Effects of temperature and its combination with high light intensity on lipid production of Monoraphidium dybowskii Y2 from semi-arid desert areas

Cold adaptation of harmful dinoflagellate facilitates their poleward colonization: Insights into extracellular polymeric substances and intracellular bio-macromolecules dynamics through in-situ FTIR imaging

While higher latitudes are becoming relatively warm ecosystem for phytoplankton, the rapid and active adaptation of harmful algal cells to cold conditions also contributes to their poleward colonization, which has scarcely been studied. We examined the adaptive mechanism to cold stress in Gymnodinium catenatum, a eurythermic species that has been recently reported to spread to higher latitudes. Using the in-situ focal plane array Fourier transform infrared spectroscopy (FPA-FTIR) imaging combined with transmission electron microscopy, we demonstrated that this dinoflagellate could adapt to cold stress by establishing two cell barriers: one consisting of the massive extracellular polymeric substances (EPS) that accumulated outside the cell and the other represented by lipid phase separation within the reshaped cellular microenvironment. Two-dimensional correlation (2D-COS) spectroscopy further revealed that intracellular bio-macromolecules (lipids, proteins, and carbohydrates) were organized in an ordered and purposeful manner to resist cold. Transcriptome analysis confirmed the inhibition of nicotinamide adenine dinucleotide (NADH) dehydrogenase and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) involved in protein and carbohydrate metabolism, in cold-treated cells. This study elucidated a flexible adaptation strategy of G. catenatum at the bio-macromolecular level and generally discussed the widespread colonization of harmful microalgae at higher latitudes.

Microalgae bioprospecting for the food industry: insights into the autotrophic biomass production and macromolecular accumulation of four microalgal species

In this study, four microalgal strains were evaluated for their biomass production capacity and macromolecule biosynthesis. These include three strains from the phylum Chlorophyta: Monoraphidium sp. LB2PC 0120, Stichococcus sp. LB2PC 0117, and Tetraselmis sp. LB2PC 0320, and one strain from the phylum Haptophyta: Isochrysis sp. LB2PC 0220. The experiments were conducted under typical laboratory-scale setups. Additionally, phylogenetic analysis based on the 18-28 S rRNA internal transcribed spacer (ITS) was performed to validate the taxonomic identity of the strains. Each strain was exposed to four different cultivation conditions based on two levels of illumination intensity [25-(LI) and 50-(HI) µmol m- 2 s- 1] and nitrogen loading [100-(LΝ) and 300-(HΝ) mg NaNO3 L- 1] in a full factorial design. All the microalgae achieved maximum biomass production under HI-HN conditions, which amounted to 1495, 919, 844, and 708 mg/L for Monoraphidium sp. LB2PC 0120, Stichococcus sp. LB2PC 0117, Tetraselmis sp. LB2PC 0320 and Isochrysis sp. LB2PC 0220, respectively, after 16 days of cultivation. Among them, Stichococcus sp. LB2PC 0117 had the highest protein content (49.9% wt.) under LI-HN conditions and Monoraphidium sp. LB2PC 0120 had the highest lipid content (44.3% wt.) under HI-LN conditions. Both Monoraphidium sp. LB2PC 0120 and Tetraselmis sp. LB2PC 0320 accumulated the highest carbohydrate content (~ 37% wt.) under LI-LN and HI-LN conditions, respectively. Based on biomass and macromolecule production, Monoraphidium sp. LB2PC 0120 was identified as the most promising candidate for upscaling studies, expecting its highly manipulatable compositional profile to support multiple applications in the food industry, rendering this microalga a valuable resource.

