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

Soil fertilization with microalgae biomass from municipal wastewater treatment causes no additional leaching of dissolved macronutrients and trace elements in a column experiment

Microalgae are a promising bio-fertilizer that can be cultivated in municipal wastewater, where the organisms perform water purification by incorporation of nutrients and contaminants. Before bio-fertilization with wastewater-grown microalgae can be put into practice, its impact on the leaching of macronutrients and trace elements needs to be evaluated. Here, we studied the leaching behavior of a microalgae-fertilized soil against a control in column percolation setup. Microalgae were grown in real municipal wastewater supplemented with bromide for the analysis of within-cell Br- accumulation by time-of-flight secondary ion mass spectrometry. Dry biomass (45.0 g N kg-1 and 28.9 g P kg-1) was added to the topmost layer of the fertilized column at a level of 3 g biomass kg-1 on a whole soil basis. Column irrigation was equivalent to 3 years of precipitation in central Germany. The leaching of macronutrients and trace elements from the fertilized and control columns was largely identical. Except for P, depth profiles confirmed very low vertical translocation within the soil. This is held for total element contents as well as for operationally defined pools, suggesting that microalgae cultivated in municipal wastewater provide a slow-release fertilizer largely resistant to leaching. Mass spectrometric imaging gave clear evidence for bromide uptake by the microalgae, and pure cultures of the genus Scenedesmus showed that it was preferentially located in the cell membrane. Therefore, bromide could potentially be employed as a mineralization tracer in future studies on the use of microalgae as a bio-fertilizer.

Resource recovery and contaminants of emerging concern mitigation by microalgae treating wastewater

This study aimed to investigate the recovery of agricultural biostimulants and biogas from microalgae treating wastewater, in the framework of a circular bioeconomy. To this end, municipal wastewater was treated in demonstrative raceway ponds, and microalgal biomass (Scenedesmus sp.) was then harvested and downstream processed to recover biostimulants and biogas in a biorefinery approach. The effect of microalgal biostimulants on plants was evaluated by means of bioassays, while the biogas produced was quantified in biochemical methane potential (BMP) tests. Furthermore, the fate of contaminants of emerging concern (CECs) over the process was also assessed. Bioassays confirmed the biostimulant effect of microalgae, which showed gibberellin-, auxin- and cytokinin-like activity in watercress seed germination, mung bean rooting, and wheat leaf chlorophyll retention. In addition, the downstream process applied to raw biomass acted as a pre-treatment to enhance anaerobic digestion performance. After biostimulant extraction, the residual biomass represented 91% of the methane yield from the raw biomass (276 mLCH4·g-1VS). The kinetic profile of the residual biomass was 43% higher than that of the unprocessed biomass. Co-digestion with primary sludge further increased biogas production by 24%. Finally, the concentration of CECs in wastewater was reduced by more than 80%, and only 6 out of 22 CECs analyzed were present in the biostimulant obtained. Most importantly, the concentration of those contaminants was lower than in biosolids that are commonly used in agriculture, ensuring environmental safety.

Sequential two-stage cultivation system using novel microalga consortia for treatment of municipal wastewater and simultaneous biomass production: Sustainable environmental management

Monoculture-based microalgae cultivation systems to treat wastewater are well-reported. Despite that, this method has some limitations in terms of nutrient removal potential, environment adaptation, and low biomass productivity. Conversely, microalgae co-cultivation and a two-stage sequential cultivation system (TSSCS) recently emerged as a promising approach to improve the treatment process and biomass productivity through better adaptation to the environment. However, no outdoor large-scale experiments were reported using this approach which hinders the viability of the process. Thus, in the present study, a sequential two-stage large-scale outdoor novel microalgae consortia experiment was developed. In first stage consortia-assisted sequential cultivation, two ratios of Tetraselmis indica (TS) and one ratio of Picochlorum sp. (PC) (2 TS:1 PC) were cultivated in a 1000-L pond containing 75%-municipal wastewater (MWW) + 25%-ASN-III, while in the second stage, 2 PC:1 TS was cultivated in two different ponds, and each containing 375-L 2 TS:1 PC-treated water + 375-L ASN-III. Outdoor parameters and nutrient removal efficiency (NRE), biomass, and biomolecule productivity such as lipid, photosynthetic pigments, astaxanthin, and β-carotene were quantified, and cost analysis was performed. At the end of the first and second stages, 2 TS:1 PC and 2 PC:1 TS showed maximum NRE of COD (68.71 and 86.40%), TN (66.98 and 94.73%), and TP (82.70 and 94.36%), respectively. Moreover, 2 TS:1 PC and 2 PC:1 TS Pond 1 and 2 produced maximum dry biomass production; 2.41 and ∼2.54 g/L contained lipid content; 36.89 and 34.90% that have 86.50 and 55.79% FAME content respectively. Similarly, 2 TS:1 PC and 2 PC:1 TS biomass exhibited valuable pigments production of astaxanthin i.e., 0.56 and 0.35 mg/g, and β-carotene; 4.65 and 2.82 mg/g, respectively. The cost analysis suggested that only microalgal-based MWW treatment was unfeasible, while valorization of produced biomass into co-products could offset the operation costs and could allow the option for the microalgal-based sustainable approach for the treatment of MWW and recovery of valuable resources.

