, the enzyme activities involved in carbon metabolism were determined. The malic enzyme (ME) and acetyl-CoA carboxylase (ACCase) were positively correlated with lipid accumulation. Phosphoenolpyruvate carboxylase (PEPC) was the negative feedback enzyme for lipid synthesis. The findings could provide valuable information for regulation mechanism of lipid accumulation and value-added products recovery by microalgae.

How can we possibly resolve the planet's nitrogen dilemma?

Nitrogen is the most crucial element in the production of nutritious feeds and foods. The production of reactive nitrogen by means of fossil fuel has thus far been able to guarantee the protein supply for the world population. Yet, the production and massive use of fertilizer nitrogen constitute a major threat in terms of environmental health and sustainability. It is crucial to promote consumer acceptance and awareness towards proteins produced by highly effective microorganisms, and their potential to replace proteins obtained with poor nitrogen efficiencies from plants and animals. The fact that reactive fertilizer nitrogen, produced by the Haber Bosch process, consumes a significant amount of fossil fuel worldwide is of concern. Moreover, recently, the prices of fossil fuels have increased the cost of reactive nitrogen by a factor of 3 to 5 times, while international policies are fostering the transition towards a more sustainable agro-ecology by reducing mineral fertilizers inputs and increasing organic farming. The combination of these pressures and challenges opens opportunities to use the reactive nitrogen nutrient more carefully. Time has come to effectively recover used nitrogen from secondary resources and to upgrade it to a legal status of fertilizer. Organic nitrogen is a slow-release fertilizer, it has a factor of 2.5 or higher economic value per unit nitrogen as fertilizer and thus adequate technologies to produce it, for instance by implementing photobiological processes, are promising. Finally, it appears wise to start the integration in our overall feed and food supply chains of the exceptional potential of biological nitrogen fixation. Nitrogen produced by the nitrogenase enzyme, either in the soil or in novel biotechnology reactor systems, deserves to have a 'renaissance' in the context of planetary governance in general and the increasing number of people who desire to be fed in a sustainable way in particular.

Effects of C/N ratio on the growth and protein accumulation of heterotrophic Chlorella in broken rice hydrolysate

BACKGROUND

Microalgae protein is considered as a sustainable alternative to animal protein in the future. Using waste for microalgal culture can upgrade low-value raw materials into high-value products, helping to offset the cost of microalgal protein production. In this study we explored the feasibility of using microalgae heterotrophic fermentation to convert broken rice hydrolysate (BRH) into protein.

RESULTS

The results showed that the increase of BRH supplemental ratio was beneficial to the increase of biomass production but not beneficial to the increase of intracellular protein content. To further improve protein production, the effect of C/N ratio on intracellular protein accumulation was studied. It was found that low C/N ratio was beneficial to the synthesis of glutamate in microalgae cells, which in turn promoted the anabolism of other amino acids and further the protein. When the C/N ratio was 12:1, the biomass productivity and protein content could reach a higher level, which were 0.90 g/L/day and 61.56%, respectively. The obtained Chlorella vulgaris biomass was rich in essential amino acids (41.80%), the essential amino acid index was as high as 89.07, and the lysine content could reach up to 4.05 g/100 g.

CONCLUSIONS

This study provides a theoretical basis and guidance for using Chlorella vulgaris as an industrial fermentation platform to convert broken rice into products with high nutritional value.

Sustainability and carbon neutrality trends for microalgae-based wastewater treatment: A review

As the global economy develops and the population increases, greenhouse gas emissions and wastewater discharge have become inevitable global problems. Conventional wastewater treatment processes produce direct or indirect greenhouse gas, which can intensify global warming. Microalgae-based wastewater treatment technology can not only purify wastewater and use the nutrients in wastewater to produce microalgae biomass, but it can also absorb CO₂ in the atmosphere or flue gas through photosynthesis, which demonstrates great potential as a sustainable and economical wastewater treatment technology. This review highlights the multifaceted roles of microalgae in different types of wastewater treatment processes in terms of the extent of their bioremediation function and microalgae biomass production. In addition, various newly developed microalgae cultivation systems, especially biofilm cultivation systems, were further characterized systematically. The performance of different microalgae cultivation systems was studied and summarized. Current research on the technical approaches for the modification of the CO₂ capture by microalgae and the maximization of CO₂ transfer and conversion efficiency were also reviewed. This review serves as a useful and informative reference for the application of wastewater treatment and CO₂ capture by microalgae, aiming to provide a reference for the realization of carbon neutrality in wastewater treatment systems.

