Sustainable production of biomass and biodiesel by acclimation of non-acidophilic microalgae to acidic conditions

Isolation and Whole-genome analysis of Desmodesmus sp. SZ-1: Novel acid-tolerant carbon-fixing microalga

Utilizing microalgae to capture flue gas pollutants is an effective strategy for mitigating greenhouse gas emissions. However, existing carbon-fixing microalgae exhibit poor tolerance towards acidic flue gas. In this study, the Desmodesmus sp. SZ-1, which can thrive in acidic environments and efficiently sequester CO2, was isolated. Desmodesmus sp. SZ-1 exhibited strong acid tolerance ability, with an average carbon fixation rate of 497.6 mg/L/d under 10 % CO2 and pH 3.5. Physiological analysis revealed that SZ-1 responded to high CO2 by increasing chlorophyll levels while coping with acidic stress by activating antioxidant enzymes. Genome analysis revealed a large number of carbon fixation and acid adaptation genes, involved in membrane lipid biosynthesis, H+ pumps, molecular chaperones, peroxidase system, amino acid synthesis, and carbonic anhydrase. This study provides a novel algal resource for mitigating acid gas emissions and a comprehensive genetic database for genetically modifying microalgae.

Exploring Extremotolerant and Extremophilic Microalgae: New Frontiers in Sustainable Biotechnological Applications

Exploring extremotolerant and extremophilic microalgae opens new frontiers in sustainable biotechnological applications. These microorganisms thrive in extreme environments and exhibit specialized metabolic pathways, making them valuable for various industries. The study focuses on the ecological adaptation and biotechnological potential of these microalgae, highlighting their ability to produce bioactive compounds under stress conditions. The literature reveals that extremophilic microalgae can significantly enhance biomass production, reduce contamination risks in large-scale systems, and produce valuable biomolecules such as carotenoids, lipids, and proteins. These insights suggest that extremophilic microalgae have promising applications in food, pharmaceutical, cosmetic, and biofuel industries, offering sustainable and efficient alternatives to traditional resources. The review concludes that further exploration and utilization of these unique microorganisms can lead to innovative and environmentally friendly solutions in biotechnology.

Development in health-promoting essential polyunsaturated fatty acids production by microalgae: a review

Polyunsaturated fatty acids (PUFAs) found in microalgae, primarily omega-3 (ω-3) and omega-6 (ω-6) are essential nutrients with positive effects on diseases such as hyperlipidemia, atherosclerosis, and coronary risk. Researchers still seek improvement in PUFA yield at a large scale for better commercial prospects. This review summarizes advancements in microalgae PUFA research for their cost-effective production and potential applications. Moreover, it discusses the most promising cultivation modes using organic and inorganic sources. It also discusses biomass hydrolysates to increase PUFA production as an alternative and sustainable organic source. For cost-effective PUFA production, heterotrophic, mixotrophic, and photoheterotrophic cultivation modes are assessed with traditional photoautotrophic production modes. Also, mixotrophic cultivation has fascinating sustainable attributes over other trophic modes. Furthermore, it provides insight into growth phase (stage I) improvement strategies to accumulate biomass and the complementing effects of other stress-inducing strategies during the production phase (stage II) on PUFA enhancement under these cultivation modes. The role of an excessive or limiting range of salinity, nutrients, carbon source, and light intensity were the most effective parameter in stage II for accumulating higher PUFAs such as ω-3 and ω-6. This article outlines the commercial potential of microalgae for omega PUFA production. They reduce the risk of diabetes, cardiovascular diseases (CVDs), cancer, and hypertension and play an important role in their emerging role in healthy lifestyle management.

