A state of the art review on the cultivation of algae for energy and other valuable products: Application, challenges, and opportunities

Petroleum or fossil fuels are considered as a depleting energy reserve against growing demand due to their non-renewable status, and unarguably poses a potential threat to the transportation sector [1]. Across all major sectors, expanding the population, increasing infrastructural and socioeconomic development would trigger fossil fuel consumption. Increasing depletion of fossil fuel reserves, uncertainty in their supply and rapid rise in petroleum prices have kindled search for alternatives to fossil fuels [2]. In addition, climate catastrophe accentuated by the potentially unfriendly gases from the fossil fuel combustion is an indisputable hazard to human society [3,4]. Owing to high fuel usage, petroleum prices were escalated up to a point where alternative fuel is competitive and needed to moderate an inevitable upward march of oil prices as well as to meet the socio-economic demand. Thus, the modern world has been urged to shift from petroleum fuel to carbon neutral, renewable, alternative fuel through multidimensional global strategies. Given this, there are vigorous research initiatives sought whilst aiming for new alternatives, which are likely to alleviate dependence on fossil fuel imports as well as to circumvent global warming calamity [5,6]. In this scenario, biofuel came to limelight specifically, microalgal based biofuel have gained increased attention as a sustainable fuel [7]. In addition to biofuel production, microalgae can be used for the extraction of various non-energy products or high-value products or industrially important co-products either direct extraction method or integrated sequential bio-refinery technique. Algae contain proteins, carbohydrates, lipids, and nucleic acids as their biochemical components and notably, free fatty acids, triglycerides, phospholipids, and glycolipids from the lipid biomolecule serve as a source for substitute energy [8]. Further, culturing and maintaining indoor (laboratory grown) microalgal strain in outdoor ponds beyond one month is difficult since the strain is failed to acclimatize an unconducive outdoor environment, and often encounter cross-contamination as well [9]. Hence, maintenance, operation, and performance features of cultivation bio-system has to be improved through in-depth research efforts [9].

The algae of the marine environment have diverse species and thus, provides wide opportunities to produce functional foods from them for sustainable development [10]. In concern with the application of algal bioactive compounds in pharmaceutical industry, marine microalgal anti-cancer compounds were least studied and many investigations were carried out with extracts or fractions of microalgae acquired through liquid-liquid partitioning or solid phase low-resolution extractions [11]. Though various fuel and non-fuel products (valued added products) are produced from algae (Fig. 1), still it faces major challenges as several algal-based technologies are in a laboratory level and thus pilot-scale study needs to be undertaken. In concern with biofuel production from algae, the technical feasibility of the process to compensate the fossil fuel is comparatively high and not energy intensive. With reference to cultivation, various obstacles need to be addressed and untangled to develop effective mass cultivation technology for microalgae and it relies on meticulous understanding on photosynthetic, structural attributes of microalgae and dominance of target strain over alien strain/bacteria contamination and unavoidably biomass productivity per unit area. In concern with macroalgae for fuel production, macroalgal biomass generation is relatively simple due to its cultivation near seashore with ample sunlight availability, seawater availability, CO₂ mitigation. But, there are significant technical and economic challenges for third-generation bioethanol/biogas from macroalgae. Usage of chemicals, thermal energy and process time are the crucial part of the fermentation production of biofuel from macroalgae. Macroalgae cultivation depends on seasonal changes in the sea environment and also differs with country wise. Only few species of the macroalgae are cultivable throughout the world based on their country's native species. Still tissue culture development for the macroalgae species under infant stage and basically used to preserve the native genome for future use. Further for biofuel production macroalgae are viable only proper fermentation technology integration with seaweed industries along with other by products separation is must to reduce the cost of production. Potential bacterial/yeast strains are essential to produce higher yield of ethanol or biogas from seaweeds. These technical challenges have to overcome in future in order to achieve biofuel production economically. To achieve commercial viability of the micro/macroalgal products, cost-effective methods should be developed and a basic understanding of the process is essential. To achieve commercial viability of the microalgal/macroalgal products, cost-effective methods should be developed.

Therefore, this review aims to provide a comprehensive view on the different cultivation systems of micro and macroalgae for energy and non-energy products. Then, different cultivation system being used for algae would be discussed in detail and merits and demerits of the cultivation types would be given to choose an optimal design. Eventually, this review analyses the reports dealing with the extraction of other products such as fatty acids, vitamins, nanoparticles and valued products from algae and their application in human health like antiangiogenic, anticancer, and anti-inflammatory properties.