A two-stage degradation coupling photocatalysis to microalgae enhances the mineralization of enrofloxacin

The coupling of photocatalytic and algal processes has been used for the removal of widespread antibiotics. The removal capacities of the individual and the combined system against enrofloxacin were tested and compared in this work. Due to the low tolerance of the algae to enrofloxacin, the target compound was barely degraded during the individual algal treatment. In the individual photocatalytic process, the mineralization efficiency (defined as the ratio between the produced carbon dioxide and the initial) reached ∼57% with the remaining formed as transformation products. In contrast, a two-stage treatment incorporating photocatalytic and algal processes removed enrofloxacin completely and increased the mineralization efficiency to ∼64% or more. The addition of the citric acid as external co-substrate further elevated the mineralization efficiency with a factor of 1.25 compared to that of the individual photocatalysis. Different degradation products in both individual and integrated processes were identified and compared. The degradation pathways were found to involve the attack of the piperazine moiety and quinolone core. The results indicated the potential application of the combined photocatalytic-algal treatment in removal of veterinary antibiotics and improved our understanding of the underlying mechanisms and pathways.

Mixotrophic Cultivation of Microalgae Using Biogas as the Substrate

Biogas utilization through biotechnology represents a potential and novel technology. We propose the microalgal mixotrophic cultivation to convert biogas to microalgae-based biodiesel, in which methanotroph was co-cultured to convert CH₄ to organic intermediate (and CO₂) for microalgal mixotrophic growth. This study constructed a co-culture of Methylocystis bryophila (methanotroph) and Scenedesmus obliquus (microalgae) with biogas feeding. Compared with the single culture of S. obliquus, higher microalgal biomass but a lower chlorophyll concentration was observed. The organic metabolism-related genes were upregulated, verifying microalgal mixotrophic growth. The stoichiometric calculation of M. bryophila culture shows that M. bryophila tends to release organic matter rather than grow under a low O₂ content. M. bryophila rarely grew under five different light intensities, indicating that M. bryophila acts as a biocatalyst in the co-culture. The organic intermediate released by methanotroph increased the maximum biomass of microalgal culture, accelerated nitrogen absorption, accumulated more monounsaturated fatty acids, and improved the adaptation to light. The co-culture of microalgae and methanotroph may provide new opportunities for microalgae-based biodiesel production using biogas as a substrate.

Ultrahigh recovery rate of nitrate from synthetic wastewater by Chlorella-based photo-fermentation with optimal light-emitting diode illumination: From laboratory to pilot plant

To achieve ultrahigh recovery rate of nitrate from synthetic wastewater by Chlorella pyrenoidosa-based photo-fermentation, light-emitting diode (LED) spectrum was firstly evaluated in 5-L glass photo-fermenter with surrounding LED panels. Results showed that warm white LED was favorable to improve biomass yield and recovery rate of nutrients than mixed white LED. When scaling up from laboratory (50-L, 500-L) to pilot scale photo-fermenter with inner LED panels, the maximum recovery rates of NO₃^- (5.77 g L^-1 d^-1) and PO₄^3- (0.44 g L^-1 d^-1) were achieved in 10,000-L photo-fermenter, along with high productivity of biomass (11.06 g L^-1 d^-1), protein (3.95 g L^-1 d^-1) and lipids (3.79 g L^-1 d^-1), respectively. This study demonstrated that photo-fermenter with inner warm white LED illumination is a superhigh-efficient system for nitrate and phosphate recovery with algal biomass coproduction, providing a promising application in pilot demonstration of wastewater bioremediation and facilitating novel facility development for green manufacturing.

Regulation of carbon source metabolism in mixotrophic microalgae cultivation in response to light intensity variation

Microalgae are one of the promising sources for renewable energy production, and the light intensity variation can affect the biofuel generation and carbon assimilation of mixotrophic microalgae. To reveal the response of carbon assimilation to light intensity, the effect of light intensity on the carbon source metabolism of Chlorella vulgaris under mixotrophic cultivation was investigated in this study. Moreover, the optimal carbon source composition for mixotrophic microalgae cultivation was evaluated using bicarbonate (HCO₃^-) and carbonate (CO₃^2-) as inorganic carbon sources, and glucose and acetate as organic carbon sources. The optimal light intensity for Chlorella vulgaris growth was at the range of 8000-12000 lux. For the accumulation of biochemical components, low light intensity was beneficial to protein accumulation, and high light intensity was advantageous for carbohydrate and lipid accumulation. With HCO₃^- and glucose, the maximum lipid content reached 37.0% at a light intensity of 12000 lux. The citrate synthase activity was negatively correlated with light intensity, showing an opposite trend to biomass production. High light intensity had a positive impact on Rubisco expression, which promoted the microalgae growth and carbon fixing. The energy produced by heterotrophic metabolic activities increased at low light intensity, and the enhancement of biomass production with high light intensity was mainly caused by the improved photoreaction efficiency during the mixotrophic cultivation.

