A fungal immobilization technique for efficient harvesting of oleaginous microalgae: Key parameter optimization, mechanism exploration and spent medium recycling

Phosphorus-supplemented seawater-wastewater cyclic system for microalgal cultivation: Production of high-lipid and high-protein algae

The reuse of wastewater after seawater cultivation is critically important. In this study, a phosphorus-supplemented seawater-wastewater cyclic system (PSSWCS) based on Chlorella pyrenoidosa SDEC-35 was developed. With the addition of phosphorus, the algal biomass and the ability to assimilate nitrogen and carbon were improved. At the nitrogen to phosphorus ratio of 20:1, the biomass productivity per mass of nitrogen reached 3.6 g g-1 (N) day-1 in the second cycle. After the third cycle the protein content reached 35.7% of dry mass, and the major metabolic substances in PSSWCS reached the highest content level of 89.5% (35.7% protein, 38.3% lipid, and 15.5% carbohydrate). After the fourth cycle the lipid content maintained at 40.1%. Furthermore, 100.0% recovery of wastewater in PSSWCS increased the nitrogen and carbon absorption to 15.0 and 396.8 g per tonne of seawater. This study achieved seawater-wastewater recycle and produced high-lipid and high-protein algae by phosphorus addition.

A fungal-algal self-flocculation system and its application to treat filter sludge leachate in the sugar industry

The efficient and economical treatment of wastewater using microalgae has attracted much attention. However, harvesting microalgae cells from treated wastewater remains challenging. In the present study, a Chlorella vulgaris suspension containing filamentous fungi Aspergillus niger and Chaetomium gracile was successfully used to construct a self-flocculating system, with a microalgae flocculation efficiency of 99.6% achieved by gravity sedimentation within 4 h. The diameter of fungi played an important role in determining flocculation efficiency, and the optimal particle size was 10 mm. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) results indicated that the sweeping action of fungal mycelia and the interaction between the functional groups of fungi and the C. vulgaris surface contributed to improve flocculation. Co-cultivation of C. vulgaris and fungi could effectively remove 83.53%, 94.45% and 76.88% of total phosphorus, total nitrogen and chemical oxygen demand, respectively, from the sludge leachate from a sugar mill. The fungal-algal biomass reached 5.75 g/L. Herein, the constructed self-flocculation system had coupled efficient flocculation of C. vulgaris with removal of pollutants from wastewater in a short period of time, and providing a green, pollution-free, low-cost method for simultaneous wastewater treatment and the production of high quality biomass.

Technologies for harvesting the microalgae for industrial applications: Current trends and perspectives

Microalgae are emerging as a promising source for augmenting the supply of essential products to meet global demands in an environmentally sustainable manner. Despite the potential benefits of microalgae in industry, the high energy consumption for harvesting remains a significant obstacle. This review offers a comprehensive overview of microalgae harvesting technologies and their industrial applications, with particular emphasis on the latest advances in flocculation techniques. These cutting-edge methods have been applied to biodiesel production, food and nutraceutical processing, and wastewater treatment. Large-scale harvesting is still severely impeded by the high cost despite progress has been made in laboratory studies. In the future, cost-effective microalgal harvesting will rely on efficient resource utilization, including the use of waste materials and the reuse of media and flocculants. Additionally, precise regulation of biological metabolism will be necessary to overcome algal species-related limitations through the development of extracellular polymeric substance-induced flocculation technology.

A novel fungal-algal coupling system for slaughterhouse wastewater treatment and lipid production

In this study, a novel fungal-algal coupling system was established for slaughterhouse wastewater treatment with Chlorella sp. DT025 and a new fungus, Penicillium sp. AHP141. With the optimization of cultivation conditions for the fungal-algal coupling system, the harvest efficiency of Chlorella sp. DT025 reached 99.79%. The mechanism of microalgae harvest of the fungal-algal system was revealed to be related to the morphological characteristics, surface charge, and the secretion of humic acid-like compounds and tryptophan on the surface of the fungi cells. For the original slaughterhouse wastewater treatment, the fungal-algal coupling system had a better removal efficiency of COD, TN, and TP than both monoculture systems. In the high-concentration artificial slaughterhouse wastewater, COD removal of the fungal-algal system reached more than 5350 mg/L. The lipid production of the fungal-algal coupling system in the high-concentration artificial slaughterhouse wastewater treatment was improved by 343.33% to 1.33 g/L compared to the microalgae monoculture treatment.

