Impact of nutrient starvation on intracellular biochemicals and calorific value of mixed microalgae

Microalgae treatment of food processing wastewater for simultaneous biomass resource recycling and water reuse

Food processing wastewater presents a considerable challenge for treatment owing to its elevated nitrogen and phosphorus levels. Nonetheless, it possesses inherent value attributed to its abundant nutrients and organic content. This study presents an innovative approach for treating food processing wastewater and reusing biomass. Initially, the secondary-treated wastewater undergoes flocculation and sedimentation, followed by reverse osmosis to ensure that the effluent meets reuse standards. Subsequently, reverse osmosis concentrates, generated at varying water recovery rates, are utilized for microalgae cultivation to recover nitrogen and phosphorus. Furthermore, this study highlights the potential of reverse osmosis concentrates in reducing the water demand for microalgae cultivation and in producing commercial-grade nutrients. The findings reveal that reverse osmosis achieves removal rates exceeding 90 % for both nitrogen and phosphorus and effluent meets reuse standards. Following seven days of cultivation, microalgae cultured in reverse osmosis concentrated water with an 80 % water recovery rate demonstrate denitrification and phosphorus removal rates of 73.88 % and 80.92 % respectively, with a biomass concentration of 563 mg/L and a protein yield of 128 mg/L. Moreover, a total volumetric energy yield of 10.08 kJ/L is obtained, facilitating energy valorization. In conclusion, this study offers practical solutions for wastewater treatment and resource recovery, enabling the attainment of zero discharge of pollutants while generating valuable resources through microalgae cultivation.

Recycling air conditioner-generated condensate water for microalgal biomass production and carbon dioxide sequestration

Air conditioners alleviate the discomfort of human beings from heat waves that are consequences of climate change caused by anthropogenic activities. With each passing year, the effects of global warming worsen, increasing the growth of air conditioning industry. Air conditioning units produce substantial amounts of non-nutritive and (generally) neglected condensate water and greenhouse gases. Considering this, the study explored the potential of using air conditioner condensate water (ACW) to cultivate Chlorella sorokiniana, producing biomass, and sequestering carbon dioxide (CO2). The maximum biomass production was obtained in the BG11 medium (1.45 g L-1), followed by ACW-50 (1.3 g L-1). Similarly, the highest chlorophyll-a content was observed in the BG11 medium (11 μg mL-1), followed by ACW-50 (9.11 μg mL-1). The ACW-50 cultures proved to be better adapted to physiological stress (Fv/Fm > 0.5) and can be suitable for achieving maximum biomass with adequate lipid, protein, and carbohydrate production. Moreover, C. sorokiniana demonstrated higher lipid and carbohydrate yields in the ACW-50 medium, while biomass production and protein yields were comparable to the BG11 medium. The lipid, protein, and carbohydrate productivity were 23.43, 32.9, and 23.19 mg L-1 d-1, respectively for ACW-50. Estimation of carbon capture potential through this approach equals to 9.5% of the total emissions which is an added advantage The results indicated that ACW could be effectively utilized for microalgae cultivation, reducing the reliance on freshwater for large-scale microalgal biomass production and reduce the carbon footprints of the air conditioning industry.

Influences of changing inorganic nitrogen concentration on accumulation and degradation of organic components in indigenous microalgae cultivated with secondary effluent