Towards maximizing biomass and lipid productivity: high-throughput screening assay for prospecting heterotrophic growth for new microalgal isolates

BACKGROUND

Microalgae have emerged as sustainable alternatives to fossil fuels and high-value petrochemicals. Despite the commercial potential of microalgae, their low biomass productivity is a significant limiting factor for large-scale production. In the photoautotrophic cultivation of microalgae, achievable cell density levels depend on the light transmittance of the production system, which can significantly decrease the photosynthetic rate and biomass production. In contrast, the mixotrophic cultivation of microalgae using heterotrophic carbon sources enables high-density cultivation, which significantly enhances biomass productivity. The identification of optimal production conditions is crucial for improving biomass productivity; however, it is typically time- and resource-consuming. To overcome this problem, high-throughput screening (HTS) system presents a practical approach to maximize biomass and lipid production and enhance the industrial applicability of microalgae.

RESULTS

In this study, we proposed a two-step HTS assay that allows effective screening of heterotrophic conditions compatible with new microalgal isolates. To confirm the effectiveness of the HTS assay, three microalgal isolates with distinctive morphological and genetic traits were selected. Suitable cultivation conditions, including various heterotrophic carbon sources, substrate concentrations, and temperatures, were investigated using a two-step HTS assay. The optimized conditions were validated at the flask scale, which confirmed a significant enhancement in the biomass and lipid productivity of each isolate. Moreover, the two-step HTS assay notably enhanced economic and temporal efficiency compared to conventional flask-based optimization.

CONCLUSIONS

These results suggest that our two-step HTS assay is an efficient strategy for investigating and optimizing microalgal culture conditions to maximize biomass and lipid productivity. This approach has the potential to enhance the industrial applicability of microalgae and facilitate the seamless transition from laboratory to field applications.

Hydrogel-based photobioreactor for Solid-State cultivation of Chlorella vulgaris

Solid-state cultivation is a promising technology for algal biomass production, achieving high productivities without the need for dewatering. However, such systems have suffered from high evaporation, and capital costs. Here is described a hydrogel photobioreactor (hPBR) with the aim of reducing water demand in solid-state cultivations. Two designs are described with "Design A" offering better humidity control overgrowth conditions. A biomass productivity of 2.41gm-2d-1, and 2.87gm-2d-1 when using physically crosslinked poly(vinyl alcohol) (pPVA) and chemically crosslinked PVA (cPVA) respectively were achieved with Chlorella vulgaris with a water demand around 0.44 kg g-1 of biomass. Over the 23 days of growth, the lipid content increased from 18.9 % to 56.6 % and 13.8 % to 43.2 % for pPVA and cPVA respectively, and the chlorophyll content decreased by more than 81 %. However, cell viability stayed high at over 98 % and surface coverage analysis showed good coverage of the gel surface.

Effects of culture temperature and light regimes on biomass and lipid accumulation of Chlamydomonas reinhardtii under carbon-rich and nitrogen-limited conditions

This study investigated the impacts of various culture temperatures and light regimes on growth and biochemical constituents of Chlamydomonas reinhardtii under carbon-supply and nitrogen-limited conditions to improve oil production in algal cells. Results displayed that under a 30 ℃ and 150 μE/m2/s regime, there was a significant increase in biomass, total lipids, and lipid productivity. Specifically, these parameters reached 1.83 g/L, 36.25 %, and 130.73 mg/L/d, respectively. Remarkably, prolonging the photoperiod further enhanced the aforementioned three parameters, reaching peak levels of 1.92 g/L, 41.10 %, and 157.54 mg/L/d, respectively, recorded at a 24/0h photoperiod. Compared with cultures grown under normal conditions, these values displayed increments of 1.21-fold, 74.88 %, and 3.01-fold, respectively. Additionally, under optimal conditions, the soluble sugar content reached 79.72 mg/g, and the biodiesel properties were improved. These findings indicate that moderately increasing temperature, light intensity, and photoperiod could achieve the co-production of biomass, lipids, and sugars in C. reinhardtii.