Concurrent enhancement of biomass production and phycocyanin content in salt-stressed Arthrospira platensis: A glycine betaine- supplementation approach

In industrial-scale cultivation of microalgae, salinity stress often stimulates high-value metabolites production but decreases biomass yield. In this research, we present an extraordinary response of Arthrospira platensis to salinity stress. Specifically, we observed a significant increase in both biomass production (2.58 g L-1) and phycocyanin (PC) content (22.31%), which were enhanced by 1.26-fold and 2.62-fold, respectively, compared to the control, upon exposure to exogenous glycine betaine (GB). The biochemical analysis reveals a significant enhancement in carbonic anhydrase activity and chlorophyll a level, concurrent with reductions in carbohydrate content and reactive oxygen species (ROS) levels. Further, transcriptomic profiling indicates a downregulation of genes associated with the tricarboxylic acid (TCA) cycle and an upregulation of genes linked to nitrogen assimilation, hinting at a rebalanced carbon/nitrogen metabolism favoring PC accumulation. This work thus presents a promising strategy for simultaneous enhancement of biomass production and PC content in A. platensis and expands our understanding of PC biosynthesis and salinity stress responses in A. platensis.

Continuous cultivation of mixed-culture microalgae using anaerobic digestion effluent in photobioreactors with different strategies for adjusting nitrogen loading rate

This study examined continuous mixed-culture microalgae cultivation for nutrient removal from anaerobic digestion (AD) effluents in photobioreactors, while altering the NH4+-N loading rate (NLR) by adjusting either the hydraulic retention time (HRT) (reactor set RH) or the influent NH4+-N concentration (reactor set RS). Both RH and RS demonstrated efficient nutrient removal and microalgae cultivation at NLRs of 4-10 mg NH4+-N/L∙d, reaching peak performance at 10 mg NH4+-N/L∙d. Within this range, RH obtained greater biomass yield and productivity, while RS maintained higher microalgal concentrations. The cultivated biomasses obtained from RH and RS had good settleability and suitable fatty acid compositions as a biodiesel feedstock, although their organic composition varied considerably with NLR and HRT. Parachlorella overwhelmingly dominated the reactors' microalgal communities throughout the experiment, co-existing with various microalgae-associated bacteria. Changes in NLR significantly influenced the bacterial community structures, underscoring its critical role in determining reactor performance and microalgal-bacterial community behavior.

Physiological responses to pH in the freshwater microalga Limnomonas gaiensis

The ecological niche of the recently described limnic microalga Limnomonas gaiensis (Chlamydomonadales) in Northern Europe remains unknown. To decipher the species tolerance capacity to pH, the effects of hydrogen ions on the physiological response of L. gaiensis were investigated. Results showed that L. gaiensis could tolerate exposure from pH 3 up to pH 11, with an optimal survival at pH 5-8. Its physiological response to pH was strain specific. Globally the southernmost strain was more alkaliphilic, had a slightly rounder shape, a slowest growth rate, and a lowest carrying capacity. Despite strain discrepancies among lakes, Swedish strains exhibited similar growth rates, faster at more acidic conditions. The extreme pH conditions affected its morphological features such as the eye spot and papilla shape, especially at acidic pH, and the cell wall integrity, at more alkaline pH. The wide range tolerance of L. gaiensis to pH would not be a hindrance to its dispersal in Swedish lakes (pH 4-8). Notably, the storage of high-energetic reserves over a wide range of pH conditions, as numerous starch grains and oil droplets, makes L. gaiensis a good candidate for bioethanol/fuel industrial production and a key resource to sustain aquatic food chain and microbial loop.

The interplay between microalgae and toxic metal(loid)s: mechanisms and implications in AMD phycoremediation coupled with Fe/Mn mineralization

Acid mine drainage (AMD) is low-pH with high concentration of sulfates and toxic metal(loid)s (e.g. As, Cd, Pb, Cu, Zn), thereby posing a global environmental problem. For decades, microalgae have been used to remediate metal(loid)s in AMD, as they have various adaptive mechanisms for tolerating extreme environmental stress. Their main phycoremediation mechanisms are biosorption, bioaccumulation, coupling with sulfate-reducing bacteria, alkalization, biotransformation, and Fe/Mn mineral formation. This review summarizes how microalgae cope with metal(loid) stress and their specific mechanisms of phycoremediation in AMD. Based on the universal physiological characteristics of microalgae and the properties of their secretions, several Fe/Mn mineralization mechanisms induced by photosynthesis, free radicals, microalgal-bacterial reciprocity, and algal organic matter are proposed. Notably, microalgae can also reduce Fe(III) and inhibit mineralization, which is environmentally unfavorable. Therefore, the comprehensive environmental effects of microalgal co-occurring and cyclical opposing processes must be carefully considered. Using chemical and biological perspectives, this review innovatively proposes several specific processes and mechanisms of Fe/Mn mineralization that are mediated by microalgae, providing a theoretical basis for the geochemistry of metal(loid)s and natural attenuation of pollutants in AMD.

