Microalgae have been widely reported for having the ability to produce various commercial bioproducts, such as bioethanol, biofuel, biodiesel, biohydrogen, biojet, and biofertilizers Table 1. The relatively high lipid content makes microalgae a candidate for vital bioenergy feedstock in the future. Several strains that produce high amounts of lipid include Botryococcus, Nannochloropsis, and Chlorella sp. Carbohydrate content such as that found in Arthrospira platensis (60%) (Markou et al., 2013) and Chlamydomonas sp. (50%) (Kim et al., 2020) is also a critical source of bioethanol. Not only limited to the energy sector, microalgae can also be used as biofertilizers, such as those found in Schenedesmus obliquus (, 2019).

Some of the criteria for suitable microalgae strains to be used as candidates for lipid sources to produce biofuels are having a high growth rate, high lipid content, being able to grow in a variety of environmental conditions, including being resistant to contamination, being easily harvested, and being able to be processed using the latest lipid extraction method and potentially developed in many biorefinery activities (Resdi et al., 2016).

Two kinds of microalgae cultivation systems are commonly used, namely open pond and photobioreactor. Open pond is a microalgae cultivation system carried out openly (Egbo et al., 2018). An open microalgae cultivation system can be applied on a large scale. This system is suitable for wastewater treatment and is sustainable. There are several types of open pond cultivation systems, including shallow lagoons, ponds, inclined systems, circular ponds, mixed ponds, and raceway ponds (Borowitzka & Moheimani, 2013).

A photobioreactor is also called a closed system because it is carried out in a closed container made of translucent material (Egbo et al., 2018). Several types of photobioreactors are tubular reactors, flat plate reactors, pyramid reactors, fermenter types, and hybrid reactors. Tubular reactors consist of vertical tubular reactors, horizontal tubular reactors, helical tubular reactors, α-shaped reactors, and polyethylene bags and sleeves (Płaczek et al., 2017).

Photobioreactors require higher production costs than that of open ponds. However, the water demand in photobioreactor, the risk of contamination, as well as the loss of CO₂ and nutrient is lower compared to that in the open pond. A photobioreactor is made of translucent materials that can be penetrated by sunlight. Photobioreactor productivity is higher compared to that in open ponds. It is easier to regulate culture temperature in a photobioreactor than it is in an open pond. However, the commercialization of open ponds on a large scale has more potential (Egbo et al., 2018).

Phycoremediation can be used as a method of environmental recovery. Phycoremediation is safe, environmentally friendly, and relatively easy to perform. In addition, the microalgae biomass produced in this process can be used as a source of biodiesel production (Asiandu & Wahyudi, 2021). Bioprospecting of various algal species has resulted in many forms of bioenergy for utilizing the variation of growth-supporting chemical content Table 1.

Biodiesel from microalgae is considered as having more potential to be developed for overcoming the scarcity of fossil-based energy. When compared to crop-based biodiesel, microalgae are more sustainable for production in the long run. Microalgae biomass obtained from microalgae cultivation activities can be extracted to produce algal oil. Algal oil can be used as a raw material for biodiesel production. Meanwhile, algal oil production produces by-products that can be processed into animal feed and other products. Biomass waste can be processed in an anaerobic digestion system to produce biogas and bioelectricity. Meanwhile, the effluent from this process can be used as a biofertilizer (Chisti, 2008).

Some efficient solvents for lipid extraction in microalgae are petroleum ether, n-hexane, and chloroform. The percentage ratio of solvents with high yield conversion values ​​is 75% ethanol and petroleum ether, 75% ethanol and n-hexane, and 75% ethanol and chloroform. The combination produced a yield of about 85%. This solvent can be used to carry out direct transesterification of Chlorella spp. biomass (Zhang et al., 2015).

Microalgae have many advantages over higher plants in the production of biofuels. Microalgae are photosynthetic and absorb CO ₂ from the air. It is one strategy to reduce greenhouse pollutants in the atmosphere. Microalgae have a rapid growth rate and biomass can be produced in a short time. Microalgae require a smaller land area than higher plants. Also, microalgae can be harvested easily (Widjaja, 2009).

