Effect of ammonium and high light intensity on the accumulation of lipids in Nannochloropsis oceanica (CCAP 849/10) and Phaeodactylum tricornutum (CCAP 1055/1)

Nitrogen (N) balancing metabolism in the toxic dinoflagellate Alexandrium pacificum against N shift revealed by physiology and N-related genes regulation

The dinoflagellate Alexandrium pacificum is responsible for harmful algal blooms and paralytic shellfish poisoning in marine environments. Its physiology is greatly affected by nitrogen (N) sources; however, the molecular mechanisms involved in N acquisition and balancing are not clearly understood. Here, we determined the full-length gene sequences of nitrate (NO3-) transporter (ApNRT), NO3- reductase (ApNR), and ammonium (NH4+) transporter (ApAMT) from the dinoflagellate A. pacificum. In addition, we examined physiological and transcriptional responses of these three genes under diverse concentrations of NO3- (0.00-8.82 mM) and NH4+ (0.00-1.76 mM). The open reading frames of ApNRT, ApNR, and ApAMT were determined as 1767 bp, 3312 bp, and 1363 bp, without introns in their genomic coding regions. Their encoded proteins were phylogenetically close to those of other photosynthetic eukaryotes. NO3- supplementation promoted cell growth, while NH4+ inhibited it. Expression of ApNRT and ApNR were correlated in both low and high N conditions. Sufficient uptake of one of the N forms (NO3- and NH4+, respectively) suppressed the regulation of the other dissolved inorganic nitrogen (DIN) transporter (ApAMT and ApNRT, respectively). These results showed that A. pacificum may have a selective mechanism for N uptake depending on the available N sources, suggesting a proliferation strategy of dinoflagellate in eutrophic environments.

Physiological and metabolic fluctuations of the diatom Phaeodactylum tricornutum under water scarcity

Water scarcity is an escalating environmental concern. The model diatom, Phaeodactylum tricornutum, holds promise as a potential cell factory for the production of high-value natural compounds. However, its dependence on saline water cultivation restricts its use in areas facing water shortages. Although numerous studies have delved into the metabolic mechanisms of plants under water stress, there is a limited understanding when it comes to microalgae. In our study, we employed polyethylene glycol (PEG) to simulate water scarcity conditions, and assessed a range of parameters to elucidate the metabolic responses of P. tricornutum. Water stress induced the generation of reactive oxygen species (ROS), curtailed the photosynthetic growth rate, and amplified lipid content. Our insights shed light on the physiology of P. tricornutum when subjected to water stress, setting the stage for potential applications of microalgae biotechnology in regions grappling with water scarcity.

Geographically-Resolved Techno-Economic and Life Cycle Assessment Comparing Microalgae-Based Renewable Diesel and Sustainable Aviation Fuel in the United States

This study integrates high-resolution thermal and biological modeling with techno-economic analysis and life cycle assessment to evaluate and compare two different microalgae biorefinery configurations targeting renewable diesel (RD) and sustainable aviation fuel (SAF) production in the United States at the county level. A dynamic engineering process model captures mass and energy balances for biomass growth, storage, dewatering, and conversion with hourly resolution. The modeled configurations enable large-scale biofuel production by supporting facilities in remote locations and cultivation on marginal lands. The two pathways under examination share identical biomass production and harvesting assumptions but differ in their conversion processes. The first pathway evaluates hydrothermal liquefaction (HTL) to produce RD, while the second pathway explores the hydroprocessed esters and fatty acids (HEFA) process to produce SAF. Results indicate that the minimum fuel selling price (MFSP) for HTL could decrease from 3.72-$7.26 to 1.48-$4.10 per liter of gasoline equivalent (LGE-1), and for HEFA from 5.79-$10.93 to 1.73-$4.48 LGE-1 under future scenarios with increased lipid content and reduced CO2 delivery costs. Optimization analyses reveal pathways to achieve a MFSP of $0.75 LGE-1 and 70% greenhouse gas emissions reductions compared to petroleum fuels for both pathways. The study also examines water footprint, land-use change emissions, and additional environmental impacts. Discussion focuses on outlining strategic research and development investments to reduce production costs and environmental burdens from microalgae biofuels.

