Synergism and mutualistic interactions between microalgae and fungi in fungi-microalgae symbiotic system

Enhancing mixed-species microalgal biofilms for wastewater treatment: Design, construction, evaluation and optimisation

Biofilm-based cultivation of microalgae is a powerful method for wastewater treatment with low harvesting costs, water and energy consumption. This article provides a detailed summary of the design, construction, evaluation, and optimisation (DCEO) of mixed-species biofilms including algal and bacteria, and discusses their relevant applications in the treatment of industrial and agricultural wastewater and new pollutants. Finally, it presents the problems faced by mixed-species microalgal biofilms, along with solutions. DCEO is a typical synthetic biology concept, in which design and construction are bottom-up, and evaluation and optimisation are top-down approaches. Detailed knowledge of the metabolic pathways and the regulation of microalgae and other microorganisms is helpful for designing mixed-species biofilms. Three dimensional bioprinting is a powerful tool for constructing structured biofilms. Further analysis after evaluation is beneficial to optimise such biofilms. This review provides a new insight into using DCEO to enhance mixed-species biofilms for wastewater treatment.

Fabrication of mycelium-gellan gum hybrids as next generation alternative protein foods produced by fermentation

The growing demand for a more sustainable and nutritious food supply has increased interest in replacing animal-derived foods with those from alternative sources, such as fermentation processes. However, creating foods entirely from ingredients generated using a single fermentation process is often challenging. Consequently, there is interest in combining different sources of fermentation-derived ingredients to create foods with improved physicochemical, sensory, and nutritional properties. In this study, we examined the potential of combining two functional ingredients obtained using microbial fermentation. Mycoprotein (MCP) is a protein-rich material derived from mycelium fermentation that can form fibrous meat-like structures and has good nutritional properties, but it has poor gelling properties, which limits its ability to create meat substitutes and analogs. High acyl gellan gum (HA-GG) is a polysaccharide derived from bacterial fermentation that has excellent gelling properties. We therefore combined MCP and HA-GG to create hybrid hydrogels suitable for formulating meat substitutes and analogs. Differential scanning calorimetry, dynamic shear rheology, texture profile analysis, and scanning electron microscopy were used to assess the thermal, rheological, textural, and structural properties of MCP + HA-GG hydrogels with different compositions. The pure MCP (10 w/w%) samples did not exhibit any strong thermal transitions when heated or cooled from 10 to 90 °C. In contrast, pure HA-GG (2 w/w%) melted when heated above 85 °C and gelled when cooled below 80 °C. The MCP + HA-GG hybrids maintained a high shear modulus during both heating and cooling, which may be useful for food applications. The hybrids had an appreciably higher gel strength (Young's modulus, hardness, and shear modulus) than the individual MCP or HA-GG samples, which was attributed to a synergistic interaction between these two components. The gel strength, breaking stress, and breaking strain increased with increasing gellan gum concentration (0.5 to 2.0 w/w%), which meant that the mechanical properties of the hybrid materials could be tailored for specific applications. This study highlights the complex interactions among ingredients from alternative sources and their significant impact on the properties of food matrices. This information may be useful for formulating meat substitutes and analogs with enhanced physicochemical and functional properties.

Symbiotic microalgae and microbes: a new frontier in saline agriculture

With the growing human population worldwide, innovative agricultural development is needed to meet food security needs. However, this has inadvertently led to problematic irrigation practices and overuse of agrochemicals. Such practices can exacerbate soil salinization, which prevents plant growth. As a progressively widespread and escalating problem, soil salinization poses a major threat to global food security. Compared with the traditional use of microalgae or microorganisms that act on plant growth, microalgae-microorganism symbiosis has significant advantages in promoting plant growth. Microalgae and microorganisms can work together to provide a wide range of nutrients required by plants, and they exhibit nutrient complementarity, which supports plant growth. Here, the development potential of microalgae-microbial symbiosis for enhancing plant salt tolerance was investigated. Our review demonstrated that the metabolic complementarity between microalgae and microorganisms can enhance plant salt tolerance. The diversity of a microalgae-microorganism symbiotic system can improve ecosystem stability and resistance and reduce the incidence of plant disease under salt stress. These systems produce bioactive substances (e.g., phytohormones) that promote plant growth, which can improve crop yield, and they can improve soil structure by increasing organic matter and improving water storage capacity and soil fertility. Exploiting the synergistic effects between microalgae and beneficial microorganisms has biotechnological applications that offer novel solutions for saline agriculture to mitigate the deleterious effects of soil salinity on plant health and yield. However, there are several implementation challenges, such as allelopathic interactions and autotoxicity. To make microalgae-bacteria consortia economically viable for agricultural applications, optimal strains and species need to be identified and strategies need to be employed to obtain sufficient biomass in a cost-effective manner. By elucidating the synergistic mechanisms, ecological stability, and resource utilization potential of microalgae-microbial symbiotic systems, this review clarifies salt stress responses and promotes the shift of saline-alkali agriculture from single bioremediation to systematic ecological engineering.