One candidate for biodiesel production is Chlorella vulgaris. This strain is capable of accumulating lipid in relatively high amounts. The treatment in the form of N-depletion initiates lipid accumulation in the microalgae to produce high lipid accumulation at the end of incubation. Lipid accumulation is more than 40% in cultures that experienced N-depletion on day 27. Cultivation on day 20 produces a biomass of 0.86 mg/L, with a lipid percentage of 29.53%, and a lipid productivity of 12.77 mg/L/day. The accumulation of lipid can be higher when implementing vacuuming at low temperatures in the extraction process (Widjaja, 2009).

Moreover, compared to other plants in lipid production, microalgae have a higher lipid conversion value. Lipid conversion value of microalgae with low lipid content reaches 58.700 L/ha/year Figure 1. Meanwhile, palm oil only reaches a conversion of 5.366 L/ha/year. This amount is much lower than that of microalgae. It means that palm oil production is not optimal as a future energy source. This is also related to lower productivity with large cultivation area (Rahman et al., 2019); (Rajvanshi & Sharma, 2012). Meanwhile, some strains have already been reported as oleaginous microalgae, as presented in Figure 2, which accumulates high amounts of lipid, such as Botryococcus brauni (> 75%), Schizotrium sp. (> 75%), Nannochloropis sp. (> 60%), to widely cultivated strain, such as Chlorella sp. (> 30%) (Chisti, 2007).

Several technologies can be used in the production of microalgae-based biofuels, including biochemical conversion, transesterification, thermochemical conversion, and direct combustion. Biochemical conversion is a biofuel production method that involves microorganism fermentation. Transesterification is a biofuel production technique using the reaction between alcohol and microalgal lipid. Thermochemical conversion is a method that uses high temperature to convert biomass into alcohol, hydrocarbon fuels, and other chemical substances. The process includes gasification, pyrolysis, and liquefaction (Culaba et al., 2020). Direct combustion is used to produce electrical energy using microalgae biomass residue. The microalgae biomass is converted through the use of heat which will produce electrical energy.

Some microalgae accumulate lipid in a high percentage which is essential in biofuel production Figure 2. One of the strains that contains high lipid is Botryococcus braunii with lipid content of 25 - 75%. Additionally, lipid accumulation in Nannochloropsis sp. peaked at 68%. Lipid content in Schizochytrium sp. is up to 77%. Other lipid-accumulating microalgae are Chlorella sp., Crypthecodinium cohnii, Dunaliella primolecta, Monallanthus salina, and Tetraselmis sueica with lipid content of 28 - 32%, 20%, 23%, > 20%, and 15 - 23%, respectively (Chisti, 2007).

Botryococcus braunii cultivated in a photobioreactor is able to reach a biomass of 4.1 g/L. Lipid accumulation in the microalgae is higher with increasing incubation time. The highest lipid accumulation reached 78%. At 42 days of incubation, the lipid extract obtained from 5 g of dry biomass was 3.8 g or 78% (Nagaraja et al., 2014). Some B. braunii isolates can be found in Miyagi, Fukui, and Kochi, Japan, at pH 6.7 - 8.0. Meanwhile, several isolates from Indonesia, namely Palangka Raya, Pundu, Bunto, Muara Teweh, and Tenggarong are found at pH 6.0 - 7.7 (Kawamura et al., 2020).

There are differences in fatty acid composition in microalgae and palm oil. Palm oil contains methyl stearate, methyl oleate, methyl palmitate, and methyl myristate as much as 5.73%, 55.43%, 37.37%, and 1.46%, respectively. Meanwhile, the contents of methyl stearate, methyl oleate, methyl palmitate, and methyl myristate in algal oil (Spirulina sp.) are 6.04%, 49.20%, 44.76%, and trace (Pradana et al., 2020). Moreover, lipid of Spirulina platensis contains C14:0, C16:0, C16:1, C18:0, C18:1, C18:2, C18:3, C20:0, C20:1. The FFA value of this lipid is relatively high, reaching 18.7%. The use of 80% alcohol in the microalgae lipid transesterification process produces a biodiesel yield of 84.7%.

