Microalgae were the basis of life into the planet, but only recently these microorganisms are exploited at a commercial scale. Thus, the production of pharmaceuticals, cosmetics, feed, and foods from microalgae is today a commercial reality increasing year by year. Additionally, microalgae have been proposed to be used to enhance the sustainability of existing industrial activities, as wastewater treatment and biofuel production. In this way, the utilization of microalgae at a large scale is considered a green revolution in the sustainability of mankind. This chapter is focused on reviewing the real contribution of microalgae to human activities. The last improvements of technologies and its uses, in addition to still existing bottlenecks for the massive exploitation of these microorganisms, are reviewed.
- food production
Microalgae and cyanobacteria are photosynthetic microorganisms performing oxygenic photosynthesis. There are more than 30,000 species catalogued and classified; however, less than a hundred have been studied, and no more than 20 are actually under commercial exploitation . These microorganisms were responsible for larger transformations into the planet as oxygen production in addition to Fe and S oxidation, which allows the explosion of life into the planet . Moreover, these microorganisms were responsible for CO2 reduction in past ages transforming it into calcium and diatom rocks, in addition to fossil fuels that we are using now. Today microalgae and cyanobacteria are responsible for most of the solar energy capture and oxygen production into the planet. Thus, these microorganisms are today the basis of food chain in aquatic systems, thus being a fundamental pillar in the sustainability of the planet. Moreover microalgae and cyanobacteria are majorly responsible for CO2 transformation to biomass into the planet, thus also contributing to the reduction of the global warming effect .
Microalgae are capable of growing in largely different ambients, from warm areas in the tropic and deserts to cold areas in the high mountains and poles. Some of the major advantages of these microorganisms are that they do not require fertile land or usable water and they can grow in contaminated waters . These capabilities allowed humans to use microalgae and cyanobacteria for centuries. Thus,
Due to the high capacity to produce biomass and its highly interesting composition, containing proteins rich in essential amino acids, high‐value lipids and fatty acids, and valuable carbohydrates, the industrial production of microalgae attracts special attention. The first reports about the production of microalgae were published in 1950  focusing on the utilization of tubular photobioreactors for the production of
In this chapter, the major factors influencing the production of microalgae and the technologies used to produce it at a large scale are summarized. Future trends and contributions of microalgae to mankind in the next years will be also discussed to show the relevance of this “green revolution.”
2. Major factors in microalgae production
Microalgae are photosynthetic microorganisms equivalent to plants but with some differences: (i) they are micro with size ranging from 2 to 20 µm and usually grow in water bodies; (ii) they grow much faster than higher plants with duplication times lower than 1 day; (iii) they do not have roots or large structures, their photosynthetic efficiency being much higher than higher plants; and (iv) they require a supply of large amounts of nutrients, mainly CO2, N, and P, to maximize their performance. Thus, on the basis of these differences, the microalgae production systems have been developed to satisfy the requirements of microalgae cells to achieve the maximal production capacity at a lower cost.
2.1. Light availability
Light is the main factor in determining the performance of microalgal cells. For the entire solar spectrum, the photosynthetic apparatus only use the light at wavelengths from 400 to 700 nm (photosynthetically active radiation, or PAR), which is being saturated at relatively low irradiances ranging from 100 to 200 µE/m2·s. Because the solar radiation achieves values more than times higher than this saturation value, the photosynthetic apparatus can be oversaturated or photoinhibited at outdoor conditions. To solve this problem and enhance the performance of microalgae cultures, the solar radiation must be “distributed” between the larger number of cells and surface as possible. Thus, different designs of photobioreactors have been proposed. Whatever the photobioreactor design, in microalgae cultures the light impinging into the reactor surface (I0) is attenuated along the culture as a function of the path length (p), biomass concentration (Cb), and extinction coefficient of the biomass (Ka). This attenuation makes that light gradients exist, the cells being exposed to different light conditions according to the light profile and mixing . Although