Simultaneous chromium removal and lipid accumulation by microalgae under acidic and low temperature conditions for promising biodiesel production

Microalgae have become the hotspot of recent researches as heavy metals (HMs) adsorbent and biodiesel production feedstock. In this study, the cell growth, lipid production and Cr⁶⁺ removal of Parachlorella kessleri R-3 at pH 3.5 and 15 °C were investigated. It was found that low concentration of Cr⁶⁺ (0.5 to 2 mg/L) promoted the algal growth, whereas Cr⁶⁺ higher than 5 mg/L inhibited the growth of P. kessleri R-3. Biomass concentration (2.40 g/L) and lipid productivity (131.79 mg/L d^-1) reached the highest at 2 mg/L Cr⁶⁺, and lipid content (61.03 %) reached the highest at 5 mg/L Cr⁶⁺. The maximum Cr⁶⁺ removal efficiency of 91 % was obtained at 0.5 mg/L Cr⁶⁺ treatment. Furthermore, fatty acid composition analysis showed that strain R-3 had a high C16-18 content of 74.88-89.21 %. This study provides new insight into the treatment of HMs and lipid production in cold regions.

Impact of Nitrate and Ammonium Concentrations on Co-Culturing of Tetradesmus obliquus IS2 with Variovorax paradoxus IS1 as Revealed by Phenotypic Responses

Mutual interactions in co-cultures of microalgae and bacteria are well known for establishing consortia and nutrient uptake in aquatic habitats, but the phenotypic changes in terms of morphological, physiological, and biochemical attributes that drive these interactions have not been clearly understood. In this novel study, we demonstrated the phenotypic response in a co-culture involving a microalga, Tetradesmus obliquus IS2, and a bacterium, Variovorax paradoxus IS1, grown with varying concentrations of two inorganic nitrogen sources. Modified Bold's basal medium was supplemented with five ratios (%) of NO₃-N:NH₄-N (100:0, 75:25, 50:50, 25:75, and 0:100), and by maintaining N:P Redfield ratio of 16:1. The observed morphological changes in microalga included an increase in granularity and a broad range of cell sizes under the influence of increased ammonium levels. Co-culturing in presence of NO₃-N alone or combination with NH₄-N up to equimolar concentrations resulted in complete nitrogen uptake, increased growth in both the microbial strains, and enhanced accumulation of carbohydrates, proteins, and lipids. Total chlorophyll content in microalga was also significantly higher when it was grown as a co-culture with NO₃-N and NH₄-N up to a ratio of 50:50. Significant upregulation in the synthesis of amino acids and sugars and downregulation of organic acids were evident with higher ammonium uptake in the co-culture, indicating the regulation of carbon and nitrogen assimilation pathways and energy synthesis. Our data suggest that the co-culture of strains IS1 and IS2 could be exploited for effluent treatment by considering the concentrations of inorganic sources, particularly ammonium, in the wastewaters.

Phenotypic changes in microalgae at acidic pH mediate their tolerance to higher concentrations of transition metals

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Acid-tolerant microalgae were grown at pH 3.5 and 6.7 in presence of heavy metals (HMs).

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HMs-induced phenotypic changes in microalgae were evaluated by ATR-FTIR spectroscopy.

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Higher HMs bioavailability affected microalgae more at pH 6.7 than pH 3.5.

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Acclimation of microalgal strains to acidic pH significantly alleviates HMs toxicity.

