Marine diatom, Navicula sp. strain JPCC DA0580 and marine green alga, Chlorella sp. strain NKG400014 as potential sources for biodiesel production

Characterizing A21: Natural Cyanobacteria-Based Consortium with Potential for Steroid Bioremediation in Wastewater Treatment

Microalga-bacteria consortia are increasingly recognized for their effectiveness in wastewater treatment, leveraging the metabolic synergy between microalgae and bacteria to enhance nutrient removal and overall treatment efficiency. These systems offer a sustainable approach to addressing pollutants such as nitrogen and phosphorus. However, their potential in removing specific contaminants like steroid hormones is less explored. In this study, a natural microbial consortium, A21, has been characterized and isolated from primary sewage treatment in Madrid and its potential for bioremediation of steroid hormone effluents has been evaluated. The A21 consortium includes Alphaproteobacteria genera Sphingopyxis and Pseudorhizobium and the Cyanobacterium Cyanobium. Sphingopyxis (31.78%) is known for biodegradation, while Pseudorhizobium (15.68%) exhibits detoxification abilities. Cyanobium (14.2%) may contribute to nutrient uptake and oxygen production. The effects of pH, nitrogen sources, and Sodium chloride concentrations on growth were evaluated. The optimal growth conditions were determined to be a pH range of 7 to 9, a salt concentration below 0.1 M, and the presence of a nitrogen source. The consortium also demonstrated effective growth across various types of wastewaters (primary, secondary, and tertiary treatment effluents). Additionally, A21 exhibited the ability to grow in the presence of steroids and transform them into other compounds, such as converting androstenedione (AD) into androsta-1,4-diene-3,17-dione (ADD) and β-estradiol into estrone.

Towards sustainable biocontrol: inhibition of soil borne fungi by microalgae from harsh environments

Using microorganisms as biocontrol agents against soilborne plant pathogens is a promising alternative to chemical pesticides. However, only some biocontrol agents have proven effective under field conditions. This study explores the potential of highly resilient microalgae isolated from harsh environments, such as Biological Soil Crusts and agricultural fields in semi-arid regions, as a novel and sustainable approach to biocontrol. Fifty-nine microalgal strains, including thirteen cyanobacteria and forty-six green algae, were isolated and identified. Dual-culture plate assays and toxicity tests of microalgal growth media were conducted to evaluate the antifungal activity of the isolates against eight representative soilborne pathogens. The results showed that many microalgae strains exhibited significant inhibitory effects on the growth of specific fungal pathogens, although the activity varied among different microalgal strains and pathogen species. Some strains even promoted the growth of certain fungi. The lack of a clear pattern in the antifungal activity highlights the complexity and specificity of the interactions between microalgae and soilborne pathogens. An "Inhibition Effectiveness" metric was developed to quantify biocontrol potential based on fungal growth inhibition. The green algal genus Desmodesmus, particularly Desmodesmus subspicatus isolates, showed higher antifungal efficacy than other genera. While the inhibitory mechanisms remain unclear, the results demonstrate the promising biocontrol capabilities of microalgae from extreme environments like BSCs. Further research could unlock novel opportunities for sustainable disease management by harnessing specific microalgal strains or synergistic strain combinations targeting soilborne pathogens.

Highly Efficient Genetic Transformation Methods for the Marine Oleaginous Diatom Fistulifera solaris

The oleaginous diatom Fistulifera solaris is a promising producer of biofuel owing to the high content of the lipids. A genetic transformation technique by microparticle bombardment for this diatom was already established. However, the transformation efficiency was significantly lower than those of other diatoms. Devoting efforts to advance the genetic modifications of this diatom is crucial to unlock its full potential. In this study, we optimized the microparticle bombardment protocol, and newly established a multi-pulse electroporation protocol for this diatom. The nutrient-rich medium in the pre-culture stage played an essential role to increase the transformation efficiency of the bombardment method. On the other hand, use of the nutrient-rich medium in the electroporation experiments resulted in decreasing the efficiency because excess nutrient salts could hamper to establish the best conductivity condition. Adjustments on the number and voltage of the poring pulses were also critical to obtain the best balance between cell viability and efficient pore formation. Under the optimized conditions, the transformation efficiencies of microparticle bombardment and multi-pulse electroporation were 111 and 82 per 108 cells, respectively (37 and 27 times higher than the conventional bombardment method). With the aid of the optimized protocol, we successfully developed the transformant clone over-expressing the endogenous fat storage-inducing transmembrane protein (FIT)-like protein, which was previously found in the genome of the oleaginous diatom F. solaris and the oleaginous eustigmatophyte Nannochloropsis gaditana. This study provides powerful techniques to investigate and further enhance the metabolic functions of F. solaris by genetic engineering.

Pulsed ultraviolet (PUV) disinfection of artificially contaminated seawater seeded with high levels of pathogen disease indicators as an alternative for the shellfish industry depuration systems

The increase in pathogen levels in seawater threatens the safety of entire aquatic ecosystems. Foodborne pathogens can potentially accumulate in shellfish, especially in filter feeders such as bivalves, requiring an efficient depuration process before consumption. Alternative approaches to promote a cost-efficient purge at depuration plants are urgently needed. A small prototype pulsed ultraviolet (PUV) light recirculation system was designed, and its depuration potential was tested in a seawater matrix artificially contaminated with high levels of microbial pathogens Escherichia coli, Staphylococcus aureus, Salmonella typhimurium, Bacillus cereus and Candida albicans. The analysis of treatment parameters including voltage, number of pulses and duration of treatment was performed to ensure the highest reduction in contaminant levels. Optimal PUV disinfection was attained at 60 pulses/min at 1 kV for 10 min (a UV output of 12.9 J/cm²). All reductions were statistically significant, and the greatest was observed for S. aureus (5.63 log₁₀), followed by C. albicans (5.15 log₁₀), S. typhimurium (5 log₁₀), B. cereus (4.59 log₁₀) and E. coli (4.55 log₁₀). PUV treatment disrupted the pathogen DNA with the result that S. aureus, C. albicans and S. typhimurium were not detectable by PCR. Regulations were reviewed to address the applicability of PUV treatment as a promising alternative to assist in the reduction of microbial pathogens at depuration plants due to its high efficiency, short treatment period, high UV dose and recirculation system as currently employed in shellfish depuration plants.