Genome-wide adenine N6-methylation map reveals epigenomic regulation of lipid accumulation in Nannochloropsis

Epigenetic marks on histones and DNA, such as DNA methylation at N6-adenine (6mA), play crucial roles in gene expression and genome maintenance, but their deposition and function in microalgae remain largely uncharacterized. Here, we report a genome-wide 6mA map for the model industrial oleaginous microalga Nannochloropsis oceanica produced by single-molecule real-time sequencing. Found in 0.1% of adenines, 6mA sites are mostly enriched at the AGGYV motif, more abundant in transposons and 3' untranslated regions, and associated with active transcription. Moreover, 6mA gradually increases in abundance along the direction of gene transcription and shows special positional enrichment near splicing donor and transcription termination sites. Highly expressed genes tend to show greater 6mA abundance in the gene body than do poorly expressed genes, indicating a positive interaction between 6mA and general transcription factors. Furthermore, knockout of the putative 6mA methylase NO08G00280 by genome editing leads to changes in methylation patterns that are correlated with changes in the expression of molybdenum cofactor, sulfate transporter, glycosyl transferase, and lipase genes that underlie reductions in biomass and oil productivity. By contrast, knockout of the candidate demethylase NO06G02500 results in increased 6mA levels and reduced growth. Unraveling the epigenomic players and their roles in biomass productivity and lipid metabolism lays a foundation for epigenetic engineering of industrial microalgae.

Impact of macronutrients and salinity stress on biomass and biochemical constituents in Monoraphidium braunii to enhance biodiesel production

Microalgal lipids are precursors to the production of biodiesel, as well as a source of valuable dietary components in the biotechnological industries. So, this study aimed to assess the effects of nutritional (nitrogen, and phosphorus) starvations and salinity stress (NaCl) on the biomass, lipid content, fatty acids profile, and predicted biodiesel properties of green microalga Monoraphidium braunii. The results showed that biomass, biomass productivity, and photosynthetic pigment contents (Chl. a, b, and carotenoids) of M. braunii were markedly decreased by nitrogen and phosphorus depletion and recorded the maximum values in cultures treated with full of N and P concentrations (control, 100%). These parameters were considerably increased at the low salinity level (up to 150 mM NaCl), while an increasing salinity level (up to 250 mM NaCl) reduces the biomass, its productivity, and pigment contents. Nutritional limitations and salt stress (NaCl) resulted in significantly enhanced accumulation of lipid and productivity of M. braunii, which represented more than twofold of the control. Furthermore, these conditions have enhanced the profile of fatty acid and biodiesel quality-related parameters. The current study exposed strategies to improve M. braunii lipid productivity for biodiesel production on a small scale in vitro in terms of fuel quality under low nutrients and salinity stress.

Seawater with Added Monosodium Glutamate Residue (MSGR) Is a Promising Medium for the Cultivation of Two Commercial Marine Microalgae

Phaeodactylum tricornutum and Nannochloropsis oceanica, with their satisfactory performance in accumulating lipids and other high-value products, have been successfully used for commercial production in recent years. However, costly chemicals in culture media greatly increase the price of the resulting bioproducts. To control the cultivation cost, this paper assessed the potential of seawater supplemented with monosodium glutamate residue wastewater at a ratio of 1/500 (S-MSGR) to serve as a growing medium for these two marine species. Compared with the standard chemical culture medium, Erdschreiber’s medium (EM), both the algal growth and metabolite accumulation of P. tricornutum and N. oceanica were greatly promoted in S-MSGR. The maximum biomass concentrations of P. tricornutum and N. oceanica reached 0.93 and 0.36 g/L, which were, respectively, 1.5 and 1.9 times higher than those in EM medium. For lipid accumulation, P. tricornutum exhibited an excellent lipid productivity of 22.9 mg/L/day in S-MSGR, a 64% increase compared to EM medium. Furthermore, the average yield coefficients indicated good performance of P. tricornutum and N. oceanica in transferring the nitrogen in S-MSGR to the biomass, at 74.8 and 174.8 mg/g of nitrogen. In addition, compared with EM, the costs of the medium for lipid production of P. tricornutum and N. oceanica cultured in S-MSGR were USD 2.3 and 5.8/(kg lipid), which saved 96.9% and 97.6%, respectively. Therefore, this paper demonstrates that S-MSGR is a suitable nutrient resource for P. tricornutum and N. oceanica, and it has a great potential to cut the cultivation cost during real commercial production.

