Sequential accumulation of starch and lipid induced by sulfur deficiency in Chlorella and Parachlorella species

A novel sulfur supply strategy for maximizing lipid production in Tribonema minus (Xanthophyceae)

Tribonema minus, a promising filamentous oleaginous microalga, was cultured under different nutrient concentrations and different culture modes (fed-batch culture, two-step culture) to study the method of rapid regulation of its lipid metabolism. In contrast to many other oleaginous microalgae, T. minus did not show that nitrogen stress promoted lipid accumulation; however, sulfur deficiency promoted rapid lipid accumulation with a maximum lipid content of 54% of dry weight. Increasing the MgSO4 concentration significantly increased nitrogen uptake and biomass (10.09 g/L). Lipid productivity was significantly increased by the two-step culture using a medium with a high concentration of MgSO4 in the first step and a sulfur-free medium in the second step. In addition, it was found that the lipid content of T. minus was negatively correlated with the intracellular sulfur content when the intracellular sulfur content was below 0.6%. This study provides a new approach for industrial lipid production in T. minus.

Efficient pH and dissolved CO2 conditions for indoor and outdoor cultures of green alga Parachlorella

Efficient pH and dissolved CO2 conditions for indoor (50-450 mL scale) and outdoor (100-500 L scale) culture of a green alga BX1.5 strain that can produce useful intracellular lipids and extracellular polysaccharides were investigated for the first time in Parachlorella sp. The cultures harvested under 26 different conditions were analysed for pH, dissolved CO2 concentration, and the biomass of extracellular polysaccharides. The BX1.5 strain could thrive in a wide range of initial medium pH ranging from 3 to 11 and produced valuable lipids such as C16:0, C18:2, and C18:3 under indoor and outdoor culture conditions when supplied with 2.0% dissolved CO2. Particularly, the acidic BG11 medium effectively increased the biomass of extracellular polysaccharides during short-term outdoor cultivation. The BG11 liquid medium also led to extracellular polysaccharide production, independent of acidity and alkalinity, proportional to the increase in total sugars derived from cells supplied with high CO2 concentrations. These results suggest Parachlorella as a promising strain for indoor and outdoor cultivation to produce valuable materials.

Description of Neochlorella semenenkoi gen. et. sp. nov. (Chlorophyta, Trebouxiophyceae), a Novel Chlorella-like Alga with High Biotechnological Potential

Despite many publications about Chlorella-like algae, their reliable and accurate identification is still difficult due to their simplicity and high phenotypic plasticity. The molecular approach has revolutionized our understanding of the diversity of ’small green balls’, and a natural classification of this group is currently being developed. This work is aimed at providing a detailed study of the phylogenetic position, morphology, ultrastructure, and physiology of the biotechnologically remarkable Chlorella-like strain IPPAS C-1210. Based on the SSU–ITS1–5.8S–ITS2 phylogeny, genetic distances, and the presence of compensatory base changes (CBCs) in ITS1 and conserved regions of ITS2 secondary structures, we describe a new genus, Neochlorella, with IPPAS C-1210 as the authentic strain of the type species, N. semenenkoi gen. and sp. nov. In addition, we justify the reassignment of the strain C. thermophila ITBB HTA 1–65 into N. thermophila comb. nov. The distinctive ultrastructural and physiological traits of the new species are discussed.

Effects of sulfur and phosphorus concentration on the lipid accumulation and fatty acid profile in Chlorella vulgaris (Chlorophyta)

Nutrient deficiency induces a variety of cellular responses, including an increase in lipid accumulation in microalgae. Nitrogen starvation is the most studied deprivation. Here, we determine the effects of phosphorus and sulfur limitation on lipid accumulation in Chlorella vulgaris. A set of 9 experiments were performed, varying the initial concentration of these nutrients (set to 0, 50, and 100% of their original composition in Bold's basal medium). According to our results, the variation of P and S modified the specific growth rate, lag phase, and cell generation time. The ratio of 50%P and 0%S significantly increased the total lipid concentration. The fatty acid profile was dominated by C16:0, C18:0, and C18:1; a considerable increase in C20:5 was observed with 0%P and 50%S and 0%P and 100%S. Regarding neutral lipids, the response surface methodology (RSM) indicates that the maximum was observed when S was between 40 and 60% and P was between 95 and 100%. Therefore, the enhanced production of lipids caused by P and S limitation may contribute to the efficient oil production useful for algal biofuels.

Enhancing biomass and lipid productivity of a green microalga Parachlorella kessleri for biodiesel production using rapid mutation of atmospheric and room temperature plasma

BACKGROUND

Microalgae, with their high adaptability to various stress conditions and rapid growth, are considered excellent biomass resources for lipid production and biodiesel feedstocks. However, lipid yield and productivity of the natural strains are common bottlenecks in their large-scale use for lipid production, which can be overcome by evolving new strains using conventional and advanced mutagenic techniques. It is challenging to generate microalgae strains capable of high lipid synthesis through natural selection. As a result, random mutagenesis is currently considered a viable option in many scenarios. The objective of this study was to explore atmospheric and room temperature plasma (ARTP) as a random mutagenesis technique to obtain high lipid-accumulating mutants of a green microalga for improved biodiesel production.