Different Responses of Photosynthesis to Nitrogen Starvation Between Highly Oil-Accumulative Diatoms, Fistulifera solaris and Mayamaea sp. JPCC CTDA0820

Highly oil-accumulative diatoms are expected to be a promising biomass for the production of biofuel. To harvest the diatom oils at high yields, it is critical to elucidate the relationship of oil accumulation with photosynthesis under fluctuating environmental conditions. Here, we characterized the physiological responses of the growth and photosynthesis in the mesophilic diatom Fistulifera solaris and the cold-tolerant one Mayamaea sp. JPCC CTDA0820 to nitrogen starvation, one of the most notable abiotic stresses, where most eukaryotic algae decrease their photosynthetic activity and accumulate oil in the cells. While F. solaris started showing growth retardation at NaNO₃ levels less than 50% of a normal F/2 artificial seawater (ASW) medium, Mayamaea sp. sustained normal growth even at a NaNO₃ level 10% of normal F/2ASW, indicating the sharp contrast of nitrogen requirement between these two diatom species. In the transition from 100 to 0% nitrogen conditions in the modified F/2ASW, F. solaris showed a clear suppression of chlorophyll (Chl)-based photosynthetic O₂ evolution rate and relative electron transport rate at photosystem II, which were negatively correlated to the capacity of non-photochemical quenching. Meanwhile, there was no change in these Chl-based parameters observed in nitrogen-starved Mayamaea sp. Instead, Mayamaea sp. showed a significant decrease in the Chl a amount per cells. These data suggested the occurrence of two types of photosynthetic responses to nitrogen starvation in oleaginous diatoms; that is, (1) suppression of photosynthetic activity per Chl with enhancing heat dissipation of excess light energy and (2) synchronous suppression of cellular photosynthetic activity with Chl amounts.

Total Phenolic Levels, In Vitro Antioxidant Properties, and Fatty Acid Profile of Two Microalgae, Tetraselmis marina Strain IMA043 and Naviculoid Diatom Strain IMA053, Isolated from the North Adriatic Sea

This work studied the potential biotechnological applications of a naviculoid diatom (IMA053) and a green microalga (Tetraselmis marina IMA043) isolated from the North Adriatic Sea. Water, methanol, and dichloromethane (DCM) extracts were prepared from microalgae biomass and evaluated for total phenolic content (TPC) and in vitro antioxidant properties. Biomass was profiled for fatty acid methyl esters (FAME) composition. The DCM extracts had the highest levels of total phenolics, with values of 40.58 and 86.14 mg GAE/g dry weight (DW in IMA053 and IMA043, respectively). The DCM extracts had a higher radical scavenging activity (RSA) than the water and methanol ones, especially those from IMA043, with RSAs of 99.65% toward 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)diammonium salt (ABTS) at 10 mg/mL, and of 103.43% against 2,2-diphenyl-1-picrylhydrazyl (DPPH) at 5 mg/mL. The DCM extract of IMA053 displayed relevant copper chelating properties (67.48% at 10 mg/mL), while the highest iron chelating activity was observed in the water extract of the same species (92.05% at 10 mg/mL). Both strains presented a high proportion of saturated (SFA) and monounsaturated (MUFA) fatty acids. The results suggested that these microalgae could be further explored as sources of natural antioxidants for the pharmaceutical and food industry and as feedstock for biofuel production.

The potential use of stickwater from a kilka fishmeal plant in Dunaliella salina cultivation

In this study, the possibility of culturing Dunaliella salina in stickwater (SW) as the main effluent of fishmeal plants was evaluated. D. salina was grown in different media obtained by replacing standard Guillard medium (F/2) with SW at 0% (control), 10%, 25%, 50%, 75%, and 100% ratios. The cell density, pigment contents, proximate composition, saponification value, and fatty acids (FAs) profiles were measured for 14 days. SW was collected from a kilka fishmeal factory in northern Iran, and the characteristics indicated high concentrations of nitrate (242.00 mg L^-1) and phosphate (11.13 mg L^-1). A significant increase in the cell density was observed in 14 days when 75% SW was used. Moreover, SW significantly affected the pigment contents. The highest contents of chlorophylls, total carotenoids, and β-carotene (3.64 μg mL^-1) were calculated in 75% SW. According to the algal proximate composition, the highest and lowest contents of lipid were accumulated in 75% and 100% SW, respectively (p < 0.05). The highest level of saturated FAs was observed in 75% SW compared with the others (p < 0.05). In conclusion, replacing F/2 with SW indicated the capability of D. salina to grow in a treated medium with 75% SW substitution as a bioremediator.

Homoeolog expression bias in allopolyploid oleaginous marine diatom Fistulifera solaris

Background

Allopolyploidy is a genomic structure wherein two or more sets of chromosomes derived from divergent parental species coexist within an organism. It is a prevalent genomic configuration in plants, as an important source of genetic variation, and also frequently confers environmental adaptability and increased crop productivity. We previously reported the oleaginous marine diatom Fistulifera solaris JPCC DA0580 to be a promising host for biofuel production and that its genome is allopolyploid, which had never previously been reported in eukaryotic microalgae. However, the study of allopolyploidy in F. solaris was hindered by the difficulty in classifying the homoeologous genes based on their progenitor origins, owing to the shortage of diatom genomic references.

Results

In this study, the allopolyploid genome of F. solaris was tentatively classified into two pseudo-parental subgenomes using sequence analysis based on GC content and codon frequency in each homoeologous gene pair. This approach clearly separated the genome into two distinct fractions, subgenome Fso_h and Fso_l, which also showed the potency of codon usage analysis to differentiate the allopolyploid subgenome. Subsequent homoeolog expression bias analysis revealed that, although both subgenomes appear to contribute to global transcription, there were subgenomic preferences in approximately 61% of homoeologous gene pairs, and the majority of these genes showed continuous bias towards a specific subgenome during lipid accumulation. Additional promoter analysis indicated the possibility of promoter motifs involved in biased transcription of homoeologous genes. Among these subgenomic preferences, genes involved in lipid metabolic pathways showed interesting patterns in that biosynthetic and degradative pathways showed opposite subgenomic preferences, suggesting the possibility that the oleaginous characteristics of F. solaris derived from one of its progenitors.

