Mixotrophic Growth of Chlorella sorokiniana on Acetate and Butyrate: Interplay Between Substrate, C:N Ratio and pH

Engineered algal systems for the treatment of anaerobic digestate: A meta-analysis

The objective of this review was to provide quantitative insights into algal growth and nutrient removal in anaerobic digestate. To synthesize the relevant literature, a meta-analysis was conducted using data from 58 articles to elucidate key factors that impact algal biomass productivity and nutrient removal from anaerobic digestate. On average, algal biomass productivity in anaerobic digestate was significantly lower than that in synthetic control media (p < 0.05) but large variation in productivity was observed. A mixed-effects multiple regression model across study revealed that biological or chemical pretreatment of digestate significantly increase productivity (p < 0.001). In contrast, the commonly used practice of digestate dilution was not a significant factor in the model. High initial total ammonia nitrogen suppressed algal growth (p = 0.036) whereas initial total phosphorus concentration, digestate sterilization, CO2 supplementation, and temperature were not statistically significant factors. Higher growth corresponded with significantly higher NH4-N and phosphorus removal with a linear relationship of 6.4 mg NH4-N and 0.73 mg P removed per 100 mg of algal biomass growth (p < 0.001). The literature suggests that suboptimal algal growth in anaerobic digestate could be due to factors such as turbidity, high free ammonia, and residual organic compounds. This analysis shows that non-dilution approaches, such as biological or chemical pretreatment, for alleviating algal inhibition are recommended for algal digestate treatment systems.

Influence of super-optimal light intensity on the acetic acid uptake and microalgal growth in mixotrophic culture of Chlorella sorokiniana in bubble-column photobioreactors

This study seeks to determine the influence of super-optimal light intensity on acetic acid uptake and its associated impact on the cellular composition of Chlorella sorokiniana in a semi-batch mixotrophic cultivation setup. Unicellular green microalga Chlorella sorokiniana is grown in a 1L bubble-column photobioreactor at light intensities from 6000 to 14,000 lx (≈81 to 189 µmol.photons.m-2.s-1). We find that microalgal acetic acid utilization reduces as illumination increases from an optimal 10,000 lx (≈135 µmol.photons.m-2.s-1) to a super-optimal zone (>10000 lx). This lowers microalgal growth (2.75 g/L) and acetic acid intake, which peak at 6 mL/L (10000 lx) and drop to 2 and 1 mL/L at 12,000 and 14,000 lx, respectively. Concurrently, the maximum lipid yield decreases from 0.66 g/L (10000 lx) to 0.54 g/L (12000 lx) and 0.42 g/L (14000 lx). Hence, super-optimal illumination not only disturbs phototrophy but also affects the heterotrophic component, creating an imbalance between the two.

Screening of microalgae strains for efficient biotransformation of small molecular organic acids from dark fermentation biohydrogen production wastewater

Dark fermentation biohydrogen production is a rapidly advancing and well-established field. However, the accumulation of volatile organic acid (VFAs) byproducts hinder its practical applications. Microalgae have demonstrated the ability to efficiently utilize VFAs while also treating waste gases and other nutrient elements. Integrating microalgae cultivation with dark fermentation is a promising approach. However, low VFAs tolerance and slow VFAs consumption restrict their application. To find suitable wastewater treatment microalgae, this work screened eight microalgae strains from five family. The results demonstrated that Chlamydomonas reinhardtii exhibited significant advantages in VFAs utilization, achieving a maximum removal of 100% for acetate and 52.5% for butyrate. Among the tested microalgae strains, CW15 outperformed in terms of photobioreactor adaptability, VFAs utilization, biomass productivity, and nutrient removal, making it the most promising microalgae for practical applications. This research demonstrates the feasibility of integrating microalgae cultivation with dark fermentation and providing a viable technical solution for integrated-biorefining.

