Can microalgae grown in wastewater reduce the use of inorganic fertilizers?

Acclimated green microalgae consortium to treat sewage in an alternative urban WWTP in a coastal area of Central Italy

This study exposed a microalgal consortium formed by Auxenochlorella protothecoides, Tetradesmus obliquus, and Chlamydomonas reinhardtii to six mixed wastewater media containing different proportions of primary (P) or secondary (S) effluents diluted in centrate (C). Algae could grow at centrate concentrations up to 50 %, showing no significant differences between effluents. After acclimation, microalgae cultivated in 50%P-50%C and 50%S-50%C grew at a rate similar to that of control cultures (0.59-0.66 d-1). These results suggest that the consortium acclimated to both sewage streams by modulating the proportion of the species and their metabolism. Acclimation also altered the photosynthetic activity of wastewater-grown samples compared to the control, probably due to partial photoinhibition, changes in consortium composition, and changes in metabolic activity. No major differences were observed between the two streams with respect to biochemical composition, biomass yield, or bioremediation capacity of the cultivated algae but algae grown in the secondary effluent showed qualitatively higher exopolysaccharides (EPS) production than algae grown in primary. Regarding wastewater remediation, microalgae grown in both WW media showed proficient nutrient removal efficiencies (close to 100 %); however, the final pH value (close to 11) would be controversial if the system were upscaled as it is over the legal limit and would cause phosphorus precipitation, so that CO2 addition would be required. The theoretical scale-up of the microalgae system could achieve water treatment costs of 0.109 €·m-3, which was significantly lower than the costs of typical activated sludge systems.

The effects of microalgae use as a biofertilizer on soil and plant before and after its anaerobic (co-)digestion with food waste

The increase in food waste generation has resulted in significant challenges for its sustainable management. Anaerobic digestion coupled with microalgae-based ponds for digestate treatment can be used as a low-cost eco-friendly technology approach. In this case, microalgal biomass harvested from the ponds may be valorized into bioenergy (biogas) and soil conditioner and/or biofertilizers. The aim of the present study was to evaluate the microalgal biomass produced from a food waste digestate treatment ponds as agricultural fertilizer. For this purpose, microalgal biomass was tested before and after anaerobic digestion and co-digestion with food waste, exploring its potential for valorization. The inorganic fertilizer urea and soil with no fertilization were also used as treatments. The experimental design consisted of applying the treatments in pots cultivated with hybrid grass Brachiaria cv. Sabiá and distributed in randomized blocks in a controlled greenhouse. Microalgal biomass was mainly composed by Scenedesmus sp.. The assessed parameters showed comparable results on plant growth (i.e. number of tillers, fresh and dry matter and Chlorophyll content index) for fresh and digested microalgal biomass and inorganic fertilizer. Furthermore, it was observed that fresh microalgae provided the highest Phosphorus content in the leaf (21 %). Additionally, there were increases of 9 % in Nitrogen and 12 % in organic matter in the soil after applying digested microalgae compared to the control group without any fertilization. Finally, experimental data obtained suggests that microalgae-based biofertilizer holds the potential to replace inorganic fertilizer as a nutrient source. Moreover, it contributes to the valorization of by-products from organic waste treatment.

Urea intercalated encapsulated microalgae composite hydrogels for slow-release fertilizers

In agriculture, hydrogels can be addressed for effective operation of water and controlled-release fertilizers. Hydrogels have a significant ability for retaining water and improving nutrient availability in soil, enhancing plant growth while reducing water and fertilizer usage. This work aimed to prepare a hydrogel composite based on microalgae and biopolymers including chitosan and starch for use as a soil conditioner. The hydrogel composite was characterized by FTIR, XRD, and SEM. All hydrogel properties were studied including swelling degree, biodegradability, water-holding capacity, water retention, and re-swelling capacity in soil and water. The urea fertilizer loading and releasing behavior of the prepared hydrogels were investigated. The results revealed that the range of the maximal urea loading was between 99 and 440%, and the kinetics of loading was fitted with Freundlich model. The urea release % exhibited 78-95%, after 30 days, and the kinetics of release was fitted with zero-order, Higuchi, and Korsmeyer-Peppas models. Furthermore, the prepared hydrogels obtained a significant water-holding capacity, after blending soil (50 g) with small amount of hydrogels (1 g), the capacity increased in the range of 99.4-101.5%. In sum, the prepared hydrogels have the potential to be applied as a soil conditioner.