Climatic changes due to emission of greenhouse gases are a global concern. These emissions occur by combustion of fossil fuels whose drought is near in which case renewable energy is the only alternative. Microalgae are promising sources of sustainable bioenergy production, and utilisation of wastewater as cultures is recommended for economical production cost. In this study, indigenous microalgae, which had adaptability for wastewater samples, were cultivated with a municipal secondary effluent, and influences of changes in inorganic nitrogen (IN) concentration, specifically IN increase, on temporal accumulation and degradation of organic components in indigenous microalgae were investigated. Indigenous microalgae accumulated total lipids and carbohydrates against reduced IN, and increase in superoxide dismutase suggested that the accumulation was possibly induced by generating reactive oxygen species. Continued cultivation of indigenous microalgae under the IN exhausted condition should be avoided because of the resulting total carbohydrate degradation. IN replenishment when IN was decreased but still existed in the culture and that when IN was exhausted in the culture triggered sharp degradation of the total carbohydrate, which possibly utilised to accumulate crude protein and/or chlorophyll a for continuous growth or regrowth. The total carbohydrate was accumulated and recovered after the degradation; meanwhile, two or three days were required for the recovery of the total carbohydrate. In addition, the IN replenishment also resulted in total lipid degradation. Therefore, to produce indigenous microalgae with high and stable total carbohydrate and lipid content, it was critical to prevent IN increase in the culture.

Revealing the role of phosphorus supply on the phosphorus distribution and lipid production in Scenedesmus obliquus UTEX 393 during nitrogen starvation

Microalgal-based processes offer promise for addressing two sustainability challenges: recovering phosphorus (P) from wastewater and producing biofuel feedstock. This study investigated the role of phosphorus supply on microalgal growth, lipid yield, and P distribution for Scenedesmus during nitrogen starvation. Extracellular polymeric substances and intracellular polymeric substances were the most important pools for inorganic phosphorus (IP) and organic phosphorus (OP), respectively. The main P pool for microalgae with low phosphorus supply was EPS, which accounted for 57 % of the total biomass phosphorus; while under high P concentrations, 79 % of the phosphorus was stored in IPS. A high concentration of orthophosphate stimulated rapid P uptake as IP and promoted the transformation of IP to OP associating with biomass synthesis. The highest P content of microalgal biomass was 6.5 % of dry weight when the phosphorus concentration in medium was 113 mg/L, and the OP content was 4.9 % of dry weight. High phosphate-P enhanced the biomass's lipid content by 60 %, and the distribution of fatty acid methyl esters was not altered by P concentrations. Collectively, high phosphate-P availability could promote microalgal biomass synthesis, lipid production and P accumulation.

Nitrogen-assisted lipid production by biofilms of aerial microalga Coccomyxa subellipsoidea KGU-D001 in the aerial phase

The aerial microalga (Coccomyxa subellipsoidea KGU-D001) was photoautotrophically cultured under aerial phase conditions, and the influence of the nitrogen source on lipid production was investigated. Coccomyxa biofilms were introduced onto cotton wool using pure water (i.e., nutrient depletion) or a nitrogen-containing solution in a Petri dish, and they were cultured for 14 days under aerial phase conditions. The biomass in the biofilm increased by more than 150% in 14 days under nutrient depleted conditions and then increased further by approximately 30% following the addition of a nitrogen source. The lipid content rose under both nutrient depletion and nitrogen-added conditions, increasing by 170 and 150% in 14 days, respectively. The protein and sugar contents were also monitored and analyzed. In the presence of a nitrogen source, C. subellipsoidea undergo cell division in a relatively short time span, and biomass and lipids can be synthesized under both nutrient depleted and nitrogen-added conditions.

Boosting β-carotene and storage materials productivities by two-stage mixed and monochromatic exposure stresses on Dunaliella salina