Growth of Scenedesmus obliquus on anaerobic soybean wastewater using different wasted organics for high biomass production and nutrients recycling

The microalgae culture in mixing sewage with different characteristics may significantly improve biomass production and nutrients recycling efficiency. In this study, three waste organic wastewater including molasses, alcohol and glycerol wastewater were mixed with anaerobic soybean wastewater as mediums for microalgae culture. The optimal mixture of molasses, alcohol and glycerol wastewater was at an initial carbon-nitrogen ratio of 7:1, 5:1 and 10:1, improving biomass production by 60.4%, 31.3% and 68.7%, respectively. The removal efficiencies of organics, ammonia nitrogen and phosphorus at optimal mixture were 54.8-62.4%, 79.5-99.1% and 49.3-61.5%, and the removal rates increased by 340-630%, 27.5-66.3% and 36.3-70.2% compared to the blank culture. In addition, the culture in mixed wastewater increased lipids contrast by 0.7-1.3 times, while achieving higher saturation in fatty acids. The results suggested that microalgae culture using mixed wastewater was a strategy for high biomass production and nutrients recycling efficiency.

Simultaneous bioremediation of petroleum hydrocarbons and production of biofuels by the micro-green alga, cyanobacteria, and its consortium

There are two major problems in the world, fuel deficiency and environmental pollution by fossil fuels. Microalgae are regarded as one of the most feasible feedstocks for the manufacturing of biofuels and are used in the degradation of fossil fuel spills. The present study was possessed to investigate the ability of green alga Chlorella vulgaris, blue-green alga Synechococcus sp, and its consortium to grow and degrade hydrocarbon such as kerosene (k) with different concentrations (0, 0.5, 1, and 1,5%), and also using algal biomasses to produce biofuel. The algal growth was estimated by optical density (O.D) at 600 nm, pigment contents such as Chlorophyll a,b carotenoid, and dry weight. The kerosene degradation was estimated by FT-IR analysis after and before the cultivation of algae and its consortium. The components of the methanol extract were determined by GC-MS spectroscopy. The results denote the best growth was determined by O.D, algae consortium with 1.5% Kerosene after ten days, meanwhile, the highest dry weight was with C. vulgaris after ten days of cultivation. The FT-IR demonstrated the algae and consortium possessed high efficacy to degrade kerosene. After 15 days of algae cultivation with 1% K, C.vulgaris produced the maximum amount of lipids (32%). The GC-MS profile of methanol extract of two algae and consortium demonstrated that Undecane was presented in high amounts, C.vulgaris (19.9%), Synechococcussp (82.16%), algae consortium (79.51%), and also were presented moderate amounts of fatty acid methyl ester in Synechococcus sp. Overall, our results indicate that a consortium of algae can absorb and remove kerosene from water, and at the same time produce biofuels like biodiesel and petroleum-based fuels.

Biomass and lipid production by the native green microalgae Chlorella sorokiniana in response to nutrients, light intensity, and carbon dioxide: experimental and modeling approach

Introduction: Microalgae are photosynthetic cells that can produce third-generation biofuels and other commercial compounds. Microalgal growth is influenced by two main parameters: light intensity and carbon dioxide concentration, which represent the energy and carbon source, respectively. For photosynthesis, the optimum values of abiotic factors vary among species. Methods: In this study, the microalga Chlorella sorokiniana was isolated from a freshwater lake. It was identified using molecular analysis of the ribosomal internal transcribed spacer. A single-factor design of experiments in 250-mL Erlenmeyer flasks was used to evaluate which concentrations of nitrogen and phosphorus increase the production of biomass and lipids. The response surface methodology was used with a 3²-factorial design (light intensity and CO₂ were used to evaluate its effect on biomass, lipid production, and specific growth rates, in 200-mL tubular photobioreactors (PBRs)). Results and Discussion: Low levels of light lead to lipid accumulation, while higher levels of light lead to the synthesis of cell biomass. The highest biomass and lipid production were 0.705 ± 0.04 g/L and 55.1% ± 4.1%, respectively. A mathematical model was proposed in order to describe the main phenomena occurring in the culture, such as oxygen and CO₂ mass transfer and the effect of light and nutrients on the growth of microalgae. The main novelties of this work were molecular identification of the strain, optimization of culture conditions for the indigenous microalgae species that were isolated, and formulation of a model that describes the behavior of the culture.

