Current Status of the Microalgae Application in Agriculture and Aquaculture

Abstract

Microalgae are organisms with great potential for the use of goods and services in areas of social and commercial interest. The application of bioproducts of microalgal origin for the purpose of feed in aquaculture and agriculture directly influences the generation or mitigation of environmental impacts. Its use in the treatment of wastewater allows the reduction of nutrients such as nitrogen, phosphorus, and CO₂, providing a system that acts in the reuse and use of water resources, generating the return of cleaner water to bodies of water and acting in the reduction of the gases greenhouse effect. Microalgae biomass is presented as an alternative to generating a wide variety of value-added products that, in recent years, given its biotechnological potential, have been gaining ground in agribusiness. This document aims to show the application and current perspectives for obtaining biomass of microalgae from crops in wastewater that can be used as raw material for the production of biofertilizers, biostimulants, feed additives that encourage reuse, transformation, recovery, and savings of resources to promote bioeconomic and circular economy processes. Generate strategies to develop practices in the agricultural sector with high productivity, quality, and efficiency, which in turn can be sustainable, friendly, and provide economic advantages, part of the need to use bioresources and implement circular bioeconomy concepts.

1. Introduction

With the arrival of the Green Revolution, in the middle of the twentieth-century, the development of techniques in the production of agricultural businesses intensified [1]. Basically, the transformations were perceived in using modern machinery and in developing chemical inputs and fertilizers. In other words, the agrarian transformation has positively impacted the increase in food production, agricultural land, irrigation, and soils, as well as helped to rapidly increase the economic development of several countries [2]. At the same time, this practice has led to serious nutritional imbalances, modifying, for example, the relationship between nitrogen (N), phosphor (P), and potassium (K), salinization, groundwater pollution, and reduction of soil fertility as time passes. In addition, these products are responsible for considerable greenhouse gas emissions in addition to requiring high-energy consumption. Therefore, it is imperative to seek more sustainable alternatives to overcome these limitations [3].

Biofertilizers fit into this context as a viable, healthy, and pollution-free option, currently being considered the best substitute for chemical fertilizers. Among the various types of biofertilizers, formulations based on microorganisms, including cyanobacteria and microalgae, have been growing substantially due to their excellent ability to increase soil fertility, improve nutrient quality and increase the productivity of different crops [4]. In parallel, the use of microalgae in the biostimulants market makes it possible to reduce the number of fertilizers needed for crops and thus indirectly increase agricultural sustainability. In this case, the application of these products at a rate of 1 L/ha can reduce the need for chemical fertilizers by more than 5% in addition to promoting increments of up to 10% in the growth and rooting of several plants [5].

Briefly, microalgae play a vital role in the sustainability of our planet, mainly because it reduces the environmental impact of transforming CO2 into O2. Microalgae are rapid-growth microorganisms, that reach high cell productivity in a short time and use mainly sunlight as an energy source. They have been studied as a source for the production of a wide range of products, such as animal feed, food, pharmaceuticals, and biofuels [6]. However, microalgae have even greater potential to boost the global bioeconomy, expanding its current production capacity and exploring new applications. These emerging applications include the production of agro-industrial products such as biofertilizers, biostimulants, biopesticides, and aquaculture. In the latter, progress has been made with new formulations based on microalgae and vegetable oils, significantly reducing the dependence on marine ingredients [7], and in turn, showing positive effects for the growth of salmonids and marine species that vary depending on the microalgal species used [8, 9].

In addition, they make the cultivation of food more sustainable when collaborating with the treatment of wastewater and its efficient reuse in aquaculture farms, soil irrigation, and plantations [5].

By exploiting their full potential, we can reap even more benefits from these microscopic organisms, promoting a more sustainable and ecologically balanced future as well as promoting economic success. This occurs mainly because microalgae are able to produce different inputs from the recovery of nutrients and significant biological material derived from different organic residues. In fact, the use of biofertilizers from waste recycling is currently considered the basis of sustainable agriculture, since it adopts modern recycling techniques, reduces the environmental impact, and reduces the carbon footprint, in addition to avoiding the high costs associated with landfills [10].