The biodiesel quality test produced on microalgae-based biodiesel has a viscosity of 4.8 mm ² /s, density of 886 kg/m ³ , flash point at 172 ^o C, cloud point at 5 ^o C, pour point at -1 ^o C, and cetane number 60.73. Considering other biodiesel, the fatty acid composition of the biodiesel is dominated by a palmitic acid of 48.35%, while the content of other fatty acids, namely mystic acid, linolenic, linoleic, palmitoleic, oleic, stearic, and lauric is 20.9%, 7.84%, 5.37%, 2.66%, 2.41%, 2.02%, and 0.7%, respectively. From this analysis, biodiesel produced by S. platensis lipid is very appropriate to be used as a biodiesel source (El-Shimi et al., 2013).

Some potential microalgae strains in biojet production are found in Malaysia, including Chlorella sp., Chlamydomonas sp., and Chlorococcum sp. Lipid content in Chlorella spp. peaks at 26.72 - 40.38%, the lipid content of Chlamydomonas sp. is 43.34%, and in  Chlorococcum sp. is 39.28%. The productivity of lipid production in Chlorella spp. is 12.71 - 30.57 (mg/L/day), Chlamydomonas sp. at 14.03%, and Chlorococcum sp. at 22.55%.

To produce 1 kg of biojet through HEFA (Hydroprocessed Esters and Fatty Acids), 4.14 - 6.25 kg of Chlorella spp. biomass, 3.86 kg of Chlamydomonas sp. biomass, and 4.26 kg of Chlorococcum sp. biomass are needed (, 2015); (, 2016). The production of biojet through HEFA can be applied by converting microalgal oil extracted from its biomass through hydrotreatment (Lundquist et al., 2010); (, 2016).

Some methods for converting microalgae biomass into biojet fuel are hydrothermal liquefaction; biomass gasification and Fischer-Tropsch, comprising pyrolysis; cracking, reforming, and upgrading; deoxygenation and decarboxylation, transesterification, and fractional distillation (Bwapwa et al., 2018). Hydroprocessing requires relatively low production costs, suitable for large-scale production.

Nannochloropsis sp. is one of the future candidates for biojet production. This strain grows well at temperatures of 15 - 25 ^o C. Lipid accumulation in the microalgae increased by 80% after being treated in the form of nutrient starvation for 3 days. The bio-oil extracted from the microalgae can then be processed through pyrolysis to produce the next oil fraction in the form of biojet. The characteristics of the biojet fuel produced from this microalgae are a heating value of 44 MJ/kg, freezing point at -30 ^o C, flash point at 68 ^o C, density at 15 ^o C which is 1.38 g/mL with total sulfur of 0.27 wt% (Bwapwa et al., 2018).

Microalgae can also be utilized in producing bioethanol due to the high accumulation of carbohydrates as a result of their photosynthesis. As an example, Cyanobacteria  Synechoccus sp. biomass contains 60% carbohydrates. Their carbohydrate compounds can be hydrolyzed and then fermented using Saccharomyces cerevisiae involving some enzymes to convert carbohydrates into ethanol. The ethanol conversion value reaches 0.22 g per one gram of dried biomass or 30 g/L. Furthermore, the sugar of carbohydrates in Synechoccus sp. includes glucose (60%), as well as galactose, xylose, arabinose, and mannose with concentrations of less than 5% (Möllers et al. 2014).

Chlamydomonas sp. can also be used in bioethanol production with an average yield reaching 0.22 g/g residual biomass. Microalgal biomass growth can be initiated by cultivating the microalgae with 80 μm photon/m ² /s light. The percentage of carbohydrates in the microalgae cells was 50.5%, with 24.2% protein and 19% lipid. The main composition of the fatty acids that make up the microalgae lipid included polyunsaturated fatty acids (PUFAs) and Monounsaturated Fatty Acids (MUFAs) dominated by C16:4 with a percentage of 25.6%, followed by C18:3 (19.4%), C18:2 (5.9%), C18:1 (7.8%), and other fatty acids. The high carbohydrate content in microalgae is a promising source of bioethanol production (Kim et al., 2020).