Growth and product formation of Dunaliella salina, a potent β-carotene source, were investigated under single and two-stage monochromic and mixed illuminations using two LEDs, each emitting red (R), blue (B), or white (W) light. Targeting cell growth in single-stage, WW, RR, and BB, as well as RB illumination, were compared and mixed RB illumination was found most supportive showing the highest cell growth of 1.81 ± 0.008 g/L. Subsequently, new two-stage illuminations (RB-BB and RB-RR) were designed to investigate growth and bio-product formation using RB illumination similarly in the 1st stage followed by separate BB and RR illuminations within the 2nd stage. RB-BB strategy resulted in enhanced productivities of lipid (7.6 mg/L/day), starch (20 mg/L/day), and β-carotene (0.4 mg/L/day) which were respectively higher by 80, 70, and 81% compared to single-stage control (WW). RB-RR strategy stimulated cell growth while it resulted in decreased productivities of products (other than chlorophyll). The highest biomass level of 2.2 g/L and nitrate removal of 80% were obtained in RB-RR while RB-BB resulted in the lowest values of 1.2 g/L and 48%, respectively. Appropriate selection of illuminations in two-stage strategies, therefore, functions to enhance the productivity of important metabolites or cell growth which can have generic applications in other microalgae.NOVELTY STATEMENTAlthough the effects of a variety of stressful conditions on microalgae product lines have been investigated so far, the effects of two-stage mixed and monochromatic exposure as a light management strategy have not yet been considered. This strategy was inspired by the fact that cell mass alongside the cell content of a product contributes to product productivity. Accordingly, the growth of Dunaliella salina was first examined under single-stage mixed and monochromatic exposure where mixed red-blue light led to the highest biomass formation. Shifting from mixed to different monochromatic exposures was then examined as a stress factor to stimulate product formation. Higher cell factories obtained under mixed exposure in the 1st stage escalated product productivities within the 2nd stage when exposed to monochromatic light.

Combined Process of Biogenic Manganese Oxide and Manganese-Oxidizing Microalgae for Improved Diclofenac Removal Performance: Two Different Kinds of Synergistic Effects

Biogenic manganese oxides (Bio-MnOx) have attracted considerable attention for removing pharmaceutical contaminants (PhCs) due to their high oxidation capacity and environmental friendliness. Mn-oxidizing microalgae (MnOMs) generate Bio-MnOx with low energy and organic nutrients input and degrade PhCs. The combined process of MnOMs and Bio-MnOx exhibits good prospects for PhCs removal. However, the synergistic effects of MnOMs and Bio-MnOx in PhCs removal are still unclear. The performance of MnOMs/Bio-MnOx towards diclofenac (DCF) removal was evaluated, and the mechanism was revealed. Our results showed that the Bio-MnOx produced by MnOMs were amorphous nanoparticles, and these MnOMs have a good Mn²⁺ tolerance and oxidation efficiency (80–90%) when the Mn²⁺ concentration is below 1.00 mmol/L. MnOMs/Bio-MnOx significantly promotes DCF (1 mg/L) removal rate between 0.167 ± 0.008 mg/L·d (by MnOMs alone) and 0.125 ± 0.024 mg/L·d (by Bio-MnOx alone) to 0.250 ± 0.016 mg/L·d. The superior performance of MnOMs/Bio-MnOx could be attributed to the continuous Bio-MnOx regeneration and the sharing of DCF degradation intermediates between Bio-MnOx and MnOMs. Additionally, the pathways of DCF degradation by Bio-MnOx and MnOMs were proposed. This work could shed light on the synergistic effects of MnOMs and Bio-MnOx in PhCs removal and guide the development of MnOMs/Bio-MnOx processes for removing DCF or other PhCs from wastewater.

Microalgae as Contributors to Produce Biopolymers

Biopolymers are very favorable materials produced by living organisms, with interesting properties such as biodegradability, renewability, and biocompatibility. Biopolymers have been recently considered to compete with fossil-based polymeric materials, which rase several environmental concerns. Biobased plastics are receiving growing interest for many applications including electronics, medical devices, food packaging, and energy. Biopolymers can be produced from biological sources such as plants, animals, agricultural wastes, and microbes. Studies suggest that microalgae and cyanobacteria are two of the promising sources of polyhydroxyalkanoates (PHAs), cellulose, carbohydrates (particularly starch), and proteins, as the major components of microalgae (and of certain cyanobacteria) for producing bioplastics. This review aims to summarize the potential of microalgal PHAs, polysaccharides, and proteins for bioplastic production. The findings of this review give insight into current knowledge and future direction in microalgal-based bioplastic production considering a circular economy approach. The current review is divided into three main topics, namely (i) the analysis of the main types and properties of bioplastic monomers, blends, and composites; (ii) the cultivation process to optimize the microalgae growth and accumulation of important biobased compounds to produce bioplastics; and (iii) a critical analysis of the future perspectives on the field.