Although the use of these biological alternatives is still relatively low compared to the use of synthetic products, representing no more than 5% of the world market, the growing practice of organic aquaculture and agriculture, the need to improve water and soil organic matter, and a regulatory framework favorable are the main factors driving this market [11]. Furthermore, with the advent of integrated pest and other disease management programs that promote the use of biological inputs, the aquaculture and agriculture sector is expected to see an increase in the use of microalgae-based processes and products in the future really close.

2. Sources of organic waste

In recent years, organic waste has been studied with great frequency due to its serious disposal problems and low efficiency of use. Among these residues are some more complex ones, such as agricultural and forest residues, and animal, industrial, and municipal residues, due to their great source of pollution, pathogens, odors, and increase of greenhouse gases [12].

It is estimated that rice, wheat, and corn straw residues (the most produced in the agricultural sector) reach more than 700, 350, and 200 million tons (Mt) per year, respectively. In addition, the global production of wood waste can exceed 4 Gt per year, of which more than 20% is caused by loss of production [13]. Most animal waste is generated from the fishing, poultry, and livestock industries, for example, due to their high production and disposal of inedible or unsuitable portions for human consumption. The methane gas (CH4) emitted by cattle (due to the enteric fermentation that occurs in the animals’ digestive process) is also one of those responsible for the greenhouse effect, which, intensified, generates global warming [14].

As for industrial waste, sewage sludge contains various hazardous materials, pathogens, heavy metals, and difficult-to-treat microplastics. Other large residual volumes come from the most diverse sectors, such as the pharmaceutical industry, chemical industry, and fermentation products. The characteristics of industrial waste are high chemical oxygen demand (COD), biological oxygen demand (BOD), and strong smell resulting from the presence of organic fillers and volatile compounds [15].

Over the years, the growing generation of municipal waste and established disposal practices, combined with the still high cost of storage, have resulted in increasing volumes of accumulated waste and, historically, in serious environmental and public health problems. Irregular disposal has caused contamination of soils, watercourses, and groundwater, and also caused many diseases in the population. The high number of nutrients and organic materials produced by, for example, cleaning products and human waste are said to generate large residual loads in municipal treatment plants, yielding around 0.10 tons/capita/year [16]. In addition, the composition of urban waste is very heterogeneous, in which gravimetric analyses reveal a significant frequency of the fraction composed of organic materials (food remains, pruning, and other putrescible), representing on average more than 50% of the total waste collected.

2.1 Integrated organic waste management

The management of organic waste prioritizes the reduction, reuse, recycling, and environmentally appropriate final disposal of waste. Another essential concept is that it is the responsibility of all of society to participate in the management of waste and the life cycle of products, as opposed to the linear model of “production-consumption-disposal” [17]. Thus, all generators, individually and collectively, in addition to those who act directly or indirectly at any stage of the product’s life cycle, are responsible for waste management, obviously considering the specificity of each one in the production chain. This topic has been discussed in the corporate environment for some years, especially after the environmental pillar gained even more prominence on the world stage because of the ESG (Environmental, Social, and Governance). Therefore, it is not possible to ignore reality, since the global trend has given more value to sustainable practices [18].

Composting and biodigestion (with or without energy conversion) are the most recommended technologies worldwide for recycling organic waste [19]. However, the World Economic Forum has pointed to the “circular economy” as a model that makes it possible to reintroduce waste into the production chain in order to reduce pressure on natural resources. This new paradigm leads to a change in the very concept of waste, which is now considered a resource, since, in large part, it can be reused. This enormous potential is already recognized by several countries, whose organic waste management is intrinsically linked to the local economy, promoting income, employment, and mitigating environmental impacts [20].

For a reuse system to be implemented, the treatment system must be complemented with operations that transform the substances into a material that can be used in the production process, as a raw material [21]. This guarantees the quality of the treated waste and its suitability for its intended use.

One way of treating industrial waste is the installation of an effluent treatment plant, where the waste can pass through the physical, chemical, or biological phase, as shown in Figure 1. In the first, the pollutants are removed, and the solid and liquid phases are separated. Already the biological treatment is carried out using bacteria and other microorganisms. These consume polluting organic matter through the respiratory process [22].