RESULTS

A green microalgal species was isolated from the Chinese Yellow Sea and identified as Parachlorella kessleri (OM758328). The isolated microalga was subsequently mutated by ARTP to obtain high lipid-accumulating mutants. Based on the growth rate and lipid content, 5 mutants (named M1, M2, M4, M5, and M8) were selected from 15 pre-selected mutants. These five mutants varied in their growth rate from 0.33 to 0.68 day^-1, with the lipid content varying between 0.25 g/L in M2 to 0.30 g/L in M8 at 10th day of cultivation. Among the mutants, M8 showed the maximum biomass productivity (0.046 g/L/day) and lipid productivity (20.19 mg/L/day), which were 75% and 44% higher than the wild strain, respectively. The triglyceride (TAG) content of M8 was found to be 0.56 g/L at 16th day of cultivation, which was 1.77-fold higher than that of the wild strain. Furthermore, M8 had the highest saturated fatty acids (C16-18) with the lowermost polyunsaturated fatty acid content, which are favorable properties of a biodiesel feedstock according to international standards.

CONCLUSION

The mutant strain of P. kessleri developed by the ARTP technique exhibited significant improvements in biomass productivity, lipid content, and biodiesel quality. Therefore, the biomass of this mutant microalga could be a potential feedstock for biodiesel production.

Coproduction of lipids and carotenoids by the novel green alga Coelastrella sp. depending on cultivation conditions

A novel green alga Coelastrella sp. D3-1 was isolated, and its unique and significant lipid and carotenoid coproduction capability was characterised depending on cultivation conditions. The main component of produced lipids was triacylglycerol under nutrient depletion conditions, in which fatty-methyl-esters made up 20-44% of the dry cell weight (DCW) and consisted of abundant C16:0 and C18:1 fatty acids. The red (orange)-stage cells also produced a large portion of carotenoids (38.5% of the DCW) involving echinenone, canthaxanthin, and astaxanthin as major components accumulated over only 5-6 days under optimal conditions. Stress tests revealed resistance of the cells to pH 2-11, high temperatures (40-60 °C), ultraviolet irradiation, drought, and H₂O₂ treatment, thereby showing a robust nature. Both green- and red (orange)-stage cell extracts also showed antioxidant and anti-inflammatory abilities, implying that they have significant functions as useful biorefinery materials.

Sulfur heterogeneity: A non-negligible factor in manipulating growth and lipid accumulation of Scenedesmus obliquus at a relatively high ratio of carbon to nitrogen

Algal biodiesel has been becoming a focus in the field of bioenergy worldwide. In this study, effects of heterogeneous sulfur (SO₄^2-, SO₃^2- and S^2-) on growth and lipid accumulation of Scenedesmus obliquus cultured in wastewater with a C/N ratio of 30 were investigated, respectively. The results shown that SO₄^2-, the optimal sulfur source, could trigger cell growth in a concentration-dependent manner. However, SO₃^2- was superior to the others in boosting carbon uptake of cells, which was subject to NH₄⁺-N concentration. Only SO₄^2- could simultaneously increase lipid content and productivity of cells with a dominant component of oleic acid (C18:1n9c) occupying approximately 40% in fatty acid profile. Additionally, the genes encoding enzymes such as CDIPT, ADPRM, DPP1, pmtA and BTA1 involved in the uppermost lipid-related pathway (glycerophospholipid metabolism) were identified facing different sulfur source regardless of the concentration changes. These findings may facilitate nutrition management efforts to enhance microalgae-based biofuel production.

Improving carbohydrate accumulation in Chlamydomonas debaryana induced by sulfur starvation using response surface methodology

Most methods that promote carbohydrate production negatively affect cell growth and microalgal biomass production. This study explores, in a two-stage cultivation strategy, in Chlamydomonas debaryana the optimization of certain culture conditions for high carbohydrate production without loss of biomass. In the first stage, the interaction between sodium bicarbonate supplementation, aeration, and different growth periods was optimized using the response surface methodology (RMS). The 3-factor Box-Behnken design (BBD) was applied, and a second-order polynomial regression analysis was used to analyze the experimental data. The results showed that 0.45 g L^-1 of sodium bicarbonate combined with a good aerated agitation (0.6 L min^-1) and a cultivation period of 18 days are optimal to produce 5.02 g L^-1 of biomass containing 43% of carbohydrates.Under these optimized growth conditions, accumulation of carbohydrates was studied using different modes of nutritional stress. The results indicated that carbohydrate content was improved and the maximum accumulation (about 60% of the dry weight) was recorded under sulfur starvation with only a 14% reduction in biomass as compared to control. This study showed promising results as to biomass production and carbohydrate yield by the microalgae C. debaryana in view of production of third-generation biofuels.

Revealing Lipid Body Formation and its Subcellular Reorganization in Oleaginous Microalgae Using Correlative Optical Microscopy and Infrared Nanospectroscopy

The purpose of this work is to develop an integrated imaging approach to characterize without labeling at the sub-cellular level the formation of lipid body droplets (LBs) in microalgae undergoing nitrogen starvation. First conventional optical microscopy approaches, gas chromatography, and turbidimetry measurements allowed to monitor the biomass and the total lipid content in the oleaginous microalgae Parachlorella kesslerii during the starvation process. Then a local analysis of the LBs was proposed using an innovative infrared nanospectroscopy technique called atomic force microscopy-based infrared spectroscopy (AFM-IR). This label-free technique assessed the formation of LBs and allowed to look into the LB composition thanks to the acquisition of local infrared spectra. Last correlative measurements using fluorescence microscopy and AFM-IR were performed to investigate the subcellular reorganization of LB and the chloroplasts.