Microalgal co-cultivation -recent methods, trends in omic-studies, applications, and future challenges

The burgeoning human population has resulted in an augmented demand for raw materials and energy sources, which in turn has led to a deleterious environmental impact marked by elevated greenhouse gas (GHG) emissions, acidification of water bodies, and escalating global temperatures. Therefore, it is imperative that modern society develop sustainable technologies to avert future environmental degradation and generate alternative bioproduct-producing technologies. A promising approach to tackling this challenge involves utilizing natural microbial consortia or designing synthetic communities of microorganisms as a foundation to develop diverse and sustainable applications for bioproduct production, wastewater treatment, GHG emission reduction, energy crisis alleviation, and soil fertility enhancement. Microalgae, which are photosynthetic microorganisms that inhabit aquatic environments and exhibit a high capacity for CO2 fixation, are particularly appealing in this context. They can convert light energy and atmospheric CO2 or industrial flue gases into valuable biomass and organic chemicals, thereby contributing to GHG emission reduction. To date, most microalgae cultivation studies have focused on monoculture systems. However, maintaining a microalgae monoculture system can be challenging due to contamination by other microorganisms (e.g., yeasts, fungi, bacteria, and other microalgae species), which can lead to low productivity, culture collapse, and low-quality biomass. Co-culture systems, which produce robust microorganism consortia or communities, present a compelling strategy for addressing contamination problems. In recent years, research and development of innovative co-cultivation techniques have substantially increased. Nevertheless, many microalgae co-culturing technologies remain in the developmental phase and have yet to be scaled and commercialized. Accordingly, this review presents a thorough literature review of research conducted in the last few decades, exploring the advantages and disadvantages of microalgae co-cultivation systems that involve microalgae-bacteria, microalgae-fungi, and microalgae-microalgae/algae systems. The manuscript also addresses diverse uses of co-culture systems, and growing methods, and includes one of the most exciting research areas in co-culturing systems, which are omic studies that elucidate different interaction mechanisms among microbial communities. Finally, the manuscript discusses the economic viability, future challenges, and prospects of microalgal co-cultivation methods.

Potential of biogenic and non-biogenic waste materials as flocculant for algal biomass harvesting: Mechanism, parameters, challenges and future prospects

In this review article, waste materials (biogenic/non-biogenic) are focused as the flocculants for harvesting of algal biomass. Chemical flocculants are widely utilized for the effective harvesting of algal biomass at a commercial scale while the high cost is a major drawback. The waste materials-based flocculants (WMBF) are started to utilize as one of the cost-effective performance for dual benefits of waste minimization and reuse for sustainable recovery of biomass. The novelty of the article is articulated with the objective that presents an insight of WMBF, classification of WMBF, preparation methods of WMBF, mechanisms of flocculation, factors affecting flocculation-mechanism, challenges and future recommendations that are required for harvesting of algae. The WMBF are shown similar flocculation mechanisms and flocculation efficiencies as chemical flocculants. Thus, the utilization of waste material for the flocculation process of algal cells minimizes the waste load into the environment and transforms the waste materials into valuable resources.

Insights into differences between spore-assisted and pellet-assisted microalgae harvesting using a highly efficient fungus: Efficiency, high-value substances, and mechanisms

To achieve efficient and low-cost microalgae harvesting, investigations were conducted on the harvesting efficiency and potential mechanisms of Chlorella sp. HQ by filamentous fungi using two strategies, fungal spore-assisted harvesting (FSH) and fungal pellet-assisted harvesting (FPH). Five of the 19 fungal species isolated from domestic sewage could form pellets, and Aspergillus niger HW8-1 comprised the highest harvesting efficiencies of 99.17 % and 88.70 % for FPH and FSH, respectively. FSH had 2-3 times more lipids and polysaccharides in fungus-alga pellets and caused richer saturated and monounsaturated fatty acids compared with FPH. Moreover, by optimizing the microalgae preculture time, glucose concentration, and microalgae initial density, the contents of high-value substances, such as lipids, polysaccharides, and proteins of fungus-alga pellets after FPH were improved from 5.96 %, 5.67 %, and 7.27 % to 20.18 %, 24.34 % and 10.48 %, respectively. Furthermore, fungal pellets secreted more extracellular polymeric substances (EPS) during FPH than those by FSH, which could chemisorb algal cells by lowering the surface potential of fungal pellets. FPH caused algal cells to cover the outside, which increased the light exposure area of algae, thereby increasing the photosynthesis rate, whereas FSH mainly captured microalgae physically through mycelium entanglement.