rigorous calculations about light profile in microalgae cultures have been performed, to approximate the average irradiance at which the cells are exposed to in whatever photobioreactor the equation proposed by Molina is really useful and comfortable (Eq. (1)) . Figure 1 shows as the higher the biomass concentration the higher are the gradients at which the cells are exposed to inside the cultures, these being higher also the higher the culture depth. Following this argument it would be recommendable to use thin‐layer reactors with low biomass concentrations to minimize the light gradients inside the cultures, but in this scenario, the production capacity is largely reduced; then an optimal solution must be found. For this, the optimal design of the reactor maximizing the light on the reactor surface (I0) while optimizing the culture depth (p) and its adequate operation to maintain the optimal biomass concentration (Cb) is the challenge:
According to the limitations of photosynthetic apparatus, it has been reported that microalgae can achieve a maximal photosynthetic efficiency (PE) of 5% from global radiation. This means that microalgae are able to accumulate up to 5000 GW/ha·year in tropic areas if production systems achieving 5% PE are operated, whereas this value reduces to 400 GW/ha·year when considering 1% PE and temperate locations with low solar radiation availability (Figure 2A). Considering the heat value of the microalgae biomass of 20 MJ/kg, this means that the amount of biomass than can be produced per unit area and year is limited by the solar radiation availability at the selected location and the photosynthetic efficiency achieved in the used production system. Figure 2B shows that biomass productivity values up to 250 t/ha·year can be obtained in tropic areas if 5% PE is achieved in used systems, whereas this productivity decreases to 20 t/ha·year in temperate areas with low solar radiation if 1% PE is achieved. These values are higher than productivities of corn (12 t/ha), wheat (8 t/ha), or soya (6 t/ha), thus showing that microalgae biomass is a realistic food alternative. Moreover, the large biomass production capacity of microalgae‐based processes does that these microorganisms were considered a real alternative to energy crops to produce biofuels . These figures confirm that although solar radiation availability is a major factor in the production of microalgae biomass, the optimization of the used system and at the end the photosynthetic efficiency achieved is also highly relevant in the final biomass production capacity.
2.2. Nutrient supply
Microalgae biomass is mainly composed of carbon (45%), nitrogen (7%), and phosphorus (1%) in addition to oxygen and hydrogen that are directly obtained from the hydrolysis of water. However, the first ones must be supplied, the required amount of these nutrients being directly proportional to the microalgae biomass capacity required. Carbon can be supplied as carbonate or bicarbonate, but the utilization of CO2 is greatly recommended because it allows at the same time to control the pH of the cultures. Stoichiometrically up to 1.8 kg of CO2 is required to produce 1.0 kg of microalgae biomass, although this value can be modified according to the precise elemental composition of produced biomass. Figure 3 shows that CO2 fixation capacity of microalgae cultures is directly a function of solar radiation availability and photosynthetic efficiency achieved in the production system. Values ranging from 190 to 450 tCO2/ha·year can be fixed in tropic areas at photosynthetic efficiencies ranging from 1 to 5%, whereas in temperate areas with low solar radiation availability, these figures reduce to values ranging from 40 to 90 tCO2/ha·year at the same photosynthetic efficiencies. Not only pure CO2 but also whatever gas containing CO2 can be used to produce microalgae; thus, it is being proposed to use flue gases from power stations, biogas from anaerobic digestion, or fermentation gas from ethanol production to produce microalgae biomass at the same time that reducing CO2 emissions of these industries . When using whatever gas to supply CO2 to microalgae cultures, two main aspects must be considered: (i) to use efficient systems capable to transfer more than 90% of CO2 contained into the flue gas to the microalgae culture and (ii) to be sure that the gas does not contain toxics that can damage the growth of microalgae cells (SOx, NOx) . Related to the supply of CO2 is the removal of oxygen because it is produced at the same rate that CO2 is consumed, it accumulating into the culture if not removed. Most of microalgae strains are inhibited by oxygen at dissolved oxygen concentration higher than 200%Sat. (c.a. 20 mg/L); thus, adequate oxygen removal systems must be installed and operated to overpass these phenomena . In general, the optimization of mass transfer capacity is a key factor in the performance of whatever microalgae production system .