Valorization of treated swine wastewater and generated biomass by microalgae: Their effects and salt tolerance mechanisms on wheat seedling growth

The extensive use of mineral fertilizers has a negative impact on the environment, whereas wastewater and microalgal biomass can provide crops with nutrients such as nitrogen, phosphorus, and potassium, and have the potential to be used as a source of fertilizers in circular agriculture. In this study, a step-by-step resource utilization study of algae-containing wastewater generated from microalgae treatment of swine wastewater was carried out. When wheat seedlings were cultivated in the effluent after microalgae separation, the root fresh weight, seedling fresh weight, and total seedling length were increased by 3.44%, 14.45%, and 13.64%, respectively, compared with that of the algae-containing wastewater, and there was no significant difference in seedling fresh weight, total seedling length, maximum quantum yields of PSII photochemistry (Fv/Fm), and performance index (PIABS) from that of the Hogland solution group, which has the potential to be an alternative liquid fertilizer. Under salt stress, microalgae extract increased the contents of GA3, IAA, ABA, and SA in wheat seedlings, antioxidant enzymes maintained high activity, and the PIABS value increased. Low-dose microalgae extract (1 mL/L) increased the root fresh weight, seedling fresh weight, longest seedling length, and total seedling length by 30.73%, 31.28%, 16.43%, and 28.85%, respectively. Algae extract can act as a plant biostimulant to regulate phytohormone levels to attenuate the damage of salt stress and promote growth.

A Holistic Approach to Circular Bioeconomy Through the Sustainable Utilization of Microalgal Biomass for Biofuel and Other Value-Added Products

Emissions from transportation and industry primarily cause global warming, leading to floods, glacier melt, and rising seas. Widespread greenhouse gas emissions and resulting global warming pose significant risks to the environment, economy, and society. The need for alternative fuels drives the development of third-generation feedstocks: microalgae, seaweed, and cyanobacteria. These microalgae offer traits like rapid growth, high lipid content, non-competition with human food, and growth on non-arable land using brackish or waste water, making them promising for biofuel. These unique phototrophic organisms use sunlight, water, and carbon dioxide (CO2) to produce biofuels, biochemicals, and more. This review delves into the realm of microalgal biofuels, exploring contemporary methodologies employed for lipid extraction, significant value-added products, and the challenges inherent in their commercial-scale production. While the cost of microalgae bioproducts remains high, utilizing wastewater nutrients for cultivation could substantially cut production costs. Furthermore, this review summarizes the significance of biocircular economy approaches, which encompass the utilization of microalgal biomass as a feed supplement and biofertilizer, and biosorption of heavy metals and dyes. Besides, the discussion extends to the in-depth analysis and future prospects on the commercial potential of biofuel within the context of sustainable development. An economically efficient microalgae biorefinery should prioritize affordable nutrient inputs, efficient harvesting techniques, and the generation of valuable by-products.

Using microalgae to reduce the use of conventional fertilizers in hydroponics and soil-based cultivation

The intensive use of agrochemicals has led to nutrient loss, greenhouse gas emissions, and resource depletion, thus the development of sustainable agricultural solutions is required. Microalgal biomass has the potential to provide nutrients such as nitrogen, phosphorus, and potassium, along with various plant growth promoters, to enhance crop productivity and impart disease resistance. This study provides a comprehensive assessment of the potential applications of microalgal extracts and biomass in the contexts of seed germination, hydroponic systems, and soil-based crop cultivation. The results revealed that the extracts from Chlorella sp. and Anabaena sp. have no significant impact on the germination of wheat seeds. High concentrations of Chlorella sp. and Anabaena sp. cell extracts in hydroponics enhanced the length of cucumber seedling stems by 81.7 % and 58.3 %, respectively. Additionally, the use of microalgal cell extracts hindered root elongation while stimulating the growth of lateral and fibrous roots. Furthermore, the study compared the performance of 5 different fertilizers: 1) inorganic fertilizer (IF), 2) organic fertilizer (OF), 3) microalgae-based biofertilizer (MF), 4) inorganic fertilizer + microalgae-based biofertilizer (IM), 5) organic fertilizer + microalgae-based biofertilizer (OM). The findings indicate that the plant growth and soil physicochemical properties in the groups supplied with different fertilizers are comparable and significantly higher than those in the control group. The levels of protein, chlorophyll A, and chlorophyll B in the MF group increased significantly by 40 %, 29.2 %, and 33.5 %, respectively, compared to the control group. However, it remained notably lower compared to groups supplied with inorganic and organic fertilizers (p < 0.05). Combining microalgae with organic fertilizer can simultaneously enhance the yield and quality of Chinese cabbage, representing a promising source of crop nutrition. In conclusion, this study suggests that it is promising to use microalgae to reduce the use of conventional fertilizers in hydroponics and soil-based cultivation.