Towards Lipid from Microalgae: Products, Biosynthesis, and Genetic Engineering

Microalgae can convert carbon dioxide into organic matter through photosynthesis. Thus, they are considered as an environment-friendly and efficient cell chassis for biologically active metabolites. Microalgal lipids are a class of organic compounds that can be used as raw materials for food, feed, cosmetics, healthcare products, bioenergy, etc., with tremendous potential for commercialization. In this review, we summarized the commercial lipid products from eukaryotic microalgae, and updated the mechanisms of lipid synthesis in microalgae. Moreover, we reviewed the enhancement of lipids, triglycerides, polyunsaturated fatty acids, pigments, and terpenes in microalgae via environmental induction and/or metabolic engineering in the past five years. Collectively, we provided a comprehensive overview of the products, biosynthesis, induced strategies and genetic engineering in microalgal lipids. Meanwhile, the outlook has been presented for the development of microalgal lipids industries, emphasizing the significance of the accurate analysis of lipid bioactivity, as well as the high-throughput screening of microalgae with specific lipids.

Optimizing light regimes for neutral lipid accumulation in Dunaliella salina MCC 43: a study on physiological status and carbon allocation

Dunaliella salina is a favourable source of high lipid feedstock for biofuel and medicinal chemicals. Low biomass output from microalgae is a significant barrier to industrial-scale commercialisation. The current study aimed to determine how photosynthetic efficiency, carbon fixation, macromolecular synthesis, accumulation of neutral lipids, and antioxidative defence (ROS scavenging enzyme activities) of D. salina cells were affected by different light intensities (LI) (50, 100, 200, and 400 µmol m-2 s-1). The cells when exposed to strong light (400 µmol m-2 s-1) led to reduction in chlorophyll a but the carotenoid content increased by 19% in comparison to the control (LI 100). The amount of carbohydrate changed significantly under high light and in spite of stress inflicted on the cells by high irradiation, a considerable increase in activity of carbonic anhydrase and fixation rate of CO2 were recorded, thus, preserving the biomass content. The high light exposed biomass when subjected to nitrogen-deficient medium led to increase in lipid content (59.92% of the dry cell weight). However, neutral lipid made up 78.26% of the total lipid while other lipids like phospholipid and glycolipid content decreased, showing that the lipid was redistributed in these cells under nitrogen deprivation, making the organism more appropriate for biodiesel/jet fuel use. Although D. salina cells had a relatively longer generation time (3.5 d) than other microalgal cells, an economic analysis concluded that the amount of carotenoid they produced and the quality of their lipids made them more suited for commercialization.

Biodiesel Production from the Marine Alga Nannochloropsis oceanica Grown on Yeast Wastewater and the Effect on Its Biochemical Composition and Gene Expression

Microalgae-based biodiesel synthesis is currently not commercially viable due to the high costs of culture realizations and low lipid yields. The main objective of the current study was to determine the possibility of growing Nannochloropsis oceanica on Saccharomyces cerevisiae yeast wastewater for biodiesel generation at an economical rate. N. oceanica was grown in Guillard F/2 synthetic medium and three dilutions of yeast wastewater (1, 1.25, and 1.5%). Biodiesel properties, in addition to carbohydrate, protein, lipid, dry weight, biomass, lipid productivity, amino acids, and fatty acid methyl ester (FAMEs) content, were analyzed and the quality of the produced biodiesel is assessed. The data revealed the response of N. oceanica to nitrogen-deficiency in the three dilutions of yeast wastewater. N. oceanica in Y2 (1.25%) yeast wastewater dilution exhibited the highest total carbohydrate and lipid percentages (21.19% and 41.97%, respectively), and the highest lipid productivity (52.46 mg L-1 day -1) under nitrogen deficiency in yeast wastewater. The fatty acids profile shows that N. oceanica cultivated in Y2 (1.25%) wastewater dilution provides a significant level of TSFA (47.42%) and can be used as a feedstock for biodiesel synthesis. In addition, N. oceanica responded to nitrogen shortage in wastewater dilutions by upregulating the gene encoding delta-9 fatty acid desaturase (Δ9FAD). As a result, the oleic and palmitoleic acid levels increased in the fatty acid profile of Y2 yeast wastewater dilution, highlighting the increased activity of Δ9FAD enzyme in transforming stearic acid and palmitic acid into oleic acid and palmitoleic acid. This study proved that the Y2 (1.25%) yeast wastewater dilution can be utilized as a growth medium for improving the quantity of specific fatty acids and lipid productivity in N. oceanica that affect biodiesel quality to satisfy global biodiesel requirements.