Conclusions

We report the detailed genomic structure and expression patterns in the allopolyploid eukaryotic microalga F. solaris. The allele-specific patterns reported may contribute to the oleaginous characteristics of F. solaris and also suggest the robust oleaginous characteristics of one of its progenitors. Our data reveal novel aspects of allopolyploidy in a diatom that is not only important for evolutionary studies but may also be advantageous for biofuel production in microalgae.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4691-0) contains supplementary material, which is available to authorized users.

Structure and properties of oil bodies in diatoms

Diatoms accumulate triacylglycerols in spherical organelles called oil bodies when exposed to nutrient deprivation conditions. Oil body biology in diatoms has attracted significant attention due to the complexity of the intracellular organelles and the unique combination of genes generated by the evolutionary history of secondary endosymbiosis. The demand for biofuel production has further increased the interest in and importance of a better understanding of oil body biology in diatoms, because it could provide targets for genetic engineering to further enhance their promising lipid accumulation. This review describes recent progress in studies of the structure and properties of diatom oil bodies. Firstly, the general features of diatom oil bodies are described, in particular, their number, size and morphology, as well as the quantity and quality of lipids they contain. Subsequently, the diatom oil body-associated proteins, which were recently discovered through oil body proteomics, are introduced. Then, the metabolic pathways responsible for the biogenesis and degradation of diatom oil bodies are summarized. During biogenesis and degradation, oil bodies interact with other organelles, including chloroplasts, the endoplasmic reticulum and mitochondria, suggesting their dynamic nature in response to environmental changes. Finally, the functions of oil bodies in diatoms are discussed.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

UV-C irradiation accelerates neutral lipid synthesis in the marine oleaginous diatom Fistulifera solaris

This study investigated the induction of oil synthesis in the oleaginous diatom, Fistulifera solaris, following irradiation with small doses of UV-C. A rapid induction of oil accumulation was confirmed within 6h following UV-C radiation of the diatom cells, with increases in cell oil body volumes after 24h of approximately 4- to 6-fold from the initial volume. Reactive oxygen species (ROS), which can be generated by a UV-C-mediated reaction, were detected in irradiated cells and the correlation between ROS generation and oil accumulation was confirmed. The smallest UV-C intensity required for oil induction in the cells was 10mJ/cm². Based on the ideal biodiesel profile, the most suitable FAME composition was obtained when UV₂₅₅ was used to irradiate the cells. The UV-C radiation method is therefore a solution for shortening the oil accumulation period and improving biodiesel productivity.

Production of eicosapentaenoic acid by high cell density cultivation of the marine oleaginous diatom Fistulifera solaris

Polyunsaturated fatty acids (PUFAs), including eicosapentaenoic acid (EPA), have attracted attention owing to their health benefits for humans, as well as their importance in aquaculture and animal husbandry. Establishing a sustainable PUFA supply based on fish oils has been difficult due to their increasing demand. Therefore, alternative sources of PUFAs are required. In this research, we examined the potential of the marine oleaginous diatom Fistulifera solaris as an alternative producer of PUFAs. Optimization of culture conditions was carried out for high cell density cultivation, and a maximal biomass productivity of 1.32±0.13g/(L·day) was achieved. By slightly adjusting the culture conditions for EPA production, the maximal EPA productivity reached 135.7±10.0mg/(L·day). To the best of our knowledge, this is the highest EPA productivity among microalgae cultured under photoautotrophic conditions. This result indicates that F. solaris is a promising candidate host for sustainable PUFA production.

Optimisation of critical medium components and culture conditions for enhanced biomass and lipid production in the oleaginous diatom Navicula phyllepta: a statistical approach

Diatoms hold great promise as potential sources of biofuel production. In the present study, the biomass and lipid production in the marine diatom Navicula phyllepta, isolated from Cochin estuary, India and identified as a potential biodiesel feedstock, were optimized using Plackett-Burman (PB) statistical experimental design followed by central composite design (CCD) and response surface methodology (RSM). The growth analyses of the isolate in different nitrogen sources, salinities and five different enriched sea water media showed the best growth in the cheapest medium with minimum components using urea as nitrogen source at salinity between 25 and 40 g kg^-1. Plackett-Burman experimental analyses for screening urea, sodium metasilicate, sodium dihydrogen phosphate, ferric chloride, salinity, temperature, pH and agitation influencing lipid and biomass production showed that silicate and temperature had a positive coefficient on biomass production, and temperature had a significant positive coefficient, while urea and phosphate showed a negative coefficient on lipid content. A 2⁴ factorial central composite design (FCCD) was used to optimize the concentration of the factors selected. The optimized media resulted in 1.62-fold increase (64%) in biomass (1.2 ± 0.08 g L^-1) and 1.2-fold increase (22%) in estimated total lipid production (0.11 ± 0.003 g L^-1) compared to original media within 12 days of culturing. A significantly higher biomass and lipid production in the optimized medium demands further development of a two-stage strategy of biomass production followed by induction of high lipid production under nutrient limitation or varying culture conditions for large-scale production of biodiesel from the marine diatom.

