Microalgae: An Exquisite Oil Producer

Abstract

With the influx in population and shortage of conventional energy-sources, an exponential-rise of the microalgal oil-production has been observed in the past two decades. The algal bio-oil is used in various industries viz. food, pharmaceutical, cosmetic and biodiesel plants. The present study is focused towards the production of oil from oleaginous microalgae in photo-bioreactors and open water systems. Moreover, microalgae can thrive in non-cultivable waters like seawater, salt water and even wastewater which make the algal technology more attractive in terms of soil and water preservation. Using sunlight and nutrients like salts of magnesium, potassium, sodium etc. the autotrophic microalgae can grow in large quantities in indoor photo-bioreactors and in open ponds. Microalgae are able to produce approximately 10,000 gallons of oil per acre as compared to the higher plants that produces only 50 gallons per acre (soy), 110 to 145 gallons per acre (rapeseed), 175 gallons per acre (Jatropha), 650 gallons per acre (palm). The biomass productivity is 10 times higher than that of the phytoplanktons and 20–30% higher than that of the terrestrial biomass. In terms of the fatty acid composition, the microalgal oil can well match with the plant-derived oil, mainly C16 and C18 fatty acids. Some microalgae are also rich in valuable polyunsaturated-fatty-acids, which have multiple health benefits.

1. Introduction

In this twentieth century due to the shortage in the conventional energy sources as well as exponentially rising trend of environmentally harmful products, microalgae have been chosen as an alternative source for a wide variety of metabolic products, viz. dietary supplements, pharmacological compounds, lipids, enzymes, biomass, polymers, toxins, pigments, wastewater treatment, and “renewable energy”.

The microalgal cultivation chiefly follows autotrophic growth. Almost all microalgae are photosynthetic in nature having chlorophyll a, chlorophyll b and bacterio-chlorophyll in some blue-green algae (cyanobacteria) [1], and thus are significant solar energy convertors, so, they are cultivated in illuminated environments either in open or closed cultivation systems [2].

For the last two decade microalgae have been recognized as a prominent alternative source for oil production. Several oleo genic species of microalgae can be manipulated to overproduce specific lipids and fatty acids through alteration of their physical and chemical properties of the culture medium. Microalgae can accumulate substantial amounts of lipids – up to 50% of dry cell weight in certain species [3]. Many microalgal species can thrive well in water with high salt concentration, viz. brackish water or seawater, thereby avoiding the demand for fresh water which has been designated as a limited resource in many parts of the world [4].

In microalgae, lipids play an important role in the synthesis of plasma membranes lipid protein lipid structure, in maintaining buoyancy and as an energy reserve during adverse growth conditions [5]. Accumulation of lipids in the microalgae can be attributed to the consumption of sugars at a rate higher than that of the rate of cell doubling, which promote conversion of excess sugar into lipids which favor the algae in its stationary phase of growth to fight the nutrient depletion [6].

Different saturated Fatty acids (SFA), monounsaturated Fatty acids (MUFA), and polyunsaturated Fatty Acids (PUFA) have been reported in microalgae. Hexadecanoic acid (C16:0) and oleic acid (C18:1) are common fatty acids in the microalgae. Omega-3 (ω-3) FA) can be traced in some [7]. Significantly, the microalgae can produce essential Fatty Acids, viz. alphalinolenic acid, which can be converted to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in mammals by metabolic pathways [8]. The concentration of EPA is insignificant in the terrestrial plants. Seaweeds, such as Palmaria palmata [9] can produce EPA, but in lesser concentration in comparison to the microalgae [10].

Microalgae have been designated as a producer of different types of renewable biofuels viz. methane produced by anaerobic digestion of the biomass [11], biodiesel derived from oil [12, 13, 14]; and biohydrogen can be produced photo-biologically [15, 16]. The concept of using microalgae as a source of fuel is not new [17], but for the past few decades it has being taken seriously because of the escalating price of petroleum and, most importantly, the emerging concern about global warming that is associated with the burning of fossil fuels [18].