In addition to carbon, nitrogen and phosphorus are the most relevant nutrients required for the production of microalgae. About 0.1 kg of N and 0.01 kg of P are required to produce 1 kg of microalgae biomass. On the basis of biomass production as a function of solar radiation and photosynthetic efficiency, Figure 3 shows that the N recovery ranges from 2 to 10 tN/ha·year in temperate climates with low solar radiation and from 5 to 25 tN/ha·year in tropic areas, when the photosynthetic efficiency modifies from 1 to 5%. In the same way, the P recovery ranges from 0.2 to 1.1 tP/ha·year in temperate climates with low solar radiation and from 0.5 to 2.5 tP/ha·year in tropic areas, when the photosynthetic efficiency modifies from 1 to 5% (Figure 3). Soluble forms of these compounds are produced at a large scale worldwide because they are pillars of the food production by agriculture. Phosphorous reservoirs are limited, and some reports are advertising about the crash of the actual food production system based on phosphorus . To transform P‐rich rocks into fertilizers, huge amount of energy is required, whereas nitrogen production systems use atmospheric nitrogen but also are consuming large amounts of energy to transform it into ammonia and nitrate by the Haber process. To avoid these problems, the recovery of nitrogen and phosphorus from wastes and residual streams is mandatory, microalgae being specialists on these processes . Thus, microalgae are capable to completely remove N and P contained in wastewater streams, only using solar energy into the process, at the same time producing valuable biomass. The development of microalgae‐based treatment processes is a key issue in this field .
2.3. Culture conditions
Microalgae as whatever other microorganisms have optimal conditions that must be known in order to maximize their performance. Optimal salinity, temperature, and pH are strain specific, and the production systems must be adequately designed/operated to maintain it at optimal values. Regarding salinity, although some microalgae can tolerate large variation of salinity, usually freshwater (i.e.,
To provide optimal culture conditions at a laboratory or small scale is quite simple although it is expensive. However, at large‐scale and outdoor conditions, to accurately control the culture conditions is simply impossible. As example to control the temperature in large reactors requires large investments and high energy usually it being disregarded, strains to be produced being selected to optimally growth at the ambient temperature prevailing in the selected location. In the case of pH, the injection of pure CO2 can summarize up to 30% of the overall biomass production cost then the utilization of flue gases or residual streams containing CO2 being recommendable . Anyway, when considering the control of culture conditions, three time scales must be considered: (i) annual basis that means the mean values of environmental conditions prevailing in the selected location, (ii) daily basis that considers the hour‐by‐hour variation of environmental conditions due to the variation of solar radiation, and (iii) mixing time that means the time to completely mix the system it influencing the existence of gradients of culture conditions along the reactor. Advanced control methods are being applied now to the industrial production of microalgae to reduce cost and improve the performance of microalgae‐based processes . Only an in‐depth analysis of main culture conditions and its optimization along the different time scales will allow to maximize the performance of whatever microalgae‐based process.
3. Photobioreactors and large‐scale facilities
Microalgae production is a process that must be adequately planed and performed. Major steps involved in whatever microalgae production process include (i) preparation of culture medium, (ii) production of biomass into photobioreactors, (iii) harvesting of biomass, (iv) treatment of used water for recirculation or disposal, and (v) stabilization of produced biomass or transformation into end products. The core of the process is the photobioreactor in which the microalgae biomass is produced. Large bibliography is already available about photobioreactor designs and operation, here only a comparison of most used technologies being included .
3.1. Open reactors
Open reactors are the most extended for the production of microalgae, more than 90% of microalgae biomass worldwide being produced in these reactors. They are basically large water reservoirs with low depth to facilitate the light penetration and increase the biomass productivity. Raceway reactors are the most extended technology but also simple open systems are also used (Figure 4). Major advantages of raceway reactors are its low cost, below 10 €/m2, and easy scale‐up, single units up to 5000 m2 being used at a commercial scale. Another advantage of this technology is its low energy consumption, below 1 W/m3; thus it is being recommended for low‐value applications and the production of biofuels . The major disadvantages of raceway reactors are related to the scarce control of culture conditions and the easy contamination of the cultures. By these reasons they are mainly used to produce strains growing under extreme conditions as high pH (
Examples of large facilities producing microalgae using raceway reactors are available worldwide. Companies such as Cyanotech (USA; www.cyanotech.com), Earthrise Nutritionals (USA; www.earthrise.com), Parry Nutraceuticals (India; www.murugappa.com), and Myanmar Spirulina Factory (Myanmar) are some of larger producers of
3.2. Closed reactors