An overview on microalgae as renewable resources for meeting sustainable development goals

The increased demands and dependence on depleted oil reserves, accompanied by global warming and climate change have driven the world to explore and develop new strategies for global sustainable development. Among sustainable biomass sources, microalgae represent a promising alternative to fossil fuel and can contribute to the achievement of important Sustainable Development Goals (SDGs). This article has reviewed the various applications of microalgal biomass that includes (i) the use in aquaculture and its sustainability; (ii) commercial value and emerging extraction strategies of carotenoids; (iii) biofuels from microalgae and their application in internal combustion engines; (iv) the use and reuse of water in microalgae cultivation; and (v) microalgae biotechnology as a key factor to assist SDGs. The future prospects and challenges on the microalgae circular bio economy, issues with regard to the scale-up and water demand in microalgae cultivation are also highlighted.

Bright as day and dark as night: light-dependant energy for lipid biosynthesis and production in microalgae

Microalgae are photosynthetic organisms functioning as the green bio-factories for various pharmaceutical and biofuel products. To date, numerous attempts have been carried out to manipulate culture conditions to maximize the production of the desired metabolites. Because light is the energy source of microalgae for their growth and metabolites biosynthesis, it has been one of the most investigated variables emphasized on the deep understanding of how microalgae respond towards light changes as an external stimulus. This review discusses the effects of different light sources, light intensities, light wavelengths and length of photoperiod on various microalgae species, especially in terms of biomass and lipid productivity. Additionally, the relationship between photoregulation processes and lipid productivity of microalgae are also deliberated. The current available approaches of microalgae mass cultivation, including different types of open and closed systems are recapitulated with the intention to highlight the significant insights for the design of future photoreactors.

A multi-omic characterization of temperature stress in a halotolerant Scenedesmus strain for algal biotechnology

Microalgae efficiently convert sunlight into lipids and carbohydrates, offering bio-based alternatives for energy and chemical production. Improving algal productivity and robustness against abiotic stress requires a systems level characterization enabled by functional genomics. Here, we characterize a halotolerant microalga Scenedesmus sp. NREL 46B-D3 demonstrating peak growth near 25 °C that reaches 30 g/m2/day and the highest biomass accumulation capacity post cell division reported to date for a halotolerant strain. Functional genomics analysis revealed that genes involved in lipid production, ion channels and antiporters are expanded and expressed. Exposure to temperature stress shifts fatty acid metabolism and increases amino acids synthesis. Co-expression analysis shows that many fatty acid biosynthesis genes are overexpressed with specific transcription factors under cold stress. These and other genes involved in the metabolic and regulatory response to temperature stress can be further explored for strain improvement.

Molecular Mechanism of Lipid Accumulation and Metabolism of Oleaginous Chlorococcum sphacosum GD from Soil under Salt Stress

The oleaginous microalgae species Chlorococcum sphacosum GD is a promising feedstock for biodiesel production from soil. However, its metabolic mechanism of lipid production remains unclear. In this study, the lipid accumulation and metabolism mechanisms of Chlorococcum sphacosum GD were analyzed under salt stress based on transcriptome sequencing. The biomass and lipid content of the alga strain were determined under different NaCl concentrations, and total RNA from fresh cells were isolated and sequenced by HiSeq 2000 high throughput sequencing technology. As the salt concentration increased in culture medium, the algal lipid content increased but the biomass decreased. Following transcriptome sequencing by assembly and splicing, 24,128 unigenes were annotated, with read lengths mostly distributed in the 200-300 bp interval. Statistically significant differentially expressed unigenes were observed in different experimental groups, with 2051 up-regulated genes and 1835 down-regulated genes. The lipid metabolism pathway analysis showed that, under salt stress, gene-related fatty acid biosynthesis (ACCase, KASII, KAR, HAD, FATA) was significantly up-regulated, but some gene-related fatty acid degradation was significantly down-regulated. The comprehensive results showed that salt concentration can affect the lipid accumulation and metabolism of C. sphacosum GD, and the lipid accumulation is closely related to the fatty acid synthesis pathway.