2. Attributes of microalgae

2.1 Cultivation of microalgae

Cultivation of microalgae for biodiesel production aims at maximizing the lipid productivity along with the growth rate of the microalgae. In the batch cultivation system, microalgae are exponentially grown in the log phase to increase their biomass and then they subjected to a starvation phase by omitting or limiting the nutrient supply towards the end of the stationary phase of growth. As, algal oil is a secondary metabolite, so nutrient deprivation can lead to a higher yield.

The two most common methods of microalgae cultivation are open cultivation systems and controlled closed cultivation systems.

Open-air cultivation systems comprise natural or artificial ponds, raceway ponds, and the inclined surface systems driven by paddle wheels, usually operating at water depths of 15–30 cm [19]. They represent the classical processes used for production of algal biomass. Although different types of open reactors have been studied since last few decades by different research groups, but the most commonly used systems are shallow big ponds, tanks, circular ponds, and raceway ponds. Some of the major advantages of an open cultivation system are minimal capital, operating costs, and lower energy requirement for culture mixing. The disadvantages are open systems require large areas to scale up, susceptibility to contamination (by birds, small insects and rotifers), adverse weather conditions, difficulty to regulate growth parameters viz. evaporation rate, culture temperature, etc. A scientific investigation reported the damaging effects of the occurrence of rotifers to the cultures of Tetraselmis, Chlorella, Nannochloropsis, Scenedesmus, and the damage caused by amoeba to diatoms.

On the other hand, closed and controlled cultivation systems employ photobioreactors to attain axenic single-species culture of microalgae. Photobioreactors are successfully used for producing large quantities of microalgal biomass [20]. The different types of photobioreactor (PBR) include horizontal or serpentine tube, flat-plate, bubble column, airlift column and stirred tank. PBRs can be designed and calibrated according to the research need and the experimental organism. This closed system utilizes relatively little space, while increasing the light availability and minimum contamination issues. However, the limitations of PBRs include bio-fouling, overheating, benthic algal growth, cleaning issues, growth limitation due to high build-up of dissolved oxygen and costlier operation [21, 22].

2.2 Oil productivity

Oleaginous microalgae (Table 1) are a promising source for the production of renewable biofuels because of their efficient photosynthetic capabilities. Moreover, microalgal growth requires less area in comparison to the terrestrial plants, and they are capable to channel the majority of the acquired energy into cell division, which increases the biomass yield [23]. Microalgae can be subdivided into four different groups depending on the carbon source (inorganic and organic), namely, autotrophic, mixotrophic, heterotrophic, and photoheterotrophic [24].

Microalgae	Oil yield (%)
Scenedesmus sp	38
Chlorella protothecoides	49
Botryococcus braunii	28
Tetraselmis elliptica	14
C. vulgaris	89
Auxenochlorella protothecoides	66
Chlamydomonas reinhardtii	59

Table 1.

Some prominent oleaginous microalgae.

The synthesis of triacylglycerol in microalgae takes place mostly in the chloroplast and endoplasmic reticulum through multiple enzymatic reactions [25]. Fatty acid synthesis in the chloroplast, assembly of glycerolipids in endoplasmic reticulum, and accumulation of TAGs into the oil bodies are the three major steps involved in the accumulation of lipids in the microalgae [26]. It has been proven that facilitate the synthesis of high amounts of lipids is influenced by different stress conditions such as physical, chemical, or environmental, individually or in combination [27]. Under the aforementioned stress conditions, microalgae can switch their metabolism towards the synthesis of neutral lipids in the form of TAGs, which serves as a form of carbon and energy storage [28, 29, 30]. Microalgae employ the de novo pathway to synthesize lipids. It starts in the chloroplast by CO2 fixation into sugars, which are further metabolized to acetyl-CoA, which acts as a precursor of fatty acid synthesis [31].

