The development of renewable energy resources has become a priority due to climate change and dwindling fossil fuel reserves. Algae holds much promise as a potential feedstock for biofuels because of their higher capacity for productivity per unit land area than conventional terrestrial feedstocks (Chisti, 2007; Wijffels et al., 2010; Georgianna and Mayfield, 2012; Khrunyk et al., 2020). Algal biofuel is immediately compatible with our existing transportation infrastructures like refineries, fuel stations, and the engines of cars (Hannon et al., 2010). If a profitable and sustainable algal biofuel process can be developed, the potential benefits of the technology are compelling including the use of non-arable land, recycling wastewater, and carbon dioxide.

An extensive research program on algal biofuels was sponsored more than 40 years ago by the US Department of Energy (DoE) at the National Renewable Energy Laboratory during the oil crisis of the 1970s. Despite being a successful demonstration of the feasibility of algal biomass as a source of oil, this Aquatic Species Program (1978–1996) was discontinued due to the decreasing federal budget and lower crude oil market (Hu et al., 2008). In the last decade, microalgae reemerged as a source of biofuel and concerted effort has been made towards isolating potential microalgal strain, strain improvement, elucidating biosynthesis pathways, optimize growth and cultivation parameter, harvesting, coproduct development, fuel extraction, refining and residual biomass utilization (Garrido-Cardenas et al., 2018; Wood, 2021). Like academic research, huge claims about the promise of algal biofuel (Chisti, 2007) and the high crude oil price at that time motivated a large number of companies to take an interest in microalgal biofuel, investing significant amounts of money to pursue that objective. Recent technoeconomic analysis has demonstrated that with existing technological readiness, algal biofuel is cost competitive with fossil fuel if combined with the production of high-value co-products (Ruiz et al., 2016; Cruce and Quinn, 2019). Today, most algae based companies have adopted parallel business plans that focus more on the expensive algae byproducts such as cosmetic supplements, nutraceuticals, specialty oils, natural color, and animal feed.

In this review, we provide a perspective on developing sustainable algal cultivation practices and bioproducts from microalgae to make the process of algal biofuel efficient and economically competitive. We have restricted this review to the photoautotrophic microalgal biomass production and to process them into biodiesel or converting them into biocrude at high temperature and pressure through hydrothermal liquefaction but does not include any other form of algal biofuel like bioethanol, biogas, or biohydrogen. The worldwide commercial production of microalgal products is also discussed.

Large capital investment in microalgae cultivation still limits economic biomass production (Acién et al., 2012; Ruiz et al., 2016; Kaur Nagi et al., 2021). Microalgae are not growing at a scale (<5 ha) that is required for the level of cost economy analysis of biofuels. To make “high-volume, low-cost” product like biofuel, microalgal production system must be increased several orders of magnitude, and it demand strategies to reduce the capital expenditure. A considerable variety of systems are available for photoautotrophic production of microalgal biomass, and they are broadly categorized into open raceway ponds and closed photobioreactor systems. Despite the challenge of biological contamination and water loss, raceway ponds are the major commercial production systems of algae biomass because of their large scalability, lower capital, and operational costs (Borowitzka and Vonshak, 2017; Schipper et al., 2021). To balance the strengths and weakness of open and closed systems, algae is cultivated in combined setup of photobioreactor and raceway, called as hybrid system. For production of microalgal biodiesel, two-stage hybrid system could be suitable where exponentially growing algal biomass is transferred from photobioreactor to open raceway ponds under nutrients replete condition to induce higher lipid yield (Liyanaarachchi et al., 2021).