Starch Rich Chlorella vulgaris: High-Throughput Screening and Up-Scale for Tailored Biomass Production

The use of microalgal starch has been studied in biorefinery frameworks to produce bioethanol or bioplastics, however, these products are currently not economically viable. Using starch-rich biomass as an ingredient in food applications is a novel way to create more value while expanding the product portfolio of the microalgal industry. Optimization of starch production in the food-approved species Chlorella vulgaris was the main objective of this study. High-throughput screening of biomass composition in response to multiple stressors was performed with FTIR spectroscopy. Nitrogen starvation was identified as an important factor for starch accumulation. Moreover, further studies were performed to assess the role of light distribution, investigating the role of photon supply rates in flat panel photobioreactors. Starch-rich biomass with up to 30% starch was achieved in cultures with low inoculation density (0.1 g L−1) and high irradiation (1800 µmol m−2 s−1). A final large-scale experiment was performed in 25 L tubular reactors, achieving a maximum of 44% starch in the biomass after 12 h in nitrogen starved conditions.

Supra-Optimal Temperature: An Efficient Approach for Overaccumulation of Starch in the Green Alga Parachlorella kessleri

Green algae are fast-growing microorganisms that are considered promising for the production of starch and neutral lipids, and the chlorococcal green alga Parachlorella kessleri is a favorable model, as it can produce both starch and neutral lipids. P. kessleri commonly divides into more than two daughter cells by a specific mechanism—multiple fission. Here, we used synchronized cultures of the alga to study the effects of supra-optimal temperature. Synchronized cultures were grown at optimal (30 °C) and supra-optimal (40 °C) temperatures and incident light intensities of 110 and 500 μmol photons m⁻² s⁻¹. The time course of cell reproduction (DNA replication, cellular division), growth (total RNA, protein, cell dry matter, cell size), and synthesis of energy reserves (net starch, neutral lipid) was studied. At 40 °C, cell reproduction was arrested, but growth and accumulation of energy reserves continued; this led to the production of giant cells enriched in protein, starch, and neutral lipids. Furthermore, we examined whether the increased temperature could alleviate the effects of deuterated water on Parachlorella kessleri growth and division; results show that supra-optimal temperature can be used in algal biotechnology for the production of protein, (deuterated) starch, and neutral lipids.

Characterization of Growth and Cell Cycle Events Affected by Light Intensity in the Green Alga Parachlorella kessleri: A New Model for Cell Cycle Research

Multiple fission is a cell cycle variation leading to the production of more than two daughter cells. Here, we used synchronized cultures of the chlorococcal green alga Parachlorella kessleri to study its growth and pattern of cell division under varying light intensities. The time courses of DNA replication, nuclear and cellular division, cell size, total RNA, protein content, dry matter and accumulation of starch were observed at incident light intensities of 110, 250 and 500 µmol photons m⁻²s⁻¹. Furthermore, we studied the effect of deuterated water on Parachlorella kessleri growth and division, to mimic the effect of stress. We describe a novel multiple fission cell cycle pattern characterized by multiple rounds of DNA replication leading to cell polyploidization. Once completed, multiple nuclear divisions were performed with each of them, immediately followed by protoplast fission, terminated by the formation of daughter cells. The multiple fission cell cycle was represented by several consecutive doublings of growth parameters, each leading to the start of a reproductive sequence. The number of growth doublings increased with increasing light intensity and led to division into more daughter cells. This study establishes the baseline for cell cycle research at the molecular level as well as for potential biotechnological applications, particularly directed synthesis of (deuterated) starch and/or neutral lipids as carbon and energy reserves.

Recombinant overexpression of the Escherichia coli acetyl-CoA carboxylase gene in Synechocystis sp. boosts lipid production

Microalgae have received continued attention as a potential source for biofuel production. However, the lack of suitable strains that provide a lipid-rich biomass and tolerate harsh condition inhibits their industrial application. This report describes an effort to transform Synechocystis sp. with genes encoding acetyl-CoA carboxylase (ACC), a key regulatory enzyme in the lipogenesis pathway, from the white mustard plant (Sinapis alba) and the bacterium Escherichia coli DH5α using chitosan nanoparticles. Although a recombinant plasmid encoding S. alba ACC failed to express, successful transformation was achieved with a recombinant plasmid encoding E. coli DH5α ACC. The successful transformant, Synechocystis sp. PAK13, exhibited increased ACC expression compared with its wild-type parent (11.8 vs. 7.2 ng), which significantly increased its lipid content (by 3.6-fold). Synechocystis sp. PAK13 also exhibited a significant (20%) reduction in photosynthetic pigments, a 1.52-fold higher glucose content and a 3.5-fold lower sucrose content than the wild-type. In conclusion, this report introduces a useful strategy to overexpress the ACC gene in microalgae, creating strains with improved lipid production that are suited to industrial applications.