Formation and Characterization of Mycelium-Potato Protein Hybrid Materials for Application in Meat Analogs or Substitutes

There is increasing interest in the development of meat analogs due to growing concerns about the environmental, ethical, and health impacts of livestock production and consumption. Among non-meat protein sources, mycoproteins derived from fungal fermentation are emerging as promising meat alternatives because of their natural fibrous structure, high nutritional content, and low environmental impact. However, their poor gelling properties limit their application in creating meat analogs. This study investigated the potential of creating meat analogs by combining mycoprotein (MCP), a mycelium-based protein, with potato protein (PP), a plant-based protein, to create hybrid products with meat-like structures and textures. The PP-MCP composites were evaluated for their physicochemical, rheological, textural, and microstructural properties using electrophoresis, differential scanning calorimetry, dynamic shear rheology, texture profile analysis, confocal fluorescence microscopy, and scanning electron microscopy analyses. The PP-MCP hybrid gels were stronger and had more fibrous structures than simple PP gels, which was mainly attributed to the presence of hyphae fibers in mycelia. Dynamic shear rheology showed that the PP-MCP hybrids formed irreversible heat-set gels with a setting temperature of around 70 °C during heating, which was attributed to the unfolding and aggregation of the potato proteins. Confocal and electron microscopy analyses showed that the hybrid gels contained a network of mycelia fibers embedded within a potato protein matrix. The hardness of the PP-MCP composites could be increased by raising the potato protein content. These findings suggest that PP-MCP composites may be useful for the development of meat analogs with more meat-like structures and textures.

Microalgae-fungal pellets as novel dual-bioadsorbents for dye and their practical applications in bioremediation of palm oil mill effluent

Microalgae-fungal pellets were applied as novel dual-biosorbents for dye removal compared to fungal pellets. Both pellet types effectively removed anionic dyes better than cationic dyes, with the maximum adsorbing efficiency being nearly 100 % at a wide pH range of 3-8. The adsorption isotherms of anionic Congo Red dye and Coomassie brilliant blue R-250 dye using both pellet types and their biosorption kinetics were intensively studied. Noteworthy, the maximum adsorption capacity and affinity of microalgae-fungal pellets were much higher than those of fungal pellets. Both fungal pellets were also applied in the bioremediation of palm oil mill effluent (POME). The repeated treatment of POME by replacing pellets every 12 h enhanced the percent removal of color, phenolic compounds, and COD up to 90.97 ± 0.36 %, 70.71 ± 0.90 % and 56.55 ± 1.98 %, respectively. This study has demonstrated the promising potential for addressing dye removal and bioremediation of colored-industrial effluent in a sustainable and economically viable manner.

Trilogy of comprehensive treatment of kitchen waste by bacteria-microalgae-fungi combined system: Pretreatment, water purification and resource utilization, and biomass harvesting

Given its profound disservice, a bacteria-microalgae-fungi combined system was designed to treat kitchen waste. Firstly, a new type of microbial agent homemade compound microorganisms (HCM) (composed of Serratia marcescens, Bacillus subtilis and other 11 strains) with relatively high bio-security were developed for pretreating kitchen waste, and HCM efficiently degraded 85.2 % cellulose, 94.3 % starch, and 59.0 % oil. HCM also accomplished brilliantly the initial nutrients purification and liquefaction conversion of kitchen waste. Under mono-culture mode (fungi and microalgae were inoculated separately in the pre - and post-stages) and co-culture mode (fungi and microalgae were inoculated simultaneously in the early stage), microalgae-fungi consortia were then applied for further water purification and resource utilization of kitchen waste liquefied liquid (KWLL) produced in the pretreatment stage. Two kinds of microalgae-fungi consortia (Chlorella sp. HQ and Chlorella sp. MHQ2 form consortia with pellet-forming fungi Aspergillus niger HW8-1, respectively) removed 79.5-83.0 % chemical oxygen demand (COD), 44.0-56.5 % total nitrogen (TN), 90.3-96.4 % total phosphorus (TP), and 64.9-71.0 % NH4+-N of KWLL. What's more, the microalgae-fungi consortia constructed in this study accumulated abundant high-value substances at the same time of efficiently purifying KWLL. Finally, in the biomass harvesting stage, pellet-forming fungi efficiently harvested 81.9-82.1 % of microalgal biomass in a low-cost manner through exopolysaccharides adhesion, surface proteins interaction and charge neutralization. Compared with conventional microalgae-bacteria symbiosis system, the constructed bacteria-microalgae-fungi new-type combined system achieves the triple purpose of efficient purification, resource utilization, and biomass recovery on raw kitchen waste through the trilogy strategy, providing momentous technical references and more treatment systems selection for future kitchen waste treatment.