A rapid and simplified method for evaluating the performance of fungi-algae pellets: A hierarchical analysis model

Microbial pellets technology has undergone extensive research recently and has increasingly matured, showing significant promise. However, the performance of microbial pellets cannot be predicted quickly by the current evaluating methods because they are complicated to operate, take a long time, and pose a risk to the environment. In this study, a representative microbial pellet, fungi-algae pellet, was selected as the research object. Eight evaluation parameters and four evaluation indices were chosen to construct the performance evaluation system of the fungal-algal pellets using the analytic hierarchy process (AHP) and weighting method. Combining the correlation analysis and expert opinion, we found that among the eight parameters selected, the adsorption saturation rate of mycelial pellets on algae had the most significant influence weight on the performance of fungi-algae pellet, followed by algal culture time and fungal incubation time. This research proposes and validates the Performance Evaluation Value (PEV) of fungi-algae pellet and its calculation method. We also discuss the effectiveness of this new evaluation system in saving time, cost, and emission reductions. The results of this paper enable the rapid evaluation of fungi-algae pellets and promote the better development of fungi-algae pellets technology and even other multi-microbial symbiotic pellet technologies.

Capturing effects of filamentous fungi Aspergillus flavus ZJ-1 on microalgae Chlorella vulgaris WZ-1 and the application of their co-integrated fungi-algae pellets for Cu(II) adsorption

Using filamentous fungi to capture unicellular microalgae is an effective way for microalgae recovery in water treatment. Here, fungi Aspergillus flavus ZJ-1 and microalgae Chlorella vulgaris WZ-1 isolated from a copper tailings pond were used to study the capture effect of ZJ-1 on WZ-1. The highest capture efficiency (97.85%) was obtained within 6 h under the optimized conditions of 30 °C, 150 rpm, fungi-algae biomass ratio of 2.24:1, and initial pH of 9.24 in microalgae medium. The formed fungi-algae pellets (FAPs) were further used to remove Cu(II) from aqueous solution. Results showed that the FAPs formed at different capture times all adsorbed Cu(II) well, and the PAFs formed within 2 h (PAFs_2 h) exhibited the highest Cu(II) adsorption capacity (80.42 mg·g^-1). SEM images showed that Cu(II) caused a change in the internal structure of PAFs_2 h from loose to compact, the mycelium shrunk, and the microalgal cells were concave. Cu(II) adsorption by PAFs_2 h was well conformed to the pseudo-second-order kinetics and the Langmuir isotherm (123.61 mg·g^-1 of theoretically maximum adsorption capacity). This work opens a way for applying FAPs in the remediation of heavy metal-contaminated wastewater, and the metal adsorption effect was determined by the capture amount of microalgae.

Bio-flocculation of Microcystis aeruginosa by using fungal pellets of Aspergillus oryzae: Performance and mechanism

Algal blooms caused by eutrophication are global phenomena that seriously threaten the sustainable use of freshwater resources. Traditional water treatment chemicals often typically lead to high levels of residue and cause damage to the morphology of algal cells. This study investigated an eco-friendly fungal bio-flocculant, Aspergillus oryzae, to remove the representative microalgae (Microcystis aeruginosa). Furthermore, it explored crucial flocculation parameters, adsorption kinetics, and thermodynamics of microalgae using A. oryzae. Accordingly, a flocculation efficiency of >95% was achieved when the fungus was cultured for six days, flocculant dosage was 11 g/L, rotation speed was 100 rpm, temperature was 25 °C, flocculation time was 5 h, and pH ranged between 4.0 and 9.0. KEGG analysis based on the genomic data, and chemical composition analysis revealed that proteins and polysaccharides were the major components of metabolites. Zeta potential analysis, scanning electron microscopy, three-dimensional fluorescence, X-ray spectroscopy, and infrared spectroscopy, electrostatic attraction revealed that electrostatic attraction promoted the destabilization and aggregation of microalgae. Additionally, hyphal surface adsorption and chemisorption from extracellular proteins and exopolysaccharides aided in the removal of microalgae. Therefore, fungi-based bio-flocculants have the potential to remove microalgae in a simple, effective, and eco-friendly manner without the complex extraction of extracellular metabolites.

Recovering rare earth elements via immobilized red algae from ammonium-rich wastewater

Biotreatment of acidic rare earth mining wastewater via acidophilic living organisms is a promising approach owing to their high tolerance to high concentrations of rare earth elements (REEs); however, simultaneous removal of both REEs and ammonium is generally hindered since most acidophilic organisms are positively charged. Accordingly, immobilization of acidophilic Galdieria sulphuraria (G. sulphuraria) by calcium alginate to improve its affinity to positively charged REEs has been used for simultaneous bioremoval of REEs and ammonium. The results indicate that 97.19%, 96.19%, and 98.87% of La, Y, and Sm, respectively, are removed by G. sulphuraria beads (GS-BDs). The adsorption of REEs by calcium alginate beads (BDs) and GS-BDs is well fitted by both pseudo first-order (PFO) and pseudo second-order (PSO) kinetic models, implying that adsorption of REEs involves both physical adsorption caused by affinity of functional groups such as -COO- and -OH and chemical adsorption based on ion exchange of Ca²⁺ with REEs. Notably, GS-BDs exhibit high tolerance to La, Y, and Sm with maximum removal efficiencies of 97.9%, 96.6%, and 99.1%, respectively. Furthermore, the ammonium removal efficiency of GS-BDs is higher than that of free G. sulphuraria cells at an initial ammonium concentration of 100 mg L^-1, while the efficiency decreases when initial concentration of ammonium is higher than 150 mg L^-1. Last, small size of GS-BDs favors ammonium removal because of their lower mass transfer resistance. This study achieves simultaneous removal of REEs and ammonium from acidic mining drainage, providing a potential strategy for biotreatment of REE tailing wastewater.