Figure 1.

Conventional wastewater treatment processes.

Briefly, conventional physical methods include screens, sedimentation, flotation, decantation, and filtration to remove suspended sediments and floating solids. The conventional chemical and physical-chemical treatment methods include coagulation-flocculation, adsorption, chemical oxidation, chemical precipitation, ion exchange, and electrochemical. After this phase, the processes become more intensive and are based on the conversion of organic materials by microbial decomposition. Here, many types of settings are presented. However, the activated sludge treatment process stands out for its low operating cost. In fact, this method consists of physically, mechanically, chemically, and biologically stimulating the aerobic biota of raw effluents [4].

Although there are necessary technologies to reduce the disposal of most of the waste generated, the high costs, poor sustainability, and lack of greater integration in the management of this waste have been pointed out by specialists as the reasons why it is still a global challenge. Thus, waste treatment based on the stimulation and enhancement of nutrients with the parallel use of its products, betting on a circular economy, has been gaining prominence.

3. Nutrients recovery of organic wastes

In production systems, not all raw materials are converted into products, as chemical reactions produce many different substances than those initially intended. For example, in agriculture, it is very common to apply substances from the petrochemical industry. Chemical fertilizers are commonly used to provide crops with the necessary nutrients, such as nitrogen and phosphorus, for their growth and the production of food. However, when these nutrients are not fully utilized, they can be lost from agricultural fields. Leaching is an unwanted process, in which soil nutrients are transported to depths beyond those occupied by plant roots, which can bring economic and environmental damage to the farmer [23].

Eutrophication is a process observed in different water bodies and characterized by an increase in nutrients and excess substances used and/or lost in nature. This process leads to hypoxia and anoxia, reduced water quality, altered food web structure, habitat degradation, loss of biodiversity, and production of harmful algae [24]. Chemical fertilization, in addition to contaminating soil and water, releases nitrogen-based gaseous compounds into the air, such as ammonia and nitrogen oxides. Thus, the amount of oxygen decreases, which causes the death of various species [23].

The high cost of products and the environmental problems generated by chemical fertilizers and inputs have stimulated approaches that emphasize integrated management strategies in production systems. Bioremediation processes have been proposed as an environmentally correct alternative for the remediation of residues from water bodies and agricultural environments [25]. Within this context, microalgae have been of great interest to many scientific researchers, as they assimilate considerable amounts of residual nutrients (COD, N, and P) with the ability to convert them into many products of commercial interest, resulting in economic gain and reduction of harmful substances [5].

Basically, as shown in Figure 2, the nutrients contained in the residues are assimilated in the microalgae cells for their growth, consuming mainly inorganic CO₂ as a carbon source. This CO₂ is first absorbed by water forming carbonic acid, dissociating into bicarbonate and hydrogen ions [22]. Microalgae produce biomass and release O2 which is then assimilated by bacteria to oxidize organic compounds into inorganic ones. Subsequently, through nitrification, the microalgae consume the oxygen released in their metabolic activity, reducing cultivation costs, while the phosphate macronutrients are integrated into the energy supply by phosphorylation. NADP+ and adenosine diphosphate (ADP) are incorporated into NADPH and adenosine triphosphate (ATP), regulating, respectively, the metabolic pathways of microalgae [26].

Figure 2.

Simplified representation of the microalgae biological phenomena during the treatment of wastewater.

Thus, both microalgae and bacteria can assimilate considerable amounts of nutrients from waste and incorporate them into their biomass. These microorganisms are capable of surviving in extreme environmental conditions and due to their distinct structural, physiological, and morphological characteristics are reflected in a high degree of processes involving their use. The biotechnological potential of microalgae is mainly due to their ability to synthesize a wide range of products such as food, pharmaceuticals, biofuels, bioenergy, animal feed, and agricultural products [6]. In addition, the cultivation of microalgae in wastewater is considered an effective tool, as it is natural and inexpensive, for the assimilation of nutrients and other contaminating compounds from the environment, and can therefore be used as an alternative for the tertiary treatment of wastewater [25].