The interactions between microplastic polyvinyl chloride and marine diatoms: Physiological, morphological, and growth effects

Microplastics are identified as a great threat to marine environments. However, knowledge of their impacts on phytoplankton, especially for the diatoms is scarce. Herein, the effects of different polyvinyl chloride (PVC) microplastic concentrations and contact times (24, 48, 72 and 96 h) on the F_v/F_m and cell density of Phaeodactylum tricornutum (B255), Chaetoceros gracilis (B13) and Thalassiosira sp. (B280) were investigated to evaluate the toxic effects of microplastics on marine diatoms. The effects of PVC microplastics on the morphology of the diatoms was observed by SEM. The order of sensitivity to 1 μm PVC microplastics among three marine diatoms was B13 > B280 > B255, showing that the toxic effects varied with different microalgae species. Furthermore, the presence of a siliceous cell wall played a minimal role in protecting cells from the physical attack of PVC microplastics, with no significant difference from the common cell wall. PVC microplastics caused dose-dependent adverse effects on three marine diatoms. High PVC concentrations (200 mg/L) reduced the chlorophyll content, inhibited F_v/F_m, and affected the photosynthesis of three marine diatoms. The PVC microplastics adsorbed and caused physical damage on the structure of algal cells. Interactions between PVC microplastics and diatoms may be the probable reason for the negative effects of PVC on diatoms.

Nitrogen Deficiency-Dependent Abiotic Stress Enhances Carotenoid Production in Indigenous Green Microalga Scenedesmus rubescens KNUA042, for Use as a Potential Resource of High Value Products

The microalgal strain Scenedesmus rubescens KNUA042 was identified in freshwater in Korea and characterized by evaluating its stress responses in an effort to increase lipid and carotenoid production. Under a two-stage cultivation process, the algal strain that generally exhibits optimal growth at a nitrate (source of nitrogen) concentration of 0.25 g L−1 was challenged to different exogenous stimuli—salinity (S), light intensity (L), combined L and S (LS), and nitrogen deficiency (C)—for 14 days. Lipid production and carotenoid concentration increased in a time-dependent manner under these physicochemical conditions during the culture periods. Lipid accumulation was confirmed by thin layer chromatography, BODIPY staining, and fatty acid composition analysis, which showed no differences in the algal cells tested under all four (C, S, L, and LS) conditions. The quality of biodiesel produced from the biomass of the algal cells met the American Society for Testing and Materials and the European standards. Total carotenoid content was increased in the LS-treated algal cells (6.94 mg L−1) compared with that in the C-, S-, and L-treated algal cells 1.75, 4.15, and 1.32 mg L−1, respectively). Accordingly, the concentration of canthaxanthin and astaxanthin was also maximized in the LS-treated algal cells at 1.73 and 1.11 mg g−1, respectively, whereas lutein showed no differences in the cells analyzed. Conversely, chlorophyll a level was similar among the C-, S-, and LS-treated algal cells, except for the L-treated algal cells. Thus, our results suggested that S. rubescens KNUA042 was capable of producing carotenoid molecules, which led to the maximum values of canthaxanthin and astaxanthin concentrations when exposed to the combined LS condition compared with that observed when exposed to the salinity condition alone. This indicates that the algal strain could be used for the production of high-value products as well as biofuel. Furthermore, this article provides the first evidence of carotenoid production in S. rubescens KNUA042.

