Efficient resource recycling from liquid digestate by microalgae-yeast mixed culture and the assessment of key gene transcription related to nitrogen assimilation in microalgae

Research Progress on the Mechanisms of Algal-Microorganism Symbiosis in Enhancing Large-Scale Lipid Production

Microalgae, characterized by their exceptional lipid content, rapid growth, and robust adaptability, represent a promising biological resource. In natural and engineered ecosystems, microalgae engage in intricate symbiotic relationships with diverse microorganisms, a dynamic interplay essential for ecological resilience and metabolic optimization. This review examines the role of symbiotic microorganisms in microalgal growth and lipid accumulation, with particular emphasis on the biological regulatory mechanisms that govern these processes. These include nutrient exchange, phytohormone-mediated growth stimulation, cofactors, and quorum-sensing-driven community coordination. The review highlights how these microbial interactions facilitate optimal lipid production by enhancing metabolic pathways, thereby improving the efficiency of lipid accumulation in microalgae. Furthermore, the review investigates horizontal gene transfer as an evolutionary driver that fortifies algal-microbial consortia against environmental stressors, enabling robust performance in fluctuating conditions. The integration of these biological insights holds transformative potential for advancing next-generation bioenergy platforms, where algal-microbial systems could play a pivotal role in enhancing biofuel production, wastewater treatment, and sustainable agriculture.

Smart proteins as a new paradigm for meeting dietary protein sufficiency of India: a critical review on the safety and sustainability of different protein sources

India, a global leader in agriculture, faces sustainability challenges in feeding its population. Although primarily a vegetarian population, the consumption of animal derived proteins has tremendously increased in recent years. Excessive dependency on animal proteins is not environmentally sustainable, necessitating the identification of alternative smart proteins. Smart proteins are environmentally benign and mimic the properties of animal proteins (dairy, egg and meat) and are derived from plant proteins, microbial fermentation, insects and cell culture meat (CCM) processes. This review critically evaluates the technological, safety, and sustainability challenges involved in production of smart proteins and their consumer acceptance from Indian context. Under current circumstances, plant-based proteins are most favorable; however, limited land availability and impending climate change makes them unsustainable in the long run. CCM is unaffordable with high input costs limiting its commercialization in near future. Microbial-derived proteins could be the most sustainable option for future owing to higher productivity and ability to grow on low-cost substrates. A circular economy approach integrating agri-horti waste valorization and C1 substrate synthesis with microbial biomass production offer economic viability. Considering the use of novel additives and processing techniques, evaluation of safety, allergenicity, and bioavailability of smart protein products is necessary before large-scale adoption.

Cultivation of yeasts on liquid digestate to remove organic pollutants and nutrients and for potential application as co-culture with microalgae

In this study, the growth of yeast and yeast-like fungi in the liquid digestate from vegetable wastes was investigated in order to remove nutrients and organic pollutants, and for their application as co-culture members with green microalgae. The studied yeast strains were characterized for their assimilative and enzymatic profiles as well as temperature requirements. In the first experimental stage, the growth dynamics of each strain were determined, allowing to select the best yeasts for further studies. In the subsequent stage, the ability of selectants to remove organic pollutants was assessed. Different cultivation media containing respectively 1:3, 1:1, 3:1 vol ratio of liquid digestate and the basal minimal medium were used. Among all tested yeast strains, Rhodotorula mucilaginosa DSM 70825 showed the most promising results, demonstrating the highest potential for removing organic substrates and nutrients. Depending on the medium, this strain achieved 50-80% sCOD, 45-60% tVFAs, 21-45% TN, 33-52% PO43- reduction rates. Similar results were obtained for the strain Candida sp. OR687571. The high nutrient and organics removal efficiency by these yeasts could likely be linked to their ability to assimilate xylose (being the main source of carbon in the liquid digestate). In culture media containing liquid digestate, both yeast strains achieved good viability and proliferation potential. In the liquid digestate medium, R. mucilaginosa and Candida sp. showed vitality at the level of 51.5% and 45.0%, respectively. These strains seem to be a good starting material for developing effective digestate treatment strategies involving monocultures and/or consortia with other yeasts or green microalgae.

A comprehensive report on valorization of waste to single cell protein: strategies, challenges, and future prospects

The food insecurity due to a vertical increase in the global population urgently demands substantial advancements in the agricultural sector and to identify sustainable affordable sources of nutrition, particularly proteins. Single-cell protein (SCP) has been revealed as the dried biomass of microorganisms such as algae, yeast, and bacteria cultivated in a controlled environment. Production of SCP is a promising alternative to conventional protein sources like soy and meat, due to quicker production, minimal land requirement, and flexibility to various climatic conditions. In addition to protein production, it also contributes to waste management by converting it into food and feed for both human and animal consumption. This article provides an overview of SCP production, including its benefits, safety, acceptability, and cost, as well as limitations that constrains its maximum use. Furthermore, this review criticizes the downstream processing of SCP, encompassing cell wall disruption, removal of nucleic acid, harvesting of biomass, drying, packaging, storage, and transportation. The potential applications of SCP, such as in food and feed as well as in the production of bioplastics, emulsifiers, and as flavoring agents for baked food, soup, and salad, are also discussed.