Various research attempted to reduce the capital as well as the expenditure cost of the production systems (Table 1). Capital investment although about one order magnitude lower in an open pond than photobioreactor and again, the construction costs can be reduced by 24–75% if self-sealing layers are developed, rather than using synthetic liners, at the soil-water interface by microalgae and associated organisms through bioclogging process (Coleman et al., 2014; Pattullo et al., 2019). Sapphire Energy demonstrated stable microalgal productivity in an unlined pond (2,000 m² surface area, 500,000 L volume, 10 cms⁻¹ flow rate) at the Las Cruces test facility, New Mexico without any issues with suspended materials or major water loss through soil (McBride and Merrick, 2014). High-value products like eicosapentaenoic acid (EPA) and omega-3 fatty acid are produced in the unlined pond of Qualitas Health in Imperial, Texas (Efroymson et al., 2020). Greenhouse gas emissions associated with synthetic pond liner manufacture and transportation could be eliminated by using unlined ponds and which eventually will make technology more sustainable (Canter et al., 2014; Greene et al., 2020). Closed raceway ponds were also designed by enclosing a normal raceway ponds with transparent cover that prevents escaping supplied CO₂ into atmosphere and consequently reduce the expenditure of CO₂. Instead of using paddle wheel, airlift-driven raceway can reduce around 80% power consumption for algal production (Kumar Singh et al., 2021).

A considerable variation exists in data in literature and opinions among experts about the selecting the suitable production system for algal bioenergy. However, a general consensus of several life cycle assessment (LCA) studies indicates raceway ponds are better than photobioreactor in terms of net energy ratio and global warming potential (Ketzer et al., 2018; Herrera et al., 2021). The use of expensive photobioreactor system can be justified for making high-value products. Furthermore, the location of the production facility is one of the divers for selecting a commercial production system. For instance, astaxanthin is produced in raceway ponds in Hawaii, United States by Cyanotech Corporation, but it is not feasible under the sun of the Arava desert due to high water evaporation. Therefore, Algatech in Israel uses photobioreactors for the same product. However, substantial energy is used for cooling the photobioreactor. To minimize the cost associated with temperature control, reactor design or strain improvement should be considered in future research. Similar to NASA’s OMEGA (Offshore Membrane Enclosures for Growing Algae) system, microalgae can be grown in floated plastic tubes in seawater which functions as a temperature buffer (Wiley et al., 2013).

An alternative to the traditional cultivation systems, biofilm-based systems have tested for biofuel production as summarized by Gross et al., in 2015 (Gross et al., 2015). Algal biofilm-based technology was first developed by Walter Adey in the 1980s called Algal Turf Scrubber™ (ATS) in which naturally seeded filamentous algae grow on a screen in a shallow basin through which water is pumped (Adey et al., 2011). This is used to treat wastewater and was commercialized it through a company Hydromentia based in Florida. Rather than in suspension, algae are attached to the surface and harvested through scraping and thus avoid the expensive harvesting procedures in traditional microalgal cultivation. Although ATS is a robust system for algal biomass production and its productivity comparable to the raceway ponds, higher ash (30–50%) and lower lipid in algal biomass are major challenges to use them for biofuel (DeRose et al., 2019). ATS could be a viable option for biofuel production coupling with wastewater treatment if the ash content can be reduced by growing desired algal communities especially by avoiding silica containing diatoms which contribute up to 65% of the total ash (Adey et al., 2011; Kim et al., 2021).

Maintaining long-term, stable, and highly productive algal biomass production in large scale outdoor conditions is the most significant barrier in algal biofuel commercialization. Much like terrestrial crops, microalgal cultivation systems are invaded by weeds, pests, and pathogens, making crop protection a major challenge in the commercialization effort. It is estimated that 30–40% of annual algal crop production is lost to pond crashes (Newby et al., 2016). In this section, we discuss integrated pest management practices (IPM) for healthy algal crop cultivation and a sustainable crop protection process against undesirable biomass losses (Figure 1).