The chemical and microbiological safety of emerging alternative protein sources and derived analogues: A review

Climate change and changing consumer demand are the main factors driving the protein transition. This shift toward more sustainable protein sources as alternatives to animal proteins is also reflected in the rapid upscaling of meat and dairy food analogues. Such changes could challenge food safety, as new food sources could result in new and unexpected food safety risks for consumers. This review analyzed the current knowledge on chemical and microbiological contamination of emerging alternative protein sources of plant origin, including soil-based (faba bean, mung bean, lentils, black gram, cowpea, quinoa, hemp, and leaf proteins) and aquatic-based (microalgae and duckweeds) proteins. Moreover, findings on commercial analogues from known alternative protein sources were included. Overall, the main focus of the investigations is on the European context. The review aimed to enable foresight approaches to food safety concerning the protein transition. The results indicated the occurrence of multiple chemical and microbiological hazards either in the raw materials that are the protein sources and eventually in the analogues. Moreover, current European legislation on maximum limits does not address most of the "contaminant-food" pairs identified, and no legislative framework has been developed for analogues. Results of this study provide stakeholders with a more comprehensive understanding of the chemical and microbiological safety of alternative protein sources and derived analogues to enable a holistic and safe approach to the protein transition.

A Comprehensive Review of Microalgae and Cyanobacteria-Based Biostimulants for Agriculture Uses

Significant progress has been achieved in the use of biostimulants in sustainable agricultural practices. These new products can improve plant growth, nutrient uptake, crop yield and quality, stress adaptation and soil fertility, while reducing agriculture's environmental footprint. Although it is an emerging market, the biostimulant sector is very promising, hence the increasing attention of the scientific community and agro-industry stakeholders in finding new sources of plant biostimulants. Recently, pro- and eucaryotic microalgae have gained prominence and can be exploited as biostimulants due to their ability to produce high-value-added metabolites. Several works revealed the potential of microalgae- and cyanobacteria-based biostimulants (MCBs) as plant growth promoters and stress alleviators, as well as encouraging results pointing out that their use can address current and future agricultural challenges. In contrast to macroalgae biostimulants, the targeted applications of MBs in agriculture are still in their earlier stages and their commercial implementation is constrained by the lack of research and cost of production. The purpose of this paper is to provide a comprehensive overview on the use of this promising new category of plant biostimulants in agriculture and to highlight the current knowledge on their application prospects. Based on the prevailing state of the art, we aimed to roadmap MCB formulations from microalgae and cyanobacteria strain selection, algal biomass production, extraction techniques and application type to product commercialization and farmer and consumer acceptance. Moreover, we provide examples of successful trials demonstrating the beneficial applications of microalgal biostimulants as well as point out bottlenecks and constraints regarding their successful commercialization and input in sustainable agricultural practices.

Microalgae harvesting for wastewater treatment and resources recovery: A review

Microalgae-based wastewater treatment has been conceived to obtain reclaimed water and produce microalgal biomass for bio-based products and biofuels generation. However, microalgal biomass harvesting is challenging and expensive, hence one of the main bottlenecks for full-scale implementation. Finding an integrated approach that covers concepts of engineering, green chemistry and the application of microbial anabolism driven towards the harvesting processes, is mandatory for the widespread establishment of full-scale microalgae wastewater treatment plants. By using nature-based substances and applying concepts of chemical functionalization in already established harvesting methods, the costs of harvesting processes could be reduced while preventing microalgae biomass contamination. Moreover, microalgae produced during wastewater treatment have unique culture characteristics, such as the consortia, which are primarily composed of microalgae and bacteria, that should be accounted for prior to downstream processing. The aim of this review is to examine recent advances in microalgal biomass harvesting and recovery in wastewater treatment systems, considering the impact of consortia variability. The costs of available harvesting technologies, such as coagulation/flocculation, coupled to sedimentation and differential air flotation, are provided. Additionally, promising technologies are discussed, including autoflocculation, bioflocculation, new filtration materials, nanotechnology, microfluidic and magnetic methods.