Co-cultivation of Phaeodactylum tricornutum and Aurantiochytrium limacinum for polyunsaturated omega-3 fatty acids production

Marine protist Aurantiochytrium limacinum produces docosahexaenoic acid (DHA) as main polyunsaturated fatty acid (PUFA) and lacks any monounsaturated fatty acids (MUFA), while eicosapentaenoic acid (EPA) and MUFA's are produced by Phaeodactylum tricornutum. The marine diatom P. tricornutum was co-cultured with A.limacinum to match the EPA:DHA ratio of fish oil. Modulation in initial cell density ratio overcame the dominance of A.limacinum during co-cultivation and led to regulated proliferation of both species. Media engineering with nitrate and glycerol concentration yielded 2:1 (56.44: 30.11) mg g^-1 and 1:1 (47.43: 49.61) mg g^-1 EPA: DHA ratio. The oil and biomass obtained from co-cultivation comprised of MUFA's such as palmitoleic acid (2.65 mg g^-1) and oleic acid (1.25 mg g^-1) along with pigments like fucoxanthin (367.18 µg g^-1), β-carotene (8.98 µg g^-1) and astaxanthin (0.77 µg g^-1). Thus, co-cultivation of P. tricornutum with A. limacinum represented a unique strategy towards achieving desired fatty acid composition.

An auxin-like supermolecule to simultaneously enhance growth and cumulative eicosapentaenoic acid production in Phaeodactylum tricornutum

Phaeodactylum tricornutum, a model alga, is well known for its ability to accumulate intracellular omega-3 eicosapentaenoic acid (EPA). However, P.tricornutum cells need to have a higher EPA content if they are to be used for industrial applications. In this study, an auxin-like supermolecule (SM) was synthesised and used for the cultivation of P. tricornutum. Results show that the addition of 1 ppm of SM significantly increased the P. tricornutum cell density and boosted the P. tricornutum biomass. The experimental group treated with 5 ppm of SM, had an EPA content of 31.7%, which was a 2.09-fold increase over the EPA content in the untreated group. Overall, our results demonstrated that SM can significantly improve the microalgal growth and EPA accumulation in P. tricornutum, providing a feasible strategy to achieve efficient and cost-effective EPA production.

Towards a Phaeodactylum tricornutum biorefinery in an outdoor UK environment

A series of commercial powdered media (Cell-Hi F2P, JWP and WP) and a hydroponics medium (FloraMicroBloom) were investigated for the cultivation of P. tricornutum, and compared with f/2 (a commonly employed laboratory cultivation medium; costlier to scale). Cell-Hi JWP showed good performance characteristics including cost-effectiveness. Outdoor cultivation of P. tricornutum in an airlift photobioreactor, using Cell-Hi JWP in the United Kingdom (UK) during September and October (average daily temperature ranging between 8 and 18 °C and natural sunlight) was comparable to cultivation indoors under controlled temperature and lighting. A strong positive correlation between fucoxanthin and chlorophyll a content, and a weak inverse correlation between eicosapentaenoic (EPA) content and temperature were observed. Commensal bacterial counts revealed a sinusoidal growth profile with a change in community dominance from Halomonas sp. to Marinobacter sp. This investigation reveals for the first time that a multi-product approach can be adopted with P. tricornutum in a UK outdoor environment using commercially viable powdered media.