The production of microalgae involves the stages of cultivation, biomass recovery, and obtaining products. The recovery stage is a production bottleneck, representing a high cost for the process. In addition, some separation systems compromise the cellular integrity of microalgae. Among the possible biomass concentration methods are sedimentation, centrifugation, flotation, flocculation, conventional filtration, and membrane separation methods, such as microfiltration, ultrafiltration, and even direct osmosis [22]. However, despite the abundance of studies on biomass harvesting, there is still no common consensus on the most appropriate and economically viable methodology for obtaining microalgae bioproducts [27].

In this context, despite the steps and types of systems used, microalgae-based processes that aim to reduce pollutants associated with biomass production as inputs of interest to agribusiness are booming markets. Today, many companies are focusing on microalgae raw materials as an agricultural commodity and replacing traditional aquaculture production methods. Thus, its applicability in these two fields of study will be discussed below.

4. Overview of aquaculture systems

Aquaculture production has become increasingly important and is growing rapidly in terms of providing a dietary source of protein for human consumption. With the increasing world population, the demand for aquaculture is high and it is estimated that the industry is expected to grow at a rate of more than 7% within 5 years, contributing more than 50% of the total weight of the aquatic animal harvest for consumption human [28, 29].

Among the most common culture units that farmers recognize in aquaculture are ponds, conduits, and aquaculture recirculation systems (RAS). Although these methods differ in terms of labor requirement, energy consumption, dependence on natural resources, simplicity, and costs, they have in common the large flow of circulating water and its indirect disposal to external sources [30]. In this context, many challenges are encountered in traditional aquaculture which is best shown in Figure 3.

Figure 3.

Challenges and solutions for aquaculture systems.

4.1 Main challenges of traditional aquaculture

Water quality is associated with the productivity and health of aquatic animals, as there are ideal development conditions that vary according to the cultivated species. Among the main factors to be monitored, the chemical demand for dissolved oxygen and excreted metabolic waste are the main problems encountered in the ecosystem [31]. Ocean and river acidification, associated with elevated carbon dioxide content, harms calcifying organisms such as mollusks and crustaceans [24]. In fact, the accumulation of particulate organic matter that accelerates microbial activity and the consumption of dissolved oxygen in culture waters increases morbidity in aquaculture. On the other hand, oxygen depletion accelerates the production of various toxins, which not only threaten the survival of aquatic species but also disturb the ecological balance of nature [32].

Aquaculture wastewater contains organic matter in the form of dissolved nutrients or suspended solids. Many studies reveal that in addition to organic matter from feed waste, excrement from aquatic animals can be considered the main factor contributing to eutrophication in aquaculture [33]. Nutrient enrichment in waterways due to excess feed used on aquaculture farms, to increase growth rates, exceeds average levels of nitrogen, organic matter, and phosphorus. Thus, eutrophication leads to harmful algal blooms, increased frequency of anoxic events, and fish death. These conditions lead to health implications, economic losses, and even ecological disasters if discarded without treatment [24].

Water scarcity and depletion of natural water resources are significant challenges facing the world today and particularly represent a major problem in the aquaculture industry. In the state of the art, the safety of aquatic organisms is closely linked to good quality water resources in order to guarantee the efficient supply of nutrients for the survival of diverse species [31]. This requires sustainable innovative practices as a possible way to solve the problems faced in the field of traditional aquaculture.

4.2 Microalgae-assisted aquaculture

Arguably worldwide, the most important application (in volume) of microalgae is in the production of fish, crustaceans, mollusks, seaweed, and others. Since, in natural environments, microalgae are the basis of a large part of the reproductive chains, larviculture, and growth, their high production becomes necessary, given the high nutritional value of its biomass [34].

In aquaculture, high loads of water used for production are usually discharged into the environment during water changes, as well as during harvesting [30]. Nutrients dissolved in aquaculture wastewater are mainly from unused feed and aquatic animal secretions. Therefore, in terms of nutrient supply, aquaculture wastewater is a promising alternative medium for microalgae cultivation.

