Cyanobacteria can be used for the bioremediation of several contaminants such as heavy metals, pesticides, crude oil, phenanthrene, naphthalene, phenol, catechol, and xenobiotics (Kesaano and Sims, 2014; Hamouda et al., 2016; Kumar et al., 2016; Singh et al., 2016). Synechococcus elongatus, Microcystis aeruginosa and Anacystics nidulans have shown potentials to remove organo-chlorine and organo-phosphorus insecticides (Vijayakumar, 2012). Cyanobacterial genera such as Oscillatoria, Synechococcus, Nodularia, Nostoc, Anabaena, and Microcystis break down lindane residues (El-Bestawy et al., 2007). Anabaena sp., Lyngya sp., Microcystis sp., Nostoc sp. and Spirulina sp. can utilize glyphosate as the source of phosphorus, and therefore, helps in removal of this herbicide from contaminated soil (Forlani et al., 2008; Lipok et al., 2009). Thus, an above mentioned cyanobacterial strains are potential candidates for their application in developing sustainable agricultural practice where they can be used for removing various contaminants from soil as well as water.

The consortia of cyanobacteria and chemotrophic bacteria have been effectively used to bioremediate wastewaters and oil-contaminated sediments (Abed and Köster, 2005). Cyanobacteria can oxidize oil components as well as other complex organic compounds such as herbicides and surfactants (Mansy and El-Bestway, 2002; Subashchandrabose et al., 2013). The consortium of Oscillatoria-Gammaproteobacteria can degrade phenanthrene, dibenzothiophene, pristine and n-octadecane (Abed and Köster, 2005). Similarly, consortia of Microcoleus chthonoplastes with organotrophic bacteria can fix atmospheric nitrogen and degrade aliphatic heterocyclic organo-sulfur compounds and hydrocarbons such as alkylated monocyclic and polycyclic aromatic compounds (Sánchez et al., 2005). An artificially designed biofilm consortium of hydrocarbon-degrading bacteria and cyanobacteria on gravel particles and glass plates has been developed that can be used for cleaning up crude-oil contamination of sea water samples (Al-Awadhi et al., 2003). The consortium of Anabaena oryzae and Chlorella kessleri can be used to biodegrade crude oil under mixotrophic condition (Hamouda et al., 2016). Cyanobacteria are also capable of removing heavy metals from water bodies and can reduce the excess of nitrate and phosphate from agricultural fields (Kesaano and Sims, 2014; Kumar et al., 2016). Consortia of cyanobacteria and bacteria have shown positive results in wastewater treatment. Cultures of Aphanocapsa sp. BDU 16, Oscillatoria sp. BDU 30501 and Halobacterium US 101 can be utilized for minimizing the amount of calcium and chloride in wastewater to a level that can support the survival of fishes (Uma and Subramanian, 1990). Similarly, Phormidium valderianum BDU 30501 and Oscillatoria boryana BDU 92181 can be utilized to remove phenol and melanoidin, respectively, from the effluents of distillery (Shashirekha et al., 1997; Kalavathi et al., 2001). Desertification is another challenge for sustainable agriculture practices which can be reversed by the application of cyanobacterial inoculums. Cyanobacteria together with bacteria, mosses, algae, lichens and fungi forms biological soil crusts (BSCs) in semiarid and arid areas of various geographical regions (Rossi et al., 2017). These BSCs play an important role in stabilization and primary colonization of desert soil surface by increasing the nutrient and moisture contents (Rossi et al., 2017). Thus, cyanobacteria can be ideally utilized for removing the various contaminants. However, for bioremediation of polluted sites, focus should be given to utilization of indigenous consortia that could potentially reduce the need of adding new microorganisms or fertilizers to the target site. Further improvement in bioremediation property can be achieved by genetic engineering of parent strains which can help in developing a maintenance-free and economical remediation technology while producing the biomass of high value for other purposes (Kuritz and Wolk, 1995; Cuellar-Bermudez et al., 2017).