Modulation of lipid biosynthesis by stress in diatoms

Diatoms are responsible for up to 40% of the carbon fixation in our oceans. The fixed carbon is moved through carbon metabolism towards the synthesis of organic molecules that are consumed through interlocking foodwebs, and this process is strongly impacted by the abiotic environment. However, it has become evident that diatoms can be used as 'platform' organisms for the production of high valuable bio-products such as lipids, pigments and carbohydrates where stress conditions can be used to direct carbon metabolism towards the commercial production of these compounds. In the first section of this review, some aspects of carbon metabolism in diatoms and how it is impacted by environmental factors are briefly described. The second section is focused on the biosynthesis of lipids and in particular omega-3 long-chain polyunsaturated fatty acids and how low temperature stress impacts on the production of these compounds. In a third section, we review the recent advances in bioengineering for lipid production. Finally, we discuss new perspectives for designing strains for the sustainable production of high-value lipids.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

Enhanced lipid and biomass production by a newly isolated and identified marine microalga

BACKGROUND

The increasing demand for microalgae lipids as an alternative to fish has encouraged researchers to explore oleaginous microalgae for food uses. In this context, optimization of growth and lipid production by the marine oleaginous V₂-strain-microalgae is of great interest as it contains large amounts of mono-unsaturated (MUFAs) and poly-unsaturated fatty acids (PUFAs).

METHODS

In this study, the isolated V₂ strain was identified based on 23S rRNA gene. Growth and lipid production conditions were optimized by using the response surface methodology in order to maximize its cell growth and lipid content that was quantified by both flow cytometry and the gravimetric method. The intracellular lipid bodies were detected after staining with Nile red by epifluorescence microscopy. The fatty acid profile of optimal culture conditions was determined by gas chromatography coupled to a flame ionization detector.

RESULTS

The phenotypic and phylogenetic analyses showed that the strain V₂ was affiliated to Tetraselmis genus. The marine microalga is known as an interesting oleaginous species according to its high lipid production and its fatty acid composition. The optimization process showed that maximum cell abundance was achieved under the following conditions: pH: 7, salinity: 30 and photosynthetic light intensity (PAR): 133 μmol photons.m^-2.s^-1. In addition, the highest lipid content (49 ± 2.1% dry weight) was obtained at pH: 7, salinity: 37.23 and photosynthetic light intensity (PAR): 188 μmol photons.m^-2.s^-1. The fatty acid profile revealed the presence of 39.2% and 16.1% of total fatty acids of mono-unsaturated fatty acids (MUFAs) and poly-unsaturated fatty acids (PUFAs), respectively. Omega 3 (ω3), omega 6 (ω6) and omega 9 (ω9) represented 5.28%, 8.12% and 32.8% of total fatty acids, respectively.

CONCLUSIONS

This study showed the successful optimization of salinity, light intensity and pH for highest growth, lipid production and a good fatty acid composition, making strain V₂ highly suitable for food and nutraceutical applications.

Isolation and characterization of bacterium producing lipid from short-chain fatty acids

Anaerobic fermentation generates propionic acid, which inhibits microbial growth and accumulates in wastewater containing increased amounts of organic matter. We therefore isolated a propionic acid-assimilating bacterium that could produce triacylglycerol, for use in wastewater treatment. Nitratireductor sp. strain OM-1 can proliferate in medium containing propionic, acetic, butyric, and valeric acids as well as glycerol, and produces triacylglycerol when both propionic and acetic acids or glycerol are present. In composite model wastewater containing acetic acid, propionic acid and glycerol, this strain shows an even higher conversion rate, suggesting that it is suitable for wastewater treatment. Further, nitrogen depletion in medium containing an acetic-propionic acid mixture resulted in the production of the light oil 2-butenoic acid 1-methylethyl ester, but not triacylglycerol. Collectively, our data indicate that strain OM-1 has the potential to reduce accumulation of activated sludge in wastewater treatment and may contribute to the production of biodiesel.

Peptide-mediated microalgae harvesting method for efficient biofuel production

BACKGROUND

Production of biofuels from microalgae has been recognized to be a promising route for a sustainable energy supply. However, the microalgae harvesting process is a bottleneck for industrialization because it is energy intensive. Thus, by displaying interactive protein factors on the cell wall, oleaginous microalgae can acquire the auto- and controllable-flocculation function, yielding smarter and energy-efficient harvesting.

RESULTS

Towards this goal, we established a cell-surface display system using the oleaginous diatom Fistulifera solaris JPCC DA0580. Putative cell wall proteins, termed frustulins, were identified from the genome information using a homology search. A selected frustulin was subsequently fused with green fluorescent protein (GFP) and a diatom cell-surface display was successfully demonstrated. The antibody-binding assay further confirmed that the displayed GFP could interact with the antibody at the outermost surface of the cells. Moreover, a cell harvesting experiment was carried out using silica-affinity peptide-displaying diatom cells and silica particles where engineered cells attached to the silica particles resulting in immediate sedimentation.

CONCLUSION

This is the first report to demonstrate the engineered peptide-mediated harvesting of oleaginous microalgae using a cell-surface display system. Flocculation efficiency based on the silica-affinity peptide-mediated cell harvesting method demonstrated a comparable performance to other flocculation strategies which use either harsh pH conditions or expensive chemical/biological flocculation agents. We propose that our peptide-mediated cell harvest method will be useful for the efficient biofuel production in the future.

Advantageous characteristics of the diatom Chaetoceros gracilis as a sustainable biofuel producer

BACKGROUND

Diatoms have attracted interest as biofuel producers. Here, the contents of lipids and photosynthetic pigments were analyzed in a marine centric diatom, Chaetoceros gracilis. This diatom can be genetically engineered using our previously reported transformation technique and has a potential to produce valuable materials photosynthetically. Sustainable culture conditions for cost-effective production of biological materials under autotrophic conditions with atmospheric carbon dioxide were investigated in the laboratory. A large-scale, open-air culture was also performed.

RESULTS

Cell population doubling time was ~10 h under continuous illumination without CO₂ enrichment, and large amounts of triacylglycerols (TAG) and fucoxanthin accumulated under a wide range of salinity and nutrient conditions, reaching ~200 and 18.5 mg/L, respectively. It was also shown that C. gracilis produced high amounts of TAG without the need for nitrogen or silica deprivation, which is frequently imposed to induce lipid production in many other microalgae. Furthermore, C. gracilis was confirmed to be highly tolerant to changes in environmental conditions, such as salinity. The diatom grew well and produced abundant lipids when using sewage water or liquid fertilizer derived from cattle feces without augmented carbon dioxide. High growth rates (doubling time <20 h) were obtained in a large-scale, open-air culture, in which light irradiance and temperature fluctuated and were largely different from laboratory conditions.