Dark fermentation and microalgae cultivation coupled systems: Outlook and challenges

The implementation of a sustainable bio-based economy is considered a top priority today. There is no doubt about the necessity to produce renewable bioenergy and bio-sourced chemicals to replace fossil-derived compounds. Under this scenario, strong efforts have been devoted to efficiently use organic waste as feedstock for biohydrogen production via dark fermentation. However, the technoeconomic viability of this process needs to be enhanced by the valorization of the residual streams generated. The use of dark fermentation effluents as low-cost carbon source for microalgae cultivation arises as an innovative approach for bioproducts generation (e.g., biodiesel, bioactive compounds, pigments) that maximizes the carbon recovery. In a biorefinery context, after value-added product extraction, the spent microalgae biomass can be further valorised as feedstock for biohydrogen production. This integrated process would play a key role in the transition towards a circular economy. This review covers recent advances in microalgal cultivation on dark fermentation effluents (DFE). BioH₂ via dark fermentation processes and the involved metabolic pathways are detailed with a special focus on the main aspects affecting the effluent composition. Interesting traits of microalgae and current approaches to solve the challenges associated to the integration of dark fermentation and microalgae cultivation are also discussed.

Biotechnological Approaches for Biomass and Lipid Production Using Microalgae Chlorella and Its Future Perspectives

Heavy reliance on fossil fuels has been associated with increased climate disasters. As an alternative, microalgae have been proposed as an effective agent for biomass production. Several advantages of microalgae include faster growth, usage of non-arable land, recovery of nutrients from wastewater, efficient CO₂ capture, and high amount of biomolecules that are valuable for humans. Microalgae Chlorella spp. are a large group of eukaryotic, photosynthetic, unicellular microorganisms with high adaptability to environmental variations. Over the past decades, Chlorella has been used for the large-scale production of biomass. In addition, Chlorella has been actively used in various food industries for improving human health because of its antioxidant, antidiabetic, and immunomodulatory functions. However, the major restrictions in microalgal biofuel technology are the cost-consuming cultivation, processing, and lipid extraction processes. Therefore, various trials have been performed to enhance the biomass productivity and the lipid contents of Chlorella cells. This study provides a comprehensive review of lipid enhancement strategies mainly published in the last five years and aimed at regulating carbon sources, nutrients, stresses, and expression of exogenous genes to improve biomass production and lipid synthesis.

Proteomics unveil a central role for peroxisomes in butyrate assimilation of the heterotrophic Chlorophyte alga Polytomella sp

Volatile fatty acids found in effluents of the dark fermentation of biowastes can be used for mixotrophic growth of microalgae, improving productivity and reducing the cost of the feedstock. Microalgae can use the acetate in the effluents very well, but butyrate is poorly assimilated and can inhibit growth above 1 gC.L-1. The non-photosynthetic chlorophyte alga Polytomella sp. SAG 198.80 was found to be able to assimilate butyrate fast. To decipher the metabolic pathways implicated in butyrate assimilation, quantitative proteomics study was developed comparing Polytomella sp. cells grown on acetate and butyrate at 1 gC.L-1. After statistical analysis, a total of 1772 proteins were retained, of which 119 proteins were found to be overaccumulated on butyrate vs. only 46 on acetate, indicating that butyrate assimilation necessitates additional metabolic steps. The data show that butyrate assimilation occurs in the peroxisome via the β-oxidation pathway to produce acetyl-CoA and further tri/dicarboxylic acids in the glyoxylate cycle. Concomitantly, reactive oxygen species defense enzymes as well as the branched amino acid degradation pathway were strongly induced. Although no clear dedicated butyrate transport mechanism could be inferred, several membrane transporters induced on butyrate are identified as potential condidates. Metabolic responses correspond globally to the increased needs for central cofactors NAD, ATP and CoA, especially in the peroxisome and the cytosol.