Two-step microalgal (Coelastrella sp.) treatment of raw piggery wastewater resulting in higher lipid and triacylglycerol levels for possible production of higher-quality biodiesel

Microalgal treatment of undiluted raw piggery wastewater is challenging due to ammonia toxicity and a deep dark color hampering photosynthesis. To overcome these problems, (1) a microalga (Coelastrella sp.) was isolated from an ammonia-rich environment, (2) the wastewater treatment was divided into two steps: a heterotrophic process followed by a mixotrophic process, and (3) a narrower transparent photobioreactor was employed with higher light intensity in the mixotrophic process. Coelastrella sp. removed 99% of ammonia, 92% of chemical oxygen demand (COD), and 100% of phosphorus during the 4-day process. Acetate in the wastewater relieved the ammonia stress on microalgae and promoted algal lipid and triacylglycerol productivity. Oxidative stability and low-temperature fluidity of triacylglycerols in lipids were improved by means of an altered fatty acid profile. Aside from the overall microalgal treatment performance, the proposed processing of piggery wastewater yielded a material suitable for possible production of algal biodiesel of better quality.

Effect of Phytohormones Supplementation under Nitrogen Depletion on Biomass and Lipid Production of Nannochloropsis oceanica for Integrated Application in Nutrition and Biodiesel

Economic viability of biodiesel production relies mainly on the productivity of biomass and microalgal lipids. In addition, production of omega fatty acids is favorable for human nutrition. Thus, enhancement of lipid accumulation with high proportion of omega fatty acids could help the dual use of microalgal lipids in human nutrition and biodiesel production through biorefinery. In that context, phytohormones have been identified as a promising factor to increase biomass and lipids production. However, nitrogen limitation has been discussed as a potential tool for lipid accumulation in microalgae, which results in simultaneous growth retardation. The present study aims to investigate the combined effect of N-depletion and 3-Indoleacetic acid (IAA) supplementation on lipid accumulation of the marine eustigmatophyte Nannochloropsis oceanica as one of the promising microalgae for omega fatty acids production. The study confirmed that N-starvation stimulates the lipid content of N. oceanica. IAA enhanced both growth and lipid accumulation due to enhancement of pigments biosynthesis. Therefore, combination effect of IAA and nitrogen depletion showed gradual increase in the dry weight compared to the control. Lipid analysis showed lower quantity of saturated fatty acids (SFA, 26.25%) than the sum of monounsaturated (MUFA) and polyunsaturated fatty acids (PUFA). Under N-depletion, SFA decreased by 12.98% compared to the control, which recorded much reduction by increasing of IAA concentration. Reduction of SFA was in favor of PUFA, mainly omega-6 and omega-3 fatty acids which increased significantly due to IAA combined with N-depletion. Thus, the present study suggests a biorefinery approach for lipids extracted from N. oceanica for dual application in nutrition followed by biodiesel production.

Effects of nitrogen and phosphorus limitations on fatty acid methyl esters and fuel properties of Dunaliella salina

This study was designed to assess the synergistic effects of nitrogen (N) and phosphorus (P) concentrations on oil content, fatty acid profile, and predicted fuel properties of Dunaliella salina. Axenic D. salina cells were grown in F/2 growth medium of salinity 34 ppt containing 33.6 g.l^-1 ultramarine synthetic sea salt. Growth dry weight, cell count, and their relationship were measured, and oils were extracted by soaking following Soxhlet extraction. Growth dry weight was markedly affected by N and P concentrations, with maximum growth dry weights of cultures grown at recommended N and P concentrations (control), half of the recommended N concentration (0.5 N) and (0.5 N/0.5P) being 0.911 g.l^-1, 0.755 g.l^-1, and 0.615 g.l^-1, respectively. Oil content showed the reverse pattern, with cultures grown in the absence of phosphorus (0.0P), full N/P starvation (0.0 N/0.0P), and control resulting in maximum oil contents of 24.86%, 22.85%, and 5.88%, respectively. The majority of fatty acid methyl esters ranged between C14 and C22. Estimated fuel properties of algal cells grown under NP stress conditions were found to meet the American Society for Testing and Materials (ASTM) and European Committee for Standardization (EN) guidelines.

