The effect of iron on growth, lipid accumulation, and gene expression profile of the freshwater microalga Chlorella sorokiniana

Enhancing microalgae-based biofuels production, wastewater treatment and bio-products generation by synergistic effect of iron and zinc addition to real municipal wastewater

The feasibility of microalgae-based biofuel production is still unclear due to the high cost and energy consumption. In order to be competitive with traditional fuels, the price per unit biofuel produced should be reduced by improving microalgal cells quality for higher biofuels productivity as well as enhancing microalgae other advantages such as wastewater treatment (WWT) and CO2 bio-fixation. In this research, the synergistic effect of iron (Fe) and zinc (Zn) addition to municipal wastewater (MWW) on Chlorella sorokiniana Pa.91 performance was investigated in terms of biomass productivity, WWT, bio-products generation and biofuel quality. According to the result, maximum biomass concentration of 1.1, 1.49, 1.36 and 1.9 mg/L were achieved after 10 days C. sorokiniana cultivation in MWW before and after addition of Fe (9 mg/L), Zn (1 mg/L) and combined Zn/Fe (6/0.5 mg/L), respectively. It was observed that the nutrients uptake ability of microalgal cells improved by pre-treatment with Fe/Zn, as the mass balance of COD, NH4 and PO43- increased from 104.5, 13.6 and 2.9 to 111.6, 16.6 and 3.78 mg per 1 g C. sorokiniana, respectively. By adding Fe/Zn, the lipid content increased from 25 to 33 % CDW, while, no significant changes were observed in the protein and carbohydrate content. The results revealed that the fatty acid composition (i.e. SFA and MUFA content) and biofuel quality of C. sorokiniana remarkability enhanced after Fe/Zn supplementation. Overall, our finding suggested that MWW enrichment with combined Fe and Zn at an appropriate dosage is a promising approach for improving microalgae performance in particular increasing biofuel production quantity and quality.

Synergistic promotion of microalgal growth and copper removal from synthetic wastewater by nanoscale zero-valent iron particles

The increasing concentrations of heavy metals in livestock wastewater pose a serious threat to the environmental safety and human health, limiting its resource utilisation. In the present study, microalgae and nanoscale zero-valent iron were selected to construct a coupled system for copper-containing wastewater treatment. The addition of 50 mg·L-1 nanoscale zero-valent iron (50 nm) was the optimal value for the experiment, which could significantly increase the biomass of microalgae. In addition, nanoscale zero-valent iron stimulated microalgal secretion of extracellular polymeric substances, increasing the contents of binding sites, organic ligands, and functional groups on the microalgal surfaces and ultimately promoting the settling of microalgae and binding of heavy metals. The coupled system could quickly adapt to copper-containing wastewater of 10 mg·L-1, and the copper removal rate reached 94.99%. Adsorption and uptake by organisms, together with the contribution of zero-valent iron nanoparticles, are the major copper removal pathways. Overall, this work offers a novel technical solution for enhanced treatment of copper-containing livestock wastewater, which will help improve the efficiency and quality of wastewater treatment.

Insight into the roles of hematite iron oxide nanoparticles on microalgae growth, urban wastewater treatment and bioproducts generation: Gompertz simulation, nutrient mass balance and gene expression

In this study, the effect of α-Fe2O3 nanoparticles spiking in urban wastewater (UWW) on growth rate, wastewater treatment ability and bioproducts generation of C. vulgaris and Spirulina was investigated and compared with pure cultivation system. The biomass concentration of C. vulgaris and Spirulina improved by 20 % and 39 % at 10 and 15 mg/L α-Fe2O3, respectively while the both microalgae growth pattern fitted better with Gompertz simulation after treatment with α-Fe2O3. The nutrients mass balance revealed that 1 g of treated C. vulgaris and Spirulina could uptake more COD, TN and TP in comparison to the untreated cells. The lipid generation increased remarkably (C. vulgaris: 45 % and Spirulina: 72 %) after α-Fe2O3 treatment. While, the addition of α-Fe2O3 showed no significant impact on the protein and carbohydrate productivity. Overall, this study evangelize the role of nanoparticles on promoting microalgae applications as a sustainable approach for UWW treatment and promising feedstock for biofuel production.