The coupling effect promotes superoxide radical production in the microalgal-fungal symbiosis systems: Production, mechanisms and implication for Hg(II) reduction

Redox transformation of mercury (Hg) is critical for Hg exchange at the air-water interface. However, the superoxide radicals (O2•─) contribution of microalgal-fungal symbiotic systems in lake water to Hg(II) reduction is mainly unknown. Here, we studied the enhanced potential for O2•─ production by the coupling effect between microalgae and fungi. The relationships between microenvironment, microorganisms, and O2•─ production were also investigated. Furthermore, the implication of O2•─ for Hg(II) reduction was explored. The results showed that the coupling effect of microalgae and fungi enhanced O2•─ generation in the symbiotic systems, and the O2•─ generation peaked on day 4 in the lake water at 160.51 ± 13.06-173.28 ± 18.21 μmol/kg FW (fresh weight). In addition, O2•- exhibited circadian fluctuations that correlated with changes in dissolved oxygen content and redox potential on the inter-spherical interface of microalgal-fungal consortia. Partial least squares path modeling (PLS-PM) indicates that O2•─ formation was primarily associated with microenvironmental factors and microbial metabolic processes. The experimental results suggest that O2•─ in the microalgal-fungal systems could mediate Hg(II) reduction, promoting Hg conversion and cycling. The findings highlight the importance of microalgae and fungal symbiotic systems in Hg transformation in aquatic environments.

Analysis of the regulatory mechanism of exogenous IAA-mediated tryptophan accumulation and synthesis of endogenous IAA in Chlorococcum humicola

The activated sludge method is widely used for the treatment of phenol-containing wastewater, which gives rise to the problem of toxic residual sludge accumulation. Indole-3-acetic acid (IAA), a typical phytohormone, facilitates the microalgal resistance to toxic inhibition while promoting biomass accumulation. In this study, Chlorococcum humicola (C. humicola) was cultured in toxic sludge extract and different concentrations of IAA were used to regulate its physiological properties and enrichment of high value-added products. Ultimately, proteomics analysis was used to reveal the response mechanism of C. humicola to exogenous IAA. The results showed that the IAA concentration of 5 × 10-6 mol/L (M) was most beneficial for C. humicola to cope with the toxic stress in the sludge extract medium, to promote the activity of rubisco enzyme, to enhance the efficiency of photosynthesis, and, finally, to accumulate protein as a percentage of specific dry weight 1.57 times more than that of the control group. Exogenous IAA altered the relative abundance of various amino acids in C. humicola cells, and proteomic analyses showed that exogenous IAA stimulated the algal cells to produce more indole-3-glycerol phosphate (IGP), indole, and serine by up-regulating the enzymes. These precursors are converted to tryptophan under the regulation of tryptophan synthase (A0A383V983), and tryptophan can be metabolized to endogenous IAA to promote the growth of C. humicola. These findings have important implications for the treatment of toxic residual sludge while enriching for high-value amino acids.

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.

Recent advances in CO2 fixation by microalgae and its potential contribution to carbon neutrality

Many countries and regions have set their schedules to achieve the carbon neutrality between 2030 and 2070. Microalgae are capable of efficiently fixing CO₂ and simultaneously producing biomass for multiple applications, which is considered one of the most promising pathways for carbon capture and utilization. This work reviews the current research on microalgae CO₂ fixation technologies and the challenges faced by the related industries and government agencies. The technoeconomic analysis indicates that cultivation is the major cost factor. Use of waste resources such as wastewater and flue gas can significantly reduce the costs and carbon footprints. The life cycle assessment has identified fossil-based electricity use as the major contributor to the global warming potential of microalgae-based CO₂ fixation approach. Substantial efforts and investments are needed to identify and bridge the gaps among the microalgae strain development, cultivation conditions and systems, and use of renewable resources and energy.

Transformation of Enzymatic Hydrolysates of Chlorella–Fungus Mixed Biomass into Poly(hydroxyalkanoates)

The production of poly(hydroxylalkanoates) (PHA) is limited by the high cost of the feedstock since various biomass wastes look attractive as possible sources for polymer production. The originality of this present study is in the biotransformation of mixed Chlorella-based substrates into PHAs. The synthetic potential of Cupriavidus necator B8619 cells was studied during the bioconversion of algae biomass in mixtures with spent immobilized mycelium of different fungi (genus Rhizopus and Aspergillus) into PHAs. The biomass of both microalgae Chlorella and fungus cells was accumulated due to the use of the microorganisms in the processes of food wastewater treatment. The biosorption of Chlorella cells by fungal mycelium was carried out to obtain mixed biomass samples (the best ratio of “microalgae:fungi” was 2:1) to convert them by C. necator B8619 into the PHA. The influence of conditions used for the pretreatment of microalgae and mixed types of biomass on their conversion to PHA was estimated. It was found that the maximum yield of reducing sugars (39.4 ± 1.8 g/L) can be obtained from the mechanical destruction of cells by using further enzymatic hydrolysis. The effective use of the enzymatic complex was revealed for the hydrolytic disintegration of treated biomass. The rate of the conversion of mixed substrates into the biopolymer (440 ± 13 mg/L/h) appeared significantly higher compared to similar known examples of complex substrates used for C. necator cells.