Advances in microalgal cell wall polysaccharides: a review focused on structure, production, and biological application

Microalgae have been shown to be useful in several biotechnological fields due to their feasible cultivation and high-value biomolecules production. Several substances of interest produced by microalgae, such as: proteins, lipids, and natural colorants, have already been explored. Based on the continuing demand for new natural molecules, microalgae could also be a valuable source of polysaccharides. Polysaccharides are extremely important in aquaculture, cosmetics, pharmaceutical, and food industries, and have great economic impact worldwide. Despite this, reviews on microalgal polysaccharide production, biological activity, and chemical structure are not abundant. Moreover, techniques of microalgal cultivation, coupled with carbohydrate production, need to be clarified in order to develop forward-looking technologies. The present review provides an overview of the main advances in microalgal cell wall polysaccharide production, as well as their associated potential biological applications and chemical structure. Several studies on future prospects, related to microalgae are presented, highlighting the key challenges in microalgal polysaccharide production.

Insight into nitrogen and phosphorus coupling effects on mixotrophic Chlorella vulgaris growth under stably controlled nutrient conditions

In water environment, nitrogen (N) and phosphorus (P) are biochemically dependent nutrients following the co-limitation concept for algae growth under mixotrophic mode. From a practical viewpoint, algae growth may not bring about significant change of the background nutrient concentration of an actual waterbody in contrast to a conventional batch system. In order to better understand the growth pattern of microalgae in aquatic environments, a series of experiments were conducted under stably controlled N-P levels for studying the N-P coupling effect on mixotrophic Chlorella vulgaris growth process, with attention paid to the physiological and biochemical characteristics. It was found that within the concentration range of N = 1-8 mg·L^-1 and P = 0.1-1.0 mg·L^-1, the variation of the N-P level slightly affected the specific growth rate, but significantly influenced nutrients uptake, biomass dry weight, chlorophyll contents of the grown C. vulgaris. The biochemical and elemental composition of the microalgae tended to be more sensitive to the N-P concentrations and ratios in the lower nutrient range (1-2 mg N·L^-1, 0.1-0.4 mg P·L^-1) in which the highest N and P conversion rates were gained as 90.18 ± 1.23% and 60.47 ± 1.59%, respectively. The P assimilation and conversion efficiencies were much affected by both N and P supplies, while the P supply showed little influence on N assimilation and conversion efficiencies. It was also noticed that the N level greatly affected the metabolic pathway involving nutrient assimilation, carbohydrate fixation and monosaccharide profile, resulting in conversion of the dominant fraction of protein at N ≤ 2 mg·L^-1 into other biochemical compositions including lipids at N ≥ 3 mg·L^-1. The fatty acid methyl esters (FAMEs) composition tended to differ with varied nutrient levels. These findings may deepen our understanding of algal growth in aquatic environment and provide perspective for eutrophication control.

Effects of different culture media on physiological features and laboratory scale production cost of Dunaliella salina

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Comparison among well-known marine microalgae culture media was performed.

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Conway medium increased cellular growth of Dunaliella salina.

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Johnson medium improved accumulation of photosynthetic pigments.

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Microalgal biomass showed a cost variation of US$4.64–301.61 per kg in lab scale.

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Monitoring of pigment profile proved to be essential in microalgal parameters.