Response of fungi-microalgae pellets to copper regulation in the removal of sulfonamides and release of dissolved organic matters

Both sulfonamides (SAs) and copper (Cu(II)) were frequently detected together in swine wastewater. In this study, the regulation of Cu(II) on SAs adsorption and release of dissolved organic matters (DOMs) by fungi-microalgae pellets (FM-pellets) were investigated. Aspergillus oryzae pellets were prepared for combination with Chlorella vulgaris and the optimal conditions were at agitation speed of 130 rpm, fungi to microalgae ratio of 10:1 and the combined time of 3 h with the highest combination efficiency of 98.65%. The results showed that adsorption was the main mechanism for SAs removal. FM-pellets exhibited a high SAs adsorption potential within 6 h, and the adsorption capacity of sulfamethazine (SMZ), sulfamonomethoxine (SMM) and sulfamethoxazole (SMX) was 1.07, 0.94 and 1.67 mg/g, respectively. Furthermore, the removal of SMX, SMZ and SMM was greatly promoted from 62.31% to 85.21%, 58.71-67.91% and 64.17-80.31%, respectively, under the presence of 2 mg/L Cu(II) through ion exchange and adsorption bridging. DOMs were analyzed by the parallel factor (PARAFAC) to demonstrate the response mechanism of FM-pellets to Cu(II). Protein-like substances and NADH in DOMs released by FM-pellets formed complexes with Cu(II) to alleviate the damage on the organism. These findings provide new insights into the mechanism and response of Cu(II) in the removal of SAs by FM-pellets.

Microalgae Chlorella vulgaris and Scenedesmus dimorphus co-cultivation with landfill leachate for pollutant removal and lipid production

In this study, landfill leachate was pre-treated with NaClO, and then diluted to 5%, 10% and 15% for microalgae growth of Chlorella vulgaris and Scenedesmus dimorphus in the mono- and co-culture modes to investigate the nutrient removal and growth characteristics of microalgae. The results revealed that landfill leachate with the 10% dilution rate was conducive for microalgae growth and exhibited robust biomass growth and the highest nutrient removal efficiency. The co-culture biomass in 10% landfill leachate achieved 0.266 g/L within 10 days and demonstrated the improved nutrient utilisation efficiency of microalgae. In addition, the chemical oxygen demand, ammonia nitrogen, total nitrate and total phosphorus removal efficiencies accordingly reached 81.0%, 80.1%, 72.1% and 86.0% in 10% landfill leachate. Meanwhile, both the enzyme activity and fluorescence parameters proved that the cell activity of co-culture was higher than that of mono-culture.

Evaluation of growth and biochemical responses of freshwater microalgae Chlorella vulgaris due to exposure and uptake of sulfonamides and copper

Sulfonamides (SAs) and heavy metals are frequently detected together in livestock wastewater. In this study, evaluations regarding their potentially adverse effects on microalgae and according removals were investigated. Results showed that the growth of C. vulgaris was inhibited by SAs and Cu. There was an obvious recovery period in photosynthetic activity (Fv/Fm), indicating that the damage to the photosystem of microalgae was reversible. The co-existence of SAs and Cu significantly affected the biochemical characteristics, including the activities of antioxidant enzyme and the contents of photosynthetic pigments, proteins and polysaccharides. The addition of Cu obviously promoted the removal efficiencies of SMZ, SMX and SMM, which might be ascribed to the bridging effect of Cu in the bioadsorption of SAs. This study is conducive to understand the changes in the biochemical responses of microalgae under the combined impacts of SAs and Cu, and provides a new insight for the simultaneous removals.

Barriers to microalgal mass cultivation

Economically successful microalgal mass cultivation is dependent on overcoming several barriers that contribute to the cost of production. The severity of these barriers is dependent on the market value of the final product. These barriers prevent the commercially viable production of algal biofuels but are also faced by any producers of any algal product. General barriers include the cost of water and limits on recycling, costs and recycling of nutrients, CO₂ utilization, energy costs associated with harvesting and biomass loss due to biocontamination and pond crashes. In this paper, recent advances in overcoming these barriers are discussed.