Marine microalgae have a higher content of PUFA in comparison to the freshwater species because they need to produce more unsaturated fatty acids to survive in the salty marine environment [32]. Thus, cultivation of marine microalgae can render higher economic interest to the cultivars. According to reported literature, the marine oleaginous diatom Fistulifera solaris when cultivated in photoautotrophic conditions can produce 135.7 mg/(L·day) EPA. On the otherhand, the heterotrophic growth of the marine diatom Nitzschia laevis, when supplemented with glucose, resulted in EPA production of 174.6 g/(L·day) [33].

2.3 Extraction of oil

During lipid extraction, the microalgal biomass is exposed to an organic eluting solvent which extracts the lipids out of the cell cytoplasm. A lipid extraction technology for microalgal oil production needs to be highly specific towards the lipids in order to avoid the co-extraction of non-lipid contaminants, viz. protein and carbohydrates. The lipid extraction technology should be more selective towards acylglycerols than other lipid fractions as they are not readily convertible to biodiesel, such as polar lipids and non-acylglycerol neutral lipids (free fatty acids, hydrocarbons, sterols, ketones, carotenes, and chlorophylls) [34]. Moreover, the technology should be efficient (both time and energy saving), non-reactive with the lipids, relatively cheap (capital cost and operating cost), and safe (environmentally and mechanically) [35]. Dewatering of the microalgal biomass beyond a paste consistency (200 g dried microalgal biomass/L culture) is energy consuming, so, it will be economically friendly if the selected lipid extraction technology is effective for the wet feedstock, i.e. concentrate or disrupted concentrate with concentrations between 10 and 200 g dried microalgal biomass/L culture [36].

2.3.1 Solvent extraction

The principles underlying solvent extraction of microalgal lipids are based on the concept of chemistry ‘like dissolving like’. The long hydrophobic fatty acid chains interact with neutral lipids through weak van der Waals forces, thus forms globules in the cytoplasm [37]. The mechanism for organic solvent extraction is depicted in Figure 1. When a microalgal cell is exposed to a non-polar organic solvent, such as hexane or chloroform, the organic solvent penetrates through the cell membrane into the cytoplasm and interacts with the neutral lipids trough van der Waals forces to form an organic solvent-lipids complex. This organic solvent–lipids complex, driven by a concentration gradient, diffuses across the cell membrane. The neutral lipids are thus extracted out of the cells and remain dissolved in the non-polar organic solvent. However, some neutral lipids remain as a complex with polar lipids in the cytoplasm. The complex is strongly linked via hydrogen bonds to the proteins in the cell membrane. The van der Waals interactions formed between non-polar organic solvent and neutral lipids are inadequate to disrupt these membrane-based lipid–protein associations. On the other hand, polar organic solvent (viz. methanol or isopropanol) is able to disrupt the lipid–protein associations by forming hydrogen bonds with the polar lipids in the complex [38].

Figure 1.

Schematic representation depicting the solvent extraction of oil from oleaginous microalgae. 1. Penetration of the solvent inside the cell cytoplasm. 2.Solvent interaction with the lipids. 3. Formation of lipid-solvent complex. 4. Diffusion of the complex through the cell membrane. 5. Diffusion of the complex in the surrounding bulk organic solvent.

Figure 2 depicts the extraction steps generally, undertaken for laboratory-scale production of microalgal oil and finally trans-esterified to biodiesel using an organic solvent mixture.

Figure 2.

Microalgal oil and further trans-esterification to bio-diesel extraction procedure.

2.4 Compositional analysis of algal oil

Algal oil is very high in unsaturated fatty acids [39]. The mentioned fatty acids are present in abundance in algal oil [40].

• Arachidonic acid (AA)

It is an unsaturated, essential long chain (C20) fatty acid and acts as a precursor in the biosynthesis of prostaglandins, thromboxanes, and leukotrienes.

• Eicospentaenoic acid (EPA)

EPA is an omega-3 fatty acid. Omega-3 fatty acid helps lower risk of heart disease, lower triglycerides in the blood, blood pressure, and inflammation.