CONCLUSIONS

The ability of this microalga to accumulate TAG without nutrient deprivation, which incurs added labor, high costs, and complicates scalability, is important for low-cost industrial applications. Furthermore, its high tolerance to changes in environmental conditions and high growth rates observed in large-scale, open-air culture implied scalability of this diatom for industrial applications. Therefore, C. gracilis would have great potential as a biofactory.

Enhancement of glycerol metabolism in the oleaginous marine diatom Fistulifera solaris JPCC DA0580 to improve triacylglycerol productivity

BACKGROUND

Microalgal oil is a promising alternative feedstock for biodiesel fuel (BDF). Mixotrophic cultivation with glycerol, the primary byproduct of BDF production, may be used to optimize BDF production. This strategy would reduce costs through glycerol recycling and improve lipid productivity and biomass productivity by overcoming the growth retardation caused by decreased light penetration in high-density culture.

RESULTS

Overexpression of the endogenous glycerol kinase (GK) gene in an oleaginous marine diatom, Fistulifera solaris JPCC DA0580, accelerates glycerol metabolism and improves lipid and biomass productivities. Two candidates were selected from a collection of 90 G418-resistant clones, based on growth and confirmation of genome integration. GK gene expression was higher in the selected clones (GK1_7 and GK2_16) than in the wild-type culture. The GK2_16 clone achieved a 12% increase in lipid productivity.

CONCLUSION

We have demonstrated the potential of metabolic engineering in oleaginous microalgae to improve lipid productivity. Metabolic engineering techniques can be used to optimize BDF production.

In vivo live cell imaging for the quantitative monitoring of lipids by using Raman microspectroscopy

A straightforward in vivo monitoring technique for biomolecules would be an advantageous approach for understanding their spatiotemporal dynamics in living cells. However, the lack of adequate probes has hampered the quantitative determination of the chemical composition and metabolomics of cellular lipids at single-cell resolution. Here, we describe a method for the rapid, direct, and quantitative determination of lipid molecules from living cells using single-cell Raman imaging. In vivo localization of lipids in the form of triacylglycerol (TAG) within oleaginous microalga and their molecular compositions are monitored with high spatial resolution in a nondestructive and label-free manner. This method can provide quantitative and real-time information on compositions, chain lengths, and degree of unsaturation of fatty acids in living cells for improving the cultivating parameters or for determining the harvest timing during large-scale cultivations for microalgal lipid accumulation toward biodiesel production. Therefore, this technique is a potential tool for in vivo lipidomics for understanding the dynamics of lipid metabolisms in various organisms.

Chloroplast-targeting protein expression in the oleaginous diatom Fistulifera solaris JPCC DA0580 toward metabolic engineering

The chloroplast plays critical roles in lipid metabolism of microalgae, thus it is recognized as an attractive target of metabolic engineering to enhance biofuel production. It has been well known that recombinant protein expression in microalgal chloroplasts needs specific signal sequence which governs the transition manner of nuclear-encoded polypeptides within the subcellular compartments. However certain microalgae, including diatoms, have complex membrane systems surrounding the chloroplast, and thus chloroplast-targeting protein expression with the signal sequence has rarely been demonstrated except for a few model non-oleaginous diatoms. In this study, we performed recombinant green fluorescence protein (GFP) expression and transportation into the chloroplast of the oleaginous marine diatom, Fistulifera solaris JPCC DA0580. The signal sequence of ATP synthetase gamma subunit, which was predicted to localize in the chloroplast according to a bioinformatics analysis pipeline, was employed as a key factor of this technique. As a result, specific localization of GFP in the chloroplast was observed. It would be useful to engineer the lipid synthesis pathways existing in the chloroplast. Furthermore, intensive gathering of GFP in the rod-like structure was also detected, which has not been observed in model diatom studies. As comparing with electron microscopic observation, the structure was estimated to be a pyrenoid.

Oleosome-associated protein of the oleaginous diatom Fistulifera solaris contains an endoplasmic reticulum-targeting signal sequence

Microalgae tend to accumulate lipids as an energy storage material in the specific organelle, oleosomes. Current studies have demonstrated that lipids derived from microalgal oleosomes are a promising source of biofuels, while the oleosome formation mechanism has not been fully elucidated. Oleosome-associated proteins have been identified from several microalgae to elucidate the fundamental mechanisms of oleosome formation, although understanding their functions is still in infancy. Recently, we discovered a diatom-oleosome-associated-protein 1 (DOAP1) from the oleaginous diatom, Fistulifera solaris JPCC DA0580. The DOAP1 sequence implied that this protein might be transported into the endoplasmic reticulum (ER) due to the signal sequence. To ensure this, we fused the signal sequence to green fluorescence protein. The fusion protein distributed around the chloroplast as like a meshwork membrane structure, indicating the ER localization. This result suggests that DOAP1 could firstly localize at the ER, then move to the oleosomes. This study also demonstrated that the DOAP1 signal sequence allowed recombinant proteins to be specifically expressed in the ER of the oleaginous diatom. It would be a useful technique for engineering the lipid synthesis pathways existing in the ER, and finally controlling the biofuel quality.

Optimisation of the critical medium components for better growth of Picochlorum sp. and the role of stressful environments for higher lipid production

BACKGROUND

Coastal countries that suffer from a scarcity of water, such as Tunisia, have to cultivate marine microalgae on non-arable land in order to produce feedstock and overcome their demands of nutrition and energy. In this framework, a green microalga, CTM 20019, was isolated, identified as Picochlorum sp. and tested for its lipid production.