Revealing the Characteristics of the Antarctic Snow Alga Chlorominima collina gen. et sp. nov. Through Taxonomy, Physiology, and Transcriptomics

Snow algae play crucial roles in cold ecosystems, however, many aspects related to their biology, adaptations and especially their diversity are not well known. To improve the identification of snow algae from colored snow, in the present study we used a polyphasic approach to describe a new Antarctic genus, Chlorominima with the species type Chlorominima collina. This new taxon was isolated of colored snow collected from the Collins Glacier (King George Island) in the Maritime Antarctic region. Microscopy revealed biflagellated ellipsoidal cells with a rounded posterior end, a C-shaped parietal chloroplast without a pyrenoid, eyespot, and discrete papillae. Several of these characteristics are typical of the genus Chloromonas, but the new isolate differs from the described species of this genus by the unusual small size of the cells, the presence of several vacuoles, the position of the nucleus and the shape of the chloroplast. Molecular analyzes confirm that the isolated alga does not belong to Chloromonas and therefore forms an independent lineage, which is closely related to other unidentified Antarctic and Arctic strains, forming a polar subclade in the Stephanosphaerinia phylogroup within the Chlamydomonadales. Secondary structure comparisons of the ITS2 rDNA marker support the idea that new strain is a distinct taxon within of Caudivolvoxa. Physiological experiments revealed psychrophilic characteristics, which are typical of true snow algae. This status was confirmed by the partial transcriptome obtained at 2°C, in which various cold-responsive and cryoprotective genes were identified. This study explores the systematics, cold acclimatization strategies and their implications for the Antarctic snow flora.

Influence of nutrient status on the biohydrogen and lipid productivity in Parachlorella kessleri: a biorefinery approach

The commercial reality of microalgal biotechnology for the production of individual bioactives is constrained by the high cost of production and requires a biorefinery approach. In this investigation, we examined the influence of different nutrient deprivation (nitrogen (N), phosphorus (P), sulphur (S) and manganese (Mn)) on growth, chlorophyll a (Chl a), biohydrogen (H₂) and fatty acid profiles in Parachlorella kessleri EMCCN 3073 under both aerobic and anaerobic conditions. Anaerobic conditions combined with the nutrient deprivation resulted in cell division blockage, reduction in Chl a and remarkable changes in pH, whereas a significant increase in the H₂ production was observed after 24 h. The highest cumulative H₂ productivity was observed in N-deficient medium (300 μL/L, day 9) followed by Mn-deficient medium (250 μL/L, day 7). The highest H₂ production rate (3.37 μL/L/h) was achieved by Mn-deficient medium after 24 h. In terms of fatty acid composition, P. kessleri exhibited a differential response to different nutrient stresses. Under aerobic conditions, N-deficient media resulted in the highest lipid content (119% compared to control, day 7), whereas earlier lipid induction at (1-3 days) was observed with Mn- and S-deficient media with 18-91% and 25-34% increase, respectively, compared with the replete control. Meanwhile, higher lipid content was observed under anaerobic conditions combined with Mn-, N-, P- and S-deprived media (day 1) with 20%, 13%, 8% and 7% increases respectively compared with the control. This investigation, for the first time clearly, highlights the potential of P. kessleri as a sustainable biorefinery platform, for H₂ and fatty acid bio-production under anaerobic conditions. KEY POINTS: • Parachlorella kessleri could provide a future sustainable biorefinery platform. • Nutrient-deprived anaerobic conditions blocked cell growth but differentially induced H₂ production. • Nutrient status, under both aerobic/anaerobic conditions, alters lipids and fatty acids profile of P. kessleri. • Nutrient-deprived (N- and Mn-) anaerobic conditions: future biorefinery platform.

Energy conservation in photosynthetic microorganisms

Photosynthesis is a biological process of energy conversion from solar radiation to useful organic compounds for the photosynthetic organisms themselves. It, thereby, also plays a role of food production for almost all animals on the Earth. The utilization of photosynthesis as an artificial carbon cycle is also attracting a lot of attention regarding its benefits for human life. Hydrogen and biofuels, obtained from photosynthetic microorganisms, such as microalgae and cyanobacteria, will be promising products as energy and material resources. Considering that the efficiency of bioenergy production is insufficient to replace fossil fuels at present, techniques for the industrial utilization of photosynthesis processes need to be developed intensively. Increase in the efficiency of photosynthesis, the yields of target substances, and the growth rates of algae and cyanobacteria must be subjects for efficient industrialization. Here, we overview the whole aspect of the energy production from photosynthesis to biomass production of various photosynthetic microorganisms.