First and second generations of biofuels have utilized feedstocks such as rapeseed, soybean, sunflower, wheat, switchgrass, peanuts, and sesame seeds. These raw materials have been used to produce different energy sources such as ethanol, propanol, butanol, and vegetable oils (Quintana et al., 2011). However, energy crops, which are used in first and second generations of biofuels production, compete with conventional food sources for water, nutrients and fertile land. Therefore, the third generation of biofuel production using microalgae has emerged as an alternative to avoid the competition between food crops and energy crops for available resources, and furthermore, cyanobacteria are one of the most promising feedstocks for the production of third generation biofuels (Quintana et al., 2011; Al-Haj et al., 2016; Rajneesh et al., 2017). Rapid growth rate and cultivation in suitable in-house bioreactors and/or on non-arable land gives cyanobacteria an advantage over plants (Singh, 2014; Sarma et al., 2016). Furthermore, cyanobacteria show higher photosynthetic efficiency (~10%), as compared to land plants (~3–4% maximum efficiency) (Lewis and Nocera, 2006; Melis, 2009). Cyanobacteria can be easier genetically manipulated than other algae, and hence, serve as a better candidate in comparison to eukaryotic algae for the production of targeted chemicals and fuels. The genome size of cyanobacteria is relatively small and till date genomes of several genera have been sequenced (Rajneesh et al., 2017). Therefore, cyanobacteria provide an exceptional opportunity to conduct genetic and metabolic engineering studies for improved biomass production which is comparatively difficult to do with eukaryotic algae (Rittmann, 2008). Cyanobacteria contain considerable amounts of lipids; primarily located in the thylakoid and plasma membranes, and show higher rates of growth and photosynthesis. Enhanced production of biofuel from cyanobacteria utilizing genetic engineering has been attempted mainly on Synechocystis sp. PCC 6803 and S. elongatus PCC 7942, whose genomes are fully sequenced and molecular techniques are well-established (Kaneko et al., 1996). Genetic engineering can be used for the production of various fuels such as 2,3-butanediol, acetone 1-butanol, ethylene, ethanol, fatty acids, isobutyraldehyde, isobutanol, 2-methyl-1-butanol, and isoprene in cyanobacteria. Therefore, genetically modified cyanobacteria might play a crucial role in reduction of crude oil dependency and CO₂ emissions, owing to direct photosynthetic fixation of CO₂ to biofuel and other valuable secondary metabolites (Ducat et al., 2011; Oliver et al., 2016).

However, cyanobacteria have some limitations for their application in biofuel production. The production of valuable chemicals in photoautotrophic cyanobacteria is always less than sugar-utilizing systems such as S. cerevisiae and E. coli (Savakis and Hellingwerf, 2014). Generally, photoautotrophic cyanobacterial chassis can produce only ~100 mg of biochemicals per liter of cell culture (Gao et al., 2016), which is far too low for any commercially viable application. Theoretical yields under heterotrophic and autotrophic growth conditions for production of several chemicals have been computed for cyanobacterial chasis to find out the limiting factors in cyanobacterial metabolic network (Gudmundsson and Nogales, 2015). However, study suggests that low yield is not due to the topology of the photosynthetic metabolic networks in cyanobacteria. Therefore, it is important to optimize the inherent biological chassis for enhancing the yield of biochemicals from cyanobacteria. In recent years, several groups have emphasized on construction, designing, and expression of the biosynthetic pathways along with development of the toolboxes for metabolic engineering in cyanobacteria which could lead to economic profitability by increasing the production of existing and novel chemicals and biofuels (Wang et al., 2012; Berla et al., 2013; Desai and Atsumi, 2013; Oliver and Atsumi, 2014; Gudmundsson and Nogales, 2015; Markley et al., 2015).

It is very expensive to produce inorganic nitrogen fertilizers due to the requirement of large amount of fossil-fuel energy. This necessitated the development of alternate, sustainable and cost-effective biologically available nitrogen resources which can fulfill the nitrogen demand of agriculture in sustainable manner (Mahanty et al., 2017). For this purpose biological systems have been identified which can fix atmospheric dinitrogen (Hegde et al., 1999; Vaishampayan et al., 2001). Biological nitrogen fixation contributes ~2 × 10² Mt of nitrogen annually (Guerrero et al., 1981). According to Metting (1988), the total nitrogen fixation can be ~90 kg N ha⁻¹ y⁻¹. Symbiotic and free-living eubacteria, including cyanobacteria, are two groups of nitrogen-fixing organisms. The free-living cyanobacteria fix <10 kg of N ha⁻¹y⁻¹, however, annually ~10–30 kg of N ha⁻¹ is fixed by dense mats of cyanobacteria (Aiyer et al., 1972; Watanabe et al., 1977). Therefore, cyanobacteria constitute an important component of naturally available biofertilizers (Vaishampayan et al., 2001; Prasanna et al., 2013). Rice production in tropical countries mainly depends on biological N₂ fixation by cyanobacteria which are a natural component of paddy fields (Vaishampayan et al., 2001). In these cultivated agriculture systems, annually ~32 Tg of nitrogen is fixed by biological nitrogen fixers (Singh et al., 2016), and cyanobacteria add about 20–30 kg fixed nitrogen ha⁻¹ along with organic matter to the paddy fields (Subramanian and Sundaram, 1986; Issa et al., 2014). Cyanobacteria also make symbiotic associations with different photosynthetic and non-photosynthetic organisms such as algae, fungi, diatoms, bryophytes, hornworts, liverworts, mosses, pteridophytes, gymnosperms, and angiosperms (Rai et al., 2000; Sarma et al., 2016).