Morphological Variability Identification of Harumanis Mango (Mangifera indica L.) Harvested from Different Location and Tree Age

Harumanis is one of the main signatures of Perlis with regards to its delightful taste, pleasant aroma and expensive price. Harumanis authenticity and productivity had become the remarks among the farmers, entrepreneurs, consumers and plant breeders due to the existence of morphological characteristics variation among the fruits and high production cost. Assessment of Harumanis morphological characteristics of natural population and different tree ages may represent a possible source of important characteristics for development and breeding purposes of Harumanis. The aim of this study is to evaluate the morphological variation of Harumanis collected from different location in Perlis and tree age. A total of 150 Harumanis fruits from 50 trees with three different stages of development (young, middle-aged and old) were characterised using 11 traits; 10 quantitative and one qualitative morphological trait. The ANOVA analyses in combination with Dunn's pairwise and Kruskal-Wallis multiple comparison test able to point out the existence of environmental factor and age influence towards the significant different of identified morphological traits except for Total Soluble Solid (TSS) and pulp percentage. Five clusters of 50 Harumanis accessions reflect a grouping pattern which not according to neither geographical region nor age. The result of Principal Component Analysis (PCA) using the first two principal components (PCs) provided a good approximation of the data explaining 84.09% of the total variance which majorly contributed by parameters of weight, fruit dimensional characteristics, peel percentage and hue angle, h. Preliminary screening of important morphological characteristics which contribute to the phenotypic diversity of Harumanis is successfully achieved. The findings can be employed by the plant breeders and farmers for the establishment of standard grading of Harumanis and advancement of breeding crop of Harumanis in future.

Simultaneous improvements on nutrient and Mg recoveries of microalgal bioremediation for municipal wastewater and nickel laterite ore wastewater

Effects of different mixing ratios between synthetic municipal wastewater (MW) and magnesium (Mg²⁺)-enriched nickel laterite ore wastewater (NLOWW) on growth of Chlorella sorokiniana (C. sorokiniana), photosynthetic activities, cellular biocomposition, nutrient and Mg²⁺ removal were investigated in photobioreactors. In the culture without NLOWW, wrinkled cells were observed with low biomass production. The culture mixed with 0.13% NLOWW obtained 1.89-fold higher biomass yield, 3.77-fold enhanced photosynthetic activity (Fv/Fm value), and improved nutrient removal (nitrogen by 102.2%, phosphorus by 39.3%). However, excessive Mg²⁺ at 100% NLOWW produced highest reactive oxygen species suppressing microalgal growth. The Mg²⁺ removal capacity increased with NLOWW loading. Moreover, microalgal assimilation primarily contributed to nutrient removal while absorption was the dominant Mg²⁺ removal pathway. Carbohydrate content in biomass increased with Mg²⁺ loading. Finally, the approach for MW/NLOWW treatment was demonstrated as economically feasible with revenue of $75.6 per kilogram biomass through a comprehensive economic model.

Optimization of light intensity and photoperiod for Isochrysis galbana culture to improve the biomass and lipid production using 14-L photobioreactors with mixed light emitting diodes (LEDs) wavelength under two-phase culture system

The optimal light intensity and photoperiod required to produce high biomass and lipid contents in Isochrysis galbana cultured in a 14-L bioreactor with LED wavelengths was studied. The cell biomass production was monitored in the first phase comprising of mixed blue (465 nm) and red (640 nm) LED wavelengths, then green (520 nm) LED were used in the second phase for lipid production. The optimal light intensity was 400 µmol/m²/s giving a maximum cell biomass of 1.05 g dcw/L and total lipid content of 65.2% (w/w) cultured under 12:12 h L/D cycle. The optimal light intensity of 400 µmol/m²/s was applied at different L/D cycles, the maximum cell biomass (1.25 g dcw/L) and lipid content (71.1% w/w) were obtained at 18:6 h L/D cycle. Stearic acid was the main fatty acid ranging from 42.91 (500 µmol/m²/s) to 65.57% w/w (100 µmol/m²/s) and 53.84 (18:6 h) to 65.44% w/w (24:0 h).