Sustainable management and valorization of biomass wastes using synthetic microbial consortia

In response to the persistent expansion of global resource demands, considerable attention has been directed toward the synthetic microbial consortia (SMC) within the domain of microbial engineering, aiming to address the sustainable management and valorization of biomass wastes. This comprehensive review systematically encapsulates the most recent advancements in research and technological applications concerning the utilization of SMC for biomass waste treatment. The construction strategies of SMC are briefly outlined, and the diverse applications of SMC in biomass wastes treatment are explored, with particular emphasis on its potential advantages in waste degradation, hazardous substances control, and high value-added products conversion. Finally, recommendations for the future development of SMC technology are proposed, and prospects for its sustainable application are discussed.

Growth of microalgae and cyanobacteria consortium in a photobioreactor treating liquid anaerobic digestate from vegetable waste

This research examines the biological treatment of undiluted vegetable waste digestate conducted in a bubble column photobioreactor. Initially, the bioreactor containing 3N-BBM medium was inoculated with Microglena sp., Tetradesmus obliquus, and Desmodesmus subspicatus mixture with a density of 1.0 × 104 cells/mL and the consortium was cultivated for 30 days. Then, the bioreactor was semi-continuously fed with liquid digestate with hydraulic retention time (HRT) of 30 days, and the treatment process was continued for the next 15 weeks. The change in the microalgal and cyanobacterial species domination was measured in regular intervals using cell counting with droplet method on a microscope slide. At the end of the experiment, Desmonostoc sp. cyanobacteria (identified with 16S ribosomal RNA genetical analysis) as well as Tetradesmus obliquus green algae along with Rhodanobacteraceae and Planococcaceae bacteria (determined with V3-V4 16sRNA metagenomic studies) dominated the microbial community in the photobioreactor. The experiment demonstrated high treatment efficiency, since nitrogen and soluble COD were removed by 89.3 ± 0.5% and 91.2 ± 1.6%, respectively, whereas for phosphates, 72.8 ± 2.1% removal rate was achieved.

Lipid Production of Schizochytrium sp. HBW10 Isolated from Coastal Waters of Northern China Cultivated in Food Waste Hydrolysate

Marine oleaginous thraustochytrids have attracted increasing attention for their great potential in producing high-value active metabolites using various industrial and agricultural waste. Food waste containing abundant nutrients is considered as an excellent feedstock for microbial fermentation. In this study, a thraustochytrid strain Schizochytrium sp. HBW10 was isolated from a water column in Bohai Bay in Northern China for the first time. Further lipid production characteristics of S. sp. HBW10 were investigated utilizing sulfuric acid hydrolysate of food waste (FWH) from two different restaurants (FWH1 and FWH2) with the initial pH value adjusted by NaOH or NaHCO3. Results showed that the highest concentration of total fatty acids (TFAs) was observed in FWH2 medium with the 50% content level on the fifth day, reaching up to 0.34 g/L. A higher initial pH promoted the growth and saturated fatty acid (SFA) accumulation of S. sp. HBW10, achieving nearly 100% of the sum of saturated and monounsaturated fatty acids (SMUFAs) in TFAs with initial pH7 and pH8 in FWH1 medium. This work demonstrates a possible way for lipid production by thraustochytrids using food waste hydrolysate with a higher initial pH (pH7~pH8) adjusted by NaHCO3.

From anaerobic digestion to single cell protein synthesis: A promising route beyond biogas utilization

The accumulation of a large amount of organic solid waste and the lack of sufficient protein supply worldwide are two major challenges caused by rapid population growth. Anaerobic digestion is the main force of organic waste treatment, and the high-value utilization of its products (biogas and digestate) has been widely concerned. These products can be used as nutrients and energy sources for microorganisms such as microalgae, yeast, methane-oxidizing bacteria(MOB), and hydrogen-oxidizing bacteria(HOB) to produce single cell protein(SCP), which contributes to the achievement of sustainable development goals. This new model of energy conversion can construct a bioeconomic cycle from waste to nutritional products, which treats waste without additional carbon emissions and can harvest high-value biomass. Techno-economic analysis shows that the SCP from biogas and digestate has higher profit than biogas electricity generation, and its production cost is lower than the SCP using special raw materials as the substrate. In this review, the case of SCP-rich microorganisms using anaerobic digestion products for growth was investigated. Some of the challenges faced by the process and the latest developments were analyzed, and their potential economic and environmental value was verified.

Enhanced nitrogen removal via Yarrowia lipolytica-mediated nitrogen and related metabolism of Chlorella pyrenoidosa from wastewater

We investigated the optimum co-culture ratio with the highest biological nitrogen removal rate, revealing that chemical oxygen demand, total nitrogen (TN), and ammoniacal nitrogen (NH₃-N) removal was increased in the Chlorella pyrenoidosa and Yarrowia lipolytica co-culture system at a 3:1 ratio. Compared with the control, TN and NH₃-N content in the co-incubated system was decreased within 2-6 days. We investigated mRNA/microRNA (miRNA) expression in the C. pyrenoidosa and Y. lipolytica co-culture after 3 and 5 days, identifying 9885 and 3976 differentially expressed genes (DEGs), respectively. Sixty-five DEGs were associated with Y. lipolytica nitrogen, amino acid, photosynthetic, and carbon metabolism after 3 days. Eleven differentially expressed miRNAs were discovered after 3 days, of which two were differentially expressed and their target mRNA expressions negatively correlated with each other. One of these miRNAs regulates gene expression of cysteine dioxygenase, hypothetical protein, and histone-lysine N-methyltransferase SETD1, thereby reducing amino acid metabolic capacity; the other miRNA may promote upregulation of genes encoding the ATP-binding cassette, subfamily C (CFTR/MRP), member 10 (ABCC10), thereby promoting nitrogen and carbon transport in C. pyrenoidosa. These miRNAs may further contribute to the activation of target mRNAs. miRNA/mRNA expression profiles confirmed the synergistic effects of a co-culture system on pollutant disposal.