IPM uses proactive strategies rather than controlling the pests in the production system. Selection of robust strain in terms of adaptability in varying water chemistry, tolerant to a wide range of field temperature is an essential criterion for any successful algal field cultivation (Lee and White, 2019; Harmon et al., 2021). A robust stain can withstand the variation of water quality parameters that is common in industrial effluents. Selection of suitable microalgal candidates is crucial for recycling flue gases that has high temperatures, fluctuating gas composition, and the presence of toxic chemicals (Kondaveeti et al., 2020). It is often argued that indigenous strains have better fitness to grow in that local environment (Winckelmann et al., 2015; Mutanda et al., 2020). Sero and colleagues reported that the microalgal strains isolated from extreme urban wastewater environments have inherent biological traits to proliferate in stress and capable of producing high biomass yield using wastewater (Sero et al., 2021). Adaptive evolution, mutagenesis, genetic engineering, and systems biology approaches have been used now for microalgal strain improvement with desired traits (Arora and Philippidis, 2021; Kumar Singh et al., 2021; LaPanse et al., 2021). Growing extremophilic microalgae have a successful commercial history. For example, Dunaliella is used for commercial β-carotene production in extreme salinity, and widely cultivated microalgal species Spirulina is grown in a highly alkaline solution. Extreme conditions help in reducing contaminations from other algal weeds or pests (Varshney et al., 2015; Lafarga et al., 2021). Alkaliphilic Chlorella used for biofuel also showed resistance to grazers in high alkaline cultures (Vadlamani et al., 2017). In addition to pest management, alkaliphilic microalgae can grow efficiently without external sparging of CO₂ as alkaline solutions scavenge atmospheric CO₂ at high rates. If a direct air capture technology is established with those microalgae, the capital expenditure, as well as around 65% of total operational cost associated with the recovery of CO₂ from flue gases and delivery to the production unit, can be eliminated (Davis et al., 2016).

Amoeba, ciliates, rotifers, flagellates and crustaceans are the commonly found grazers in microalgal cultivation (Rajvanshi and Sayre, 2020). Infections from fungi like chytrid, bacteria and virus may affect productivity (Grivalský et al., 2021). Crop rotation over the year is quite essential to prevent pest population buildup. The development of a microalgal cell line resistant to pests is another preventive approach that was demonstrated in Synechococcus elongatus against amoeba attack (Simkovsky et al., 2012). Pesticides or chemicals are often used to mitigate the challenges of contamination like traditional agriculture. Extensive application of insecticide to maintain productivity causes burden of the maintenance cost, development of insecticide resistant pest and water quality loss of nature. More than 550 species of insects were found resistant to insecticide in agriculture, albeit there is no report from algal cultivation (Whalon et al., 2008; Smith and Crews, 2014). However, commercial production of microalgae has the capacity to repeat the same environmental damage if we do not follow the IPM practice. Thus, identification of pests and understanding their life cycle are primary steps to develop a controlling measure. It is possible to forecast pest attacks and take preventive measures if we have clear knowledge about pest biology and their interactions with algae and the environment. Pest monitoring through microscopy is a common practice in algae cultivation. On several occasions, algae crashes were reported within 2–5 days after detection of pests. Thus, increasing the detection sensitivity and developing early detection tools are essential for algal crop protection techniques. Besides monitoring pests through molecular techniques, algal phenotypic response to pests was employed for early detection. For example, rapid decline in quantum yield (Fv/Fm) and non-photochemical quenching in microalgae were reported prior to pond crashes due to parasitic and grazers attack, respectively (McBride et al., 2014; Deore et al., 2020). Infochemicals released by Microchloropsis salina due to the grazers attack was used as a marker for pond health monitoring (Reese et al., 2019; Roccuzzo et al., 2020).

The use of mechanical or biological control rather than conventional chemical treatment is the essential component of IPM strategy (Lee and White, 2019; Al-Jabri et al., 2021). The pest types and their density often determine control operation. Mechanical treatments like pump cavitation or filtering through plankton net are applied to remove larger grazers like rotifers (Kim et al., 2017). Selective feeding of invertebrate consumers can be used to control grazers in algal ponds. Smith and colleagues experimentally demonstrated that introducing zooplanktivorous fish can control the negative impact of grazers and increase lipid productivity in open raceway ponds (Smith et al., 2010). This concept stems from the trophic cascade principle of ecology, which posits that the biomass of primary producers can be maintained by top predators that reduce the population density of primary consumers (Shurin et al., 2014). Invasion of undesired algal strains affects the community structure and alters the biomass productivity and composition that have an impact on the biorefinery process. Maintaining high density cultures in the field is an effective approach to protect against invading algal weed in field cultivation (Richmond et al., 1990). Like pest prevention, biological control was applied to treat small unicellular contaminants such as Chlorella vulgaris and Monoraphidium minutum in Spirulina culture. Herbivorous rotifer Brachionus plicatilis that can selectively ingest only small single-celled algae because of their small mouth opening were introduced in long filamentous Spirulina culture (Mitchell and Richmond, 1987).