Coproduction of lipids and extracellular polysaccharides from the novel green alga Parachlorella sp. BX1.5 depending on cultivation conditions

A novel strain of microalga Parachlorella sp. BX1.5 was isolated and its unique properties of producing lipids and extracellular polysaccharides (EPS) characterized. The cells could extracellularly produce a large amount of acidic EPS, when cultured in nitrogen-deficient BG11₀ medium (BG11–N) with 2 % CO₂-air supply. The main component of intracellularly accumulated lipids was triacylglycerol (TAG), depending on the different cultivation conditions of BG11, BG11–N, BG11–P (phosphate depleted), and BG11–N–P (nitrogen and phosphate depleted). Fatty-methyl-esters (FAMEs), methyl-esterification of total lipids, consisted of abundant saturated C16 and unsaturated C18 fatty acids under the culture conditions. Cell spot assays on BG11 plates revealed the resistance of cells to pH 2–11, high temperatures of 50–70 °C, ultraviolet irradiation, and drought, under different culture conditions, thereby suggesting the biological significance of lipid and EPS accumulation. The prospects of BX1.5 as a dual producer has also been discussed for biorefineries.

Storage of starch and lipids in microalgae: Biosynthesis and manipulation by nutrients

Microalgae accumulate starch and lipid as storage metabolites under nutrient depletion, which can be used as sustainable feedstock for biorefinery. Omics analysis coupled with enzymatic and genetic verifications uncovered a partial picture of pathways and important enzymes or regulators related to starch and lipid biosynthesis as well as the carbon partitioning between them under nutrient depletion conditions. Depletion of macronutrients (N, P, and S) resulted in considerable enhancement of starch and/or lipid content in microalgae, but the accompanying declined photosynthesis hampered the achievements of high concentrations. This review summarized the current knowledge on the pathways and the committed steps as well as their carbon allocation involved in starch and lipid biosynthesis, and focused on the manipulation of different nutrients and the alleviation of oxidative stress for enhanced storage metabolites production. The biological and engineering approaches to cope with the conflict between biomass production and storage metabolites accumulation are proposed.

Identification of significant proxy variable for the physiological status affecting salt stress-induced lipid accumulation in Chlorella sorokiniana HS1

BACKGROUND

Current efforts on the optimization of the two-stage cultivation using stress-induced lipid accumulation have mostly focused only on the lipid induction stage. Although recent studies have shown that stress-induced lipid accumulation is affected by the physiological status of the cells harvested at the preceding cultivation stage, this issue has hardly been examined hitherto. Such a study needs to be carried out in a systematic way in order to induce lipid accumulation in a consistent and predictable manner with regard for variances seen at the cultivation stage.

RESULTS

After a photoautotrophic cultivation of Chlorella sorokiniana HS1 in a modified BG11, harvested cells were re-suspended in the fresh medium, and then NaCl was added as the sole stress inducer with light illumination to induce additional accumulation of lipid. Effects of culture temperature on the lipid accumulation were analyzed by the Kruskal-Wallis test. From the microscopic observation, we had observed a definite increase in lipid body induced by the stress since the cell entered a stationary phase. A multiple linear regression model was developed so as to identify significant parameters to be included for the estimation of lipid induction. As a result, several key parameters at the end of cultivation, such as cell weight, total lipid content, chlorophyll a in a cell, and Fv/Fm, were identified as the important proxy variables for the cell's physiological status, and the modeling accuracy was achieved by 87.6%. In particular, the variables related to Fv/Fm were shown to have the largest influence, accounting for 65.7% of the total variance, and the Fv/Fm had an optimal point of maximum induction at below its average. Clustering analysis using the K-means algorithm indicated that the algae which are 0.15 pg cell^-1 or less in chlorophyll concentration, regardless of other conditions, had achieved high induction results.

CONCLUSION

Experimental results showed that it usually achieves high lipid induction after the cells naturally end their division and begin to synthesize lipid. The amount of lipid induction could be estimated by the selected proxy variables, and the estimation method can be adapted according to practical situations such as those with limited measurements.

Cell Cycle Arrest by Supraoptimal Temperature in the Alga Chlamydomonas reinhardtii

Temperature is one of the key factors affecting growth and division of algal cells. High temperature inhibits the cell cycle in Chlamydomonas reinhardtii. At 39 °C, nuclear and cellular divisions in synchronized cultures were blocked completely, while DNA replication was partly affected. In contrast, growth (cell volume, dry matter, total protein, and RNA) remained unaffected, and starch accumulated at very high levels. The cell cycle arrest could be removed by transfer to 30 °C, but a full recovery occurred only in cultures cultivated up to 14 h at 39 °C. Thereafter, individual cell cycle processes began to be affected in sequence; daughter cell release, cell division, and DNA replication. Cell cycle arrest was accompanied by high mitotic cyclin-dependent kinase activity that decreased after completion of nuclear and cellular division following transfer to 30 °C. Cell cycle arrest was, therefore, not caused by a lack of cyclin-dependent kinase activity but rather a blockage in downstream processes.

Regulatory carbon metabolism underlying seawater-based promotion of triacylglycerol accumulation in Chlorella kessleri

Chlorella kessleri accumulates triacylglycerol usable for biodiesel-fuel production to >20% dry cell weight in three days when cultured in three-fold diluted seawater, which imposes the combinatory stress of hyperosmosis and nutrients limitation. The quantitative behavior of major C-compounds, and related-gene expression patterns were investigated in Chlorella cells stressed with hyperosmosis, nutrients limitation, or their combination, to elucidate the C-metabolism for economical seawater-based triacylglycerol accumulation. Combinatory-stress cells showed repressed protein synthesis with initially accumulated starch being degraded later, the C-metabolic flow thereby being diverted to fatty acid and subsequent triacylglycerol accumulation. This C-flow diversion was induced by cooperative actions of nutrients-limitation and hyperosmosis. Semi-quantitative PCR analysis implied positive rewiring of the diverted C-flow into triacylglycerol in combinatory-stress cells through upregulation of gene expression concerning fatty acid and triacylglycerol synthesis, and starch synthesis and degradation. The information of regulatory C-metabolism will help reinforce the seawater-based triacylglycerol accumulation ability in algae including Chlorella.