Real-time response counterattack strategy of tolerant microalgae Chlorella vulgaris MBFJNU-1 in original swine wastewater and free ammonia

This work was the first time to systematically clarify the potential tolerance mechanism of an indigenous Chlorella vulgaris MBFJNU-1 towards the free ammonia (FA) during the original swine wastewater (OSW) treatment by transcriptome analysis using C. vulgaris UETX395 as the control group. The obtained results showed that C. vulgaris MBFJNU-1 was found to be more resistant to the high levels of FA (115 mg/L) and OSW in comparison to C. vulgaris UETX395 (38 mg/L). Moreover, the transcriptomic results stated that some key pathways from arginine biosynthesis, electron generation and transmission, ATP synthesis in chloroplasts, and glutathione synthesis of C. vulgaris MBFJNU-1 were greatly related with the OSW and FA. Additionally, C. vulgaris MBFJNU-1 in OSW and FA performed similar results in the common differentially expressed genes from these mentioned pathways. Overall, these obtained results deliver essential details in microalgal biotechnology to treat swine wastewater and high free ammonia wastewater.

Single-Cell Protein Production as a Strategy to Reincorporate Food Waste and Agro By-Products Back into the Processing Chain

Food waste is a serious problem with negative environmental and economic consequences. Unused food (either as waste or by-products and referred to as food residues in the present work) is a source of carbohydrates, lipids, proteins, vitamins, minerals and bioactive compounds that could be used in an alternate or secondary life cycle to avoid discarding it. The present work reviews the potential use of food residues for the bioengineering of single-cell protein (SCP), addressing aspects of production, nutrition and safety, as well as the main challenges and perspectives. SCP is obtained from various microorganisms, including fungi, bacteria, yeasts and algae, in pure or mixed form. SCP generally contains a higher percentage of protein (30-80%) compared to soy (38.6%), fish (17.8%), meat (21.2%) and whole milk (3.28%). SCP is a source of essential amino acids, including methionine, threonine and lysine. The use of food residues as substrates for the production of SCP would reduce production costs (35-75%); however, optimization and industrial scaling are some of the main challenges to its sustainable production. The use food waste and agro by-products from the food industry could be a promising alternative to obtain protein according to a circular production scheme.

Two-stage hybrid microalgal electroactive wetland-coupled anaerobic digestion for swine wastewater treatment in South China: Full-scale verification

Constructed wetlands have been widely used for organic wastewater treatment owing to low operating costs and simple maintenance. However, there are some disadvantages such as unstable efficiency in winter. In this study, a microalgal electroactive biofilm-constructed wetland was coupled with anaerobic digestion for full-scale treatment of swine wastewater. In a 12-month outdoor trial, the overall removal efficiencies of chemical oxygen demand, ammonium, nitrate, total nitrogen, total phosphorus, and nitrite reached 98.26%/95.14%, 97.96%/92.07%, 85.45%/66.04%, 95.07%/91.48%, 91.44%/91.52%, and 85.45%/84.67% in summer/winter, respectively. Hydrolytic bacteria were dominant in the anaerobic digestion part, and Cyanobium, Shewanella, and Azoarcus were enriched in the microalgal electroactive biofilm. The operating cost of the entire system was approximately 0.118 $/m³ of wastewater. These results confirm that the microalgal electroactive biofilm significantly enhances the efficiency and stability of constructed wetlands. In conclusion, the anaerobic digestion-microalgal electroactive biofilm-constructed wetland is technically and economically feasible for the treatment of swine wastewater.

Effects of three surfactants on co-conversion of endogenous carbon and nitrogen of dairy wastewater in mesophilic hydrolytic acidification coupled microalgae culture system

To determine the feasibility of enhancing co-conversion of endogenous C and N in dairy wastewater by surfactants, the effects of varying dosages of sodium lignosulfonate (LS), sodium dodecyl sulfate (SDS), and Tween-80 on mesophilic hydrolytic acidification coupled microalgae culture system were investigated. Tween-80 had a positive effect on hydrolytic acidification, while LS had no clear effect and SDS had a negative effect. Tween-80 significantly increased the C/N ratio in hydrolytic acidification liquor (HAL) (maximum HAc/NH₄⁺-N rate reached 7.90 in 2.9% Tween-80 test). Pyrosequencing analysis demonstrated that community richness and diversity decreased and the proportion of acidobacteria increased with increasing Tween-80 dosage. Furthermore, the effect of Tween-80-enhanced HAL on microalgae (Chlorella pyrenoidosa) growth and nitrogen removal and the assimilation of C and N on the microalgae culture system were investigated. The biomass concentration and a C and N assimilation yield of 4.8% Tween-80 test (1.75 g/L, 825.01 mg/L, 126.68 mg/L) were significantly higher than those of both the low-concentration Tween-80 tests and control. The yield of protein and carbohydrates and higher heating value in the microalgae culture system were also promoted by Tween-80.