Microalgal consortia have the potential to offer crop protection and increase the stability of yields (Smith et al., 2010; Newby et al., 2016; Mattsson et al., 2021). The use of consortia makes the biorefinery process more complex for extracting any species-specific product, however, managing microbial consortia could be a viable industrial practice for biofuel with higher productivity and stability. Consortia benefit from the “portfolio effect”, whereby some species populations will increase in response to pest or environmental fluctuations even if others decline (Shurin et al., 2014). Algal consortia could enhance the nutrients-use efficiency, eventually reducing the fertilizer cost of algal biomass production (Mandal et al., 2018b). In addition, consortia increase productivity in the field through niche partitioning, facilitation, and complementarity (Cardinale et al., 2007; Mandal et al., 2018a). However, random inclusion of species in algal consortia showed success or failures in previous algal biofuel studies. We urge here to design consortia based on the algal complementary traits. Whether it is intentional or nuisance, microbial consortia is the reality for open pond raceway and even in a photobioreactor. Molecular 16S and ITS2 regions analysis of year-long cultivation of industrial microalgal cultivation showed how the diversity of prokaryotic and eukaryotic communities changes over time, and pond productivity and stability positively linked with eukaryotic species diversity of the pond (Beyter et al., 2016). Analogous to rhizosphere in plants, phycosphere is proposed, but it is not studied systematically in commercial production (Wirth et al., 2020). Besides parasitic microbes, many microbes observed in the cultivation have a mutualistic relationship with microalgae and provide essential vitamins for microalgal growth (Kazamia et al., 2012; Yao et al., 2019; Kaur Nagi et al., 2021). Thus, careful management of microbial food-web structure can maximize crop protection and improve crop yield for industrial algal biofuels production (Yun et al., 2016).

In IPM, strong record-keeping and making a correlation of data between yield and operational activities over seasons is a common practice. The factors that determine pest pressure must be identified to predict the pest development time in future operations. The evaluation of the effectiveness of pest control treatments guides selecting the best crop protection strategy. Importantly, translating laboratory-scale results to farm-scale production is a shortcoming in the scaling up of algae cultivation. Field cultivation faces different selection pressures like variable irradiance, temperature, and additional biological challenges—most of which are not seen in the bench-scale studies. To close the lab-to-field yield gap for reliable biomass production, those variables can be tested at a small laboratory scale in more controlled environments before tested at a pilot scale.

Considering the large amounts of wastewater generated globally, around 28–38% of wastewater is treated in developing countries and it became down to almost 8% in underdeveloped ones (Sato et al., 2013). Nitrogen, phosphorus, other macro or micronutrients, the organic carbon in wastewaters is being used for the growth of microalgae. To produce each metric tonne of dry algal biomass requires around 88 kg of elemental N and 12 kg of elemental P, which in turn puts a significant impact on the economy of algal biomass production (Pate et al., 2011). Algal cultivation and wastewater treatment can be integrated to accomplish improved environmental and economic stability. This will not only save the cost of the nutrients of algal biomass production but also surplus the wastewater treatment cost. Techno-economic suggests the production cost can be reduced to more than five times when coupled with wastewater treatment (Acién et al., 2012).

The concept of treating municipal wastewater using microalgae was initiated in the 1950s by Oswald and colleagues at the University of California, Berkeley using high-rate algal ponds with shallow depth and paddlewheel mixed (Oswald and Golueke, 1960; Benemann, 1980; Benemann et al., 1980). Later, it advanced to different types of cultivations systems like photobioreactors, earthen lagoons, concrete tanks, corrugated raceway ponds, biocoils, for use (Craggs et al., 1997; Park et al., 2011; Posadas et al., 2015; Randrianarison and Ashraf, 2017). The advantages and limitations of using an algal turf scrubber system for treating wastewater and biofuel production were described in earlier section. The nutrient removal efficiency of different microalgal strains and their productivities varied in different cultivation systems and wastewater types as illustrated in Table 2. The treatment efficiency of algae-based system and biomass productivity can be improved by operating parameters such as mode of cultivation (batch or continuous), aeration, changing water chemistry (pH, adding require nutrients) (González-Camejo et al., 2021). Further, different stresses like pH, temperature, salinity changes or nutrients reduction in growth media have been suggested to increase lipid yield for biofuel production (Bélanger-Lépine et al., 2018).