Biochemical and proteomic response of the freshwater green alga Pseudochlorella pringsheimii to iron and salinity stressors

BACKGROUND

Pseudochlorella pringsheimii (Ppr) is a green unicellular alga rich with chlorophyll, carotenoids, and antioxidants. As a widespread organism, Ppr must face, and adapt to, many environmental stresses and these are becoming more frequent and more extreme under the conditions of climate change. We therefore focused on salinity induced by NaCl and iron (Fe) variation stresses, which are commonly encountered by algae in their natural environment.

RESULTS

The relatively low stress levels improved the biomass, growth rate, and biochemical components of Ppr. In addition, the radical-scavenging activity, reducing power, and chelating activity were stimulated by lower iron concentrations and all NaCl concentrations. We believe that the alga has adapted to the stressors by increasing certain biomolecules such as carotenoids, phenolics, proteins, and carbohydrates. These act as antioxidants and osmoregulators to protect cell membranes and other cellular components from the harmful effects of ions. We have used SDS-PAGE and 2D-PAGE in combination with tandem mass spectrometry to identify responsive proteins in the proteomes of stressed vs. non-stressed Ppr. The results of 2D-PAGE analysis showed a total of 67 differentially expressed proteins, and SDS-PAGE identified 559 peptides corresponding to 77 proteins. Of these, 15, 8, and 17 peptides were uniquely identified only under the control, iron, and salinity treatments, respectively. The peptides were classified into 12 functional categories: energy metabolism (the most notable proteins), carbohydrate metabolism, regulation, photosynthesis, protein synthesis, stress proteins, oxido-reductase proteins, transfer proteins, ribonucleic-associated proteins, hypothetical proteins, and unknown proteins. The number of identified peptides was higher under salinity stress compared to iron stress.

CONCLUSIONS

A proposed mechanism for the adaptation of Ppr to stress is discussed based on the collected data. This data could serve as reference material for algal proteomics and the mechanisms involved in mediating stress tolerance.

Introducing of high rich lysine, arginine, and unsaturated fatty acids microalga as a food supplement

Microalgae are powerful source for nutritionally valuable components as proteins, carbohydrates and especially unsaturated fatty acids. Microalgae may be employed in pharmaceutical, food, cosmetic, health industries, and biofuels. In this study for looking at high-level unsaturated fatty acids species, from 31 strains, by comparing growth curves, the best strain with a high growth rate and lipid content was selected by red Nile staining. It was determined by molecular identification that this strain belongs to the genus Chlorella sp. and is deposited into the Agricultural Biotechnology Research Institute of Iran Culture collection with culture collection number ABRIICC 30,041. Biomass analysis after growth optimization by response surface methodology showed that the selected strain had a specific growth rate of 0.216 ± 0.008 d-1, biomass productivity of 142.58 ± 4.41 mg/Ld, and lipid content of 13.9 ± 0.26% with a high level of unsaturated fatty acids of 53.15%. It also included 51.3 ± 0.53% protein with a very high quality essential amino acids of 40.36%, the most lysine (8.77%) and arginine (13.31%) has been reported until now, and 26.9 ± 0.23% carbohydrates in photoautotroph condition. By MTT assay, there is no effect of cytotoxicity. This research introduces a potent native strain comparable with commercial strains that can be a hopeful source for food supplements and valuable bioactive ingredients in functional foods.

Iron and nitrogen regulate carbon transformation in a methanotroph-microalgae system

Nutrient supply is important for maintaining a methanotroph and microalgae (MOB-MG) system for biogas valorization. However, there is a lack of understanding regarding how key elements regulate the growth of a MOB-MG coculture. In this study, a MOB-MG coculture with high protein content (0.47 g/g biomass) was established from waste activated sludge using synthetic biogas. An increase in iron availability substantially stimulated the specific growth rate (from 0.18 to 0.62 day-1) and biogas conversion rate (from 26.81 to 106.57 mg-C L-1 day-1) of the coculture. Moreover, the protein content remained high (0.51 g/g biomass), and the total lipid content increased (from 0.09 to 0.14 g/g biomass). Nitrogen limitation apparently constrained the specific growth rate (from 0.64 to 0.28 day-1) and largely reduced the protein content (from 0.51 to 0.31 g/g biomass) of the coculture. Intriguingly, the lipid content remained unchanged after nitrogen was depleted. The eukaryotic community was consistently dominated by MG belonging to Chlorella, while the populations of MOB shifted from Methylococcus/Methylosinus to Methylocystis due to iron and nitrogen amendment. In addition, diverse non-methanotrophic heterotrophs were present in the community. Their presence neither compromised the performance of the coculture system nor affected the protein content of the biomass. However, these heterotrophs may contribute to high carbon conversion efficiency by utilizing the dissolved organic carbon released by MOB and MG. Overall, the findings highlight the vital roles of iron and nitrogen in achieving efficient conversion of biogas, fast growth of cells, and optimal biomass composition in a MOB-MG coculture system.