Microalgal cultivation system depends on optimal parameters obtained from laboratory conditions to overcome high-value compounds production and decrease cost. In this study, the laboratory-scale cultivation of Dunaliella salina was performed to explore the cost and biochemical effects on biomass cultivated using F/2, Conway, and Johnson media. D. salina cultivation was monitored by cell counting, dry biomass measurements and biochemical analyses. Photosynthetic pigment profiles were identified and quantified through high-performance liquid chromatography (HPLC) whereas economic evaluation was based on media cost. Cellular growth parameters were significantly improved by Conway medium, whereas maximum accumulation of pigments, especially violaxanthin (0.855 mg g⁻¹) and chlorophyll a (14.255 mg g⁻¹) were observed when Johnson medium was used. Conway-biomass showed 43 % of total lipid content. This value represents 1.5 and 1.4 times higher than lipid content found in Johnson- and F/2-biomass, respectively. Furthermore, media cost presented a variation of US$1.17–49.62 for each 10³ L of media mainly due to NaNO₃, KNO_3, and cyanocobalamin supplementation. Thus, biomass production showed a cost variation of US$4.64–301.61 per kg on a dry weight basis. These outcomes emphasize the suitability of laboratory parameters and cost comparison among biomass produced by different media.

Using polyethylene glycol to promote Nannochloropsis oceanica growth with 15 vol% CO2

CO₂ capture with microalgae has been put forward in response to global concern on greenhouse gas emission. However, the short residence time and slow diffusion of CO₂ in water limits the growth of microalgae. In order to improve CO₂ transfer from gas phase to liquid phase and utilization by algal cells, polyethylene glycol 200 (PEG 200) was used as CO₂ absorbent to promote growth of Nannochloropsis oceanica with the bubbling of 15 vol% CO₂. Total inorganic carbon (TIC) absorbed in culture medium remained constant at 5.6 mM when 15 vol% CO₂ was bubbled continuously. PEG 200 in the medium provided additional CO₂ absorption from 0.6 to 4.8 mM when PEG 200 concentration increased from 0.5 to 4 mM. The specific growth rate of N. oceanica reached the maximum (1.41 d^-1) with 1 mM PEG 200 in culture medium, which was 21.5% higher than the specific growth rate without PEG 200. About 79% of the increase in biomass was attributed to the increased TIC with more CO₂ dissolution in culture medium because of PEG 200, and about 21% was attributed to PEG 200 itself utilized as an organic carbon source. In conclusion, PEG 200 as a CO₂ absorbent can effectively capture flue-gas CO₂ for algal growth.

Microalgae proteins: production, separation, isolation, quantification, and application in food and feed

Many countries have been experienced an increase in protein consumption due to the population growth and adoption of protein-rich dietaries. Unfortunately, conventional-based protein agroindustry is associated with environmental impacts that might aggravate as the humankind increase. Thus, it is important to screen for novel protein sources that are environmentally friendly. Microalgae farming is a promising alternative to couple the anthropic emissions with the production of food and feed. Some microalgae show protein contents two times higher than conventional protein sources. The use of whole microalgae biomass as a protein source in food and feed is simple and well-established. Conversely, the production of microalgae protein supplements and isolates requires the development of feasible and robust processes able to fractionate the microalgae biomass in different value-added products. Since most of the proteins are inside the microalgae cells, several techniques of disruption have been proposed to increase the efficiency to extract them. After the disruption of the microalgae cells, the proteins can be extracted, concentrated, isolated or purified allowing the development of different products. This critical review addresses the current state of the production of microalgae proteins for multifarious applications, and possibilities to concatenate the production of proteins and advanced biofuels.

Resource Recovery from Waste Coffee Grounds Using Ultrasonic-Assisted Technology for Bioenergy Production

Biodiesel is a proven alternative fuel that can serve as a substitute for petroleum diesel due to its renewability, non-toxicity, sulphur-free nature and superior lubricity. Waste-based non-edible oils are studied as potential biodiesel feedstocks owing to the focus on the valorisation of waste products. Instead of being treated as municipal waste, waste coffee grounds (WCG) can be utilised for oil extraction, thereby recovering an energy source in the form of biodiesel. This study evaluates oil extraction from WCG using ultrasonic and Soxhlet techniques, followed by biodiesel conversion using an ultrasonic-assisted transesterification process. It was found that n-hexane was the most effective solvent for the oil extraction process and ultrasonic-assisted technology offers a 13.5% higher yield compared to the conventional Soxhlet extraction process. Solid-to-solvent ratio and extraction time of the oil extraction process from the dried waste coffee grounds (DWCG) after the brewing process was optimised using the response surface methodology (RSM). The results showed that predicted yield of 17.75 wt. % of coffee oil can be obtained using 1:30 w/v of the mass ratio of DWCG-ton-hexane and 34 min of extraction time when 32% amplitude was used. The model was verified by the experiment where 17.23 wt. % yield of coffee oil was achieved when the extraction process was carried out under optimal conditions. The infrared absorption spectrum analysis of WCG oil determined suitable functional groups for biodiesel conversion which was further treated using an ultrasonic-assisted transesterification process to successfully convert to biodiesel.