Physiological Changes of Parachlorella Kessleri TY02 in Lipid Accumulation under Nitrogen Stress

In order to study the effects of nitrogen stress on the lipid synthesis of Parachlorella kessleri TY02 and to understand the changes in growth, photosynthetic pigments, total protein and total carbohydrate contents during lipid accumulation, the cells of the strain were cultured in nitrogen-deficient (N⁻) and nitrogen-rich (N⁺) media for one week. Changes in cell growth, chlorophyll content, chlorophyll fluorescence parameters, neutral lipid and total lipid content, total protein content and total carbohydrate content were measured and analyzed. The results showed that, under nitrogen stress, the algal strain grew slowly, and chlorophyll and total protein contents decreased, while total carbohydrate and total lipid contents increased. This indicated that, under nitrogen stress, most of the carbon flowed to the synthesis of lipids and carbohydrates. Meanwhile, reducing the nitrogen content was a relatively economical and easy to operate method of promoting lipid accumulation.

Biomass and lipid production from Chlorella vulgaris UTEX 26 cultivated in 2 m3 raceway ponds under semicontinuous mode during the spring season

A Chlorella vulgaris UTEX 26 semicontinuous culture was implemented in 2000 L raceways with M medium during spring season at greenhouse conditions. Areal biomass productivities between 20 and 26 g m^-2 d^-1 were reached on the third day. The maximal areal lipid productivity obtained was 6.1 g m^-2 d^-1 and an increment in the saturated fatty acids (SFA) proportion (C14-C18) was favored in comparison with the fatty acids obtained with M medium in photobioreactors of 1 L and photoperiod light:darkness 12:12 h. After the eighth day of the culture or biomass concentrations above 0.25 g L^-1, the microalgal cultures were prone to contamination by ciliates and amoebae, due to the sugars excreted by C. vulgaris UTEX 26. The periodical addition of NH₄HCO₃ to the microalgal culture maintained the ammonium concentration between 25 and 50 mg L^-1, which contributed to diminish the contamination risks by protozoa.

Coupling of abiotic stresses and phytohormones for the production of lipids and high-value by-products by microalgae: A review

Microalgae can produce lipids and high-value by-products under abiotic stress conditions, including nutrient starvation, high light intensity, extreme temperature, high salinity and the presence of heavy metals. However, the growth and development of microalgae and the accumulation of metabolites may be inhibited by adverse stresses. In recent years, phytohormones have emerged as a topic of intense focus in microalgae research. Phytohormones could sustain the growth of microalgae under abiotic stress conditions. In addition, the combination of plant hormones and abiotic stresses could further promote the biosynthesis of metabolites and improve the ability of microalgae to tolerate abiotic stresses. This review primarily focuses on the regulatory effects of exogenous phytohormones on the biosynthesis of metabolites by microalgae under adverse environmental conditions and discusses the mechanisms of phytohormone-mediated cell growth, stress tolerance and lipid biosynthesis in microalgae under abiotic stress conditions.

Optimizing culture conditions for heterotrophic-assisted photoautotrophic biofilm growth of Chlorella vulgaris to simultaneously improve microalgae biomass and lipid productivity

In order to solve the technical bottleneck that the biomass yield and lipid accumulation cannot be increased simultaneously during microalgae growth, a heterotrophic-assisted photoautotrophic biofilm (HAPB) growth mode of Chlorella vulgaris was constructed. The light penetration capability of the microalgae biofilm formed through heterotrophic-assisted photoautotrophic growth was 64% stronger than that formed by photoautotrophic growth. Due to the different demands of autotrophic and heterotrophic growth of microalgae, the nutrient environment and growth conditions were optimized to fully utilize the advantages and potentials of the HAPB culture model. An optimized molar ratio of total inorganic carbon (CO₂) to total organic carbon (glucose) (20:1) and a molar ratio of total carbon to total nitrogen (72:1) were obtained. The maximum specific growth rate of Chlorella vulgaris increased by 78% compared to that before optimization. Meanwhile, the lipid content and yield increased by 120% and 147%, respectively, up to 47.53% and 41.95 g m^-2.