Arthrospira platensis also contains a high carbohydrate percentage which is nearly 60%. Meanwhile, the protein content is more than 20%, and lipid content is less than 10%. The yield of bioethanol produced from Arthrospira platensis  biomass reaches 16.27% using 0.5 N H ₂ SO ₄ , with a bioethanol yield of 16% g/g of dried biomass (Markou et al., 2013).

Also, Nannochloropsis oculata contains carbohydrates of 252.84 mg/g dried biomass. The yield of carbohydrates produced from the mixotrophic cultivation is 268.53 mg/g, while from the phototrophic cultivation the carbohydrates produced is 177.73 mg/g of dried biomass. The reducing sugar content is 232.39 mg/g at a concentration of 3% H ₂ SO ₄ with a saccharification percentage of 92.2%. The sugar is dominated by galactose, fucose, rhamnose, rabinose, xylose, and mannose (Fetyan et al., 2021).

Nannochloropsis gaditana can be cultivated using municipal wastewater. The yield of bioethanol produced in municipal wastewater is 30%, equal to 94.3 mg/g dried biomass with an accumulation of carbohydrates of 17.7%. The biomass produced on the 14 ^th day of incubation is 2.33 g/L (Onay, 2018). Meanwhile, to optimize the production of biomass and carbohydrates in microalgae like Arthrospira platensis, optimum K ₂ HPO ₄ and NaHCO ₃ concentrations are required, because the accumulation of lipid in the microalgae is influenced by the concentration of the two kinds of nutrients in the medium (Tourang et al., 2019).

The integration of bioremediation and microalgae-based biofuel production can also be carried out in bioethanol production. Microalgae Parachlorella kessleri, Desmodesmus sp., and Chlamydomonas sp. isolated from pig farms can remediate swine wastewater. The biomass of each microalgae cultivated in a mixotrophic culture during an incubation period of 8 days is 5 g/L, 3.1 g/L, and 6.3 g/L, respectively. The carbohydrate content in each isolate is 40%, 37%, and 49%. Meanwhile, the percentage of COD reduction in each isolate reaches 47%, 38%, and 47%. The enzymes involved in the production of bioethanol are glucoamylase, α-amylase, β-glucosidase, endoglucanase, and cellobiohydrolase. Meanwhile, in Chlamydomonas sp. the maximum ethanol concentration obtained was 61 g/L (Qu et al., 2020). Another potential isolate in bioethanol production is Schizocytrium sp. with a yield of 11.8 g/L (Kim et al., 2012), Chlorella minutissima with carbohydrates of 0.125 g/L (Margarites & Costa, 2014).

Microalgae are also pivotal in biohydrogen production. One of many species, Scenedesmus obliquus produces high-purity biohydrogen through fermentation of about 67.1 mL H ₂ /g VS and 167.8 mL H ₂ /mL FM. The microalgae can be cultivated using brewery wastewater to minimize the production cost as well as remediate the environment (, 2019).   The production of biohydrogen can also be carried out using Chlorella sp. through the process of anaerobic biomass metabolism without the presence of sulfur. Microalgae cells are known of their capability in carrying out anaerobic metabolic processes by using carbon sources in their cells. However, the accumulation of hydrogen increases with the addition of glucose in the medium in the anaerobic phase (Song et al., 2011).

Additionally, hydrogen gas can also be produced through fermentation which utilizes bacteria and microalgae biomass. Scenedesmus obliquus biomass can be fermented by Enterobacter aerogenes and Clostridium butyricum to produce energy in the form of hydrogen gas which can be converted into electrical energy. Dry biomass fermentation of Scenedesmus obliquus using Clostridium butyricum produces a biohydrogen yield of 113.1 mL H ₂ /g VS algae. Meanwhile, the yield of biohydrogen produced through the fermentation of E. aerogenes is 57.6 mL H ₂ /g VS algae. In addition, fermentation can also be carried out using wet biomass with lower resulting. However, the production costs and time by using wet biomass are more prospective (Batista et al., 2014).