Acclimatory phenotypic response is a common phenomenon in microalgae, particularly during heavy metal stress. It is not clear so far whether acclimating to one abiotic stressor can alleviate the stress imposed by another abiotic factor. The intent of the present study was to demonstrate the implication of acidic pH in effecting phenotypic changes that facilitate microalgal tolerance to biologically excess concentrations of heavy metals. Two microalgal strains, Desmodesmus sp. MAS1 and Heterochlorella sp. MAS3, were exposed to biologically excess concentrations of Cu (0.50 and 1.0 mg L^‒1), Fe (5 and 10 mg L^‒1), Mn (5 and 10 mg L^‒1) and Zn (2, 5 and 10 mg L^‒1) supplemented to the culture medium at pH 3.5 and 6.7. Chlorophyll autofluorescence and biochemical fingerprinting using FTIR-spectroscopy were used to assess the microalgal strains for phenotypic changes that mediate tolerance to metals. Both the strains responded to acidic pH by effecting differential changes in biochemicals such as carbohydrates, proteins, and lipids. Both the microalgal strains, when acclimated to low pH of 3.5, exhibited an increase in protein (< 2-fold) and lipid (> 1.5-fold). Strain MAS1 grown at pH 3.5 showed a reduction (1.5-fold) in carbohydrates while strain MAS3 exhibited a 17-fold increase in carbohydrates as compared to their growth at pH 6.7. However, lower levels of biologically excess concentrations of the selected transition metals at pH 6.7 unveiled positive or no effect on physiology and biochemistry in microalgal strains, whereas growth with higher metal concentrations at this pH resulted in decreased chlorophyll content. Although the bioavailability of free-metal ions is higher at pH 3.5, as revealed by Visual MINTEQ model, no adverse effect was observed on chlorophyll content in cells grown at pH 3.5 than at pH 6.7. Furthermore, increasing concentrations of Fe, Mn and Zn significantly upregulated the carbohydrate metabolism, but not protein and lipid synthesis, in both strains at pH 3.5 as compared to their growth at pH 6.7. Overall, the impact of pH 3.5 on growth response suggested that acclimation of microalgal strains to acidic pH alleviates metal toxicity by triggering physiological and biochemical changes in microalgae for their survival.

Heavy metal detoxification mechanisms by microalgae: Insights from transcriptomics analysis

Heavy metal pollution in ecosystem is a global threat. The associated toxicity and carcinogenic nature of heavy metals/metalloids such as mercury, cadmium, lead, and arsenic are imposing a severe risk to both ecological diversity and human lives. Harnessing the adaptive feature of microalgae for remediating toxic heavy metal has reached a milestone in past few decades. Transcriptomics analyses have provided mechanistic insights to map the dynamics of cellular events under heavy metal stress, thus deciphering the strategic responses of microalgae. Here, the present review comprehensively addresses the elicited molecular responses of microalgae to detoxify the heavy metal stress. The review highlights the intricate role of biochemical components and signaling networks mediating stress responsive transitions of microalgae at physiological level. Furthermore, the differential gene expression signifying the transporters involved in uptake, distribution/sequestration, and efflux of heavy metal has also been reviewed. In a nutshell, this study provided a comprehensive understanding of the molecular mechanisms adopted by microalgae at transcriptome level to nullify the oxidative stress while detoxifying the heavy metals.

The Upcoming 6Li Isotope Requirements Might Be Supplied by a Microalgal Enrichment Process

Lithium isotopes are essential for nuclear energy, but new enrichment methods are required. In this study, we considered biotechnology as a possibility. We assessed the Li fractionation capabilities of three Chlorophyte strains: Chlamydomonas reinhardtii, Tetraselmis mediterranea, and a freshwater Chlorophyte, Desmodesmus sp. These species were cultured in Li containing media and were analysed just after inoculation and after 3, 12, and 27 days. Li mass was determined using a Inductively Coupled Plasma Mass Spectrometer, and the isotope compositions were measured on a Thermo Element XR Inductively Coupled Plasma Mass Spectrometer. The maximum Li capture was observed at day 27 with C. reinhardtii (31.66 µg/g). Desmodesmus sp. reached the greatest Li fractionation, (δ⁶ = 85.4‰). All strains fractionated preferentially towards ⁶Li. More studies are required to find fitter species and to establish the optimal conditions for Li capture and fractionation. Nevertheless, this is the first step for a microalgal nuclear biotechnology.