Previous studies have shown that some species of microalgae grow well in aquaculture wastewater to sufficiently remove most of the contained nutrients. Ansari et al. [35] reported the use of different species of microalgae (Chlorella sorokiniana, Scenedesmus obliquus, and Ankistrodesmus falcatus) to treat aquaculture wastewater, in which they achieved removal efficiencies between 75.5 and 98.5% of nitrate (NO₃) and phosphorus in 15 days of cultivation. Likewise, high nitrate, carbon dissolved, and phosphorus removal efficiencies were found for Parachlorella kessleri at the inoculation concentration of only 0.1 g/L [36]. These same authors report NO3 removal efficiency (85.7–97.1%) and total phosphorus removal efficiency between 90.2–98.9% for Chlorella vulgaris, Scenedesmus quadricauda, Chlorococcum sp., and Scenedesmus obliquus. In addition to removal efficiencies, the use of microalgae biomass as a supplement for aquatic animals has grown substantially, as at the same time that it increases their growth, it decreases feed consumption, thus consequently lowering production costs. This type of diet increases the nutritional value and disease resistance of most fish and crustaceans [33].

Briefly, the concept of microalgae-assisted aquaculture involves different stages: (i) Microalgae are inoculated in ponds or aquatic systems. Here, they digest part of the organic waste secreted by the animals, improving the self-purification of the aquaculture system; (ii) The effluent water from the systems is used to cultivate microalgae and thus absorb the nutrients contained therein in separate tanks using various methods; (iii) The microalgae are then harvested in a cost-effective and environmentally friendly way; (iv) Freshly harvested biomass will be used as value-added feed and food, thus reducing the cost of rearing and production in aquaculture, and (v) treated effluent is recycled for use again in aquaculture systems [37].

Among the various microalgae cultivation systems developed, the Raceway system and the Rotating Algal Biofilm (RAB) are the two main systems used in microalgae-assisted aquaculture. The first involves pond systems in a circulation channel from suspended cell culture, reaching average productivity between 1 and 10 g/L for raceway tanks and photobioreactors, respectively [38]. Despite the low operational and investment costs due to its simplicity, the low cell concentration makes this type of system unfeasible, since the demand for time increases energy consumption and the harvesting and dehydration processes become more expensive (Ahmad et al., 2021). Thus, the microalgae cultivation system by biofilm becomes promising because it involves the growth and production of microalgae directly adhered to the surface of a support, in which its biomass is simply obtained by scraping. Thus, the cost of the harvesting process is practically non-existent [37].

Although the integration of microalgae in aquaculture systems alongside the treatment of effluents results in several technical, sustainable, and economic advantages, there are still several challenges that must be faced mainly in the use of microalgae biomass as an aquatic feeding medium [31]. It is mentioned that the cellulose-rich cell walls of microalgae make it difficult for aquatic animals to digest and therefore their structures must first be broken down for nutrients to be efficiently absorbed in their organism. The antinutrient effects and the bioaccumulation of heavy metals in microalgae from the treatment of highly polluting waste can also negatively affect the environment in which they live [39]. Thus, new technologies are being studied to eliminate these disadvantages and allow better assimilation of microalgae biomass as value-added products in aquaculture in the coming years. The main advantages and disadvantages of microalgae-assisted aquaculture are presented below (Table 1).

Advantages	Disadvantages
Wastewater remediation	Energy consumption
Water replacement	Deficiency in digestibility
Recycling and water quality	Anti-nutritional factors
High-value food	Bioaccumulation of heavy metals
Healthier diets	Harvest and dehydration expenses
Reduced feed costs
Prevention of oxygen depletion
Prevention of toxic microorganisms

Table 1.

Main advantages and disadvantages of microalgae-assisted aquaculture.

5. Microalgae for sustainable agricultural practices

The use of microalgae in modern agriculture has aroused great interest due to the wide range of benefits they can provide, both in increasing yields and crop health, while preserving the environmental balance [40]. Although several products can be used to improve crop productivity, microalgae biomass has high levels of nutrients that provide beneficial effects to plants. It is important to point out that different biological compounds can improve agricultural productivity through different modes of action and applicability, as shown in Figure 4. For example, microalgae contain plant hormones that function as plant growth promoters and at the same time protect against pests and pathogens. The concomitant assimilation of wastewater treatment, as previously discussed, also brings many benefits, mainly by keeping soils and water bodies free of contamination and consequently promoting the circular bioeconomy, by using its biomass as an organic matter [41].