Improved lipid productivity in Nannochloropsis gaditana in nitrogen-replete conditions by selection of pale green mutants

BACKGROUND

Nannochloropsis gaditana is a photosynthetic unicellular microalgae considered one of the most interesting marine algae to produce biofuels and food additive due to its rapid growth rate and high lipid accumulation. Although microalgae are attractive platforms for solar energy bioconversion, the overall efficiency of photosynthesis is reduced due to the steep light gradient in photobioreactors. Moreover, accumulation of lipids in microalgae for biofuels production is usually induced in a two-phase cultivation process by nutrient starvation, with additional time and costs associated. In this work, a biotechnological approach was directed for the isolation of strains with improved light penetration in photobioreactor combined with increased lipids productivity.

RESULTS

Mutants of Nannochloropsis gaditana were obtained by chemical mutagenesis and screened for having both a reduced chlorophyll content per cell and increased affinity for Nile red, a fluorescent dye which binds to cellular lipid fraction. Accordingly, one mutant, called e8, was selected and characterized for having a 30% reduction of chlorophyll content per cell and an almost 80% increase of lipid productivity compared to WT in nutrient-replete conditions, with C16:0 and C18:0 fatty acids being more than doubled in the mutant. Whole-genome sequencing revealed mutations in 234 genes in e8 mutant among which there is a non-conservative mutation in the dgd1 synthase gene. This gene encodes for an enzyme involved in the biosynthesis of DGDG, one of the major lipids found in the thylakoid membrane and it is thus involved in chloroplast biogenesis. Lipid biosynthesis is strongly influenced by light availability in several microalgae species, including Nannochloropsis gaditana: reduced chlorophyll content per cell and more homogenous irradiance in photobioreactor is at the base for the increased lipid productivity observed in the e8 mutant.

CONCLUSIONS

The results herein obtained presents a promising strategy to produce algal biomass enriched in lipid fraction to be used for biofuel and biodiesel production in a single cultivation process, without the additional complexity of the nutrient starvation phase. Genome sequencing and identification of the mutations introduced in e8 mutant suggest possible genes responsible for the observed phenotypes, identifying putative target for future complementation and biotechnological application.

Phaeodactylum tricornutum: A Diatom Cell Factory

A switch from a petroleum-based to a biobased economy requires the capacity to produce both high-value low-volume and low-value high-volume products. Recent evidence supports the development of microalgae-based microbial cell factories with the objective of establishing environmentally sustainable manufacturing solutions. Diatoms display rich diversity and potential in this regard. We focus on Phaeodactylum tricornutum, a pennate diatom that is commonly found in marine ecosystems, and discuss recent trends in developing the diatom chassis for the production of a suite of natural and genetically engineered products. Both upstream and downstream developments are reviewed for the commercial development of P. tricornutum as a cell factory for a spectrum of marketable products.

Effects of light and nitrogen availability on photosynthetic efficiency and fatty acid content of three original benthic diatom strains

Microalgal biotechnology has gained considerable importance in recent decades. Applications range from simple biomass production for food and animal feed to valuable products for fuel, pharmaceuticals, health, biomolecules and materials relevant to nanotechnology. There are few reports of the exploration of wider microalgae biodiversity in the literature on high value microalgal compounds, however, because it is believed that there is little to be gained in terms of biomass productivity by examining new strains. Still, without diversity, innovation in biotechnology applications is currently limited. Using microalgal diversity is a very promising way to match species and processes for a specific biotechnological application. In this context, three benthic marine diatom strains (Entomoneis paludosa NCC18.2, Nitzschia alexandrina NCC33, and Staurosira sp NCC182) were selected for their lipid production and growth capacities. Using PAM fluorometry and FTIR spectroscopy, this study investigated the impact of nitrogen repletion and depletion as well as light intensity (30, 100, and 400 μmol.photons.m^-2.s^-1) on their growth, photosynthetic performance and macromolecular content, with the aim of improving the quality of their lipid composition. Results suggest that under high light and nitrogen limitation, the photosynthetic machinery is negatively impacted, leading cells to reduce their growth and accumulate lipids and/or carbohydrates. However, increasing lipid content under stressful conditions does not increase the production of lipids of interest: PUFA, ARA and EPA production decreases. Culture conditions to optimize production of such fatty acids in these three original strains led to a balance between economic and ecophysiological constraints: low light and no nitrogen limitation led to better photosynthetic capacities associated with energy savings, and hence a more profitable approach.