Selecting the optimal nutrients recovery application for a biogas slurry based on its characteristics and the local environmental conditions: A critical review

Digestate (effluent of biogas plants) became the main bottleneck for biogas industry expansion because it often exceeds the capacity of surrounding croplands as fertilizer. Nutrients recovery from digestate is a promising solution for closing nutrients cycles and generating high value-added byproducts. In fact, numerous nutrients recovery technologies were reported and utilized for that purpose. However, each technology has optimum working conditions, while digestates have different characteristics due to the different substrates, digestion conditions, and handling methods. On the other hand, no protocol has been reported yet for selecting the optimal nutrients recovery technology or sequenced technologies for different digestates regarding their characteristics and the surrounding environmental conditions. In this study, an interactive flowchart was suggested and discussed for selecting the most appropriate technology or sequential techniques among the different alternatives. The whole digestate utilization technologies, solid-liquid separation technologies, liquid and solid processing technologies were included.

A review on co-culturing of microalgae: A greener strategy towards sustainable biofuels production

There is a growing global recognition that microalgae-based biofuel are environment-friendly and economically feasible options because they incur several advantages over traditional fossil fuels. Also, the microalgae can be manipulated for extraction of value-added compounds such as lipids (triacylglycerols), carbohydrates, polyunsaturated fatty acids, proteins, pigments, antioxidants, various antimicrobial compounds, etc. Recently, there is an increasing focus on the co-cultivation practices of microalgae with other microorganisms to enhance biomass and lipid productivity. In a co-cultivation strategy, microalgae grow symbiotically with other heterotrophic microbes such as bacteria, yeast, fungi, and other algae/microalgae. They exchange nutrients and metabolites; this helps to increase the productivity, therefore facilitating the commercialization of microalgal-based fuel. Co-cultivation also facilitates biomass harvesting and waste valorization, thereby help to build an algal biorefinery platform for bioenergy production along with multivariate high value bioproducts and simultaneous waste bioremediation. This article comprehensively reviews various microalgae cultivation practices utilizing co-culture approaches with other algae, fungi, bacteria, and yeast. The review mainly focuses on the impact of several binary culture strategies on biomass and lipid yield. The advantages and challenges associated with the procedure along with their respective cultivation modes have also been presented and discussed in detail.

Treatment and resource utilization of dairy liquid digestate by nitrification of biological aerated filter coupled with assimilation of Chlorella pyrenoidosa

Ammonia inhibition is considered a key issue when using liquid digestate for microalgae cultivation. To study the effect of pretreatment with a biological aerated filter (BAF) on microalgae culture with dairy liquid digestate, nitrification characteristics of BAFs under different hydraulic retention time (HRT) and the growth characteristics of Chlorella pyrenoidosa in effluents of BAFs were investigated. Results showed that the BAFs can rapidly nitrify ammonia nitrogen and significantly improve the light transmittance of liquid digestate (the maximum promotion rate was ~260%), and the effect improved as the HRT increased. Pretreatment of liquid digestate with BAFs can eliminate ammonia inhibition for C. pyrenoidosa. Furthermore, lipid, crude protein, and higher heating value (HHV) output were also not affected by HRT. The similar removal of nitrate nitrogen in microalgae culture systems using effluents with 6-h and 12-h HRT (21.59% and 21.07%, respectively) were recorded. The results suggested that BAF coupled with microalgae culture is a novel option on the resource utilization of dairy liquid digestate.

Hydrothermal co-hydrolysis of corncob/sugarcane bagasse/Broussonetia papyrifera blends: Kinetics, thermodynamics and fermentation

Biorefinery of biomass blends can achieve sustainable development of biofuel production. Herein, three lignocellulosic wastes with significant differences in chemical composition-namely corncob (CC), sugarcane bagasse (SB), and Broussonetia papyrifera (BP)-were selected to investigate their hydrothermal co-hydrolysis kinetics and thermodynamics of different biomass blends. Activation energies of hemicellulose decomposition (Ea₁, 90.59 kJ/mol) for CC/SB were lower than those for CC (126.12 kJ/mol) and CC/SB/BP (153.62 kJ/mol). BP (having a high content of nitrogen sources) loading weakened the acidic autohydrolysis of CC/SB hemicellulose, but yielded stable products as indicated by the negative entropy value for CC/SB/BP hydrolysis. Cumulative feedback inhibition occurred among different biomass, and it could be minimized by controlling the blending ratio. The highest total xylose yield was 83.64% for CC/SB with a mass ratio of 2:1. Moreover, biomass blend of CC/SB/BP enabled complete utilization of hexose, pentose and amino acids by co-production of ethanol and microalga biomass.