Night illumination using monochromatic light-emitting diodes for enhanced microalgal growth and biodiesel production

The present study investigated the effect of monochromatic light-emitting diodes (LEDs) on the growth and biodiesel yield of the green microalga Scenedesmus obliquus. Different LEDs were applied individually or in combination during the night period. Among different individual treatments, red and blue illumination showed the highest biomass and lipid productivity due to stimulation of pigmentation and photosystem II, respectively. Microalgal growth, lipid production and biodiesel recovery significantly increased under combined blue-red illumination. In addition, saturated and monounsaturated fatty acids proportions increased in favor of polyunsaturated ones. Moreover, blue-red LEDs enhanced the net biodiesel energy output over the control. The total increase in net energy output represented 5.1, 3.8 and 10.8 MJ using red, blue and blue-red light, respectively. In conclusion, application of blue-red LEDs during the night period is an economical technology for microalgae cultivation, which might have a potential impact on the future of commercial biodiesel production from microalgae.

Three-dimensional ultrastructure and hyperspectral imaging of metabolite accumulation and dynamics in Haematococcus and Chlorella

Phycology has developed alongside light and electron microscopy techniques. Since the 1950s, progress in the field has accelerated dramatically with the advent of electron microscopy. Transmission electron microscopes can only acquire imaging data on a 2D plane. Currently, many of the life sciences are seeking to obtain 3D images with electron microscopy for the accurate interpretation of subcellular dynamics. Three-dimensional reconstruction using serial sections is a method that can cover relatively large cells or tissues without requiring special equipment. Another challenge is monitoring secondary metabolites (such as lipids or carotenoids) in intact cells. This became feasible with hyperspectral cameras, which enable the acquisition of wide-range spectral information in living cells. Here, we review bioimaging studies on the intracellular dynamics of substances such as lipids, carotenoids and phosphorus using conventional to state-of-the-art microscopy techniques in the field of algal biorefining.

Characterization of Chlorella sorokiniana, UTEX 1230

This paper characterizes the strain Chlorella sorokiniana UTEX 1230 within a laboratory setting using a 1 L bubble column. The findings show that productivity can be trebled under mixotrophic conditions (from 0.2 g·L⁻¹·d⁻¹ to 0.66 g·L⁻¹·d⁻¹) with the addition of sodium acetate. The results also indicate that both the growth rate and final yield increase with the cultivation temperature, with most parameters showing an optimum in the range of 30–35 °C. The maximum specific growth rate was found to be in the region of 0.12 h⁻¹ at a surface irradiance between 100–500 µE·m⁻²·s⁻¹. This high growth rate makes the strain particularly suited to the rapid production of biomass, suitable for either whole cell bioprocessing or bioremediation. However, the relatively low lipid productivity (9.2 mg·L⁻¹·d⁻¹) confirms previous findings which would indicate poor applicability for biodiesel production. The strain shows greater promise in wastewater treatment applications with removal rates of nitrogen and phosphorus in the region of 37 and 30 mg·L⁻¹·d⁻¹ respectively. Furthermore, the findings show that a fed-batch strategy to inorganic nutrient loading can increase the final yield by around 50% compared to a conventional batch run. This is particularly interesting as fed-batch production techniques are rarely used within microalgal cultivation, so provide an interesting avenue for further investigation. Overall, the findings show that C. sorokiniana UTEX 1230 is a robust and fast-growing microalgal strain suitable both for the laboratory and scale-up.

A novel galactolipase from a green microalga Chlorella kessleri: purification, characterization, molecular cloning, and heterologous expression

We have identified an enzyme, galactolipase (ckGL), which hydrolyzes the acyl ester bond of galactolipids such as digalactosyldiacylglycerol (DGDG), in the microalga Chlorella kessleri. Following purification of the enzyme to electrophoretic homogeneity from cell-free extract, the maximum activity toward DGDG was observed at pH 6.5 and 37 °C. ckGL was Ca²⁺-dependent enzyme and displayed an apparent molecular mass of approx. 53 kDa on SDS-PAGE. The substrate specificity was in the order: DGDG (100%) > monogalactosyldiacylglycerol ≈ phosphatidylglycerol (~ 40%) > sulfoquinovosyldiacylglycerol (~ 20%); the enzyme exhibited almost no activity toward glycerides and other phospholipids. Gas chromatography analysis demonstrated that ckGL preferably hydrolyzed the sn-1 acyl ester bond in the substrates. The genomic DNA sequence (5.6 kb) containing the ckGL gene (designated glp1) was determined and the cDNA was cloned. glp1 was composed of 10 introns and 11 exons, and the 1608-bp full-length cDNA encoded a mature ckGL containing 475 amino acids (aa), with a presequence (60 aa) containing a potential chloroplast transit peptide. Recombinant functional ckGL was produced in Escherichia coli. Although the deduced aa sequence of ckGL contained the typical GXSXG motif of serine hydrolases together with conserved histidine and aspartate residues which would form part of the catalytic triad of α/β-hydrolases, ckGL showed no significant overall similarity with known lipases including GLs from Chlamydomonas reinhardtii and Aspergillus japonicus, indicating that ckGL is a novel GL. ckGL, with high specificity for DGDG, could be applicable to food processing as an enzyme capable of improving material textures.