Enhancing sustainability through microalgae cultivation in urban wastewater for biostimulant production and nutrient recovery

Microalgae can produce biostimulants in form of phytohormones, which are compounds that, even if applied in low concentrations, can have stimulant effects on plants growth and can enhance their quality and their resistance to stress. Considering that microalgal biomass can grow recovering nutrients from wastewater, this circular approach allows to use residues for the production of high added value compounds (such as phytohormones) at low cost. The interest on biostimulants production from microalgae have recently raised. Scientists are focused on the direct application of these cellular extracts on plants, while the number of studies on the identification of bioactive molecules, such as phytohormones, is very scarce. Two cyanobacteria strains (Synechocystis sp. (SY) and Phormidium sp. (PH)) and a chlorophyte (Scenedesmus sp. (SC)) were cultured in laboratory-scale PBRs with a working volume of 2.5 L in secondary urban wastewater varying N:P ratio in the cultures to obtain the highest productivity. The variation of N:P ratio affects microalgae growth, and SY and PH presented higher productivities (73 and 48 mg L-1 d, respectively) under higher N:P ratio (> 22:1). Microalgal biomass was freeze-dried and phytohormones content was measured with ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The three microalgae showed similar phytohormones profiles, being the auxin (indole-3-acetic acid, IAA) the most abundant (72 ng g-1DW in SY). Proteins were major macronutrient for all strains, reaching 48 %DW in PH culture. To optimize the biostimulants production, a balance between the production of such compounds, biomass productivity and nutrients removal should be taken into consideration. In this sense, SC was the most promising strain, showing the highest N and P removal rates (73 % and 59 %, respectively) while producing phytohormones.

Overview of microalgae and cyanobacteria-based biostimulants produced from wastewater and CO2 streams towards sustainable agriculture: A review

For a long time, marine macroalgae (seaweeds) have been used to produce commercial biostimulants in order to ensure both productivity and quality of agricultural crops under abiotic stress. With similar biological properties, microalgae have slowly attracted the scientific community and the biostimulant industry, in particular because of their ability to be cultivated on non-arable lands with high biomass productivity all year long. Moreover, the recent strategies of culturing these photosynthetic microorganisms using wastewater and CO2 opens the possibility to produce large quantity of biomass at moderate costs while integrating local and circular economy approaches. This paper aims to provide a state of the art review on the development of microalgae and cyanobacteria based biostimulants, focusing on the different cultivation, extraction and application techniques available in the literature. Emphasis will be placed on microalgae and cyanobacteria cultivation using liquid and gaseous effluents as well as emerging green-extraction approaches, taking in consideration the actual European regulatory framework.

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.

Phycobiliprotein recovery coupled to the tertiary treatment of wastewater in semi-continuous photobioreactors. Tracking contaminants of emerging concern

This study evaluated a tertiary wastewater treatment technology using cyanobacteria to recover value-added phycobiliproteins. The presence of contaminants of emerging concern (CECs) in wastewater, cyanobacteria biomass and pigments recovered were also analyzed. For this, a wastewater-borne cyanobacterium (Synechocystis sp. R2020) was used to treat secondary effluent from a municipal wastewater treatment plant, with and without nutrients supplementation. Then, the stability of phycobiliprotein production was assessed by operating the photobioreactor in semi-continuous mode. Results showed similar biomass productivity with and without nutrients supplementation (153.5 and 146.7 mg L^-1 d^-1, respectively). Upon semi-continuous operation, the phycobiliprotein content was stable and reached up to 74.7 mg gDW^-1. The phycocyanin purity ratio ranged from 0.5 to 0.8, corresponding to food grade (> 0.7). Out of 22 CECs detected in secondary effluent, only 3 were present in the phycobiliprotein extract. In order to identify applications, prospective research should focus on CECs removal during pigment purification.