Treatment of Synthetic Ammonium Sulfate Wastewater by Mixed Culture of Chlorella pyrenoidosa and Enriched Nitrobacteria

Ammonium sulfate wastewater can cause eutrophication and black odor of water body. Although ammonia nitrogen can be used as nutrient of microalgae, high ammonia nitrogen levels could inhibit the growth of microalgae. Nitrobacteria can transform ammonia nitrogen into nitrate nitrogen. In this study, mono Chlorella pyrenoidosa culture (mono-C.py), synchronous mixed culture (mixed-a), and asynchronous mixed culture (mixed-b) systems were examined for their ability to treat ammonium sulfate wastewater. Nitrogen removal rate of mixed-b at the end of culture (52.96%) was higher than that of the mono-C.py (46.37%) and the mixed-a (39.11%). Higher total suspended solid concentration (2.40 g/L), crude protein yield (0.76 g/L), and heating value yield (35.73 kJ/L) were obtained in mixed-b, meanwhile with excellent settlement performance (91.43 ± 0.51%). Mechanism analysis of settlement showed that the relative abundance of floc-forming-related bacteria Sphingopyxis and Acidovorax were increased generally, while nitrification/denitrifying members were decreased in mixed-b along with the culture proceeding.

Effect of NaCl addition on the production of welan gum with the UV mutant of Sphingomonas sp

Because of its excellent stability, non-toxicity, biodegradability and unique rheology, welan gum can be widely used in various fields, such as petroleum, biomedicine and food products. In this study, a high-yield mutant strain FM01-S09 was screened through two rounds of UV mutagenesis. Remarkably, the production of welan gum could be further increased by adding 4 mM NaCl at 32 h fermentation, reaching 30.12 ± 0.25 g/L (28.66% higher than no adding), and the NaCl-WG solution had stronger structural, impact resistance, and temperature resistance than H₂O₂-WG and WG solutions. Furthermore, the mechanism by which NaCl promotes welan gum synthesis was also investigated. It was found that cell membrane characteristics, intracellular microenvironment makeup, and key enzyme gene expression levels were significantly altered in different fermentation stages. Therefore, the addition of NaCl could effectively promote the growth and fermentation performance of Sphingomonas sp., providing a novel strategy for cost-effective welan gum production.

Improving β-glucosidase and xylanase production in a combination of waste substrate from domestic wastewater treatment system and agriculture residues

Cellulase and hemicellulase activities are considered to the major bottlenecks in the lignocellulosic biorefinery process, especially in an enzyme cocktail lacking β-glucosidase (BGL) and xylanase (XYL). In view of this issue, higher levels of BGL and XYL activities were obtained in the presence of wastewater and activated sludge as an induction medium mixed with 5% of rice straw by Hypocrea sp. W₆₃. The analysis of the ionic content showed that a relatively low sludge dose could enhance the production of BGL and XYL. Most importantly, compared to a medium using freshwater, the proportion of 1:10 sludge to wastewater, which contained nutrient elements, led to 3.4-fold BGL and 3.7-fold XYL production improvements. This research describes the reuse of substrates that are largely and continuously generated from domestic wastewater treatment systems and agriculture residues, which consequently leads to the development of a simultaneous enzyme production process for sustainable biorefinery practices.

High-yield production of biomass, protein and pigments by mixotrophic Chlorella pyrenoidosa through the bioconversion of high ammonium in wastewater

To achieve a high consumption rate of ammonium with biomass coproduction, the mixotroph Chlorella pyrenoidosa was cultivated in high ammonium-high salinity wastewater medium in this study. The initial cell density, glucose and ammonium concentrations, and light intensity were optimized in shake flasks. A 5-L fermenter with surrounding LED (Light Emitting Diode) and a 50-L fermenter with inlet LED were employed for batch and semicontinuous cultivation. The results demonstrated that the highest contents of protein (56.7% DW) and total pigments (4.48% DW) with productivities of 5.62 g L^-1 d^-1 and 0.55 mg L^-1 d^-1, respectively, were obtained in 5-L photofermenter, while the maximum NH₄⁺ consumption rate (1,800 mg L^-1 d^-1) and biomass yield (23.6 g L^-1) were achieved in 50-L photofermenter. This study developed a novel strategy to convert high ammonium in wastewater to high-protein algal biomass, facilitating wastewater bioremediation and nitrogen recycling utilization by the mixotroph C. pyrenoidosa in photofermentation.

Phosphorus-Doped Iron Nitride Nanoparticles Encapsulated by Nitrogen-Doped Carbon Nanosheets on Iron Foam In Situ Derived from Saccharomycetes Cerevisiae for Electrocatalytic Overall Water Splitting

It is vitally essential to propose a novel, economical, and safe preparation method to design highly efficient electrocatalysts. Herein, phosphorus-doped iron nitride nanoparticles encapsulated by nitrogen-doped carbon nanosheets are grown directly on the iron foam substrate (P-Fe₃ N@NC NSs/IF) by in situ deriving from Saccharomycetes cerevisiae (S. cerevisiae), where anion elements of C, N, and P all from S. cerevisiae replace the hazardous CH₄ , NH₃ , and H₃ P. The diffusion pattern of N, P in S. cerevisiae and contact form between metal and S. cerevisiae observably affect the composition and phase of the product during high-temperature calcination. The obtained P-Fe₃ N@NC NSs/IF demonstrates superior electrocatalytic performance for the hydrogen evolution reaction and oxygen evolution reaction, also satisfying durability. Theoretical calculation confirms that Fe sites of P-Fe₃ N serve as the active center, and N sites and P doping regulate the hydrogen binding strength to enhance catalytic ability. Additionally, the two-electrode electrolyzer assembled by P-Fe₃ N@NC NSs/IF as both anode and cathode electrodes needs only 1.61 V to reach 10 mA cm^-2 for overall water splitting with a superb stability. The S. cerevisiae-based process presents a feasible approach for synthesis of nitrides, carbides, phosphides, and electrocatalytic applications.