Several heterocystous cyanobacterial genera such as Nostoc, Anabaena, Nodularia, Scytonema, Cylindrospermum, Mastigocladus, Calothrix, Anabaenopsis, Aulosira, Tolypothrix, Haplosiphon, Camptylonema, Stigonema, Fischerella, Gloeotrichia, Chlorogloeopsis, Rivularia, Nostochopsis, Westiellopsis, Wollea, Westiella, Chlorogloea, and Schytonematopsis have been shown to be efficient N₂ fixers (Venkataraman, 1993). Table 1 contains a list of potential cyanobacteria which can be used as biofertilizers in agricultural fields (Vaishampayan et al., 2001). For the first time, Fritsch (1907) studied the abundance and importance of cyanobacteria with respect to maintenance of soil fertility of paddy fields through biological nitrogen fixation, which was afterwards recognized by several other workers (Singh, 1950; Fogg and Stewart, 1968; Holm-Hanson, 1968). Generally, for algalization of the rice fields, mixed cyanobacterial cultures of free-living forms are used (Venkataraman, 1972; Roger and Kulasooriya, 1980). The water fern Azolla harbors Anabaena azollae in its fronds and the cyanobacterium releases ammonium into the water when paddy fields are inoculated with foam-immobilized A. azollae strains (Kannaiyan et al., 1997). Significant increase in grain yield, biomass and nutritive value of rice can be achieved by inoculating Anabaena doliolum and A. fertilissima in paddy fields with or without urea (Dubey and Rai, 1995). Several cyanobacterial species such as Anabaena iyengarii var. tenuis, A. fertilissima, Nostoc commune, N. ellipsosporum, N. linckia, and Gloeotrichia natans are known to contribute to the productivity of rice fields in Chile (Pereira et al., 2009). Generally, application of 12.5 kg ha⁻¹ of cyanobacterial biofertilizer has been recommended for quantitative and qualitative improvements in rice production (Dubey and Rai, 1995).

In addition to rice crop, cyanobacterial biofertilizers can also enhance the yield, shoot/root length, and dry weight of wheat crops (Spiller and Gunasekaran, 1990; Obreht et al., 1993; Karthikeyan et al., 2009). Inoculation of soil with various cyanobacterial strains like Nostoc carneum, N. piscinale, Anabaena doliolum and A. torulosa results in significantly higher acetylene reducing activity (Prasanna et al., 2013). Additionally, the acetylene reducing activity is highest at harvest stage when wheat fields are inoculated with an Anabaena-Serratia biofilm along with rock phosphate (Swarnalakshmi et al., 2013). The cyanobacteria based biofertilizers are cost-effective as they cost one third to that of chemical fertilizers (Prasanna et al., 2013). In addition to nitrogen fixation, cyanobacteria also contribute to mobilization of inorganic phosphates through excretion of organic acids and extracellular phosphatases (Bose and Nagpal, 1971; Rai and Sharma, 2006). Cyanobacteria solubilize and mobilize the insoluble organic phosphates and improve the availability of phosphorus to the crop (Dorich et al., 1985; Cameron and Julian, 1988). The humus content generated after death and decay of cyanobacteria creates strong reducing condition in soil which improve the soil structure and fertility (Abdel-Raouf et al., 2012). Different cyanobacterial strains are known to produce plant growth hormones and siderophores, and therefore, cyanobacteria can affect the development and productivity of crops (Rodriguez et al., 2006; Rastogi and Sinha, 2009). The EPS secreted by cyanobacteria induces aggregation of soil particles which improve the soil structure and fertility by enhancing the accumulation of organic content and water accumulation. These findings collectively support the importance of cyanobacteria as biofertilizers, and methods have been developed for their cultivation and utilization in fertilizer industry (Brouers et al., 1987; Shi and Hall, 1988; Vaishampayan et al., 2000).