Figure 4.

Main applications of microalgae-based products in agriculture.

Biofertilizers are biologically based compounds that promote agricultural processing, improving the availability of nutrients in the environment [42]. In addition to beneficial microorganisms, it is possible to find in biofertilizers various nutrients and plant hormones that, once applied to the soil, colonize the rhizosphere and even the interior of plants. Biostimulants are capable of improving the photosynthetic metabolism of plants due to respiratory control, ion absorption, and nucleic acid synthesis. Biopesticides act as biological controllers of pathogenic fungi and bacteria due to their antimicrobial, antioxidant, antiviral, and antifungal functions [40].

Although still under study and development, microalgae-based biofertilizers have proven to be a viable alternative to traditional, costly, and unsustainable agricultural techniques. Among the main benefits that biofertilizers offer to plants and the surrounding environment are: (i) an increase in organic matter in the soil; (ii) improvement in soil porosity; (iii) increased supply of oxygen in the rhizosphere; (iv) increase in soil water retention; (v) release of plant growth promoting substances; (vi) improvement in salinity and control of soil acidity; (vii) prevention of weed growth, and (viii) increased availability of phosphates for plants [38]. These and other benefits are further presented below.

5.1 Fertility and soil quality

Cropland (i.e. plantation and rangeland), which occupies over 40% of Earth’s land, is a major target of soil-based carbon sequestration. Scientists have estimated that soils – particularly agricultural soils – can sequester over an additional billion tons of carbon each year [43]. However, the conversion of natural ecosystems to agricultural land disrupts soil structure, releasing much of this stored carbon and contributing significantly to climate change.

The soils are increasingly impoverished due to extensive agricultural practices carried out in recent years. The excessive use of chemical defenders, nitrogen fertilizers, and the frequent use of heavy machinery alter the soil structure, resulting in disturbances, erosion, nutrient depletion, and difficulties in water maintenance [44].

From the point of view of health and quality, microalgae-based biofertilizers can increase the stock of organic carbon in soil through the excretion of carbon (exopolysaccharides) that are previously absorbed by the metabolism of microalgae through photosynthesis [45]. Reports have evaluated the potential of various types of biofertilizers, and the results have shown that compared to the control, microalgae-based biofertilizers are an advance for and renewal of organic carbon stocks in soils and negative biological emission of gases [43].

Considering the biological nitrogen fixation capacity, it is possible to conclude that microalgae can replace chemical nitrogen fertilizers, benefiting the economy in agricultural production [41]. In fact, microalgae have an advanced organism that fixes nitrogen gas by a cell called heterocyst, which creates a micro anaerobic environment where the enzyme nitrogenase can work smoothly. Even though it is the main constituent of our atmosphere (more than 75%), gaseous nitrogen (N₂) is not assimilable by most organisms, therefore, microalgae do not compete with plants for nitrogen demand, which ends up helping soil availability [46].

In the process of biological fixation, fixing microorganisms convert nitrogen into ammonia. Once fixed, one of the main mechanisms for transferring nitrogen from microalgae to plants is through the mineralization of the biomass after its death [47]. As the amount of ammonium salt is low in the soil, this fixation becomes extremely important for the maintenance of the environmental nitrogen cycle.

Other elements such as phosphorus, calcium, magnesium, zinc, iron, and potassium can also be found in microalgae biomass. These elements are typically involved in redox reactions, playing an important role in plant metabolism and growth [41].

5.2 Promotion of plant growth, disease, and pest control

As the environment changes and the crop passes through different stages of development, the ability to stimulate specific and natural responses in the plantation can make a significant contribution to achieving the goals of growth and integrated plantation management. Biostimulants, which have a very definite result, can provide the farmer with a balanced, protected, and proactively prepared plantation for all stages of development [41].

Biostimulants or phytohormones produced from microalgae contain complex bioactive molecules that have varied functionalities depending on their growth conditions [48]. Among the main intracellular hormones of microalgae, cytokinins are widely studied molecules, however, there are other substances with the ability to promote plant growth and development, such as auxin, polyamines, and jasmonic acid. These are excreted from microalgae for in vitro growth and regeneration of valuable plants and this interaction occurs through the interaction between microalgae biomass and plant roots [15].