Improving Fucoxanthin Production in Mixotrophic Culture of Marine Diatom Phaeodactylum tricornutum by LED Light Shift and Nitrogen Supplementation

Fucoxanthin (Fx), a kind of primary carotenoids in brown seaweeds and diatoms, has attractive efficacy in human's healthcare including loss weight, the prevention of diabetes and Alzheimer's disease. Marine diatom Phaeodactylum tricornutum is now realized as a promising producer for commercial Fx production due to its higher content of Fx than brown seaweeds with easily artificial cultivation and Fx extraction. In the present study, to improve Fx production in P. tricornutum, the mixotrophic cultures were applied to optimize initial cell density, light intensity, light regime and nitrogen supplementation. The results showed that the higher initial cell density (1 × 10⁷ cells mL^-1) and lower light intensity (20 μmol m^-2 s^-1) were favorable for biomass production and Fx accumulation. The maximal Fx content [16.28 mg g^-1 dry weight (DW)] could be achieved under blue light (BL), but the highest biomass concentration (5.53 g L^-1) could be attained under red: blue light (R: B, 6:1) in the batch culture. A novel two-phase culture approach was developed to increase the biomass concentration to the highest value (6.52 g L^-1) with the maximal productivity of Fx (8.22 mg L^-1 d^-1) through light shift from R:B ratio (6:1) in phase 1 to R:B ratio (5:1) by enhancing BL and tryptone addition in phase 2. The content and intracellular amount of Fx were also increased 8% and 12% in phase 2 compared to phase 1. The expression levels analysis revealed that genes encoding phytoene synthase (PSY), zeaxanthin epoxidase (ZEP), and fucoxanthin-chlorophyll-protein b (FCPb) were upregulated significantly, with downregulation of the gene encoding violaxanthin de-epoxidase (VDE), leading to the improvement of Fx in phase 2. The present study demonstrated the two-phase culture strategy could promote Fx productivity through enhancing biomass production and increasing Fx content, indicating that strengthening BL coupled with adding tryptone were effective to facilitate Fx production by mixotrophic cultivation of marine diatom P. tricornutum.

Performance of a microalgal-bacterial consortium system for the treatment of dairy-derived liquid digestate and biomass production

To enhance the treatment performance of dairy-derived liquid digestate (DLD) using microalgal-bacterial consortium system composed of Chlorella vulgaris and indigenous bacteria (CV), activated sludge was introduced to form a new microalgal-bacterial consortium system (Co-culture). The activated sludge shortened the lag phase and increased the specific growth rate of C. vulgaris (0.56 d-1). The biomass yield in the Co-culture was 2.72 g L-1, which was lower than that in the CV (3.24 g L-1), but the Co-culture had an improved COD (chemical oxygen demand) removal (25.26%) compared to the CV (13.59%). Quantitative PCR and metagenomic analyses demonstrated that microalgae also promoted bacterial growth, but influenced differently on the bacterial communities of indigenous bacteria and activated sludge. Compared with indigenous bacteria, activated sludge was more prone to forming a favorable symbiosis with C. vulgaris. These findings contribute to the construction of efficient microalgal-bacterial consortium system in wastewater treatment.

Organic degrading bacteria and nitrifying bacteria stimulate the nutrient removal and biomass accumulation in microalgae-based system from piggery digestate

The microalgae-based system has been applied in anaerobic digestate treatment for nutrient removal and biomass production. To optimize its performance in treating piggery digestate, here, commercial bacterial agents, including organic degrading bacteria (Cb) and nitrifying bacteria (Nb), were inoculated into the microalgae-based system dominated by Desmodesmus sp. CHX1 (D). Reactor DN (inoculated with D and Nb) and DCN (inoculated with D, and Cb to Nb at a ratio of 1:2) have better performance on NH₄⁺-N removal, with a final efficiency at 40.26% and 39.87%, respectively, and no NO₃^--N or NO₂^--N accumulations. The final total chlorophyll concentration, an indicator of microalgal growth, reached 4.74 and 5.47 mg/L in DN and DCN, respectively, three times more than that in D. These results suggested that high NH₄⁺-N removal was achieved by the assimilation into high microalgal biomass after the inoculation with functional bacteria. High-throughput sequencing showed that the richness of microbial community decreased but the evenness increased by inoculating functional microorganisms. Microalgae aggregating bacteria were Cellvibrio, Sphingobacterium, Flavobacterium, Comamonas, Microbacterium, Dyadobacter, and Paenibacillus. This study revealed that the inoculation with functional bacteria reconstructed the microbial community which benefited for the microalgal growth and nutrient removal, providing a promising strategy for treating highly-concentrated digestate.