Adverse effects of iron-based nanoparticles on freshwater phytoplankton Scenedesmus armatus and Microcystis aeruginosa strains

Zero-valent nano-iron particles (nZVI) are increasingly present in freshwater aquatic environments due to their numerous applications in environmental remediation. However, despite the broad benefits associated with the use and development of nZVI nanoparticles, the potential risks of introducing them into the aquatic environment need to be considered. Special attention should be focused on primary producer organisms, the basal trophic level, whose impact affects the rest of the food web. Although there are numerous acute studies on the acute effects of these nanoparticles on photosynthetic primary producers, few studies focus on long-term exposures. The present study aimed at assessing the effects of nZVI on growth rate, photosynthesis activity, and reactive oxygen activity (ROS) on the freshwater green algae Scenedesmus armatus and the cyanobacteria Microcystis aeruginosa. Moreover, microcystin production was also evaluated. These parameters were assessed on both organisms singly exposed to 72 h-effective nZVI concentration for 10% maximal response for 28 days. The results showed that the cell growth rate of S. armatus was initially significantly altered and progressively reached control-like values at 28 days post-exposure, while M. aeruginosa did not show any significant difference concerning control values at any time. In both strains dark respiration (R) increased, unlike net photosynthesis (Pn), while gross photosynthesis (Pg) only slightly increased at 7 days of exposure and then became equal to control values at 28 days of exposure. The nZVI nanoparticles generated ROS progressively during the 28 days of exposure in both strains, although their formation was significantly higher on green algae than on cyanobacteria. These data can provide additional information to further investigate the potential risks of nZVI and ultimately help decision-makers make better informed decisions regarding the use of nZVI for environmental remediation.

Dissolution rate and growth performance reveal struvite as a sustainable nutrient source to produce a diverse set of microbial protein

To provide for the globally increasing demand for proteinaceous food, microbial protein (MP) has the potential to become an alternative food or feed source. Phosphorus (P), on the other hand, is a critical raw material whose global reserves are declining. Growing MP on recovered phosphorus, for instance, struvite obtained from wastewater treatment, is a promising MP production route that could supply protein-rich products while handling P scarcity. The aim of this study was to explore struvite dissolution kinetics in different MP media and characterize MP production with struvite as sole P-source. Different operational parameters, including pH, temperature, contact surface area, and ion concentrations were tested, and struvite dissolution rates were observed between 0.32 and 4.7 g P/L/d and a solubility between 0.23 and 2.22 g P-based struvite/L. Growth rates and protein production of the microalgae Chlorella vulgaris and Limnospira sp. (previously known as Arthrospira sp.), and the purple non‑sulfur bacterium Rhodopseudomonas palustris on struvite were equal to or higher than growth on conventional potassium phosphate. For aerobic heterotrophic bacteria, two slow-growing communities showed decreased growth on struvite, while the growth was increased for a third fast-growing one. Furthermore, MP protein content on struvite was always comparable to the one obtained when grown on standard media. Together with the low content in metals and micropollutants, these results demonstrate that struvite can be directly applied as an effective nutrient source to produce fast-growing MP, without any previous dissolution step. Combining a high purity recovered product with an efficient way of producing protein results in a strong environmental win-win.

Iron-dependent mutualism between Chlorella sorokiniana and Ralstonia pickettii forms the basis for a sustainable bioremediation system

