1 Introduction
The global population is expected to increase by 9 billion in 2050 [
1], which will lead to more fossil fuel consumption with increasing greenhouse gas emissions (GHGs) [
2]. Therefore, seeking another energy source is a necessity for reducing GHGs emissions, minimizing the dependence on fossil fuels, and maintaining environmental sustainability [
3,
4]. Recently, there has been considerable attention to microalgae as a source of third-generation liquid biofuels. The utilization of microalgae for biofuel production offers several benefits. Firstly, they can grow in different waters [
5], including wastewater, and double their biomass in periods as short as 3.5 h. Secondly, microalgae can be cultivated without the need for herbicides or pesticides application [
6] due to their ability to live on non-arable lands [
7]. Thirdly, they are employed in wastewater treatment to remove nutrients such as phosphorus, nitrogen, carbon, and other toxic compounds [
8,
9]. Finally, different bio-products other than biofuel can be produced from microalgae, such as cosmetic products, animal feed, and valuable pharmaceuticals [
10,
11]. Biofuels produced from microalgae are generated via processes, including ethanol production by carbohydrates saccharification [
12‐
15], biodiesel production by lipid transesterification [
16,
17], and crude bio-oil production by thermochemical conversion [
18]. Researchers further investigated microalgae as a gas fuel source, such as biohydrogen [
19] and biogas [
20].
Biodiesel is a renewable and biodegradable fuel produced from animal fats or vegetable oils reactions with short-chain alcohol (methanol or ethanol) through the transesterification process in the existence of a catalyst. Catalysts are incorporated at 1.5 h and 60 °C under 101,325 Pa, in order to reduce response time and rising conversion rate [
21]. Transesterification switches over fresh to dense microalgae lipids to decrease corrosive alkyl esters or the subatomic weight of fats, triacylglycerols, or safe unsaturated fats [
22]. Several catalysts are employed in the process of transesterification such as alkaline, acid, enzymes, and heterogeneous inorganic catalyst [
23]. In a comparison with conventional diesel, biodiesel can equilibrate the negative balance generated by the emission into the atmosphere. Further, it reduces carbon monoxide (CO), sulfur compounds (SO
x), and particulate matter (PM) emissions and having better lubricity and renewability [
24]; however, it contributes to increasing nitrogen oxide (NO
x) emissions [
25].
In the past few decades, microalgae species have been utilized to produce biodiesel. For instance,
Chlorella vulgaris [
26],
Chlamydomonas reinhardtii [
27],
Scenedesmus dimorphus,
Scenedesmus obliquus [
28,
29],
Nannochloropsis granulata,
Nannochloropsis oculata [
30],
Scenedesmus dimorphus [
31], and
Caulerpa prolifera [
13] were evaluated for biodiesel production with lipid content 6, 49–52, 10, 30–50, 28.5, 45, 14.71 ± 0.26, and 10%, respectively. However, several challenges are facing microalgal technologies despite their participation in global energy demand. It is essential to understand the optimum growth condition of the selected microalgae to enable a large-scale production system. Such condition enhances lipid accumulation or promotes high biomass production, which leads to a reduction in the production cost. Several studies have shown an effective method to enhance lipid accumulation via sulfur and nitrogen limitation [
32,
33].
Arthrospira platensis (formerly
Spirulina platensis) is a species of microalga with high protein content.
A. platensis was first isolated by Turpin in 1827 [
34]. It has a spiral-shaped trichoma with 100–110 μm long and 8–10 μmm wide. It is a vital microalga as it contains high protein, pigments, gamma-linolenic acid (GLA), vitamins, fatty acids, and other valuable metabolites [
35]. It is worth mentioning that five fatty acids are most abundant in the
A. platensis, namely palmitic acid, palmitoleic acid
, oleic acid, linoleic acid, and γ-linolenic acid [
36,
37].