Cyanobacteria are a well-known non-conventional source of protein, healthy lipids, minerals, antioxidants, and vitamins (Pulz and Gross, 2004; Singh et al., 2005; Gantar and Svircev, 2008; Rosenberg et al., 2008). The cyanobacterium Spirulina (Arthrospira) has been used by the Aztec population as food supplement for a long time (Pulz and Gross, 2004). The various strains of Nostoc, Spirulina, and Anabaena are used as food supplements in Peru, Mexico, Philippines, and Chile. Cyanobacteria generally have low lipid content but 6–13% lipid on dry weight basis has been reported in Spirulina (Cohen, 1997). Spirulina is a good source of rare polyunsaturated fatty acid, i.e., gamma-linolenic acid (GLA), which has several pharmaceutical properties. GLA is 170 times more effective than linoleic acid, which is the main constituent of majority of polyunsaturated oils (Huang et al., 1982).

Arthrospira platensis is a best example of non-conventional source of food supplement which is mass cultivated in outdoor ponds or bioreactors, and commercially available in the form of flakes, powder, capsules or pills. It is very nutrient rich, containing more than 60% of proteins. Arthrospira is a rich source of thiamine, riboflavin, beta-carotene, and it is also considered as an excellent whole-food source of vitamin E and vitamin B₁₂ (Abed et al., 2009). Arthrospira (~20 g) can provide the required regular doses of vitamin B₁₂ and 50% of vitamin B₂ (riboflavin), 70% of B₁ (thiamine) and 12% of B₃ (niacin) in humans (Switzer, 1981). It also contains a fair amount of tocopherol (vitamin E), which is almost three times that of pure wheat germ (Hudson and Karis, 1974). The native population of the lake Chad region of Africa uses Spirulina (Arthrospira) as hardened, sun dried mat (Dihé) as a food supplement rich in proteins, minerals, carotenoids, vitamin E, folate and lipids containing essential unsaturated fatty acids (Carcea et al., 2015). Spirulina (Arthrospira) is generally termed as “magic agent” because of its wide applications as feeds, food, anticancer agent and cosmetics (Vonshak, 1997). However, Spirulina proteins are low in methionine, lysine and cysteine contents, and therefore, inferior to milk or meat protein but it is superior to plant proteins such as legumes (Ciferri, 1983). Thus, Spirulina could be utilized as a source of protein and other nutrients in countries where starvation and malnutrition is the main problem for human civilization. Spirulina can also be used for feeding hens because it stimulates egg production with intense yolk color, while Spirulina-fed fish develop a pink yellow meat of high commercial value (Sharma et al., 2011). Tilapia fish shows high growth rates in outdoor and indoor cultures when fed with marine cyanobacteria (Mitsui et al., 1983). P. valderianum is non-toxic and possesses high nutritional value, and therefore, it can be used as a complete aquaculture feeds (Steinberg et al., 1998). Thus, cyanobacteria can be used as a quality food in aquaculture and poultry farming which increases the quality and quantity of products.

Several cyanobacteria such as Anabaena flos-aquae, A. hassali, Nostoc punctiforme, Phormidium bijugatum, and Microcystis pulverana are good source of vitamin B-complex, pentothene and nicotinic acid (Robbins et al., 1951; Koptera, 1970). Similarly, N. commune is a rich sources of fibers and proteins (Abed et al., 2009). Additionally, cyanobacteria are source of pigments such as phycobiliproteins, chlorophyll a, and carotenoids. Cyanobacteria synthesize UV screening compounds such as mycosporine-like amino acids and scytonemins which have their applications in cosmetic and pharmaceutical industry (Rajneesh et al., 2017). However, several species of cyanobacteria are known to produce various cyanotoxins such as hepatotoxins, neurotoxins, cytotoxins, and dermatotoxins, and therefore, care should be taken when toxin producing cyanobacteria are used for the production of edible chemicals (Rajneesh et al., 2017). Cyanobacteria-based diet can be also used in reducing the cholesterol levels; cyanobacterial diet has been found to reduce the lipoproteins and cholesterol levels in blood serum of rats (Nakaya et al., 1988; Hori et al., 1994). Detailed studies suggest that cyanobacterial feeding increases the lipase enzyme which helps in lowering of triglycerides (Iwata et al., 1990). The consumption of cyanobacterium A. flos-aquae decrease the blood cholesterol level, stimulate liver functioning and accelerate the recovery from mild traumatic brain injury (Vlad et al., 1995; Valencia and Walker, 1999).