Bioconversion of yellow wine wastes into microbial protein via mixed yeast-fungus cultures

The potential for microbial protein production in the mixture of yellow wine lees and rice soaking wastewater was examined. Strong symbiotic effect was observed in fermentation with yeast-fungus mixed culture of Candida utilis and Geochichum candidum at a ratio of 1:1 (v/v). The maximum specific biomass yield of 4.91 ± 0.48 g _{final biomass}/g _{initial biomass} with a protein content of 68.5 ± 1.0% was achieved at inoculum-to-substrate ratio of 10% (v/v) and aeration rate of 1.0 volume_air/volume_liquid/min. The essential amino acids contents of the derived protein were comparable to commercial protein sources with high amounts of methionine (2.87%, based on total protein). The reduction in soluble chemical oxygen demand of 79.4 ± 0.4% was mainly due to uptake of carbohydrate, soluble protein, volatile fatty acids, amino acids, etc. The application of mixed yeast-fungus technology provides a new opportunity for microbial protein production from these low-value organic residue streams.

Digestate management for high-solid anaerobic digestion of organic wastes: A review

Digestate management for anaerobic digestion (AD) is becoming a bottleneck of the sustainability of AD plants when the use of digestate for agricultural application is restricted due to nutrient surplus and low market acceptance. Digestate quality and treatment in high solid anaerobic digestion (HSAD) can be better than conventional low-solid system. The rheological behavior of digestate in high solid anaerobic digestion (HSAD) can have a great impact on the energy consumption of digestate management. After post-conditioning guided by rheological parameters, the solid digestate can be further treated based on the integrated solutions to enhance the energy efficiency or nutrients recovery. The environmental impacts for some core parts of those integrated systems were also evaluated in this study. This article presented a critical review of recent investigations of digestate management for HSAD, especially focusing on the rheology of HSAD digestate, integrated solutions and their environmental performances.

Effect of organic carbon and nutrient supplementation on the digestate-grown microalga, Parachlorella kessleri

Digested effluents are usually deprived of the appropriate levels of organic carbon or macro- and micro-nutrients to effectively sustain microalgal growth. In this regard, Parachlorella kessleri was cultivated in an agro-waste digestate supplemented with different glucose concentrations, magnesium and trace metals and alternatively with cheese whey (CW), with view to enriching digestate with organic and inorganic constituents and decreasing freshwater demand. Between the conditions tested, CW addition resulted in the highest biomass concentration, 2.68 g L^-1 within 18 days of cultivation. Chlorophyll content significantly decreased under 5 g L^-1 glucose addition, in contrast to MgSO₄ co-addition and CW supplementation. The latter also induced high photosynthetic activity and better-preserved vitality of the photosynthetic apparatus, compared to sole glucose addition. Concerning lipid accumulation, in the presence of high glucose concentration, % of total fatty acids decreased, and the saturated fraction increased over polyunsaturated fatty acids (PUFAs).

A promising strategy for nutrient recovery using heterotrophic indigenous microflora from liquid biogas digestate

Nutrient overloading resulting from digestate (effluent of anaerobic digestion process) application has become a major bottleneck for the development of the biogas industry and raised environmental concerns in regions with intensive animal husbandry. Due to this, it is imperative to find low cost and effective alternative to export nutrient from digestate. Among the numerous applications, indigenous microflora has recently been utilized successfully as a biofloc technology in aquatic systems for controlling ammonia and subsequent reduction of feeding cost. Accordingly, performance of the indigenous microflora in undiluted liquid digestate of chicken manure was evaluated in this study to recover nutrients and produce high-value biomass under aerobic heterotrophic mode in batch shaking experiments. The results showed that 68% of phosphate was recovered and 97% of total nitrogen was removed from the liquid digestate. Additionally, >6 g L^-1 of dry biomass was simultaneously produced and featured with up to 65% crude protein without pathogens, 10.9% lipids, 10.7% ash and 19.6 MJ kg^-1 gross energy. Therefore, the produced biomass could be used either as an alternative sustainable source for animal or fish feeding or as a substrate for energy applications.

Development of CRISPR/Cas9 system in Chlorella vulgaris FSP-E to enhance lipid accumulation

Microalgae biorefinery is an alternative, sustainable and promising trend to solve the problem of fossil oil depletion and carbon dioxide emission. However, considering the innate limitation of cell growth and oil content in microalgae, to accelerate metabolic balance by CRISPR/Cas9 system is attractive. At first, plasmid based from Agrobacterium tumefaciens and a fragment of mGFP was transformed into Chlorella sorokiniana and Chlorella vulgaris FSP-E by electroporation, respectively. Selected colonies were tested by spectrophotometer and inverted fluorescence microscopy (IFM), and an increase of fluorescent was observed by 67% compared with that in wild type, which proved the Agrobacterium-mediated plasmid is suitable for gene insertion in Chlorella species. Consequently, plasmid with similar structure as mentioned previously containing fragment of Cas9 with sgRNA designed on omega-3 fatty acid desaturase (fad3) gene was constructed and showed a higher accumulation of lipid content by 46% (w/w) in C. vulgaris FSP-E. This is first-time to use CRISPR/Cas9 based technology for gene manipulation in Chlorella.