Microalgal-bacterial consortia unveil distinct physiological changes to facilitate growth of microalgae

Physiological changes that drive the microalgal-bacterial consortia are poorly understood so far. In the present novel study, we initially assessed five morphologically distinct microalgae for their ability in establishing consortia in Bold's basal medium with a bacterial strain, Variovorax paradoxus IS1, all isolated from wastewaters. Tetradesmus obliquus IS2 and Coelastrella sp. IS3 were further selected for gaining insights into physiological changes, including those of metabolomes in consortia involving V. paradoxus IS1. The distinct parameters investigated were pigments (chlorophyll a, b, and carotenoids), reactive oxygen species (ROS), lipids and metabolites that are implicated in major metabolic pathways. There was a significant increase (>1.2-fold) in pigments, viz., chlorophyll a, b and carotenoids, decrease in ROS and an enhanced lipid yield (>2-fold) in consortia than in individual cultures. In addition, the differential regulation of cellular metabolites such as sugars, amino acids, organic acids and phytohormones was distinct among the two microalgal-bacterial consortia. Our results thus indicate that the selected microalgal strains, T. obliquus IS2 and Coelastrella sp. IS3, developed efficient consortia with V. paradoxus IS1 by effecting the required physiological changes, including metabolomics. Such microalgal-bacterial consortia could largely be used in wastewater treatment and for production of value-added metabolites.

Acid-adapted microalgae exhibit phenotypic changes for their survival in acid mine drainage samples

Phenotypic plasticity or genetic adaptation in an organism provides phenotypic changes when exposed to the extreme environmental conditions. The resultant physiological and metabolic changes greatly enhance the organism's potential for its survival in such harsh environments. In the present novel approach, we tested the hypothesis whether acid-adapted microalgae, initially isolated from non-acidophilic environments, can survive and grow in acid-mine-drainage (AMD) samples. Two acid-adapted microalgal strains, Desmodesmus sp. MAS1 and Heterochlorella sp. MAS3, were tested individually or in combination (co-culture) for phenotypic changes during their growth in samples collected from AMD. The acid-adapted microalgae in AMD exhibited a two-fold increase in growth when compared with those grown at pH 3.5 in BBM up to 48 h and then declined. Furthermore, oxidative stress triggered several alterations such as increased cell size, granularity, and enhanced lipid accumulation in AMD-grown microalgae. Especially, the apparent limitation of phosphate in AMD inhibited the uptake of copper and iron in the cultures. Interestingly, growth of the acid-adapted microalgae in AMD downregulated amino acid metabolic pathways as a survival mechanism. This study demonstrates for the first time that acid-adapted microalgae can survive under extreme environmental conditions as exist in AMD by effecting significant phenotypic changes.

Carbon flow conversion induces alkali resistance and lipid accumulation under alkaline conditions based on transcriptome analysis in Chlorella sp. BLD

Alkaline environments are abundant globally and cause damage to most organisms, while some microalgae can grow well and accumulate lipids under those conditions. Here the mechanisms of alkali resistance and lipid accumulation in the alkaliphilic microalgae Chlorella sp. BLD were explored using physiological-biochemical and transcriptome analysis. When cultivated at alkaline pH, Chlorella sp. BLD exhibited good alkali-resistance ability and increased biomass (0.97 g L^-1). The biochemical composition of Chlorella sp. BLD changed significantly (lipid content increased 39% and protein content decreased 19.5%) compared with pH 7.5. Through transcriptome analysis, we found that pathways related to carbon metabolism such as photosynthesis, glycolysis, and the TCA cycle were significantly regulated under alkaline conditions. Genes that encoding the key enzyme in carbon-related metabolism such as Rubisco, AMY, PK, ME, CS, ACAT, KAS, and DGAT were identified. Transcriptional regulation of these genes results in carbon flow switching from starch and protein to cell wall metabolism, organic acid synthetic and lipid accumulation in response to alkaline conditions. These results reveal the alkali resistance mechanism of Chlorella sp. BLD and provide a theoretical basis for microalgae oil production under alkaline conditions.