Towards a general kinetic microalgae model: Extending a semi-deterministic green microalgae model for the cyanobacterium Arthrospira platensis and red alga Porphyridium purpureum

Mathematical models for microalgae and cyanobacteria are seldomly validated for different algal species, as such limiting their applicability. Therefore, in this research, a previously developed kinetic model describing the growth of the green microalgae species Chlorella vulgaris was used to simulate the growth of the cyanobacterium Arthrospira platensis and the red alga Porphyridium purpureum. Based on a global sensitivity analysis, the model parameter µ_max,A was calibrated using respirometric-titrimetric data. Calibration yielded values of 5.76 ± 0.17 d^-1, 2.06 ± 0.16 d^-1 and 1.06 ± 0.09 d^-1 for Chlorella vulgaris, Arthrospira platensis and Porphyridium purpureum, respectively. Model simulations revealed that the biological growth equations in this model are adequate. However, increased light intensities triggered a survival mechanism for Arthrospira platensis, which is currently not taken into account by the model, leading to bad model accuracy under these circumstances. Future work should address the most important survival mechanisms and include those in the model to widen its applicability.

Long-term semi-continuous production of carbohydrate-enriched microalgae biomass cultivated in low-loaded domestic wastewater

The production of carbohydrate-enriched biomass from waste streams as a sustainable biofuel precursor is a noteworthy endeavor. This study investigates the long-term microalgae cultivated under low domestic wastewater loads and different hydraulic retention times (HRT) in a semi-continuous photobioreactor. The influence of operational conditions, the microalgae interaction with carbon, nutrients availability, and microbial population in terms of carbohydrate content were elucidated. The results revealed that the operation at similar low nutrients and carbon loads maintained at three different hydraulic retention times (HRT) of 10, 8, and 6 days caused different patterns in nutrients uptake and biomass composition. Particularly, the carbohydrate accumulation was greatly influenced by the unbalance in the N:P ratios than complete depletion of the nutrients. Hence, during the period operated at HRT of 10 d, high nutrients removal efficiencies were observed while gradually increasing carbohydrate content up to 57% in dry cell weight (DCW). Afterward, the decrease to 8 and 6 d of HRT showed lower nutrient consumption with depleted alkalinity, reaching an appreciably high carbohydrate accumulation of up to 46%, and 56%, respectively. The biomass concentration decreased in the order of HRT of 10, 8, and 6 days. This study demonstrated that microalgae adapted to low carbon and nutrient loads could still accumulate high carbohydrate at shorter HRT using domestic wastewater as substrate.

Carbon-negative and high-rate nutrient removal using mixotrophic microalgae

Mixotrophic microalgae have demonstrated great potential for wastewater nutrient removal. How autotrophy/heterotrophy shares affect nutrient removal as well as carbon budget has not been understood. In this study, the autotrophy/heterotrophy shares in mixotrophy were quantified, and N removal rate and carbon budget under different mixotrophic autotrophy/heterotrophy shares were modeled. The results showed that mixotrophic N removal rate reached 2.09 mg L^-1h^-1, which was 53.18% and 37.98% higher than removal rates in autotrophic (0.97 mg L^-1h^-1) and heterotrophic (1.25 mg L^-1h^-1) controls. Mixotrophic-autotrophy and mixotrophic-heterotrophy contributed 1.15 mg L^-1h^-1 and 0.94 mg L^-1h^-1 in N removal, respectively. Model disclosed that at balanced share of 6:4, more than 2 mg L^-1h^-1N removal could be achieved, similar to bacterial nitrogen removal rate but with a negative carbon budget of 6.21 mg L^-1h^-1. Nutrient removal using mixotrophic microalgae would lead to carbon negative sustainable wastewater treatment and resource recycling.

Mixotrophic cultivation of microalgae coupled with anaerobic hydrolysis for sustainable treatment of municipal wastewater in a hybrid system of anaerobic membrane bioreactor and membrane photobioreactor

This study investigated the possibility of coupling anaerobic hydrolysis in an anaerobic membrane bioreactor (AnMBR) with mixotrophic microalgae cultivation in a membrane photobioreactor (MPBR) for the sustainable treatment of municipal wastewater. Using the hydrolyzed wastewater discharged from AnMBR, Chlorella pyrenoidosa in MPBR grew in a mixotrophic mode and realized rapid growth. During the stable operation, MPBR achieved average carbon capture rate of 42.82 mg L^-1 d^-1 and algal lipid production rate of 19.66 mg L^-1 d^-1. The average reduction in TN, TP, and TOC during stable operation was 96.7%, 98.0%, and 95.9%, respectively. Mass balance analysis showed that the overall system captured 14.76 mg of carbon from the atmosphere per liter of wastewater treated. Therefore, this AnMBR-MPBR hybrid system simultaneously realized advanced treatment of municipal wastewater, efficient production of algal lipid, and carbon capture from atmosphere, and thus has a good potential in the sustainable treatment of municipal wastewater.