Phototrophic communities of autotrophic microalgae and heterotrophic bacteria perform complex tasks of nutrient acquisition and tackling environmental stress but remain underexplored as a basis for the bioremediation of emerging pollutants. In industrial monoculture designs, poor iron uptake by microalgae limits their productivity and biotechnological efficacy. Iron supplementation is expensive and ineffective because iron remains insoluble in an aqueous medium and is biologically unavailable. However, microalgae develop complex interkingdom associations with siderophore-producing bacteria that help solubilize iron and increase its bioavailability. Using dye degradation as a model, we combined environmental isolations and synthetic ecology as a workflow to design a simplified microbial community based on iron and carbon exchange. We established a mutualism between the previously non-associated alga Chlorella sorokiniana and siderophore-producing bacterium Ralstonia pickettii. Siderophore-mediated increase in iron bioavailability alleviated Fe stress for algae and increased the reductive iron uptake mechanism and bioremediation potential. In exchange, C. sorokiniana produced galactose, glucose, and mannose as major extracellular monosaccharides, supporting bacterial growth. We propose that extracellular iron reduction by ferrireductase is crucial for azoreductase-mediated dye degradation in microalgae. These results demonstrate that iron bioavailability, often overlooked in cultivation, governs microalgal growth, enzymatic processes, and bioremediation potential. Our results suggest that phototrophic communities with an active association for iron and carbon exchange have the potential to overcome challenges associated with micronutrient availability, while scaling up bioremediation designs.

Transcriptomic Analysis of the Molecular Response Mechanism of Microcystis aeruginosa to Iron Limitation Stress

Iron is an essential micronutrient for cyanobacteria. It is involved in physiological activities such as photosynthesis, respiration, and the synthesis of pigments. The impact of iron limitation on planktonic algae growth occurs in surface oceans globally, as well as in freshwater ecosystems. However, the molecular and physiological effects and response mechanism of cyanobacteria under iron-limited conditions have not been reported in detail. In this study, the effects of iron limitation on the cell density, chlorophyll content, and photosynthetic activity of Microcystis aeruginosa were determined, and transcriptome sequencing was undertaken. In a severely iron-deficient environment, the cell density and chlorophyll-a content of M. aeruginosa were significantly lower than in the iron-rich group (a 55.42% and 83.51% reduction, respectively). Similarly, the photosynthetic efficiency of M. aeruginosa was also inhibited by iron deficiency, and the maximum photochemical efficiency (Fv/Fm) of the severe iron deficiency group was only 66.72% of the control group. The transcriptome results showed that to cope with the iron-deficient environment, most genes involved in iron absorption and transport in M. aeruginosa were up-regulated. In particular, the fur and perR genes that regulate the iron uptake regulatory protein (Fur) were both up-regulated. Due to the high demand for iron in the photosynthetic electron transport chain of M. aeruginosa, most photosynthesis-related genes were down-regulated, for example, petJ, which regulates iron-containing cytochrome c6. In contrast, most of the genes related to glycolysis and respiration were up-regulated. These changes in gene expression may be a survival strategy for M. aeruginosa to cope with a long-term iron-deficient environment. This study provides insights into the molecular response mechanism of M. aeruginosa under iron limitation stress.

Bioavailability and effect of α-Fe2O3 nanoparticles on growth, fatty acid composition and morphological indices of Chlorella vulgaris

The wide application of α-Fe₂O₃ nanoparticles (NPs) in different fields has resulted in release and accumulation of these materials into the aquatic ecosystem. Therefore, it is important to understand the potential impact of these NPs on aquatic organisms especially primary producers i.e., microalgae. Present study aimed to investigate the bioavailability and the effect of α-Fe₂O₃ NPs on growth of iron deprived cells of Chlorella vulgaris. Results showed that α-Fe₂O₃ NPs are not available as iron source to support the growth of C. vulgaris. Moreover，α-Fe₂O₃ NPs induced stress condition to C. vulgaris, which were reflected in its growth rates, total lipid contents, fatty acid profile and cell morphology. Specifically, low concentrations of α-Fe₂O₃ NPs (0.1, 0.5, 2.5, 5, 10 mg/L) showed similar growth profile and total lipid contents at both exponential and stationary growth phases. At 50 and 100 mg/L α-Fe₂O₃ NPs concentrations biomass reduced by 41.2% and 83.7% whereas total lipid contents increased by 39.7% and 25.5% respectively at exponential growth phase along with reduction in fatty acids. The results illustrated novel insights into the microalgal interaction with nanoparticles, providing fundamental knowledge for the development of future microalgae ecology and cultivation technology.