The phycobiliproteins produced by cyanobacteria, e.g., phycocyanin, phycoerythrin, and allophycocyanin, have several applications in food, cosmetics, medicine, and diagnostics industry. Phycocyanin from Spirulina has been used in lipsticks, eyeliner and eye shadow (Sharma et al., 2011). In Japan, phycocyanin extracted from A. platensis is marketed as a colorant of cosmetics and food (Prasanna et al., 2007). Phycocyanin is highly stable in the presence of preservatives, high temperature, and light; however, reluctance in consumption of blue foods has limited the interest of industries in utilization of phycocyanin as food colorant (Jespersen et al., 2005; Mishra et al., 2008). Phycocyanin can be also used as a rich source of protein in commercially available dietary supplements. Apart from the nutritional importance, the cyanobacteria also possess anti-inflammatory, antioxidant, anti-viral, and anti-cancer properties (Jensen et al., 2001). Several biologically active compounds have been reported from cyanobacteria (Table 2), which may contribute to the improved health on consumption of cyanobacteria based diet, and furthermore, these compounds can be also used in the development of new drugs (Rajneesh et al., 2017). The consumption of cyanobacterial diet increases the level of IgA in serum, and therefore, cyanobacterial diet can boost our immune system to fight against different allergens (Hayashi et al., 1998). Therefore, cyanobacterial consumption may help in reducing allergies, chronic inflammatory conditions, autoimmune diseases and diseases involving a suppressed immune system.

Several designs of sunlight dependent open cultivation systems have been proposed and constructed (Oswald, 1988; Chaumont, 1993; Pushparaj et al., 1997). Generally, the raceway or circular type shallow open ponds are used for mass cultivation of cyanobacteria and microalgae (Cañedo and Lizárraga, 2016). These systems are usually large raceways or open ponds where natural light is the source of energy (Figure 3A). The most advantageous aspect of this system is that the source of light, i.e., solar radiation is free of cost. However, several major disadvantages compensate this advantage and limit the overall biomass production. Contamination by algae, grazers and other microorganisms in open systems is unpreventable which compromise the net productivity. However, contamination problems could be avoided for organisms requiring unique growth conditions which generally prevent the growth of other organisms, however, this strategy limits the application of open systems for cultivation of only selected organisms (Costa et al., 2006; De Morais and Costa, 2007; Ugwu et al., 2008).

The outdoor open cultivation systems are also subject to diurnal fluctuation of different environmental conditions such as quality and quantity of light, temperature, evaporation, and rain, which significantly control the productivity and survival of the system. Apart from these limitations, open cultivation systems have been proven successful for the production of specific biomaterials producing tons of dried biomass (Lee, 1997). For example Spirulina has been extensively cultivated in Mexico, USA, China, and Thailand using open system due to its abovementioned unique growth requirement (Li and Qi, 1997; Vonshak, 1997).

Several cultivation systems have been designed which are closed and utilize solar radiation as a source of energy (Qiang and Richmond, 1994; Spektorova et al., 1997; Grima et al., 1999). In these systems, transparent material made up of plastic or glass is used for making vessels which are placed outdoors in the natural light for illumination (Figure 3B) (Khatoon and Pal, 2015). The cost of these systems is increased significantly by application of transparent materials but the closed system helps in preventing contamination of grazers and competitors. Such systems are designed in a manner which provides a higher surface to volume ratio, and therefore, cell densities obtained are often higher than in open systems (Carvalho et al., 2006).

Outdoor systems are subjected to aforementioned abiotic factors that may affect the reproducibility of their outcome. Additionally, the removal of photosynthetically produced oxygen and maintenance of optimum temperature are other factors which need to be critically observed in closed systems. These factors, if not maintained optimally, can cause a sudden crash of cultures. Techniques have been developed to maintain these parameters; however, the associated costs usually offset the cost advantage of using natural sunlight (Khatoon and Pal, 2015).

Several designs have been developed for closed (indoor) cultivation of photosynthetic microorganisms which utilize artificial light as source of energy (Ratchford and Fallowfield, 1992; Wohlgeschaffen et al., 1992; Lee and Palsson, 1994). These designs/vessels are similar to conventional fermenters in principle and are referred to as photobioreactors; however, these photobioreactors are driven by light unlike the fermenters, which are driven by an organic carbon source (Figure 3C). These photobioreactors empower real-time control and optimization of culture parameters using software, and therefore, could be suitable for the cultivation of various organisms (Ratchford and Fallowfield, 1992).

The cost of photobioreactor-based biomass production increases significantly in comparison to outdoor systems due to utilization of plastic or glass material in making the vessel and power consumption. However, the cost can be offset by the production of high quality and quantity biomass using these systems. Photobioreactors serve as an important tool for the production of high-value products such as stable-isotope-labeled biochemicals (Apt and Behrens, 1999). These systems are also ideal for the cultivation of genetically modified organisms because their release in the environment from closed photobioreactors is limited but not guaranteed. Therefore, analysis of the associated risks of spreading of genetically modified organism into the surrounding is suggested before starting a large-scale production.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.