Impact of abiotic factors on biodiesel production by microalgae

Highlights

• Microalgae are considered as the reliable resources of biodiesel production.

• Produce third generation biofuel with high productivity value.

• Promising impacts of physical pretreatment on microalgal biomass.

• Review towards physical abiotic factors and pretreatment of microalgae.

Abstract

The most advantageous renewable energy source which is still being explored for the source of renewable energy is biodiesel. But the major hindrance factor in the part of commercialization is the raw material cost under large scale various. Microalgae derived biofuels have drawn compelling interest as a sustainable and potential fossil fuel. The cellular mechanism of microalgae has been found to alter by the effect of some biotic and abiotic factors. Above all, those environmental parameters and production conditions can also act as a critical factor in altering the component quantity and quality of the biodiesel produced by microalgae. The lipids are accumulated inside the oleaginous microbial cells in the form of lipid droplet, and hence the cell disruption is essential for lipid extraction to enhance the yield. Huge varieties of physicochemical and mechanical pretreatment methods are available to facilitate extraction by effective cell membrane degeneration of oleaginous microbes. The present review is to exploit the probable magnitude and advantages of abiotic factors towards microalgal growth and biodiesel production. Additionally, this study tried to evaluate the possible certainties of the microalgal pretreatment processes in lipid accumulation.

Introduction

The microalgae are the distinct species that can synthesize an ample amount of TAGs (triacylglycerols) as lipid globules during unfavorable environmental conditions [1]. These species can able to grow very fast and produce greater biomass with higher lipid and carbohydrates content when compared with trees and other terrestrial crops [2]. Recently, research-based on algal derived biodiesel is drawing substantial attention in applied and basic areas of research. The vital factor concerning algae for biodiesel production is shifting their inherent evolutionary characters like nutritional mode, availability of substrate and light. While concerning the substrate, algae could consume organic molecules with micronutrients and can fix atmospheric CO₂. A few algae with utilizing higher energy from light and fixing atmospheric CO₂ are autotrophs. In such mode, limited biomass yield, need of large surface with shallow area cultivation system so as to access light is the innate disadvantages. If the process of photosynthesis from where the algae could gain energy would be suppressed, then the algae would probably derive energy from alternate organic processes like converting sugars to lipids [3]. Even though substantially denser growth with worthy yield was possible in algae, the limitation in light penetration is the vital drawback. And hence, the heterotrophic nutritional mode by utilizing organic molecules as both carbon and energy source would facilitate higher biomass productivity with economic friendliness for large scale production [4]. Additionally, simple operation, easy maintenance and cost-effectiveness are the desirable factors of attraction during heterotrophic cultivation [4].

The growth conditions both in biotic and abiotic are considered for algal growth. But while under extreme environments, they generate specific metabolites to adapt and overcome the stressful environment. Microalgae are strictly adaptable to some abiotic stress factors and resulted in some valuable metabolites production. This unique characteristic of microalgae could be investigated for the metabolite products along with the biorefineries for sustainable progress. The employment of microalgae has been now progressed from a modest food source to the high-value energy products recently. Enhancement in lipid and carbohydrate accumulation by microalgae was achieved under persistent stress conditions; nevertheless, the growth rate has to be compromised with a reduction in overall productivities and thus finally results in the inclination of biodiesel production cost [5].

Several abiotic factors are responsible for the lipid accumulation in microalgae. Among them, nutrient deficiency or stress, especially nitrogen or phosphate [6], is thoroughly being acknowledged for the increase in microalgal TAG accumulation [7]. Lipid productivity of the microalgal strains is species-specific, which confides on the abiotic factors like salinity⁷, the intensity of incident light and temperature [8]. Earlier studies confess already that the dilution of light would reveal prominent enhancement in biomass productivity [9]. This is because of the light radiation provided on the surface of the culture that results in an increase in the dark cycle frequency of the cells, thus affording higher light impulse rate per reaction center [9]. The vital components essential for the growth of eukaryotic organisms are polyunsaturated fatty acids (PUFA). Amid them, docosahexaenoic acid (C22:6) and eicosapentaenoic acid (C20: 5) are considered to be vital due to its pharmaceutical and nutraceutical applications. Microalgae produces numerous compounds along with PUFAs under abiotic stress environment and thus survive in a tremendous environment by acclimatization. Higher biomass productivity with economically viable PUFAs (EPA and DHA) production was achieved during stress conditions, including optimal concentrations of carbon, nitrogen, phosphorus, salt, the intensity of light [10] with UV-radiation under organized pH and temperature [11].

The cultivation efficiency of the microalgae depends significantly on the limitation of light, evaporation of water, fluctuation of temperature, the minimum intensity in mixing and insufficient carbon dioxide availability. The growth parameters like carbon dioxide, temperature and pH could be optimized and controlled well in the operation of photobioreactor with the production cost [12]. It has been significantly reported that the abiotic stress factors are responsible for the formation of ROS (Reactive Oxygen Species) and lipid peroxidation; thus, the abiotic factors are corresponding to the increased content of PUFA. These PUFAs likely engage in the repair mechanisms of the cellular membrane by scavenging free radicals present.