Although with development, practices characterized by successive planting have made the agriculture industry increasingly susceptible to disease and pests due to the plentiful availability of food, breeding sites, and chemical pesticide residues that are not easily decomposed, and thus remain in the soil as waste toxic [49]. Given this situation, microalgae can be identified as articulators of defense mechanisms in plants, such as an increase in RNA, and also in enzymatic activities described as antifungal, insecticides, nematicides, and herbicides [44].

Finally, microalgae biomass has many benefits when compared to traditional synthetic agricultural media. Many studies have already been reported in the literature and mainly involve the use of microalgae-based biofertilizers. Its main effects can be better observed in Table 2.

Strain	Target Crop/Soil	Benefits	Reference
Anabaena sp.	Rice	Improvement of iron and copper deficiency	[44]
Anabaena variabilis	Rice	Improves phosphate bioabsorption	[44]
Anabaena torulosa	Wheat	Increase in nitrogen availability in the soil	[50]
Chlorella sp.	Corn	Increased leaf length and weight	[51]
Chlorella sp.	Maize	Increased soil organic carbon	[52]
Chlorella vulgaris	Okra	Increased germination and yield; Faster maturity	[53]
Dunaliella salina	Tomato	Increase in carotenoids; Better enzymatic activities of plants	[54]
Dunaliella salina	Wheat	Increase in tolerance to salt stress	[55]
Nannochloropsis oculata	Tomato	Increase of N, P in the leaves; Increase in sugars/carotenoids in fruits	[56]
Nannochloropsis oculata	Tomato	Increase in ammonium, phosphorus, and potassium in soil; Increase in sugar and carotenoids contents in fruit	[56]
Nostoc sp.	Rice	Increase in grain yields comparable to those obtained with chemical fertilizers	[57]
Oscillatoria augustissima	Pea	Reduces the use of chemical fertilizers by 50%; Improved nutritional value of pea seeds	[58]
Scenedesmus quadricauda	Pea	Increased protein, chlorophyll, and carotenoid content; Better enzymatic activities in the leaves	[59]
Scenedesmus sp.	Lettuce	Inhibition of fungal growth	[60]
Spirulina platensis	Corn	Increased grain yield	[61]

Table 2.

Benefits of using microalgae-based products in agriculture.

6. Conclusions

The need to adopt sustainable practices in agribusiness is increasingly evident worldwide. Many producers still fear that ecological methods do not meet their production needs or are unaware of their benefits. The current consensus is that we need to urgently transition from a petrol-based market to a sustainable circulating bioeconomy.

Reducing the effects of chemical products, including traditional fertilizers, was already a goal set by the UN (United Nations) over 20 years ago. However, the main target imposed has not yet been reached. More than ever, reducing the use of chemical inputs is a more than immediate need to preserve the integrity of the environment and public health.

Microalgae have long been considered a rich source of value-added products and an attractive business opportunity. The agriculture and aquaculture sectors have been important, as microalgae biomass has shown remarkable results in improving aquafeed, soil quality, and crop development, growth, and yield.

To date, microalgae systems and processes have been well explored. In contrast, microalgae bioproducts, although known to have positive effects, their commercial implementation is limited by high-energy demand and production costs. In this sense, the treatment of organic waste using microalgae is a valuable strategy, as in addition to obtaining environmental benefits (such as cleaning watercourses, reduction of degrading compounds, and greenhouse gas emissions), there is a parallel advantage in the economy through the recovery of nutrients and conversion of its biomass for use in several areas of application.

Finally, the challenges related to microalgae processes associated with organic waste still need extensive field research, further evaluation, and, above all, increasing awareness of its benefits in the areas of social interest and the global economy. Thus, bearing in mind that biologically based products can improve the quality of life on Earth, we can be sure that the commercialization and production of microalgae-based products will be an imminent success in the future.

Acknowledgments

To the R&D Department of the Technological University of Uruguay. Josh 1:9.