Regulation of microclimate and shading effects of microalgal photobioreactors on rooftops: Microalgae as a promising emergent for green roof technology

Urban areas remarkably affect global public health due to their emissions of greenhouse gases and poor air quality. Although urban areas only cover 2% of the Earth's surface, they are responsible for 80% of greenhouse gas emissions. Dense buildings limit vegetation, leading to increased air pollution and disruption of the local and regional carbon cycle. The substitution of urban gray roofs with microalgal green roofs has the potential to improve the carbon cycle by sequestering CO2 from the atmosphere. Microalgae can fix 15-50 times more CO2 than other types of vegetation. Advanced microalgal-based green roof technology may significantly accelerate the reduction of atmospheric CO2 in a more effective way. Microalgal green roofs also enhance air quality, oxygen production, acoustic isolation, sunlight absorption, and biomass production. This endeavor yields the advantage of simultaneously generating protein, lipids, vitamins, and a spectrum of valuable bioactive compounds, including astaxanthin, carotenoids, polysaccharides, and phycocyanin, thus contributing to a green economy. The primary focus of the current work is on analyzing the ecological advantages and CO2 bio-fixation efficiency attained through microalgal cultivation on urban rooftops. This study also briefly examines the idea of green roofs, clarifies the ecological benefits associated with them, discusses the practice of growing microalgae on rooftops, identifies the difficulties involved, and the positive aspects of this novel strategy.

Biochar and Chlorella increase rice yield by improving saline-alkali soil physicochemical properties and regulating bacteria under aquaculture wastewater irrigation

The combined effects of biochar and Chlorella under aquaculture wastewater irrigation in improving saline-alkali soil physicochemical properties, microbial communities, and rice yield, is not yet clear. This study utilized soil physicochemical indicators and gene sequencing to examine the effect of salinity stress, biochar and Chlorella under aquaculture wastewater irrigation on soil properties, bacterial community compositions, and rice production. Treatments included three factors in a randomized complete block design with three replications: (i) Biochar - 40 tons ha -1 (BW) versus no-biochar (BN); (ii) Salinity - 3‰ salinity (SH) versus 1‰ salinity (SL); and (iii) Chlorella - with 107 cells mL -1 Chlorella (CW) versus no-Chlorella (CN). The results revealed that increased salinity adversely affected the soil nutrients (TOC, NO3⁻-N, NH4+-N, Olsen-P), and enzyme activity (urease, sucrase, catalase), resulting in a 9.67% reduction in rice yield compared to SL treatment. However, the close correlation between alterations in soil bacterial communities, functions, and soil physicochemical properties, as well as rice yield, indicated that biochar and Chlorella promoted rice yield by enhancing the physicochemical properties of saline-alkali soil and bacterial community when irrigated with aquaculture wastewater: (1) addition of biochar increased the146.05% rice yield by increasing TOC content, the complexity of bacterial co-occurrence patterns, nitrogen fixation potential, and nitrification potential, (2) addition of Chlorella increased TOC, NO3⁻-N, NH4+-N, enhanced urease, sucrase, catalase activity, and nitrification potential to increased rice yield by 60.29%, and (3) compared with the treatment T3 (SHBNCN), the treatments with biochar (BW) and Chlorella (CW) increased the yield by 561.30% and 445.03% under 1‰ and 3‰ salinity, respectively. These findings provide novel perspectives on the capacity of biochar and Chlorella to improve saline-alkali soil properties and increase rice yield irrigated with aquaculture wastewater.

Nitrogen dynamics and biological processes in soil amended with microalgae grown in abattoir digestate to recover nutrients

The use of microalgae for nutrient recovery from wastewater and subsequent conversion of the harvested biomass into fertilizers offers a sustainable approach towards creating a circular economy. Nonetheless, the process of drying the harvested microalgae represents an additional cost, and its impact on soil nutrient cycling compared to wet algal biomass is not thoroughly understood. To investigate this, a 56-day soil incubation experiment was conducted to compare the effects of wet and dried Scenedesmus sp. microalgae on soil chemistry, microbial biomass, CO2 respiration, and bacterial community diversity. The experiment also included control treatments with glucose, glucose + ammonium nitrate, and no fertilizer addition. The Illumina Mi-Seq platform was used to profile the bacterial community and in-silico analysis was performed to assess the functional genes involved in N and C cycling processes. The maximum CO2 respiration and microbial biomass carbon (MBC) concentration of dried microalgae treatment were 17% and 38% higher than those of paste microalgae treatment, respectively. NH4+ and NO3- released slowly and through decomposition of microalgae by soil microorganisms as compared to synthetic fertilizer control. The results indicate that heterotrophic nitrification may contribute to nitrate production for both microalgae amendments, as evidenced by low amoA gene abundance and a decrease in ammonium with an increase in nitrate concentration. Additionally, dissimilatory nitrate reduction to ammonium (DNRA) may be contributing to ammonium production in the wet microalgae amendment, as indicated by an increase in nrfA gene and ammonium concentration. This is a significant finding because DNRA leads to N retention in agricultural soils instead of N loss via nitrification and denitrification. Thus, further processing the microalgae through drying or dewetting may not be favorable for fertilizer production as the wet microalgae appeared to promote DNRA and N retention.