RESULTS

The dry weight of Picochlorum sp. is composed of 163 g kg(-1) lipids, 225 g kg(-1) total sugars, 440 g kg(-1) proteins and 112 g kg(-1) ash rich in potassium, calcium, iron, magnesium and zinc. Gas chromatography-mass spectrometry analysis showed that the main fatty acids were palmitic acid (29%), linolenic acid (26.5%), linoleic acid (23.5%), hexadecatrienoic acid (11%) and hexadecadienoic acid (8.5%). As it is known that culture conditions greatly influence the composition of microalgae, the experiments were designed to optimise the composition of the medium in order to increase Picochlorum sp. growth from OD680nm = 0.53 to OD680nm = 2.2 and lipid accumulation from 163 g kg(-1) to 190 g kg(-1) . The highest lipid contents of 570 and 585 g kg(-1) were achieved under phosphate starvation and sodium carbonate supplementation, respectively. Under these conditions, the fatty acid profile is dominated by mono-unsaturated and polyunsaturated acids, and is therefore suitable for aqua-culture feeding. However, under high salinity, growth and lipid synthesis are inhibited, and the fatty acids are saturate, and the product is therefore suitable for biodiesel.

CONCLUSION

This high lipid content rich in essential fatty acids, omega-6 and omega-3, endorses this wild strain of Picochlorum sp. as a promising feedstock for aqua-culture and human nutrition or for the production of biodiesel. © 2013 Society of Chemical Industry.

Profiling of polar lipids in marine oleaginous diatom Fistulifera solaris JPCC DA0580: prediction of the potential mechanism for eicosapentaenoic acid-incorporation into triacylglycerol

The marine oleaginous diatom Fistulifera solaris JPCC DA0580 is a candidate for biodiesel production because of its high lipid productivity. However, the substantial eicosapentaenoic acid (EPA) content in this strain would affect the biodiesel quality. On the other hand, EPA is also known as the essential health supplement for humans. EPAs are mainly incorporated into glycerolipids in the microalgal cell instead of the presence as free fatty acids. Therefore, the understanding of the EPA biosynthesis including the incorporation of the EPA into glycerolipids especially triacylglycerol (TAG) is fundamental for regulating EPA content for different purposes. In this study, in order to identify the biosynthesis pathway for the EPA-containing TAG species, a lipidomic characterization of the EPA-enriched polar lipids was performed by using direct infusion electrospray ionization (ESI)-Q-TRAP-MS and MS/MS analyses. The determination of the fatty acid positional distribution showed that the sn-2 position of all the chloroplast lipids and part of phosphatidylcholine (PC) species was occupied by C16 fatty acids. This result suggested the critical role of the chloroplast on the lipid synthesis in F. solaris. Furthermore, the exclusive presence of C18 fatty acids in PC highly indicated the biosynthesis of EPA on PC. Finally, the PC-based acyl-editing and head group exchange processes were proposed to be essential for the incorporation of EPA into TAG and chloroplast lipids.

Identification of a frustule-associated protein of the marine pennate diatom Fistulifera sp. strain JPCC DA0580

Among the proteins localized on the cell wall (frustule) of diatoms (frustule-associated proteins), several proteins tightly associated with the cell wall have been implicated in frustule formation. These proteins include diatom-specific unique serine- and lysine-rich sequences represented by silaffins. Taking advantage of available genome information, we used a recently described bioinformatics approach to screen silaffin-like proteins rich in serine and lysine from the genome of the marine pennate diatom Fistulifera sp. strain JPCC DA0580 and identified 7 proteins. All of the proteins shared a sequence motif called the XGXG domain, which was also confirmed in a silaffin-like protein identified in other diatoms. In vivo localization analysis revealed that one of the identified proteins, G7408, occurs throughout the frustule with a slightly uneven distribution. This novel frustule-associated protein could be a useful tool to elucidate the mechanism of biosilica formation in diatoms and to functionalize this strain for future biotechnological applications.

Evaluation of indigenous microalgal isolate Chlorella sp. FC2 IITG as a cell factory for biodiesel production and scale up in outdoor conditions

The present study reports evaluation of an indigenous microalgal isolate Chlorella sp. FC2 IITG as a potential candidate for biodiesel production. Characterization of the strain was performed under photoautotrophic, heterotrophic, and mixotrophic cultivation conditions. Further, an open-pond cultivation of the strain under outdoor conditions was demonstrated to evaluate growth performance and lipid productivity under fluctuating environmental parameters and in the presence of potential contaminants. The key findings were: (1) the difference in cultivation conditions resulted in significant variation in the biomass productivity (73-114 mg l⁻¹ day⁻¹) and total lipid productivity (35.02-50.42 mg l⁻¹ day⁻¹) of the strain; (2) nitrate and phosphate starvation were found to be the triggers for lipid accumulation in the cell mass; (3) open-pond cultivation of the strain under outdoor conditions resulted in biomass productivity of 44 mg l⁻¹ day⁻¹ and total lipid productivity of 10.7 mg l⁻¹ day⁻¹; (4) a maximum detectable bacterial contamination of 7 % of the total number of cells was recorded in an open-pond system; and (5) fatty acid profiling revealed abundance of palmitic acid (C16:0), oleic acid (C18:1) and linoleic acid (C18:2), which are considered to be the key elements for suitable quality biodiesel.

Biology and Industrial Applications of Chlorella: Advances and Prospects

Chlorella represents a group of eukaryotic green microalgae that has been receiving increasing scientific and commercial interest. It possesses high photosynthetic ability and is capable of growing robustly under mixotrophic and heterotrophic conditions as well. Chlorella has long been considered as a source of protein and is now industrially produced for human food and animal feed. Chlorella is also rich in oil, an ideal feedstock for biofuels. The exploration of biofuel production by Chlorella is underway. Chlorella has the ability to fix carbon dioxide efficiently and to remove nutrients of nitrogen and phosphorous, making it a good candidate for greenhouse gas biomitigation and wastewater bioremediation. In addition, Chlorella shows potential as an alternative expression host for recombinant protein production, though challenges remain to be addressed. Currently, omics analyses of certain Chlorella strains are being performed, which will help to unravel the biological implications of Chlorella and facilitate the future exploration of industrial applications.