Selection and re-acclimation of bioprospected acid-tolerant green microalgae suitable for growth at low pH

For mass culture of photosynthetic green microalgae, industrial flue gases can represent a low-cost resource of CO₂. However, flue gases are often avoided, because they often also contain high levels of SO₂ and/or NO₂, which cause significant acidification of media to below pH 3 due to production of sulfuric and nitric acid. This creates an unsuitable environment for the neutrophilic microalgae commonly used in large-scale commercial production. To address this issue, we have looked at selecting acid-tolerant microalgae via growth at pH 2.5 carried out with samples bioprospected from an active smelter site. Of the eight wild samples collected, one consisting mainly of Coccomyxa sp. grew at pH 2.5 and achieved a density of 640 mg L^-1. Furthermore, three previously bioprospected green microalgae from acidic waters (pH 3-4.5) near abandoned mine sites were also re-acclimated down to their in-situ pH environment after approximately 4 years spent at neutral pH. Of those three, an axenic culture of Coccomyxa sp. was the most successful at re-acclimating and achieved the highest density of 293.1 mg L^-1 and maximum daily productivity of 38.8 mg L^-1 day^-1 at pH 3. Re-acclimation of acid-tolerant species is, therefore, achievable when directly placed at their original pH, but gradual reduction in pH is recommended to give the cells time to acclimate.

Enhanced Lipid Production in Chlamydomonas reinhardtii Caused by Severe Iron Deficiency

Microalgae are used as a source of lipids for the production of biofuels. Most algae produce neutral lipids under stress conditions. Here, lipid accumulation by the unicellular alga Chlamydomonas reinhardtii was examined during cultivation under iron-limiting conditions. Severe iron stress caused the cells to accumulate a significant amount of lipid, specifically triacylglycerols (TAGs), by compromising the growth. Semi-quantitative measurements by Fourier transform infrared (FTIR) spectroscopy showed an increase in both carbohydrate and lipid content in iron-stressed C. reinhardtii cells compared to control. Analysis by flow cytometry and thin layer chromatography confirmed that severe iron deficiency-induced TAG accumulation to fourfold higher than in iron-replete control cells. This accumulation of TAGs was mostly degraded from chloroplast lipids accompanied by overexpression of diacylglycerol acyltransferase (DGAT2A) protein. Furthermore, liquid chromatography-mass spectrometry (LC-MS) analysis demonstrated significantly enhanced levels of C16:0, C18:2, and C18:3 fatty acids (FAs). These results indicate that iron stress triggers the rapid accumulation of TAGs in C. reinhardtii cells. The enhanced production of these lipids caused by the iron deficiency may contribute to the efficient production of algal biofuels if we escalate to the photobioreactor's growth conditions.

Sustainable Iron Recovery and Biodiesel Yield by Acid-Adapted Microalgae, Desmodesmus sp. MAS1 and Heterochlorella sp. MAS3, Grown in Synthetic Acid Mine Drainage

Sustainable resource recovery is the key to manage the overburden of various waste entities of mining practices. The present study demonstrates for the first time a novel approach for iron recovery and biodiesel yield from two acid-adapted microalgae, Desmodesmus sp. MAS1 and Heterochlorella sp. MAS3, grown in synthetic acid mine drainage (SAMD). Virtually, there was no difference in the growth of the strain MAS3 both in Bold's basal medium (control) and SAMD. Using the IC50 level (200 mg L-1) and a lower concentration (50 mg L-1) of iron in SAMD, the cell granularity, exopolysaccharide (EPS) secretion, iron recovery, and biodiesel were assessed in both the strains. Both cell granularity and accumulation of EPS were significantly altered under metal stress in SAMD, resulting in an increase in total accumulation of iron. Growth of the microalgal strains in SAMD yielded 12-20% biodiesel, with no traces of heavy metals, from the biomass. The entire amount of iron, accumulated intracellularly, was recovered in the residual biomass. Our results on the ability of the acid-adapted microalgal strains in iron recovery and yield of biodiesel when grown in SAMD indicate that they could be the potential candidates for use in bioremediation of extreme habitats like AMD.