The potential of wastewater grown microalgae for agricultural purposes: Contaminants of emerging concern, heavy metals and pathogens assessment

In the coming years, the use of microalgal biomass as agricultural biofertilizers has shown promising results. The use of wastewater as culture medium has resulted in the reduction of production costs, making microalgae-based fertilizers highly attractive for farmers. However, the occurrence of specific pollutants in wastewater, like pathogens, heavy metals and contaminants of emerging concern (CECs), such as pharmaceuticals and personal care products may pose a risk on human health. This study presents an holistic assessment of the production and use of microalgal biomass grown in municipal wastewater as biofertilizer in agriculture. Results showed that pathogens and heavy metals concentrations in the microalgal biomass were below the threshold established by the European regulation for fertilizing products, except for cadmium. Regarding CECs, 25 out of 29 compounds were found in wastewater. However, only three of them (hydrocinnamic acid, caffeine, and bisphenol A) were found in the microalgae biomass used as biofertilizer. Agronomic tests were performed for lettuce growth in greenhouse. Four treatments were studied, comparing the use of microalgae biofertilizer with a conventional mineral fertilizer, and also a combination of both of them. Results suggested that microalgae can help reducing the mineral nitrogen dose, since similar fresh shoot weights were obtained in the plants grown with the different assessed fertilizers. Lettuce samples revealed the presence of cadmium and CECs in all the treatments including both negative and positive controls, which suggests that their presence was not linked to the microalgae biomass. On the whole, this study revealed that wastewater grown microalgae can be used for agricultural purposes reducing mineral N need and guaranteeing health safety of the crops.

Roles of microalgae-based biofertilizer in sustainability of green agriculture and food-water-energy security nexus

For years, agrochemical fertilizers have been used in agriculture for crop production. However, intensive utilization of chemical fertilizers is not an ecological and environmental choice since they are destroying soil health and causing an emerging threat to agricultural production on a global scale. Under the circumstances of the increasing utilization of chemical fertilizers, cultivating microalgae to produce biofertilizers would be a wise solution since desired environmental targets will be obtained including (1) replacing chemical fertilizer while improving crop yields and soil health; (2) reducing the harvest of non-renewable elements from limited natural resources for chemical fertilizers production, and (3) mitigating negative influences of climate change through CO₂ capture through microalgae cultivation. Recent improvements in microalgae-derived-biofertilizer-applied agriculture will be summarized in this review article. At last, the recent challenges of applying biofertilizers will be discussed as well as the perspective regarding the concept of circular bio-economy and sustainable development goals (SDGs).

Microalgae as next generation plant growth additives: Functions, applications, challenges and circular bioeconomy based solutions

Sustainable agriculture practices involve the application of environment-friendly plant growth promoters and additives that do not negatively impact the health of the ecosystem. Stringent regulatory frameworks restricting the use of synthetic agrochemicals and the increase in demand for organically grown crops have paved the way for the development of novel bio-based plant growth promoters. In this context, microalgae biomass and derived agrochemicals offer novel sources of plant growth promotors that enhance crop productivity and impart disease resistance. These beneficial effects could be attributed to the presence of wide range of biomolecules such as soluble amino acid (AA), micronutrients, polysaccharides, phytohormones and other signaling molecules in microalgae biomass. In addition, their phototrophic nature, high photosynthetic efficiency, and wide environmental adaptability make them an attractive source of biostimulants, biofertilizers and biopesticides. The present review aims to describe the various plant growth promoting metabolites produced by microalgae and their effects on plant growth and productivity. Further, the effects elicited by microalgae biostimulants with respect to different modes of applications such as seed treatments, foliar spray and soil/root drenching is reviewed in detail. In addition, the ability of microalgae metabolites to impart tolerance against various abiotic and biotic stressors along with the mechanism of action is discussed in this paper. Although the use of microalgae based biofertilizers and biostimulants is gaining popularity, the high nutrient and water requirements and energy intensive downstream processes makes microalgae based technology commercially unsustainable. Addressing this challenge, we propose a circular economy model of microalgae mediated bioremediation coupled with biorefinery approaches of generating high value metabolites along with biofertilizer applications. We discuss and review new trends in enhancing the sustainability of microalgae biomass production by co-cultivation of algae with hydroponics and utilization of agriculture effluents.