Closed reactors are now being used to produce microalgae strains that do not tolerate extreme conditions but contain valuable compounds, thus its price being high. Several designs have been proposed as bubble columns, helical systems, or flat panels, but from all of them, the most extended at a commercial scale are the tubular photobioreactors (Figure 4). The basic principle of whatever close reactor is to isolate the culture from the surrounding ambient, thus minimizing contamination problems and avoiding a better control of culture parameters. In the tubular reactors, the culture is continuously recirculated along the solar receiver, which is designed to maximize the interception and utilization of solar radiation. These reactors allow to produce almost whatever strain, including sensible strains such as
Tubular reactors are used mainly to produce high‐value biomass for human applications. Thus, companies such as I’age vertd (France; www.agevert.com), SECIL (Portugal), and Roquette Klötze (Germany) produce
4. Microalgae applications
4.1. Microalgae‐based market analysis
Microalgae‐based products include a large portfolio of applications, some of them only potentially indicated whereas others being realistic at a commercial scale. These applications can be divided in four main groups related to the safety requirements of different markets: (i) production of energy, mainly biofuels; (ii) products for agriculture, such as biostimulants, biopesticides, and bioplastics; (iii) production of feeds for farms and aquaculture; and (iv) products for human consumption, mainly foods and nutraceuticals . When comparing the market size of the different markets, it is observed that biofuels market is requiring enormous productions, higher than 107 t/year, that today are far from the actual microalgae biomass production capacity, up to 104 t/year (Figure 5). The actual production capacity is closely related to human applications, requiring around 104 t/year, this being slightly lower than required capacity of agriculture uses, up to 105 t/year, and feed applications, of around 106 t/year. Regarding market price, the actual microalgae biomass production cost ranges from minimum 5 €/kg in raceway reactors to 12 €/kg in tubular photobioreactors ; the market price must be higher than production cost to be economically feasible. Results show as only human uses, the production of feed additives and some applications related to agricultural uses, as the production of biostimulants and biopesticides, have market prices higher than the actual production cost (Figure 5). From this analysis, it is easily concluded that only these applications are realistic today. Thus, the market value of human‐related products exceeds 103 M€/year, for agriculture‐related products 104 M€/year, and for aquaculture‐related products up to 105 M€/year, demonstrating the relevance of these sectors in the future (Figure 5).
To expand the application of microalgae‐based processes to other fields, the microalgae biomass production cost must be reduced by one order of magnitude, whereas the production capacity must be increased by at least three orders of magnitude; that is not an easy challenge. The microalgae biomass production cost can be greatly reduced if the productivity of the actual production systems is enhanced, the facilities are scaled‐up, and the coupling with other processes as waste treatment is performed, thus this being the challenge in the future [21, 33]. Regarding the increase of production capacity, only the development of new schemes, using more robust and scalable technologies, in addition to the utilization of more resistant and productive microalgae strains will really allow to significantly increase the production capacity.
4.2. High‐value applications of microalgae
High‐value applications of microalgae are mainly related to direct human consumption as foods, nutraceuticals, cosmetics, or pharmaceuticals . Microalgae biomass contains proteins, lipids, and carbohydrates, all of them of high quality for human consumption. Thus, microalgae biomass contains large amounts of essential amino acids and polyunsaturated fatty acids, in addition to sterols and carotenoids with antioxidant activity, thus this biomass being considered as a superfood . In this sense, in 2012, the EU adopted a strategy focused to innovate through the impulse of bioeconomy sector, the “Blue bioeconomy” being one of the pillars of this strategy which is being directly related to the production of microalgae as a source of high‐value molecules for human uses . Microalgae have been reported to be a “sustainable” source of food and nutraceuticals for human uses, by its higher nutritional and functional properties versus conventional crops as cereals and vegetables and its lower land requirement also reducing the risks related to food insecurity supply in the world .
Microalgae biomass can be used as food directly, in different mixtures with other foods, or alternatively by consuming extracts of valuable compounds. Dry biomass of
Major concern about the incorporation of microalgae biomass to foods is related to EU regulation. In spite of largely reported advantages of microalgae biomass for human consumption, only the microalgae now generally recognized as safe (GRAS) can be sold for human consumption. These only include
A wide analysis of microalgae‐based products for the food and feed sector in Europe has been recently published . According to this review, the global marine biotechnology market in 2011, with microalgae as its main component, was estimated to be €2.4bn, with an expected yearly growth of 10%. Most of this market is related to the health food market as dietary supplements; by these reasons large companies in the food ingredients market as BASF, Unilever, and Dow Chemical are now involved in projects related to microalgae production.