The essentials for evaluation of wastewater treatment schemes involving algae include a clear understanding of the standard steps of treatment to justify the expense of such developmental efforts and more importantly, the characteristics of the wastewater with large flows (Laurens, 2017). These wastewaters are highly turbid, often polluted by algal growth inhibitors like organic compounds in highly toxic concentrations, salt accumulations, and allelopathic agents excreted by algae themselves (Bacellar Mendes and Vermelho, 2013). Be it a monoculture or polyculture of microalgal strains, efficient pilot harvesting of biomass is vital, especially when the treated wastewater must be brought to re-use. Leaving back the traditional concept of drying, solvent extraction of lipids, and transesterification for the production of fatty acid methyl esters, all the time more interest is being directed towards the hydrothermal liquefaction (HTL) process for bio-oil productions (Al-Jabri et al., 2021; Chen and Quinn, 2021). This aqueous phase from the HTL process contains high concentrations of nutrients like nitrogen, phosphorus, and other elements that can be recycled for microalgal growth. While varying compositions of the algal biomass in wastewater is a shortcoming in the refinery process, converting the carbohydrates, proteins, and lipids agnostically to bio-oil would be a feasible choice.

Amongst various non-conventional sources, microalgae are promising microorganisms that play a key role in the biobased economy, since they serve as a continuous and reliable source of several bioactive natural products (Fabris et al., 2020). Microalgae are factories for producing various compounds other than the only lipid for making biofuel. Lipid is converted into biodiesel through transesterification process in which triglycerides react with alcohol in the presence catalyst. In thermochemical process like pyrolysis, gasification, combustion or hydrothermal liquefaction biomass is thermally breakdown into organic chemicals which reform into various types of biofuels (Figure 2). The biochemical conversion involves the hydrolysis of biomass by bacteria into fermentable sugars which is converted into bioethanol, biogas and biohydrogen (Saad et al., 2019). As shown in Figure 2, the algal biomass residue after high-value co-products and biodiesel production can be route into thermochemical or biochemical process for maximal valorization of algal materials. However, most microalgal companies focus on single product development. Recently, the focus of microalgae biomass delivering a single product is shifting towards delivering multiple products along with lipid derived biofuels in a biorefinery approach (Wijffels et al., 2010; Ansari et al., 2017). The current production of microalgae derived products (more than 75%) are finding their way towards food, feed, nutraceutical, and cosmetic industries (Nethravathy et al., 2019; Rahman, 2020). Thus, if algal biofuel is combined with the production of bulk chemicals, food, and feed ingredients, the cost gap between biofuel and fossil fuel would be closed (Shukla and Kumar, 2018). The proposed biorefinery approach, on other hand, may cause the market saturation of high-value products. Thus, the market niche and demand of such algal high-value products must be analyzed critically. However, nowadays people are trending towards natural products, especially in the COVID era. In the following section, we analyzed the algal products that can zeal with algal biofuel, their potential, current industrial situation, future market (Table 3).