Microalgae and immobilized TiO2/UV-A LEDs as a sustainable alternative for winery wastewater treatment

This work intends to promote the growth of microalgae biomass with simultaneous remediation of an agro-industrial wastewater. Winery wastewater (WW) was used as growth media for the cyanobacteria Arthrospira maxima and the green microalgae Scenedesmus obliquus, Auxenochlorella protothecoides and Chlorella vulgaris, under mixotrophic and heterotrophic conditions. The latter species stands out under mixotrophic conditions, with removals of TOC and TN above 90%. Biomass production and pollutant removal were influenced by the initial WW concentration. Maximum removal values within 8 days of incubation were 92, 91, 49 and 40% for COD, TN, polyphenols and P-PO₄, respectively, and 147.5 mg L^-1 d^-1 of biomass productivity. C. vulgaris biomass showed higher carotenoid content (maximum of 8.7 mg/g) when grown in WW, compared to autotrophic conditions (6.5 mg/g), making the bioremediation process more viable with the production of valuable by-products such as pigments. As the pollutant load removed by the microalgae does not allow reach the legal limits of release treated waters in natural water courses, a tertiary treatment process was applied. A post-treatment by photocatalysis in a UV LEDs photoreactor with TiO₂-supported in Raschig rings was proposed for the removal of COD and polyphenols from a high loaded WW. The heterogeneous photocatalytic process was efficient in removing 80% of total polyphenols and 40% of COD, allowing the release of the treated water in superficial water courses since complies with the legal limits (COD below 150 mg L^-1).

Model based analysis of carbon fluxes within microalgae-bacteria flocs using respirometric-titrimetric data

With the emerging need of nutrient recycling in resource recovery facilities, the use of microalgae-bacteria flocs has received considerable attention in the past few years. However, although the main biological processes are already known, the complex interactions occurring between algae and bacteria are not fully understood. In this work, a combined respirometric-titrimetric unit was used to assess the microorganisms' kinetics within microalgae-bacteria flocs under different growth regimes (i.e. photoautotrophic, heterotrophic and mixotrophic) and different ratios of inorganic (IC) to organic carbon (OC) (IC:OC-ratios). Using this respirometric-titrimetric data, a new model was developed, calibrated and successfully validated. The model takes into account the heterotrophic growth of bacteria, the photoautotrophic, heterotrophic and mixotrophic growth of algae and the production and consumption of extracellular polymeric substances (EPS) by both bacteria and algae. As such, the model can be used for detailed analysis of the carbon fluxes within microalgae-bacteria flocs in an efficient way. Model analysis revealed the high importance of the EPS regulatory mechanism. Firstly, under heterotrophic growth conditions, OC-uptake occurred during the first 10-15 min. This was linked with internal OC storage (49% of added OC) and EPS production (40%), as such providing carbon reserves which can be consumed during famine conditions. Moreover, the algae were able to compete with bacteria for OC. Secondly, under photoautotrophic conditions, algae used the added IC to grow (57% of added IC) and to produce EPS (29%), which consecutively stimulated heterotrophic bacteria growth (20%). Finally, under mixotrophic conditions, low IC:OC-ratios resulted in an extensive OC-storage and EPS production (50% of added C) and an enhanced microalgal CO₂ reuse, resulting in an increased algal growth of up to 29%.

Municipal Wastewater: A Sustainable Source for the Green Microalgae Chlorella vulgaris Biomass Production

The need to reduce the costs associated with microalgae cultivation encouraged scientific research into coupling this process with wastewater treatment. Thus, the aim of this work was to assess the growth of Chlorella vulgaris (Chlorophyta) in different effluents from a municipal wastewater treatment plant (WWTP), namely secondary effluent (SE) and sludge run-off (SR). Assays were performed, under the same conditions, in triplicate with 4 dilution ratios of the wastewaters (25%, 50%, 75% and 100%) with the standard culture medium bold basal medium double nitrated (BBM2N) as a control. The capability of C. vulgaris for biomass production, chlorophyll synthesis and nutrients removal in the SE and SR was evaluated. The 25% SE and 25% SR showed increased specific growth rates (0.47 and 0.55 day−1, respectively) and higher biomass yields (8.64 × 107 and 1.95 × 107 cells/mL, respectively). Regarding the chlorophyll content, the 100% SR promoted the highest concentration of this pigment (2378 µg/L). This green microalga was also able to remove 94.8% of total phosphorus of SE, while in 50% SR, 31.2% was removed. Removal of 73.9% and 65.9% of total nitrogen in 50% and 100% SR, respectively, was also observed. C. vulgaris growth can, therefore, be maximized with the addition of municipal effluents, to optimize biomass production, while cleansing the effluents.