Circadian rhythm promotes the biomass and amylose hyperaccumulation by mixotrophic cultivation of marine microalga Platymonas helgolandica

Background

Microalgal starch can be exploited for bioenergy, food, and bioplastics. Production of starch by green algae has been concerned for many years. Currently commonly used methods such as nutrient stress will affect cell growth, thereby inhibiting the production efficiency and quality of starch production. Simpler and more efficient control strategies need to be developed.

Result

We proposed a novel regulation method to promote the growth and starch accumulation by a newly isolated Chlorophyta Platymonas helgolandica. By adding exogenous glucose and controlling the appropriate circadian light and dark time, the highest dry weight accumulation 6.53 g L⁻¹ (Light:Dark = 12:12) can be achieved, and the highest starch concentration could reach 3.88 g L⁻¹ (Light:Dark = 6:18). The highest production rate was 0.40 g L⁻¹ d⁻¹ after 9 days of production. And this method helps to improve the ability to produce amylose, with the highest accumulation of 39.79% DW amylose. We also discussed the possible mechanism of this phenomenon through revealing changes in the mRNA levels of key genes.

Conclusion

This study provides a new idea to regulate the production of amylose by green algae. For the first time, it is proposed to combine organic carbon source addition and circadian rhythm regulation to increase the starch production from marine green alga. A new starch-producing microalga has been isolated that can efficiently utilize organic matter and grow with or without photosynthesis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13068-022-02174-2.

Effect of pH on biomass production and carbohydrate accumulation of Chlorella vulgaris JSC-6 under autotrophic, mixotrophic, and photoheterotrophic cultivation

Microalgal biomass, known as the third generation feedstock for biofuels production, is currently being explored mainly for lipids and functional components. However, the potential of microalgal carbohydrates has not been evaluated. In this investigation, Chlorella vulgaris JSC-6 was used for carbohydrates production from CO₂ and fatty acids under different cultivation strategies to meet the requirements of a CO₂-neutral and clean fermentation system for biofuel production. Autotrophic cultivation resulted in better carbon assimilation and carbohydrate accumulation; about 1.4 g CO₂ could be converted to 1 g biomass, of which 50% are carbohydrates. Assimilation of fatty acids in photoheterotrophic and mixotrophic modes was influenced by pH, and pH 7-7.5 supported butyrate and acetate assimilation. The maximum carbohydrate content (49.86%) was attained in mixotrophic mode, and the ratio of the simple sugars glucose-xylose-arabinose was 1:0.11:0.02. The higher glucose content makes the microalgal biomass a suitable feedstock for sugar-based fermentations.

Microalgae production for nitrogen recovery of high-strength dry anaerobic digestion effluent

Dry anaerobic digestion (D-AD) generates nitrogen-rich effluents that are normally neglected in the circular bioeconomy. The high turbidity and ammonium content hamper nitrogen recovery from these effluents via biological processes, such as microalgae culture. The goal of this study was to demonstrate microalgae growth viability in high-strength D-AD effluents in order to recover nitrogen (N) as microalgae biomass. According to the experimental factorial design conducted in batch reactors, ammonium was identified as the critical inhibitory compound for microalgae growth while turbidity did not exhibit a significantly negative effect. Instead, turbidity resulted advantageous since it promoted high nitrogen uptake rates and biomass production. The presence of organic turbidity resulted in a positive effect that boosted Chlorella growth in a stream with higher ammonium (350 mg NH₄⁺-N L^-1) and turbidity (175 NTU) than the inhibition thresholds reported in the literature, reaching 98.7% of N recovery as microalgae biomass. When microalgae culture was scaled up in a photobioreactor operated in continuous mode, microalgae biomass was effectively produced while recovering 100% of N at a hydraulic retention time of 10 days. By imposing long exposure times and high turbidity, Chlorella adaptation to high-strength D-AD effluent resulted in high N uptake and biomass production. This study demonstrated not only the most influencing factor and the optimal NH₄⁺-N and turbidity combination, but also the viability of using D-AD effluents as culture media for microalgae biomass production.