The effect of medium and light wavelength towards Stichococcus bacillaris fatty acid production and composition

Introduction of novel species will highlight technical feasibility of microalgae-based biofuels for commercial applications. This paper reports the effect of culture medium and light wavelength on biomass and fatty acid production of S. bacillaris which holds some advantages as short life cycle, easy cultivation, high lipid content, diversity of fatty acids and stability under harsh environmental conditions. The results displayed that, soil extract (SE) greatly enhance growth rate of cultures. Maximum biomass and lipid productivity were achieved in TAP medium as 81 mg/L·day, 19.44 mg/L·day; respectively. Light wavelength didn't significantly change growth kinetics but played a critical role on chlorophyll-a accumulation. C14:0, C16:0 and C18:0 fatty acids were abundant which are suitable for biodiesel conversion. Interestingly, blue and red light increased longer chain fatty acids content. These results indicated that; S. bacillaris holds potential for further development of biodiesel production and feasibility of algal biodiesel for fundamental and applied sciences.

Mechanistic understanding towards the effective lipid production of a microalgal mutant strain Scenedesmus sp. Z-4 by the whole genome bioinformation

Currently, the complex mechanism of lipid production in microalgal cells is still unclear, and the platform suitable for microalgal genetic transformation is urgent to be established. In this study, the whole genome of a lipid-rich microalgal mutant strain Scenedesmus sp. Z-4 and a lipid-poor wild strain Scenedesmus sp. MC-1 were sequenced, and results revealed that the sequences of 1,256 genes were changed and 148 differential genes related to glucose and lipid metabolism were identified. Especially, gene differentiation of acetyl-CoA carboxylase (ACCase) and phosphoenolpyruvate carboxylase (PEPC) in mutant strain Z-4 and wild strain MC-1, which played key roles in lipid synthesis, were evaluated. Furthermore, to investigate whether mutated ACCase and PEPC genes affect the lipid production, two genes from mutant strain Z-4 were transformed into the expression system of wild strain MC-1. Nine transformants with higher lipid content were successfully obtained, in which the optimal transformant with 28.6% more intracellular lipid than wild strain MC-1 was isolated by overexpression of mutated ACCase gene, demonstrating the important role of ACCase in lipid accumulation of microalgal cells. These results could provide a better understanding of the superior lipid production of mutant strain Scenedesmus sp. Z-4.

Acid-tolerant microalgae can withstand higher concentrations of invasive cadmium and produce sustainable biomass and biodiesel at pH 3.5

Two acid-tolerant microalgae, Desmodesmus sp. MAS1 and Heterochlorella sp. MAS3, originally isolated from non-acidophilic environment, were tested for their ability to withstand higher concentrations of an invasive heavy metal, cadmium (Cd), at an acidic pH of 3.5 and produce biomass rich in biodiesel. The growth analysis, in terms of chlorophyll, revealed that strain MAS1 was tolerant even to 20 mg L^-1 of Cd while strain MAS3 could withstand only up to 5 mg L^-1. When grown in the presence of 2 mg L^-1, a concentration which is 400-fold higher than that usually occurs in the environment, the microalgal strains accumulated >58% of Cd from culture medium at pH 3.5. FTIR analysis of Cd-laden biomass indicated production of significant amounts of biodiesel rich in fatty acid esters. This is the first study that demonstrates the capability of acid-tolerant microalgae to grow well and remove Cd at acidic pH.

Potential of acid-tolerant microalgae, Desmodesmus sp. MAS1 and Heterochlorella sp. MAS3, in heavy metal removal and biodiesel production at acidic pH

Metals in traces are vital for microalgae but their occurrence at high concentrations in habitats is a serious ecological concern. We investigated the potential of two acid-tolerant microalgae, Desmodesmus sp. MAS1 and Heterochlorella sp. MAS3, isolated from neutral environments, for simultaneous removal of heavy metals such as copper (Cu), iron (Fe), manganese (Mn) and zinc (Zn), and production of biodiesel when grown at pH 3.5. Excepting Cu, the selected metals at concentrations of 10-20 mg L^-1 supported good growth of both the strains. Cellular analysis for metal removal revealed the predominance of intracellular mechanism in both the strains resulting in 40-80 and 40-60% removal of Fe and Mn, respectively. In-situ transesterification of biomass indicated enhanced biodiesel yield with increasing concentrations of metals suggesting that both these acid-tolerant microalgae may be the suitable candidates for simultaneous remediation, and sustainable biomass and biodiesel production in environments like metal-rich acid mine drainages.