Development of a Strategy for Enhancing the Biomass Growth and Lipid Accumulation of Chlorella sp. UJ-3 Using Magnetic Fe3O4 Nanoparticles

In this study, magnetic Fe₃O₄ nanoparticles (NPs) were used as an effective enhancer to increase the biomass and total lipid production of Chlorella sp. UJ-3. It was found that the biomass of algal cells increased significantly when they were exposed to low concentrations of Fe₃O₄ NPs (20 mg/L), while the best total lipid content of algal cells was achieved when they were exposed to high concentrations of Fe₃O₄ NPs (100 mg/L). Therefore, we established a strategy to promote the growth and lipid accumulation of microalgae by initially exposing the algal cells to low concentrations of Fe₃O₄ NPs and then treating them with an increased concentration of Fe₃O₄ NPs after 12 days of culture. For this strategy, the biomass and total lipid production of algal cells increased by 50% and 108.7%, respectively, compared to the untreated control. The increase in lipid production and change in the fatty acid composition of Chlorella cells were found to help them to cope with the increased number of reactive oxygen species produced due to oxidative stress in alga cells after the addition of Fe₃O₄ NPs. This study provided a highly efficient way to improve the lipid production of microalgae using nanoparticles.

Impact of heavy metals in the microalga Chlorella sorokiniana and assessment of its potential use in cadmium bioremediation

The chlorophyte microalga Chlorella sorokiniana was tested for the bioremediation of heavy metals pollution. It was cultured with different concentrations of Cu²⁺, Cd²⁺, As (III) and As (V), showing a significant inhibition on its growth at concentrations of 500 µM Cu²⁺, 250 µM Cd²⁺, 750 µM AsO₃^3- and 5 mM AsO₄^3- or higher. Moreover, the consumption of ammonium was also studied, showing significant differences for concentrations higher than 1 mM of Cu²⁺ and As (III), and 5 mM of As (V). The determination of intracellular heavy metals concentration revealed that Chlorella sorokiniana is an outstanding Cd accumulator organism, able to accumulate 11,232 mg kg^-1 of Cd, and removing 65% of initial concentration of this heavy metal. Finally, antioxidant enzymes, such as catalase (CAT) and ascorbate peroxidase (APX), and enzymes involved in the production of glutamate and cysteine, such as glutamine syntethase (GS), glutamate dehydrogenase (GDH), O-acetylserine (thiol) lyase (OASTL) and NAD-isocitrate dehydrogenase (NAD-IDH) were studied both at gene expression and enzymatic activity levels. These enzymes exhibited different grades of upregulation, especially in response to Cd and As stress. However, GS expression was downregulated when Chlorella sorokiniana was cultured in the presence of these heavy metals.

Strategy for Managing Industrial Anaerobic Sludge through the Heterotrophic Cultivation of Chlorella sorokiniana: Effect of Iron Addition on Biomass and Lipid Production

Microalgae provides an alternative for the valorization of industrial by-products, in which the nutritional content varies substantially and directly affects microalgae system performance. Herein, the heterotrophic cultivation of Chlorella sorokiniana was systematically studied, allowing us to detect a nutritional deficiency other than the carbon source through assessing the oxygen transfer rate for glucose or acetate fermentation. Consequently, a mathematical model of the iron co-limiting effect on heterotrophic microalgae was developed by exploring its ability to regulate the specific growth rate and yield. For instance, higher values of the specific growth rate (0.17 h^-1) compared with those reported for the heterotrophic culture of Chlorella were obtained due to iron supplementation. Therefore, anaerobic sludge from an industrial wastewater treatment plant (a baker's yeast company) was pretreated to obtain an extract as a media supplement for C. sorokiniana. According to the proposed model, the sludge extract allowed us to supplement iron values close to the growth activation concentration (K_Fe ~12 mg L^-1). Therefore, a fed-batch strategy was evaluated on nitrogen-deprived cultures supplemented with the sludge extract to promote biomass formation and fatty acid synthesis. Our findings reveal that nitrogen and iron in sludge extract can supplement heterotrophic cultures of Chlorella and provide an alternative for the valorization of industrial anaerobic sludge.

Toward the Enhancement of Microalgal Metabolite Production through Microalgae-Bacteria Consortia

Engineered mutualistic consortia of microalgae and bacteria may be a means of assembling a novel combination of metabolic capabilities with potential biotechnological advantages. Microalgae are promising organisms for the sustainable production of metabolites of commercial interest, such as lipids, carbohydrates, pigments, and proteins. Several studies reveal that microalgae growth and cellular storage of these metabolites can be enhanced significantly by co-cultivation with growth-promoting bacteria. This review summarizes the state of the art of microalgae-bacteria consortia for the production of microalgal metabolites. We discuss the current knowledge on microalgae-bacteria mutualism and the mechanisms of bacteria to enhance microalgae metabolism. Furthermore, the potential routes for a microalgae-bacteria biorefinery are outlined in an attempt to overcome the economic failures and negative energy balances of the existing production processes.