Characterization and optimization of endogenous lipid accumulation in Chlorella vulgaris SDEC-3M ability to rapidly accumulate lipid for reversing nightly lipid loss

BACKGROUND

During inevitable light/dark cycle, lipid productivity of outdoor microalgae photoautotrophic cultivation is lowered by nightly biomass and lipid loss. To minimize, or even reverse the nightly lipid loss, it was expected that lipid accumulation would not cease, even if at night. Without relying on photosynthesis and organic matter in media, endogenous lipid accumulation that consumes energy and carbon sources derived from cells themselves, namely endogenous accumulation, is the only way for lipid production. The main aims of the present study was to characteristic endogenously accumulated lipid, confirm feasibility to reverse nightly lipid loss, and determine optimal conditions and its quality suitability for biodiesel feedstock production under stress conditions.

RESULTS

Chlorella vulgaris SDEC-3M ability to rapidly accumulated lipid under stress conditions was cultivated for 12 h in darkness, and the effects of various conditions on lipid accumulation and biomass loss were analyzed. Under non-stress conditions, lipid contents dropped. Under certain stress conditions, conversely, the lipid contents were substantially improved so that net nightly endogenous lipid accumulation was observed. Under the optimal conditions (aeration mode with 0.10 vvm and 15% CO₂, 5-10 mg L^-1 of NO₃ ^--N, 30-35 °C, approximate 2500 mg L^-1 of biomass), the lipid content was doubled and increased lipid was approximately 180 mg L^-1. Among stress conditions, N-deficiency had the most significant effect on endogenous lipid accumulation, and the optimum results were characterized under relatively low-N concentrations. Higher consistency between loss in carbohydrate and gain in lipid confirmed accumulated lipid endogenously conversed from carbohydrate. Based on the analyses of fatty acids profiles and prediction of kinematic viscosity, specific gravity, cloud point, cetane number and iodine value, it was confirmed that the quality of lipid obtained under optimal conditions complied with biodiesel quality standards.

CONCLUSION

Via triggering endogenous lipid accumulation by stress conditions, even in darkness, SDEC-3M can synthesize enough lipid suitable for biodiesel feedstock. It implies that the lipid accumulation phase in two-phase strategy can be scheduled at night, and following biomass production stage in light, which should be a solution to improve the lipid yield and quality of large-scale outdoor photoautotrophic microalgae cultivation for biodiesel production.

Microalgae for biofuel production

Microalgae have been used commercially since the 1950s and 1960s, particularly in the Far East for human health foods and in the United States for wastewater treatment. Initial attempts to produce bulk chemicals such as biofuels from microalgae were not successful, despite commercially favorable conditions during the 1970s oil crisis. However, research initiatives at this time, many using extremophilic microalgae and cyanobacteria (e.g., Dunaliella and Spirulina), did solve many problems and clearly identified biomass productivity and harvesting as the two main constraints stopping microalgae producing bulk chemicals, such as biofuels, on a large scale. In response to the growing unease around global warming, induced by anthropogenic CO₂ emissions, microalgae were again suggested as a carbon neutral process to produce biofuels. This recent phase of microalgae biofuels research can be thought to have started around 2007, when a very highly cited review by Chisti was published. Since 2007, a large body of scientific publications have appeared on all aspects of microalgae biotechnology, but with a clear emphasis on neutral lipid (triacylglycerol) synthesis and the use of neutral lipids as precursors for biodiesel production. In this review, the key research on microalgal biotechnology that took place prior to 2007 will be summarized and then the research trends post 2007 will be examined emphasizing the research into producing biodiesel from microalgae.