In the marine ecosystem, the microalgae are the primary producers of polyunsaturated fatty acids (PUFAs) which can grow under autotrophic, heterotrophic and mixotrophic environmental conditions [13], have shorter harvesting time than plants, can fix atmospheric CO₂ [14], and can utilize light energy and CO₂ into carbohydrates, protein and lipid-rich biomass. Triglycerides mainly composed of PUFA are considered as the vital microbial oil component, in which high unsaturated fatty acids (C16 and C18) are similar chemically to vegetable oils like rapeseed, palm, soybean oil, etc. are observed [15]. These properties of the microbial oil are mainly responsible and indicative of biodiesel quality.

The vital challenge encountered in the biomass utilization while bioconversion is the complex structural characteristics of the cell wall components for which pretreatment is essential before bioconversion. These pretreatment processes are helpful to ease the usage of components for numerous applications, like biomass valorization. Above all, the biomass is converted to biofuels and fermentable sugars by pretreatment processes. Numerous studies are found available in demonstrating the excellent productivity of pretreated biomass than the untreated biomass [16], [17]. The lipids are usually accumulated intracellularly, thus causes the lipid recovery processes more complicating on both lab and large-scale [18]. Lipid extraction with organic solvents is commonly followed by the lysed microbial biomass by cell disruption or pretreatment methods [19]. Dewatering or drying processes of the microbial biomass, makes the cellular disruption methods more energy demanding and expensive [20].There are various pretreatment methods like autoclaving, bead beating, high-speed homogenization, high-pressure homogenization, ultrasonication, microwave irradiation and thermolysis are followed for extraction [21]. Above all, the microbial conversion into renewable energy can be made possible by a huge number of thermochemical pretreatment methods such as liquefaction, gasification, combustion, pyrolysis and torrefaction [22]. The major hindrance in the biomass conversion is the complex cellular structure and complexity of cell wall components. In accordance with the microalgal cell wall, the primary component is a polysaccharide with proteins, calcified structures and biopolymers [23]. The cell wall-bounded organic compounds are made up of hemicellulose and cellulose, which is responsible for the resistance to biodegradability. Similarly, brown algae cellulose is replaced by the glucan in the form of laminarin. The cell wall rigidity based on cellulose and hemicellulose confides the polymer biodegradability. And hence biomass pretreatment is pivotal to improve the cell wall digestibility and production of bioproducts like biofuel. The four major techniques are commonly employed for biomass pretreatment like mechanical, thermal, biological and chemical. Among which mechanical and thermal are considered more proficient in the disruption of the microalgal cell wall.

Even though the huge number of reviews concerning the beneficiary effects in the utilization of microalgae for biodiesel production is available [8], [9], [10], [11], [12]. The current review tried to explore the recent advances with the environmental and nutritional parameters involved in the production of high value-added products like biofuel.

Greater oil yield was possible in microalgae with higher lipid content accumulation under considerable stress conditions. A few heavy metals like Mn, Ni, Fe, Cu and Zn acts like micronutrients for essential biological processes such as precursors for vitamins, cofactors, metalloenzymes and some structural proteins present in the cell membrane [24]. Some metal ions possess numerous physiological functions that have a significant role in microalgal metabolic activity, along with lipid accumulation [25]. The calcium ions act as a universal messenger for environmental signaling and stimuli development [26]. However, Mg²⁺ ions act vitally as a signaling factor by activating and mediating various biochemical reactions like carbon fixation regulation of the Calvin cycle, which occurs in the chloroplast [27]. Above all, an increase in Mg2 + ions aids the function of fatty acid synthesis regulators like Acetyl-CoA carboxylase, thus results in enhancement of microalgal lipid content [25].

Moreover, the cultivation conditions involve in lipid, and TAG synthesis triggers a concurrent increase in protein biodegradation along with limitations in biomass accumulation and cell growth. Thus the overall lipid productivity of the species affected significantly based on the various magnitudes involved [28]. Even though the productivity of lipids is greatly strain-specific, the cultivation parameters (physical and chemical) like temperature, light and cultivating environment can be altered so as to increase the lipid yield of the strains [29], [30], [31]. But focusing on the optimization of culture conditions to enhance overall lipid productivity exhibits limitations because of the diverging cultivation conditions employed to incline lipid content and biomass productivity. Thus, a two-stage method of cultivation usually non– reactive on either stage is implemented during production in which greater biomass is attained on the first stage of development, which transforms the cultivation conditions suitable for triglycerides production by depleting nitrogen [32]. A huge number of factors are found to be pivotal for microalgal biomass production and lipid production. Those parameters could be largely categorized into physical and chemical parameters (Fig. 1).