Carbamazepine toxicity and its co-metabolic removal by the cyanobacteria Spirulina platensis

Bioremediation of pharmaceutical-contaminated wastewater using microalgae has attracted increasing attention. Cyanobacteria, which are important prokaryotic microalgae, are widely distributed in different water environments, and have the advantages of simple culture and a fast growth rate. However, studies on either the toxicity of pharmaceutical contaminants (PhCs) to cyanobacteria or the removal of PhCs by cyanobacteria are scarce. In this study, carbamazepine (CBZ) and Spirulina platensis were selected as model PhCs and cyanobacteria, respectively. CBZ (>1 mg/L) had toxicity effects on S. platensis, showing maximal growth inhibition (34.0%) at 100 mg/L after 10 days of cultivation. At CBZ < 25 mg/L, S. platensis showed a trend similar to that of eukaryotic microalgae in increasing superoxide dismutase and catalase activities and content of chlorophylls, carotenoids, carbohydrates, and lipids. These results indicated that S. platensis had a similar protective mechanism to CBZ toxicity as that of the eukaryotic microalgae. Increasing CBZ concentration (50-100 mg/L) significantly decreased these biochemical characteristics and photosynthetic activity owing to the serious damage of the structure and function of S. platensis. However, with increasing cultivation time, the growth and photosynthetic activity of S. platensis recovered from the toxicity of CBZ. S. platensis showed a maximum of 30.97 ± 1.30% removal of CBZ (1 mg/L), mainly through biodegradation. Addition of 0.3 mg/L glucose enhanced this removal efficiency to 50.13 ± 2.51% via co-metabolism. These findings indicated that S. platensis can be used for the removal of CBZ or other PhCs from wastewater.

Exploration of two-stage cultivation strategies using nitrogen starvation to maximize the lipid productivity in Chlorella sp. HS2

In this work, the two-stage cultivation of Chlorella sp. HS2 for enhancing the lipid productivity was optimized by adjusting the duration of nitrogen-replete (N+) and -deplete (N-) stages within a 9 day period using urea as nitrogen source. The highest lipid content of 36.7% and productivity of 216.9 mg L^-1 d^-1 were obtained under five days of N+ followed by four days of N- conditions. Replenishing phosphorus and other nutrients (N-P+O+) at the beginning of the nutrient-starvation resulted in 1.55 and 1.68-folds improvement in lipid productivities compared to the single stage and zero nutrient controls (N-P-O-), respectively. The estimated biodiesel properties based on the fatty acid profiles met all criteria of international standards. The findings of this study indicate that properly adjusting the period of nitrogen availability as well as the presence of other nutrients is highly important in order to maximize the biofuel productivity in two-stage microalgal cultivation.

Effect of ammonium nitrogen on microalgal growth, biochemical composition and photosynthetic performance in mixotrophic cultivation

To enhance microalgal growth and optimize ammonium utilization, the effect of ammonium on microalgal growth, biochemical composition and photosynthetic performance were investigated by mixotrophic cultivation of microalga Spirulina platensis comparing with autotrophic cultivation. The results indicated that elevated ammonium significantly affected the microalgal growth, but the microalga in mixotrophic cultivation showed better growth and stronger tolerance to higher ammonium. The microalgal proteins were increased by increasing nitrogen concentration. The synthesis of microalgal carbohydrates was inhibited by higher ammonium, especially in mixotrophic cultivation. The addition of ammonium decreased the microalgal lipids in autotrophic cultivation but increased microalgal lipids in mixotrophic cultivation. Ammonium negatively affected the microalgal photosynthetic performance. The inhibition was intensified by elevated ammonium, inducing stronger photosystem protection mechanism, particularly in mixotrophic cultivation. The rate of ammonium inhibition to the microalgal photosystem was quick in the early stage by decreasing electron transport rate of PS II.

