•Ultrasound assisted in-situ transesterification efficiently produces the biodiesel.•Ultrasound treatment enhances the mass transfer characteristics of reaction mixture.•Ultrasound increases the ...reaction rate with shorter reaction time.•Limited ultrasound biodiesel studies in microalgae necessitates further investigation.
The application of ultrasound in biodiesel production has recently emerged as a novel technology. Ultrasound treatment enhances the mass transfer characteristics leading to the increased reaction rate with short reaction time and potentially reduces the production cost. In this review, application of ultrasound-assisted biodiesel production using acid, base and enzyme catalysts is presented. A critical assessment of the current status of ultrasound in biodiesel production was discussed with the emphasis on using ultrasound for efficient microalgae biodiesel production. The ultrasound in the biodiesel production enhances the emulsification of immiscible liquid reactant by microturbulence generated by cavitation bubbles. The major benefit of the ultrasound-assisted biodiesel production is a reduction in reaction time. Several different methods have been discussed to improve the biodiesel production. Overall, this review focuses on the current understanding of the application of ultrasound in biodiesel production from microalgae and to provide insights into future developments.
•UV mutagenesis of Scenedesmus sp. can increase both biomass and lipid content.•Oxidative stress by H2O2 in the UV mutant improves the total lipid production.•FTIR results confirms the changes of ...lipid and protein content in H2O2 treated mutant.•UV mutagenesis and H2O2 treatment do not affect the quality of methyl ester.
The high potential UV mutagenized Scenedesmus sp. was obtained in which the cells had a higher biomass and lipid content than the wild type with an increase from 1.9 to 2.4g/L and from 40 to 55% of dry cell weight respectively after 12days. Oxidative stress imposed by H2O2 treatment decreased the biomass of both the wild type and the mutant. The H2O2 treated mutant when grown in BG11 medium showed an increase in biomass which was in contrast to a decreased biomass observed in the H2O2 treated wild type. A 3-fold increase in lipid yield of 1.63g/L was obtained in the oxidative stress-induced mutant compared to the wild type. Overall results indicate that prior treatment of UV-mutagenized Scenedesmus with oxidative stress can increase the total lipid production which, due to its derived methyl ester having acceptable biodiesel properties, can be potentially utilized for biodiesel production.
Abstract
The photoautotrophic Synechocystis sp. PCC 6803 (hereafter Synechocystis) is known for its α-polyglucan (glycogen) synthesis to serve as a carbon storage compound. In this study, the glgC- ...and glgA-overexpressing Synechocystis strain with the disruption of polyhydroxybutyrate (PHB) synthesis (▴GCAX-ΔBK) showed an increased glycogen production. This engineered strain had a high glycogen content of 38.3% (g g–1 dry cell weight) as compared with 27.4% in the phaA knockout strain (ΔBK) and 34.8% in the glgC/glgA-overexpressing strain (▴GCAX) after 20 d growth. Under nitrogen-deprived growth conditions for 3 d, the ▴GCAX-ΔBK strain showed a further increase in glycogen content from 27.0% to 36.0%. Furthermore, the engineered strains grown under ionic, osmotic or oxidative stress conditions had an increase of glycogen accumulation, whereas no increase was observed in the wild type. The maximum glycogen content was 54.0% in the ▴GCAX-ΔBK strain treated with 3 mM H2O2. The overall results indicated that in the absence of PHB synthesis, Synechocystis cells redirected the carbon flow towards the synthesis of glycogen as an alternative physiological responsive compound especially under stress conditions.
Sustainable production of bioplastics by heterotrophic microbes has been restricted by the limited resources of organic substrates and the energy required for biomass harvest. Here, the ...easy-to-harvest cyanobacterium (Chlorogloea fritschii TISTR 8527), from which the biomass instantaneously settled to the bottom of liquid culture, was utilized to produce poly-3-hydroxybutyrate (PHB) using a two-stage cultivation strategy. The cells were first pre-grown under normal photoautotrophy to increase their biomass and then recultivated under a heterotrophic condition with a single organic substrate to produce the product. Through optimization of this two-stage cultivation, the mass conversion efficiency of acetate substrate to PHB was obtained at 51 ± 7% (w/w), the comparable level to the theoretical biochemical conversion efficiency of acetate to PHB. This two-stage cultivation that efficiently converted the substrate to the product, concurrent with a reduced culture biomass, may be applicable for the production of other biopolymers by cyanobacteria.