Seasonal variation of biomass and oil production of the oleaginous diatom Fistulifera sp. in outdoor vertical bubble column and raceway-type bioreactors

To evaluate the feasibility of industrial biodiesel production, outdoor mass cultivation of the marine oleaginous diatom, Fistulifera sp. strain JPCC DA0580, was conducted in bench-scale photobioreactors (∼200 L, raceway- and column-types) and seasonal variation of biomass and oil content were monitored. Through three seasons (from spring to autumn), the microalgae showed steady growth and oil accumulation in both reactors in spite of fluctuating temperature and solar irradiation. When comparing the both reactors, the column-type bioreactor was better with regard to energy conversion efficiency compared to the raceway-type bioreactor. The areal oil productivity of 3.23 g/m(2)/day is comparable or even higher level as compared with the one from other oleaginous microalgae prepared in outdoor mass cultivation. Furthermore, repeated batch culture experiments resulted in success at least 5 cycles. Through the experimental period, little bacterial contamination was observed while protozoal contamination was a fatal issue. The microalgal cell was robust enough to be handled by an automated pump system in inoculation and harvesting processes, and cell adhesion to the bioreactor wall was not observed. These beneficial features could realize ease of oil production and system maintenance. These findings ensure promising innovation by means of outdoor mass cultivation with this strain toward biodiesel production.

Reconstruction of the lipid metabolism for the microalga Monoraphidium neglectum from its genome sequence reveals characteristics suitable for biofuel production

BACKGROUND

Microalgae are gaining importance as sustainable production hosts in the fields of biotechnology and bioenergy. A robust biomass accumulating strain of the genus Monoraphidium (SAG 48.87) was investigated in this work as a potential feedstock for biofuel production. The genome was sequenced, annotated, and key enzymes for triacylglycerol formation were elucidated.

RESULTS

Monoraphidium neglectum was identified as an oleaginous species with favourable growth characteristics as well as a high potential for crude oil production, based on neutral lipid contents of approximately 21% (dry weight) under nitrogen starvation, composed of predominantly C18:1 and C16:0 fatty acids. Further characterization revealed growth in a relatively wide pH range and salt concentrations of up to 1.0% NaCl, in which the cells exhibited larger structures. This first full genome sequencing of a member of the Selenastraceae revealed a diploid, approximately 68 Mbp genome with a G + C content of 64.7%. The circular chloroplast genome was assembled to a 135,362 bp single contig, containing 67 protein-coding genes. The assembly of the mitochondrial genome resulted in two contigs with an approximate total size of 94 kb, the largest known mitochondrial genome within algae. 16,761 protein-coding genes were assigned to the nuclear genome. Comparison of gene sets with respect to functional categories revealed a higher gene number assigned to the category "carbohydrate metabolic process" and in "fatty acid biosynthetic process" in M. neglectum when compared to Chlamydomonas reinhardtii and Nannochloropsis gaditana, indicating a higher metabolic diversity for applications in carbohydrate conversions of biotechnological relevance.

CONCLUSIONS

The genome of M. neglectum, as well as the metabolic reconstruction of crucial lipid pathways, provides new insights into the diversity of the lipid metabolism in microalgae. The results of this work provide a platform to encourage the development of this strain for biotechnological applications and production concepts.

Biosynthesis of polyunsaturated fatty acids in the oleaginous marine diatom Fistulifera sp. strain JPCC DA0580

Studies of polyunsaturated fatty acid (PUFA) biosynthesis in microalgae are of great importance for many reasons, including the production of biofuel and variable omega 3-long chain PUFAs. The elucidation of the PUFA biosynthesis pathway is necessary for bioengineering to increase or decrease PUFA content in certain microalgae. In this study, we identified the PUFA synthesis pathway in the oleaginous marine diatom, Fistulifera sp. strain JPCC DA0580, a promising candidate for biodiesel production. The data revealed not only the presence of the desaturases and elongases involved in eicosapentaenoic acid (EPA) synthesis, but also the unexpected localization of ω3-desaturase expression in the chloroplast. This suggests that this microalga might perform the final step of EPA synthesis in the chloroplast and not in the endoplasmic reticulum (ER) like other diatoms. The detailed fatty acid profile suggests that the EPA was synthesized only through the ω6-pathway in this strain, which was also different from other diatoms. Finally, the transcriptome analysis demonstrated an overall down-regulation of desaturases and elongases over incubation time. These genetic features might explain the decrease of PUFA percentage over incubation time in this strain. The important insights into metabolite synthesis acquired here will be useful for future metabolic engineering to control PUFA content in this diatom.

Coagulation-flocculation of marine Chlorella sp. for biodiesel production

Harvesting of marine Chlorella sp. by autoflocculation and flocculation by addition of coagulant with pH adjustment was investigated in this study. Autoflocculation provided low efficiency. Response surface methodology was employed to optimize the coagulant dosage and pH for flocculation. Aluminium sulfate and ferric chloride were investigated coagulants. The empirical models from RSM are in a good agreement with the experimental results. The optimum flocculation was achieved at ferric chloride dosage 143 mg/L, pH 8.1 and settling time 40 min. Biomass concentration also presented the significant effect on harvesting efficiency. Lipid extracted from marine Chlorella sp. cultivated in urea fertilizer medium with hexane as a solvent is suitable to produce biodiesel according to it contains high proportion of saturated fatty acids. The crude lipid should be purified to remove some impurities before making biodiesel. As the free fatty acid content was higher than 1% a two-step biodiesel production is recommended.

Identification and functional analysis of delta-9 desaturase, a key enzyme in PUFA Synthesis, isolated from the oleaginous diatom Fistulifera

Oleaginous microalgae are one of the promising resource of nonedible biodiesel fuel (BDF) feed stock alternatives. Now a challenge task is the decrease of the long-chain polyunsaturated fatty acids (PUFAs) content affecting on the BDF oxidative stability by using gene manipulation techniques. However, only the limited knowledge has been available concerning the fatty acid and PUFA synthesis pathways in microalgae. Especially, the function of Δ9 desaturase, which is a key enzyme in PUFA synthesis pathway, has not been determined in diatom. In this study, 4 Δ(9) desaturase genes (fD9desA, fD9desB, fD9desC and fD9desD) from the oleaginous diatom Fistulifera were newly isolated and functionally characterized. The putative Δ(9) acyl-CoA desaturases in the endoplasmic reticulum (ER) showed 3 histidine clusters that are well-conserved motifs in the typical Δ(9) desaturase. Furthermore, the function of these Δ(9) desaturases was confirmed in the Saccharomyces cerevisiae ole1 gene deletion mutant (Δole1). All the putative Δ(9) acyl-CoA desaturases showed Δ(9) desaturation activity for C16∶0 fatty acids; fD9desA and fD9desB also showed desaturation activity for C18∶0 fatty acids. This study represents the first functional analysis of Δ(9) desaturases from oleaginous microalgae and from diatoms as the first enzyme to introduce a double bond in saturated fatty acids during PUFA synthesis. The findings will provide beneficial insights into applying metabolic engineering processes to suppressing PUFA synthesis in this oleaginous microalgal strain.