Volatile fatty acids as novel building blocks for oil-based chemistry via oleaginous yeast fermentation

Microbial oils are proposed as a suitable alternative to petroleum-based chemistry in terms of environmental preservation. These oils have traditionally been studied using sugar-based feedstock, which implies high costs, substrate limitation, and high contamination risks. In this sense, low-cost carbon sources such as volatile fatty acids (VFAs) are envisaged as promising building blocks for lipid biosynthesis to produce oil-based bioproducts. VFAs can be generated from a wide variety of organic wastes through anaerobic digestion and further converted into lipids by oleaginous yeasts (OYs) in a fermentation process. These microorganisms can accumulate in the form of lipid bodies, lipids of up to 60% wt/wt of their biomass. In this context, OY is a promising biotechnological tool for biofuel and bioproduct generation using low-cost VFA media as substrates. This review covers recent advances in microbial oil production from VFAs. Production of VFAs via anaerobic digestion processes and the involved metabolic pathways are reviewed. The main challenges as well as recent approaches for lipid overproduction are also discussed.

Glutamine Synthetase (GS): A Key Enzyme for Nitrogen Assimilation in The Macroalga Gracilariopsis lemaneiformis (Rhodophyta)

This study aimed to address the importance of glutamine synthetase II (GSII) during nitrogen assimilation in macroalga Gracilariopsis lemaneiformis. The cDNA full-length sequence of the three glGSII genes was revealed to have the 5' m⁷ G cap, 5'-untranslated region, open reading frame (ORF), 3'-untranslated region, and a 3' poly (A) tail. The three glGSIIs were classified into plastid glGS2 and cytosolic glGS1-1 and glGS1-2, having conserved GSII domains but different cDNA sequences. The complicated 5' end flanking region indicates complex function of glGS genes. glGS1 genes were significantly up-regulated under the different NH₄⁺ : NO₃^- ratio (i.e., 40:10, 25:25, 10:40, and 0:50) except glGS2 which dramatically up-regulated under the low NH₄⁺ : NO₃^- ratio (i.e., 10:40 and 0:50) during different cultivation times. These different expression patterns perhaps are due to the different biological roles of GS1 and GS2 in the gene family. Furthermore, hypothetical working model of nitrogen assimilation pathway exhibiting the role of glGS1 and glGS2 is proposed. Finally, glGS2 was expressed in Escherichia coli BL21 (DE3), and the optimal conditions for culture (15°C, overnight), purification (500 mM imidazole washing), and activity (pH 7.4, 37°C) were established. This study lays a very important foundation for exploring the role of GS in nitrogen assimilation in algae and plants.

Co-culturing of oleaginous microalgae and yeast: paradigm shift towards enhanced lipid productivity

Oleaginous microalgae and yeast are the two major propitious factories which are sustainable sources for biodiesel production, as they can accumulate high quantities of lipids inside their bodies. To date, various microalgal and yeast species have been exploited singly for biodiesel production. However, despite the ongoing efforts, their low lipid productivity and the high cost of cultivation are still the major bottlenecks hindering their large-scale deployment. Co-culturing of microalgae and yeast has the potential to increase the overall lipid productivity by minimizing its production cost as both these organisms can utilize each other's by-products. Microalgae act as an O₂ generator for yeast while consuming the CO₂ and organic acids released by the yeast cells. Further, yeast can break complex sugars in the medium, which can then be utilized by microalgae thereby opening new options for copious and low-cost feedstocks such as agricultural residues. The current review provides a historical and technical overview of the existing studies on co-culturing of yeast and microalgae and elucidates the crucial factors that affect the symbiotic relationship between these two organisms. Furthermore, the review also highlighted the advantages and the future perspectives for paving a path towards a sustainable biodiesel product.

Co-cultivation of Rhodotorula glutinis and Chlorella pyrenoidosa to improve nutrient removal and protein content by their synergistic relationship

With the continuous development of the livestock breeding industry, the amount of piggery wastewater discharged increases year by year, and the pressure of controlling environmental pollution continuously increases. A novel method using a co-culture of Chlorella pyrenoidosa and Rhodotorula glutinis in piggery wastewater was proposed in this study, which was aimed at treating piggery wastewater and producing useful products. The results showed that the optimal inoculum ratio of algae to yeast was 3 : 1 in the wastewater, which achieved the removal efficiencies of 58.53%, 36.07%, 33.20% and 56.25% for ammoniacal nitrogen (NH₃-N), total nitrogen (TN), total protein (TP) and chemical oxygen demand (COD), respectively, after 6 d. The synergistic relationship of C. pyrenoidosa and R. glutinis was preliminarily validated using the oxygen/carbon dioxide exchange balance and scanning electron microscopy images. The co-cultivation system gained 59.8% (w/w) protein within 5 d which can be used as a feed additive, and produces aquatic animals with better growth and quality. Thus, the 1000 litre pilot scale bioreactor was used indoors and removed 82.65% of TN, 53.51% of TP, 93.48% of NH₃-N and 85.44% of COD in 21 d which gave a better performance for TN (p < 0.05) than the bench scale results. This system improves the nutrition removal and protein production efficiencies, and is a promising method for piggery wastewater treatment and the pig breeding industry.

Aiming at treating piggery wastewater and producing useful products, a novel method using a co-culture of Chlorella pyrenoidosa and Rhodotorula glutinis in piggery wastewater was proposed in this study to improve nutrient removal and the protein content in the feed produced.