•The presence of PHB production in divergent cyanobacteria.•The specific nutrient conditions that increased PHB accumulations in the specific strains.•Experimental mean to optimize efficiency of ...energy conversion from solar energy to PHB proceeded by cyanobacteria.
The cellular PHB content was determined in 137 strains of cyanobacteria representing 88 species in 26 genera under six photoautotrophic nutrient conditions. One hundred and thirty-four strains were PHB producers. The PHB contents of these 134 strains were subtle under normal growth condition, but were significantly increased in 63 strains under nitrogen deprivation (–N), a higher frequency than with phosphate and/or potassium and all-nutrient deprivation. A high PHB accumulation was not associated with any particular evolutionary groups, but was strain specific. The filamentous Calothrix scytonemicola TISTR 8095 produced 356.5±63.4mg/L PHB under –N from a biomass of 1396.6±66.1mg/L, giving a PHB content of 25.4±3.5% (w/w dry weight). This PHB productivity is equivalent to the CO2 consumption of 729.2±129.8mg/L. The maximum energy conversion from solar energy to PHB obtained by C. scytonemicola TISTR 8095 was 1.42±0.30%.
The integrative aspect on carbon fixation and lipid production is firstly implemented in cyanobacterium Synechocystis sp. PCC 6803 using metabolic engineering approach. Genes related to ...Calvin-Benson-Bassham (CBB) cycle including rbcLXS and glpD and free fatty acid recycling including aas encoding acyl-ACP synthetase were practically manipulated in single, double and triple overexpressions via single homologous recombination. The significantly increased growth rate and intracellular pigment contents were evident in glpD-overexpressing (OG) strain among all strains studied under normal growth condition. The triple aas_glpD_rbcLXS-overexpressing (OAGR) strain notably gave the highest contents of both intracellular lipids and extracellular free fatty acids (FFAs) of about 35.9 and 9.6% w/DCW, respectively, when compared to other strains at day 5 of cultivation. However, the highest intracellular lipid titer and production rate were observed in OA strain at day 5 (228.7 mg/L and 45.7 mg/L/day, respectively) and OG strain at day 10 (358.3 mg/L and 35.8 mg/L/day, respectively) due to their higher growth. For fatty acid (FA) compositions, the main saturated fatty acid of palmitic acid (C16:0) was dominantly found in both intracellular lipid and secreted FFAs fractions. Notably, intracellular FA proportion of myristic acid (C14:0) was induced in all engineered strains whereas the increase of stearic acid (C18:0) composition was found in extracellular FFAs fraction. Altogether, these overexpressing strains efficiently produced higher lipid production via homeostasis balance on both its lipid synthesis and FFAs secretion.
•Native biosynthetic genes for PHB production overexpressed in Synechocystis.•Overexpression of phaA and phaB resulted in the highest level of PHB.•Metabolic engineering leads to increased production ...of the valuable polymer PHB.
Synechocystis sp. PCC 6803 strains overexpressing pha genes were constructed and characterized for poly-3-hydroxybutyrate (PHB) production. These pha overexpressing strains showed slightly reduced growth rates. Under N-deprived condition, the strains overexpressing (OE) phaAB, phaEC and phaABEC showed significantly higher PHB contents than the wild type. The maximum PHB content, a 2.6-fold increase producing 26% PHB (dcw), was observed in OE phaAB cells grown for 9days in N-deprived medium. Under this condition, these OE phaAB cells increased PHB production to 35% PHB (dcw) upon addition of 0.4% (w/v) acetate. Higher PHB granules in OE phaAB cells were clearly visualized by both Nile red staining and TEM imaging. All OE strains under N-deficient condition had increased glgX transcript levels. Overall results demonstrate an enhanced PHB production in Synechocystis cells overexpressing pha genes, particularly phaA and phaB, when grown in N-deprived medium containing 0.4% (w/v) acetate.