Arthrospira platensis is extensively cultured at a mass commercial scale and is considered as one of the most famous blue-green algae or cyanobacteria [
38,
39]. Worldwide, the production of
Spirulina has increased to over 89 thousand tons in 2016 [
40]. It is widely utilized in many biotechnological applications such as biofertilizers, pharmaceuticals, pigments, human food supplements, omega-3-fatty acids, and animal feed additives [
38,
41‐
44].
Recent studies have concluded that due to the high biomass and lipid productivity of
A. platensis, it is considered one of the best feedstock for biodiesel production [
39,
44‐
46]. As confirmed by literature, the extracted biodiesel from
A. platensis is within the recommended specifications of the international standards of Europe (EN14214) and the USA (ASTM675103) [
43,
44,
46]. For instance, the cultivation of
A. platensis in an iron-enriched medium has been investigated by Cepoi et al. [
47] who stated that the adaptation of the
Spirulina’s biomass to copper-containing effluents was more noticeable in Cu/Fe systems since in multicomponent systems, the process was more complex and varied based on the metal type, amount, and concentration. The spectra of iron hydroxides in
A. platensis biomass differed from those of iron complexes put into Zarrouk’s growth medium with ethylenediaminetetraacetic acid. The saturation limit of
A. platensis trichomes with iron in the form of ferrihydrite in the culture medium was found to be 5 g/mL (0.09 mol/mL) Fe [
48]. By utilizing Mössbauer spectroscopy, Wan et al. [
36] identified the effects of iron on
Chlorella sorokiniana growth and lipid synthesis, as well as the enzymes and metabolic pathways that may be altered in response to variations in iron levels in the environment. When compared to unsupplemented controls, the addition of iron up to 10
5 mol L
−l boosted ultimate cell densities by approximately twofold at 2.3 10
7 cells/mL, growth rate by twofold, and the length of the exponential phase by 5 days, while 10
3 mol L
−1 iron was hazardous. The lipid content increased from 12% for unsupplemented cultures to 33% at 10
−4% mol L
1 iron, with the maximum total lipid output of 179 mg L
−1. To the best of our knowledge, studies have not fully investigated the effects of different ferrous sulfate concentrations on the fatty acid profile, biomass, and pigments of the blue-green alga
Arthrospira platensis. Thus, this study evaluated
A. platensis for biodiesel production by cultivating it in various ferrous sulfate (FeSO
4) concentrations to find the optimal conditions. In addition, the assessment procedures involve studying the effects of the different cultivation conditions on some fatty acids profile, the growth rate, and the doubling time
. Furthermore, the effect of FeSO
4 on the biochemical composition of the alga as a nutritional source was evaluated.
4 Conclusions
Recently, there has been considerable attention among the scientific community with regard to microalgae to produce third-generation liquid biofuels. The present study examined the potential of A. platensis to produce biodiesel. Therefore, five concentrations of FeSO4 (0, 0.005, 0.01 (control), 0.05, and 0.1 g/L) were utilized. The fatty acid profile of the A. platensis revealed that palmitic, oleic acid, linoleic acid, γ-linolenic acid, and docosahexaenoic acid were most prevalent. It was also found that the maximum-minimum yields of fatty acid individuals were achieved with 0.1–0 g/L FeSO4, respectively. The study indicated that when the concentrations of FeSO4 were between 0.05 and 0.1 g/L, palmitic acid, oleic acid, linoleic acid, γ-linolenic acid, and docosahexaenoic acid were increased, and at low concentrations, these fatty acids decreased. The maximum induction rate of the total fatty acids was obtained in the modified media with 0.1 g/L FeSO4. The results revealed that the highest growth rate and the shortest doubling time were recorded with the FeSO4 concentration of 0.1 g/L. The best concentration used to increase the yields of chlorophyll-a, phycocyanin, allophycocyanin, and phycobiliproteins is 0.1 g/L FeSO4, and carotenoids is 0.1 g/L FeSO4. Finally, increasing the FeSO4 concentration to 0.1 g/L has led to the increase in fatty acid individuals, hence, the potential enhancement of the biodiesel production from the point of view of quality and quantity.
Publisher's note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.