Fecitrate converted from Fe2O3 particles in coal-fired flue gas promoted microalgal biomass and lipid productivities

In order to reutilize Fe₂O₃ particles in flue gas from coal-fired power plant as a ferrum nutrient for improving microalgae growth, Na-Citrate was proposed to chelate FeCl₃ derived from Fe₂O₃ and HCl reactions to promote biomass and lipid productivities of Chlorella PY-ZU1. Fe-Citrate gave much higher biomass and lipid productivities than FeCl₃, Fe-EDTA, Fe-DTPA and Fe-HEDTA, because organic chelator prevented Fe³⁺ from depositing, lower stability constant resulted in easier dissociation of ferric chelate, smaller chelate facilitated Fe²⁺ (reduced from Fe³⁺) transportation through cell membranes. The biomass growth and photosynthetic capacity of Chlorella PY-ZU1 cultivated with Fe-Citrate (converted from Fe₂O₃ particles) medium were similar to those with commercial ferric ammonium citrate medium. The biomass and lipid productivities of Chlorella PY-ZU1 cultivated with 5 mg L^-1 Fe-Citrate medium were 1.30 and 1.72 times, respectively, higher than those with FeCl₃ growth medium.

Biomass and Lipid Productivity by Two Algal Strains of Chlorella sorokiniana Grown in Hydrolysate of Water Hyacinth

Hydrolysate prepared from the chemical hydrolysis of water hyacinth biomass contains a high amount of solubilised carbohydrate and nutrients. This hydrolysate was utilised as a medium for the cultivation of two strains of Chlorella sorokiniana, isolated from a municipal wastewater treatment plant using two different media, i.e., BG-11 and Knop’s medium. Different light intensities, light–dark cycles, and various concentrations of external carbon sources (monosaccharides and inorganic carbon) were used to optimise the microalgal growth. For the accumulation of lipids and carbohydrates, the microalgal strains were transferred to nutrient amended medium (N-amended and P-amended). It was observed that the combined effect of glucose, inorganic carbon, and a 12:12 h light–dark cycle proved to be the optimum parameters for high biomass productivity (~200 mg/L/day). For Chlorella sorokiniana 1 (isolated from BG-11 medium), the maximum carbohydrate content (22%) was found in P-amended medium (N = 0 mg/L, P: 3 mg/L), whereas, high lipid content (17.3%) was recorded in N-amended medium (N = 5 mg/L, P = 0 mg/L). However, for Chlorella sorokiniana 2 (isolated from the Knop’s medium), both lipid (17%) and carbohydrate accumulation (12.3%) were found to be maximum in the N-amended medium. Chlorella sorokiniana 2 showed a high saturated lipid accumulation compared to other strains. Kinetic modelling of the lipid profile revealed that the production rate of fatty acids and their various constituents were species dependent under identical conditions.

Improving ‘Lipid Productivity’ in Microalgae by Bilateral Enhancement of Biomass and Lipid Contents: A Review

Microalgae have received widespread interest owing to their potential in biofuel production. However, economical microalgal biomass production is conditioned by enhancing the lipid accumulation without decreasing growth rate or by increasing both simultaneously. While extensive investigation has been performed on promoting the economic feasibility of microalgal-based biofuel production that aims to increase the productivity of microalgae species, only a handful of them deal with increasing lipid productivity (based on lipid contents and growth rate) in the feedstock production process. The purpose of this review is to provide an overview of the recent advances and novel approaches in promoting lipid productivity (depends on biomass and lipid contents) in feedstock production from strain selection to after-harvesting stages. The current study comprises two parts. In the first part, bilateral improving biomass/lipid production will be investigated in upstream measures, including strain selection, genetic engineering, and cultivation stages. In the second part, the enhancement of lipid productivity will be discussed in the downstream measure included in the harvesting and after-harvesting stages. An integrated approach involving the strategies for increasing lipid productivity in up- and down-stream measures can be a breakthrough approach that would promote the commercialization of market-driven microalgae-derived biofuel production.