A large number of microalgae isolated from Thailand were screened for their hydrogen production capacity. The selected highly efficient microalga, identified as
Nostoc
sp. CU2561, was investigated ...for the conditions under which the cells had maximal hydrogen production rate.
Nostoc
sp. CU2561 showed highest hydrogen production rate when grown in BG11 medium deprived of nitrogen and sulfur (BG11-N-S). To further improve hydrogen production, newly invented agar beads were used as matrix for cell immobilization. The highest hydrogen production rate by 1.5% (w/v) agar bead immobilized cells was obtained using cell concentration of 0.125 mg dry wt mL
−1
. Agar bead–immobilized cells showed superior hydrogen production rate, 1.5-fold higher when compared with agar cube–immobilized cells, and 5- and 10-fold higher when compared with those in alginate bead–immobilized and alginate bead–suspended cells, respectively. Supplementation of 0.5% (w/v) fructose increased hydrogen production rate of agar bead–immobilized cells approximately 1.7-fold, whereas the reducing agent β-mercaptoethanol increased hydrogen production rate by about 8.2-fold. Overall,
Nostoc
sp. CU2561 immobilized in agar bead showed the highest hydrogen production rate when incubated in BG11-N-S containing 5 mM β-mercaptoethanol with the highest hydrogen production rate of 18.78 ± 1.44 μmol H
2
mg
−1
chl
a
h
−1
. In addition, agar bead–immobilized cells could continuously produce hydrogen for 3 cycles. The hydrogen production by immobilized cells could be prolonged up to 120 h during the first cycle. This study provides a potential of new immobilization strategy using cyanobacteria immobilized in agar bead for hydrogen production which can be applied to scale up at industrial level in the future.
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Considering the present energy demand and industrial growth, finding a potential sustainable energy source is crucial. Among renewable fuels, hydrogen is considered to be the primary ...choice as it offers substantial benefits over other conventional fuels. Hydrogen can be generated from waste substrates, is cleaner, and has the highest energy density when compared to conventional fuels. Hydrogen production, particularly biological hydrogen production, is believed to be cost-efficient as it can be successfully performed in ambient conditions with easy operational techniques in an environmental-friendly manner. This review summarizes the different routes of biological hydrogen production including biophotolysis, indirect photolysis, dark fermentation, photofermentation, and microbial electrolysis. Further, leading microorganisms involved in biohydrogen production, such as Clostridium spp., Enterobacter spp., Bacillus spp., Escherichia coli, thermophilic lactic acid bacteria, and Klebsiella spp., along with the molecular approaches employed for the enhancement of biohydrogen production are discussed. In addition, a thorough techno-economic analysis of factors involved in the scale-up of hydrogen production is carried out.
Microalgae have received much interest as a biofuel feedstock. However, the economic feasibility of biofuel production from microalgae does not satisfy capital investors. Apart from the biofuels, it ...is necessary to produce high-value co-products from microalgae fraction to satisfy the economic aspects of microalgae biorefinery. In addition, microalgae-based wastewater treatment is considered as an alternative for the conventional wastewater treatment in terms of energy consumption, which is suitable for microalgae biorefinery approaches. The energy consumption of a microalgae wastewater treatment system (0.2 kW/h/m
) was reduced 10 times when compared to the conventional wastewater treatment system (to 2 kW/h/m
). Microalgae are rich in various biomolecules such as carbohydrates, proteins, lipids, pigments, vitamins, and antioxidants; all these valuable products can be utilized by nutritional, pharmaceutical, and cosmetic industries. There are several bottlenecks associated with microalgae biorefinery. Hence, it is essential to promote the sustainability of microalgal biorefinery with innovative ideas to produce biofuel with high-value products. This review attempted to bring out the trends and promising solutions to realize microalgal production of multiple products at an industrial scale. New perspectives and current challenges are discussed for the development of algal biorefinery concepts.
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