4.3. Low‐value applications of microalgae
Low‐value applications of microalgae are related to biofuel and biofertilizer production, but all of them are only sustainable if coupling with wastewater treatment . Wastewater treatment is a crucial challenge for the sustainability of human activities. The release of wastewater is continuously increasing by the increase of population and healthy habits. However, wastewater is not always adequately treated; thus, worldwide more people die by diseases related to water contamination that is caused by violence including wars. Moreover, the release of untreated wastewater to environments causes eutrophication problems which are seriously damaging ecosystems. To avoid these problems, the wastewater must be adequately treated to remove pollutants and release water in safe way . Conventional systems based on activated sludge consist of a series of operation units focused on transforming the organic matter into CO2 that is emitted to the atmosphere, nitrogen and phosphorus being also released to the atmosphere as N2 or otherwise it being accumulated into the sludge that is finally subject to anaerobic digestion to produce biogas, normally without recovering N or P. Moreover, to perform this process, a large amount of energy is required, up to 0.5 kWh/m3, the cost of the overall treatment summarizing up to 0.2 €/m3. The concern about environmental protection is forcing the governments to reduce the limits of N and P content in wastewater for safe release to the environment; then additional treatment processes are necessary, all of them consuming larger amounts of energy and imposing higher costs.
As an example, a company as Aqualia from FCC Group, which is operating more than 250 wastewater treatment plants in Europe, is treating up to 500 Mm3/year of wastewater. The business related to this activity summarizes more than 100 M€ per year and consumes up to 250 GWh/year, equivalent to the overall electricity consumption of Spain in one day. Moreover, this energy and its CO2‐related emissions are mainly used to dissipate to the environment more than 25,000 tN/year and 5000 tP/year. This large amount of nutrients is sufficient to produce more than 0.5 Mt/year of microalgae biomass, 20 times larger than the actual worldwide microalgae production. The coupling of microalgae production with wastewater treatment allows to reduce the energy and cost of wastewater treatment at the same time that recovers the nutrients contained in wastewater and reduces the production cost of microalgae biomass, to increase the performance of actual technology used being a major challenge in the future [18, 40, 41].
Microalgae can perform the treatment of wastewater in consortia with bacteria. In this technology microalgae perform photosynthesis producing the oxygen required by bacteria to degrade the organic matter to inorganic carbon, nitrogen, and phosphorus that is at the end assimilated by microalgae as valuable biomass . If aeration is not required, the energy cost of wastewater treatment is reduced to half, moreover producing microalgae biomass the net amount of energy obtained at the end of the process being higher than at the beginning by including solar energy, thus being an “energy positive” process. To couple the production of microalgae with wastewater treatment is not a new idea, and it was proposed by Oswald in the 1960s . However, very few real applications of this technology have been carried out at a commercial scale . There are several reasons for that, but the most relevant is the low efficiency of existing technologies, especially requiring large hydraulic retention times of up to 10 days, thus enormous land requirements being imposed. The improvement of operation conditions and the utilization of new photobioreactor designs as thin‐layer cascade have been proposed to improve the performance of microalgae‐based systems . Recent advances in the design and operation of raceway reactors, coupled with the reduction of energy consumption and hydraulic retention time required to achieve complete removal of contaminants from wastewater, allow Aqualia to develop the first commercial plant based on microalgae for wastewater treatment with up to 10 ha and be able to treat the wastewater of 80,000 inhabitants in Chiclana (Spain) within the ALLGAS project.
Microalgae can be also used to treat other wastewaters from farms, aquaculture, anaerobic digestion, and industry [42, 45–47]. The development of especially designed microalgae‐based processes for these sectors, including urban wastewater, is a challenge that can transform the actual energy/resources consuming conventional treatment processes into energy positive and productive systems in a revolutionary transformation of wastes sector. Moreover, the produced biomass is suitable to be used in the production of biofertilizers and feed for animals, thus largely increasing the sustainability of food production now related to the consumption of large amounts of fertilizers, land, deforestation, and water consumption [48, 49].
Although microalgae are known for centuries, only recently they are being studied and produced at a commercial scale. The feasibility of these microorganisms to grow at largely different environmental conditions and its high productivity make it as highly relevant for mankind. The knowledge of the main factors governing the production of microalgae allows developing industrial production processes at a commercial scale. Because still the production capacity is low and the production cost is excessive, the applications of microalgae are mainly related to human consumption. However, the improvement of the actual production systems, and especially the development of new technologies and the “domestication” of highly productive strains, will largely increase the production capacity and the portfolio of microalgae‐related applications in the future.