Augmenting the expression of accD and rbcL genes using optimized iron concentration to achieve higher biomass and biodiesel in Chlorella vulgaris

Chlorella vulgaris is a form of microalgae commonly employed as a biological source of oil for biodiesel production. Major algal cultivation strategies are focused on stimulating growth rate and lipid content. In the present study, the algal growth media was supplemented with iron (III) chloride (FeCl₃), as a stimulating factor for growth and lipid production, in three iron concentrations including 90, 200, and 500 µM. The turbidity of algal cells was measured on different days, to determine the growth rate. In optimum iron concentration, this measurement experienced a 2.1-fold increase. Next, the lipid content was extracted, and the amount of lipid produced in each treatment was calculated, which demonstrated a 4.57-fold increase in lipid productivity. The expression of genes corresponding to the metabolic enzymes (i.e. acetyl-CoA carboxylase (accD) and ribulose bisphosphate carboxylase large chain (rbcL)) was evaluated using real-time PCR under different initial iron feeds. As demonstrated in the results, the initial iron feed of 90 µM was an optimum concentration that obtained the highest growth rate, more cell density, and increased lipid production. In 90 µM initial iron concentration, the expression of accD and rbcL genes showed a 4.8- and 35-fold increase, respectively, compared to that of the control genes. Based on the results, this optimum iron concentration could satisfy the industrial interest in biodiesel production from C. vulgaris as a potential stimulating factor. However, higher levels of iron (e.g. 200 and 500 µM) failed to act as positive stress for increasing biodiesel production. Finally, in this paper, different mechanisms where iron affects acetyl-CoA carboxylase (ACCase) and 1,5-ribulose bisphosphate carboxylase/oxygenase (RuBisCo) are illustrated.

Improvement on lipid production by Scenedesmus obliquus triggered by low dose exposure to nanoparticles

Carbon nanotubes (CNTs), α-Fe₂O₃ nanoparticles (nano Fe₂O₃) and MgO nanoparticles (nano MgO) were evaluated for the effects on algae growth and lipid production. Nano Fe₂O₃ promoted cell growth in the range of 0-20 mg·L^-1. CNTs, nano Fe₂O₃ and nano MgO inhibited cell growth of Scenedesmus obliquus at 10, 40 and 0.8 mg·L^-1 respectively. Neutral lipid and total lipid content increased with the increasing concentration of all tested nanoparticles. The maximum lipid productivity of cultures exposed to CNTs, nano Fe₂O₃ and nano MgO was observed at 5 mg·L^-1, 5 mg·L^-1 and 40 mg·L^-1, with the improvement by 8.9%, 39.6% and 18.5%. High dose exposure to nanoparticles limited increase in lipid productivity, possibly due to the repression on cell growth caused by nanoparticles-catalyzed reactive oxygen species (ROS) generation, finally leading to reduction in biomass and lipid production. Reduced accumulation of fatty acids of C18:3n3, C18:3n6 and C20:2 was observed in cells exposed to nanoparticles.

Use of De Novo Transcriptome Libraries to Characterize a Novel Oleaginous Marine Chlorella Species during the Accumulation of Triacylglycerols

Marine chlorophytes of the genus Chlorella are unicellular algae capable of accumulating a high proportion of cellular lipids that can be used for biodiesel production. In this study, we examined the broad physiological capabilities of a subtropical strain (C596) of Chlorella sp. “SAG-211-18” including its heterotrophic growth and tolerance to low salt. We found that the alga replicates more slowly at diluted salt concentrations and can grow on a wide range of carbon substrates in the dark. We then sequenced the RNA of Chlorella strain C596 to elucidate key metabolic genes and investigate the transcriptomic response of the organism when transitioning from a nutrient-replete to a nutrient-deficient condition when neutral lipids accumulate. Specific transcripts encoding for enzymes involved in both starch and lipid biosynthesis, among others, were up-regulated as the cultures transitioned into a lipid-accumulating state whereas photosynthesis-related genes were down-regulated. Transcripts encoding for two of the up-regulated enzymes—a galactoglycerolipid lipase and a diacylglyceride acyltransferase—were also monitored by reverse transcription quantitative polymerase chain reaction assays. The results of these assays confirmed the transcriptome-sequencing data. The present transcriptomic study will assist in the greater understanding, more effective application, and efficient design of Chlorella-based biofuel production systems.