• Docasahexaenoic acid (DHA)

DHA is a long-chain, highly unsaturated omega-3 fatty acid. It affects cell and tissue physiology and function by altering the membrane structure. It also has prominent roles in membrane protein function, cellular signaling and production of lipid mediator.

• Gamma-linolenic acid (GLA)

GLA is an essential fatty acid belonging to omega-6 fatty acid. Omega-6 fatty acids play a crucial role in brain function, growth and development, stimulate skin and hair growth, maintain bone health, regulate metabolism, and maintain the reproductive system.

• Linoleic acid (LA)

It is an essential fatty acid belonging to the omega-3 group. It has been reported to inhibit the synthesis of prostaglandin resulting in reduced inflammation and prevention of certain chronic diseases.

2.4.1 Elemental (CHNO) analysis of the algal oil

CHNO analysis of the algal oil depicts the elemental constitution in order to assess its economic and environmental significance. The Carbon (C), Hydrogen (H), Nitrogen (N) and Oxygen (O) percentage (wt/wt) are reported as 64.2, 16.11, 0.87, and 21.8% respectively in the oil obtained from C.vulgaris [41]. The higher heating value (HHV) of the algal oil has been reported as high as 40.43 MJ/kg [42]. The high fraction of carbon and HHV ensure large energy content of the oil that can be further trans-esterified to bio-diesel or any bio-oil. A negligible fraction of nitrogen (0.89%) and no sulfur content advocate that there is probably no or minimal chance of NO_x and SO_x emission during the utilization of the algal oil for energy generation.

2.4.2 Analysis of lipids as total fatty acid methyl ester (FAME), protein, carbohydrate, and ash content of the microalgae

According to Table 2 the composition of different microalgal species viz. S. acutus, C. vulgaris, and N. granulata varies significantly depending on the growth-stage and the growth environment. As per a scientific investigation [43], longer cultivation time and nutrient depletion, the lipid content increases and protein content decreases. The carbohydrate content in N. granulata and S. acutus increased from early to mid growth stage, while decreased from mid to late stage. On the other hand, an increase in carbohydrate content has been noted in C. vulgaris in the starvation phase.

Microalgae	Growth phase (batch cultivation)	FAME lipids	Ash	Carbohydrates	Proteins
N. granulata	Early	12.30	14.2	8.92	32.7
	Mid	25.52	13.6	11.12	23.1
	Late	57.33	5.1	10.89	9.4
S.acutus	Early	9.15	4.5	16.88	46.3
	Mid	17.03	1.8	49.71	17.4
	Late	38.55	2.2	39.42	7.85
C.vulgaris	Early	12.07	6.7	11.12	43.27
	Mid	15.02	4.4	35.69	24.00
	Late	23.14	5.3	38.00	15.2

Table 2.

Compositional analysis of the microalgae.

3. Conclusion

The present study advocated the different attributes of oil production through micro-algal route. Conventional algal cultivation systems include raceway ponds with lower investment cost but also renders lower yields while high value products (microalgae and oil) are produced with closed photobioreactors with high cost investments. However, fresh water demand can be reduced employing the photobioreactors, using waste water, brackish water or seawater as the culture broth.

The presence of high amount of polyunsaturated fatty acids gives micro-algal biofuel good flow properties under low temperatures, thus, reducing the risk of cold filter plugging and making it suitable as aviation fuel. Most importantly, the oil composition can be manipulated by selecting the desired strain and the cultivation parameters. This significant feature of the algal oil probably can be a solution to overcome the falling economy. Micro-algal biofuel production can possibly be solutions to reduce conventional energy demand. But increase in yields by enhanced reactor designs, optimized process control, adapted separation technologies as well as integral concepts considering biology, process design for cultivation, and downstream processing to meet the different requirements for the specific target markets should be addressed in biorefinery concepts to be integrated with coal-based power plants to ensure a cleaner and greener tomorrow.

Conflict of interest

The authors declare no conflict of interest.

Thanks

A heartiest thank to my daughter, Vivita Mukherjee for her awesome co-operation in my work.