The roles of starch and lipid in Chlorella sp. during cell recovery from nitrogen starvation

The influence of N-replenishment on cell growth, chlorophyll content, nitrogen uptake, and accumulation of starch and lipid was studied in Chlorella sp. N-starved algal cells were fully recovered within 2days after N-replenishment. Stored starch in N-deficient culture was degraded immediately during recovery process. However, lipid response had a period of delay when suffered from long starvation. During the recovery process, neutral lipid was reduced accompanied by an increase in membrane lipid. It was speculated that starch served as a source of carbon and energy catabolized to support quick recovery of the culture, while lipid preferred to play a structural role that specific fatty acid species were released from storage lipid and as building blocks for quick synthesis of membrane lipid. In light of rapid growth recovery and no net degradation in total fatty acids, a semi-continuous cultivation process might be a potential way to enhance lipid productivity.

Strategies for Lipid Production Improvement in Microalgae as a Biodiesel Feedstock

In response to the energy crisis, global warming, and climate changes, microalgae have received a great deal of attention as a biofuel feedstock. Due to a high lipid content in microalgal cells, microalgae present as a promising alternative source for the production of biodiesel. Environmental and culturing condition variations can alter lipid production as well as chemical compositions of microalgae. Therefore, application of the strategies to activate lipid accumulation opens the door for lipid overproduction in microalgae. Until now, many original studies regarding the approaches for enhanced microalgal lipid production have been reported in an effort to push forward the production of microalgal biodiesel. However, the current literature demonstrates fragmented information available regarding the strategies for lipid production improvement. From the systematic point of view, the review highlights the main approaches for microalgal lipid accumulation induction to expedite the application of microalgal biodiesel as an alternative to fossil diesel for sustainable environment. Of the several strategies discussed, the one that is most commonly applied is the design of nutrient (e.g., nitrogen, phosphorus, and sulfur) starvation or limitation. Other viable approaches such as light intensity, temperature, carbon dioxide, salinity stress, and metal influence can also achieve enhanced microalgal lipid production.

A pivotal role of vacuolar H(+)-ATPase in regulation of lipid production in Phaeodactylum tricornutum

Microalgal lipids have been considered as a promising source for biodiesel production. Alkaline pH can induce neutral lipid accumulation in microalgae cells. However, whether and how proton pumps, especially vacuolar H(+)-ATPase (V-ATPase), function in these processes is not well known. In this study, we treated Phaeodactylum tricornutum with V-ATPase specific inhibitor bafilomycin A1 (BFA1) to determine its role in lipid production. Firstly, V-ATPase activity was increased in the latter phase of microalgae growth. BFA1 treatment decreased the cell density and lipid contents. Further analysis showed that BFA1 treatment reduced the number and size of oil bodies. GC-MS analysis showed that lipid components were not affected by BFA1 treatment. Intracellular pH was decreased and nitrogen depletion was delayed after BFA1 treatment. RNA-Seq analysis showed that expression of genes involved in calcium signaling, sulfur metabolism, cell cycle, glycolysis, pentose phosphate pathway, porphyrin, chlorophyll metabolism and lipid catabolic metabolism were upregulated, while expression of genes involved in ion transmembrane transport, ubiquitin mediated proteolysis, SNARE interactions in vesicular transport, fatty acid biosynthesis were downregulated under BFA1 treatment. Our findings provided insights into the molecular mechanisms underlying lipid accumulation and the key genes involved in lipid metabolism in Phaeodactylum tricornutum in response to BFA1.

Deciphering the relationship among phosphate dynamics, electron-dense body and lipid accumulation in the green alga Parachlorella kessleri

Phosphorus is an essential element for life on earth and is also important for modern agriculture, which is dependent on inorganic fertilizers from phosphate rock. Polyphosphate is a biological polymer of phosphate residues, which is accumulated in organisms during the biological wastewater treatment process to enhance biological phosphorus removal. Here, we investigated the relationship between polyphosphate accumulation and electron-dense bodies in the green alga Parachlorella kessleri. Under sulfur-depleted conditions, in which some symporter genes were upregulated, while others were downregulated, total phosphate accumulation increased in the early stage of culture compared to that under sulfur-replete conditions. The P signal was detected only in dense bodies by energy dispersive X-ray analysis. Transmission electron microscopy revealed marked ultrastructural variations in dense bodies with and without polyphosphate. Our findings suggest that the dense body is a site of polyphosphate accumulation, and P. kessleri has potential as a phosphate-accumulating organism.