Proteomics analysis of oil body-associated proteins in the oleaginous diatom

For biodiesel production from microalgae, it is desirable to understand the entire triacylglycerol (TAG) metabolism. TAG accumulation occurs in oil bodies, and although oil body-associated proteins could play important roles in TAG metabolism, only a few microalgal species have been studied by a comprehensive analysis. Diatoms are microalgae that are promising producers of biodiesel, on which such proteomics analysis has not been conducted to date. Herein, we identified oil body-associated proteins in the oleaginous diatom Fistulifera sp. strain JPCC DA0580. The oil body fraction was separated by cell disruption with beads beating and subsequent ultracentrifugation. Contaminating factors could be removed by comparing proteins from the oil body and the soluble fractions. This novel strategy successfully revealed 15 proteins as oil body-associated protein candidates. Among them, two proteins, which were parts of proteins predicted to have transmembrane domains, were indeed confirmed to specifically localize to the oil bodies in this strain by observation of GFP fusion proteins. One (predicted to be a potassium channel) was also detected from the ER, suggesting that oil bodies might originate from the ER. By utilizing this novel subtraction method, we succeeded in identifying the oil body-associated proteins in the diatom for the first time.

A process design and productivity evaluation for oil production by indoor mass cultivation of a marine diatom, Fistulifera sp. JPCC DA0580

The present study involved the designing of a culture process and the evaluation of productivity of oil products from a highly oleaginous marine diatom, Fistulifera sp. JPCC DA0580, which had been cultured in a commercial-scale factory. The culture facility had a capacity of 48,000 L and held 96 flat-type 500-L photobioreactors (PBRs) equipped with artificial light, which secures a stable, perennial supply of the products. A 10 days culture that had reached a cell density of 6.5 g dry weight L(-1) possessing a cellular oil content of 48% (wt/wt) was found to provide the highest oil yield. On considering a production area of 1500 m(2), annual algal mass and oil productivity is 68.7 and 33.3 t ha(-1) year(-1), respectively. This study thus provides a reproducible prediction of a theoretical maximum oil yield from a highly oleaginous microalgal strain based on industrially practical production area.

Establishment of a genetic transformation system for the marine pennate diatom Fistulifera sp. strain JPCC DA0580--a high triglyceride producer

A genetic transformation system for the marine pennate diatom, Fistulifera sp. JPCC DA0580, was established using microparticle bombardment methods. Strain JPCC DA0580 has been recently identified as the highest triglyceride (60 % w/w) producer from a culture collection of 1,393 strains of marine microalgae, and it is expected to be a feasible source of biodiesel fuel. The transformation conditions for strain JPCC DA0580 were optimised using the green fluorescent protein gene (gfp) and the gene encoding neomycin phosphotransferase II (nptII). The most efficient rate of transformation was attained when tungsten particles (0.6 μm in diameter) were used for microparticle bombardment. The effect of endogenous and exogenous promoters on the expression of nptII was examined. Endogenous promoters were more efficient for obtaining transformants compared with exogenous promoters. Southern hybridisation analysis suggested that nptII integrated into the nuclear genome. This genetic manipulation technique should allow us to understand the mechanisms of high triglyceride accumulation in this strain, thereby contributing to improving BDF production.

Glycosylceramides from marine green microalga Tetraselmis sp

Glycosylceramides are ubiquitous and important components of the plasma membrane in most eukaryotic cells and a few bacteria. They play significant roles in a variety of cellular functions. Their molecular structures are well recognized in animals, higher plants, and fungi, but are poorly characterized in lower plants. In this study, a high glycosylceramide-producing microalgal strain Tetraselmis sp. NKG 400013 was found. TLC and MS analyses established the presence of glycosylceramides, GT1 and GT2, in this strain. Their chemical structures were determined by NMR spectroscopy and GC/MS, and were identified as glycosylceramides consisting of the typical botanical sphingoid base ([4E, 8E]-sphinga-4, 8-dienine) and 2-hydroxy-Δ3-unsaturated fatty acyl chains, respectively. To our knowledge, the occurrence of glycosylceramides in microalga of the class Prasinophyceae was previously unknown.

High-throughput pyrosequencing of the chloroplast genome of a highly neutral-lipid-producing marine pennate diatom, Fistulifera sp. strain JPCC DA0580

The chloroplast genome of the highly neutral-lipid-producing marine pennate diatom Fistulifera sp. strain JPCC DA0580 was fully sequenced using high-throughput pyrosequencing. The general features and gene content were compared with three other complete diatom chloroplast genomes. The chloroplast genome is 134,918 bp with an inverted repeat of 13,330 bp and is slightly larger than the other diatom chloroplast genomes due to several low gene-density regions lacking similarity to the other diatom chloroplast genomes. Protein-coding genes were nearly identical to those from Phaeodactylum tricornutum. On the other hand, we found unique sequence variations in genes of photosystem II which differ from the consensus in other diatom chloroplasts. Furthermore, five functional unknown ORFs and a putative serine recombinase gene, serC2, are located in the low gene-density regions. SerC2 was also identified in the plasmids of another pennate diatom, Cylindrotheca fusiformis, and in the plastid genome of the diatom endosymbiont of Kryptoperidinium foliaceum. Exogenous plasmids might have been incorporated into the chloroplast genome of Fistulifera sp. by lateral gene transfer. Chloroplast genome sequencing analysis of this novel diatom provides many important insights into diatom evolution.