Commercial microalgae production was started with the cultivation Chlorella for the single-cell protein in the early 1960s in Japan, followed by Spirulina in the US, and then in China and Thailand (Lee, 1997; Borowitzka, 2013). In the 1980s, efforts were initiated to produce microalgal pigments, predominantly beta-carotene and astaxanthin, through the cultivation of Dunaliella sp. and Haematococcus sp. In the 1980s, commercial production of PUFAs, especially EPA and DHA, was started for nutraceuticals application. In the early of this century around 2005, a number of companies like Algenol, Cellana, Origin Oil, Aurora Biofuel, PetroAlgae, PowerFuel.de, Shell Oil, Solix Biofuel, Sapphire Energy, and Solazyme raised remarkable private sector investment with a promise of producing algal biofuel competitive with the fossil fuel (Waltz, 2009). In India, Reliance Industries invested with Algenol, United States to recycles carbon dioxide into fuels through its direct-to-ethanol process near Jamnagar petroleum refinery. Sapphire Energy used hydrothermal liquefaction technology to make “crude-like oil” that can be refined into gasoline or jet fuel. The estimated minimum price for algae biofuel was $2.1 per liter which weighs high than regular gasoline (Gu et al., 2020). Today, most algae companies except for ExxonMobil and Synthetic Genomics shifted their business model into high-value products. ExxonMobil and Synthetic Genomics reported the doubling of lipid production without compromising growth through genetic modification in Nannochloropsis gaditana using CRISPR–Cas9 genome editing techniques (Ajjawi et al., 2017). Their joint algae biofuel research program targets to produce 10,000 barrels of algae biofuel per day by 2025 using genetically modified strain. However, the phenotype stability of engineered strain in the field along with the concern of environmental risk growing often raised the question of the genetically engineering approach for microalgal biofuel research. The finding of the first field trial of genetically modified algae by researchers from the University of California San Diego and Sapphire Energy, United States was encouraging as genetically modified Acutodesmus dimorphus conserved the genetically modified phenotypes in field cultivation without impacting the phytoplankton communities in native lakes (Szyjka et al., 2017).

After few years from 2005 when microalgae did not reach the economic target, the potential of microalgae for biofuel was debated and called “hype” (Waltz, 2009). In a reply to the news feature “Biotech’s green gold?” in Nature Biotechnology (Waltz, 2009), Stephens and colleagues demonstrated microalgae are capable of producing ∼60–100 kl oil ha⁻¹ y⁻¹ on a practical conservative scale (Stephens et al., 2010). Algae-based transportation fuels have already demonstrated their ability to drive personal automobiles, fly commercial planes, and power Navy ships. In our view, it not about the potential of algae, it is all about technological readiness to compete with fossil fuel.

Although algal biofuel didn’t reach yet the economic target, Life cycle assessment (LCA) of algal biofuel from the pilot-scale facility of Sapphire Energy was found to have lower greenhouse gas (GHG) emissions than corn ethanol and petroleum fuels (Liu et al., 2013). Interestingly, Energy Return on Energy Investment (EROI) which is the key for measuring the sustainability of any energy technology was above one in their pilot-scale analysis and varied between one and four depending on the scale of production.

Microalgal biofuel remains in an early stage of development. In our view, we invested only a few years from capital injection to demonstrating large-scale commercial production. Venture capitalists should consider the challenges and barriers that need to be overcome before this technology is commercialized. Even, the demonstration plants (at <5 ha) that were used for estimating the cost analysis were well weighed below the size threshold for economic viability. To make “high volume, low-cost product” like biofuel, scaling up microalgal cultivation system to a commercial level is a key in the process development. Although microalgae are an excellent feedstock of multiple products, maintaining the stability of all chemicals with their bioavailability is critical challenge in adapting biorefinery approach. A substantial innovation is required in downstream processing steps like milder cell disruption technologies, solvents or supercritical fluid extraction to ensure the functionality of the products reserve in the process. Research should be carried out to find the appropriate sequence of products extraction from microalgal biomass in refinery process. As of now, current market values of algal nutraceutical are quite high when the global production of microalgae is inadequate. Apparently integrating biofuel systems with industrial commodities production looks economically sustainable but replacing only a part of fossil fuel with biofuel can make a surge of high-value products in the market and affect product prices. Awareness among people about algal products must be improved to reach a sustainable biorefinery. Improving downstream processing certainly is an essential step, but to produce enough biomass to feed the process is more critical in algal biofuel commercialization. Successful algae cultivation in the field demands a more ecological approach rather than industrial microbiology. Algae need to be considered as an agricultural crop, and robust agronomic and integrated pest management practices must be developed as cheaply as possible. Indeed, the 2018 Farm Bill classified algae as a crop in US policy and support the algae program. Research investment, policy development, and new scientific discoveries will pave the way for the development of viable microalgal biofuel platforms in the near future. In the meantime, valorizing high-value co-products is a feasible option for microalgal biofuel commercialization.