Primitive red alga Cyanidioschyzon merolae accumulates storage glucan and triacylglycerol under nitrogen depletion

Most microalgae accumulate neutral lipids, including triacylglycerol (TAG), into spherical structures called lipid bodies (LBs) under environmental stress conditions such as nutrient depletion. In green algae, starch accumulation precedes TAG accumulation, and the starch is thought to be a substrate for TAG synthesis. However, the relationship between TAG synthesis and the starch content in red algae, as well as how TAG accumulation is regulated, is unclear. In this study, we cultured the primitive red alga Cyanidioschyzon merolae under nitrogen-depleted conditions, and monitored the formation of starch granules (SGs) and LBs using microscopy. SGs stained with potassium iodide were observed at 24 h; however, LBs stained specifically with BODIPY 493/503 were observed after 48 h. Quantitative analysis of neutral sugar and cytomorphological semi-quantitative analysis of TAG accumulation also supported these results. Thus, the accumulation of starch occurred and preceded the accumulation of TAG in cells of C. merolae. However, TAG accumulation was not accompanied by a decrease in the starch content, suggesting that the starch is a major carbon storage sink, at least under nitrogen-depleted conditions. Quantitative real-time PCR revealed that the mRNA levels of genes involved in starch and TAG synthesis rarely changed during the culture period, suggesting that starch and TAG synthesis in C. merolae are not controlled through gene transcription but at other stages, such as translation and/or enzymatic activity.

Effect of Nutrient Starvation under High Irradiance on Lipid and Starch Accumulation in Chlorella fusca (Chlorophyta)

The effect of nitrogen and sulphur limitation under high irradiance (PAR) was studied in the green microalga Chlorella fusca (Chlorophyta) in order to follow lipid and/or starch accumulation. Growth, biomass composition and the changes in photosynthetic activity (in vivo chlorophyll a fluorescence) were followed in the trials. The full nutrient culture showed high biomass production and starch accumulation at Day 1, when photosynthetic activity was high. Gradual deprivation (no nutrients added) became evident when photosynthesis was significantly suppressed (Day 3 onwards), which entailed a decrease of maximum relative electron transport rate (rETRmax) and increase of non-photochemical quenching (NPQ), accompanied by the onset of lipid accumulation and decline in starch content. In N- and S-starved cultures, rETRmax significantly decreased by Day 3, which caused a substantial drop in biomass production, cell number, biovolume and induction of lipid and starch accumulation. High starch content (45-50 % of DW) was found at the initial stage in full nutrient culture and at the stationary phase in nutrient-starved cultures. By the end of the trial, all treatments showed high lipid content (~30 % of DW). The full nutrient culture had higher biomass yield than starved treatments although starch (~0.2 g L(-1) day(-1)) and lipid (~0.15 g L(-1) day(-1) productivities were fairly similar in all the cultures. Our results showed that we could enrich biomass of C. fusca (% DW) in lipids using a two-stage strategy (a nutrient replete stage followed by gradual nutrient limitation) while under either procedure, N- or S-starvation, both high lipid and starch contents could be achieved.

Highly efficient lipid production in the green alga Parachlorella kessleri: draft genome and transcriptome endorsed by whole-cell 3D ultrastructure

BACKGROUND

Algae have attracted attention as sustainable producers of lipid-containing biomass for food, animal feed, and for biofuels. Parachlorella kessleri, a unicellular green alga belonging to the class Trebouxiophyceae, achieves very high biomass, lipid, and starch productivity levels. However, further biotechnological exploitation has been hampered by a lack of genomic information.

RESULTS

Here, we sequenced the whole genome and transcriptome, and analyzed the behavior of P. kessleri NIES-2152 under lipid production-inducing conditions. The assembly includes 13,057 protein-coding genes in a 62.5-Mbp nuclear genome. Under conditions of sulfur deprivation, lipid accumulation was correlated with the transcriptomic induction of enzymes involved in sulfur metabolism, triacylglycerol (TAG) synthesis, autophagy, and remodeling of light-harvesting complexes.

CONCLUSIONS

Three-dimensional transmission electron microscopy (3D-TEM) revealed extensive alterations in cellular anatomy accompanying lipid hyperaccumulation. The present 3D-TEM results, together with transcriptomic data support the finding that upregulation of TAG synthesis and autophagy are potential key mediators of the hyperaccumulation of lipids under conditions of nutrient stress.

Biomass and lipid production of a local isolate Chlorella sorokiniana under mixotrophic growth conditions

A local Chlorella sp. isolate with 97% rbcL sequence identity to Chlorella sorokiniana was evaluated in terms of its biomass and lipid production under mixotrophic growth conditions. Glucose-supplemented cultures exhibited increasing growth rate and biomass yield with increasing glucose concentration. Highest growth rate and biomass yield of 1.602 day(-1) and 687.5 mg L(-1), respectively, were achieved under 2 g L(-1) glucose. Nitrogen starvation up to 75% in the 1.0 g L(-1) glucose-supplemented culture was done to induce lipid accumulation and did not significantly affect the growth. Lipid content ranges from 20% to 27% dry weight. Nile Red staining showed more prominent neutral lipid bodies in starved mixotrophic cultures. C. sorokiniana exhibited enhanced biomass production under mixotrophy and more prominent neutral lipid accumulation under nitrogen starvation with no significant decrease in growth; hence, this isolate could be further studied to establish its potential for biodiesel production.