Biohydrogen production using Chlorella vulgaris can also be enriched by using Valoniopsis pachynema extract as a nutrition source for the microalgae. The extract is added to the C. vulgaris growth medium to form hydrogen gas through the metabolism of the microalgal cells. The maximum biohydrogen produced from the culture was 0.002 g/hour/L (Lakshmikandan & Murugesan, 2016).

Microalgae biomass or waste can also be used in the production of biofertilizers. Biomass contains various essential nutrients that can be utilized by plants to enhance their growth. The use of microalgae-based biofertilizers is potentially developed considering the massive use of chemical fertilizers that are harmful to the environment. The use of fertilizer itself is getting higher due to the increasing growth of the human population in the world. To meet agricultural needs, agricultural activities as well as the demands for fertilizers will be higher in the future (Dineshkumar et al., 2019).

Extracts of blue-green algae are reported to be able to increase several chemical properties and the resistance of a plant to pathogens. The microalgae extract enhances the growth promoters, amino acids, peptides, and proteins, and even triggers antibacterial and antifungal properties of these plants (Dineshkumar et al., 2019). Microalgae also enhance the nutrient richness of the soil. Microalgae increase the carbon and nitrogen content and balance the pH and electrical conductivity of the soil. Some species of blue-green algae are important in the process of nitrogen fixation, for example, Anabaena, Nostoc, Aulosira, and Tolypothrix (Karthikeyan et al., 2007); (Dineshkumar et al., 2019).

Spirullina and Chlorella are two microalgal strains essential in enriching the soil to increase the yield of agricultural activities without causing side effects like common chemical fertilizers. Moreover, microalgae are also considered potential candidates for producing biopesticides, such as Chlorella, Dunaliella sp., Coscinodiscus sp., Tetraselmis sp.,and Spirullina sp. (Dineshkumar et al., 2019).

Scenedesmus obliquus cultivated using brewery wastewater is capable of producing various kinds of essential chemical products. It is also able to reduce the content of NH ₃ , total nitrogen, P-PO ₄ ^3- , and COD in the waste with concentrations of 81%, 88.2%, 29.7%, and 70.8% reduction, respectively. Based on chemical properties analysis, the microalgae cells incubated using brewery wastewater until the stationary phase contain 31.4% protein, 30.2% carbohydrates, 17.9% lipid, and 4.4% FAMEs. The fatty acid composition of the microalgae is dominated by C16:0 of ​​23.03% (, 2019).

Furthermore, Scenedesmus obliquus biomass cultivated using brewery wastewater increases the germination rate of wheat and barley seeds. Germinated seeds fed with S. obliquus biomass has longer roots, higher in size, and better color saturation. It triggers growth promoters in the seeds of the two samples provided with the microalgae biomass. Therefore, the microalgae can be considered as growth initiators of the plant (, 2019).

The lipid-free residue of Scenedesmus sp. is also able to enhance the availability of N, P, and K in the soil. The provision of biomass residue can be used as a slow-release organic matter in the soil (Nayak et al., 2019).

C. vulgaris and S. platensis are also reported to be able to enrich the soil to support the onion growth rate. The soil compounds administered with C. vulgaris biomass contain N, P, K, Fe, Zn, and Mn with concentrations of 18, 14.22, 58.82, 3.46, 1.03, and 2.01 mg/kg, respectively. Meanwhile, the content of N, P, K, Fe, Zn, and Mn in soil administered with S. platensis biomass are 18.2, 14.24, 57.86, 3.42, 1.13, 2.06 mg/kg, respectively. The nutrient value estimated after the onion harvesting on the tested soil is shown to be higher than the control. After planting for 100 days, the height of the onions treated with C. vulgaris becomes more than 45 cm and almost 50 cm for soil treated with  S. platensis. The administration of the two microalgae biofertilizers is also reported to increase chlorophyll and carotenoid contents in onions, as well as an increase in the content of total solute sugars, amino acids, and total phenols (Dineshkumar et al., 2020). In addition, the use of cow dung added with S. platensis or C. vulgaris also increases growth, yields, chemical components, and minerals in corn (Dineshkumar et al., 2017).