Excessive phosphorus caused inhibition and cell damage during heterotrophic growth of Chlorella regularis

A high phosphorus concentration is widely accepted as favorable for enhancing both microalgae growth and lipid accumulation; however, excessively high P could be counter-productive. In this study, we investigated the effects of increasing P levels (5.4, 25, 45, 150, and 250 mg-P L^-1) on the heterotrophic cultivation of Chlorella regularis. Microalgae growth was inhibited and cells were severely damaged in response to highly excessive P levels (≥150 mg-P L^-1). In particular, 250 mg-P L^-1 resulted in a ∼40% decrease in cell density and a ∼70% loss of cell viability. Microalgae damage induced by excessive phosphorus included enlarged cell size, deformation of cell walls, and disorganization of organelles. These negative effects were associated with the over-accumulation of polyphosphates within cells, which may further cause binding of P to intracellular components. Although P is an essential nutrient, excessive P lowers cell growth and viability.

Effect of nitrogen limitation on biochemical composition and photosynthetic performance for fed-batch mixotrophic cultivation of microalga Spirulina platensis

In this study, the effect of nitrogen limitation on microalgal growth, biochemical composition and photosynthetic performance was investigated in fed-batch mixotrophic cultivation of microalga Spirulina platensis, compared with that in autotrophic cultivation. The microalgal biomass productivity was greatly enhanced by mixotrophic cultivation. With nitrogen limitation, the mixotrophic culture accelerated the degradation of microalgal pigments and proteins to supply intracellular nitrogen for maintaining higher biomass productivity, simultaneously accumulating more carbohydrates. The mixotrophic cultivation amplified the adverse effect of nitrogen limitation on the microalgal photosynthetic performance in comparison with autotrophic cultivation. This fed-batch mixotrophic cultivation is an effective strategy for enhancing biomass productivity and total carbohydrates yield under nitrogen limited conditions.

Bicarbonate supplementation enhances growth and biochemical composition of Dunaliella salina V-101 by reducing oxidative stress induced during macronutrient deficit conditions

The unicellular marine alga Dunaliella salina is a most interesting green cell factory for the production of carotenes and lipids under extreme environment conditions. However, the culture conditions and their productivity are the major challenges faced by researchers which still need to be addressed. In this study, we investigated the effect of bicarbonate amendment on biomass, photosynthetic activity, biochemical constituents, nutrient uptake and antioxidant response of D. salina during macronutrient deficit conditions (N⁻, P⁻ and S⁻). Under nutrient deficit conditions, addition of sodium bicarbonate (100 mM) significantly increased the biomass, carotenoids including β-carotene and lutein, lipid, and fatty acid content with concurrent enhancement of the activities of nutrient assimilatory and carbonic anhydrase enzymes. Maximum accumulation of carotenoid especially β-carotene (192.8 ± 2.11 µg/100 mg) and lipids (53.9%) was observed on addition of bicarbonate during nitrate deficiency compared to phosphate and sulphate deficiency. Supplementation of bicarbonate reduced the oxidative stress caused by ROS, lowered lipid peroxidation damage and improved the activities of antioxidant enzymes (SOD, CAT and APX) in D. salina cultures under nutrient stress.

Effective role of medium supplementation in microalgal lipid accumulation

The present study investigated the interaction between starch and lipid accumulation in a green microalgae enrichment culture. The objective was to optimize the lipid content by manipulation of the medium in regular batch culture. Two medium designs were evaluated: First a high ortho-P concentration with vitamin supplementary (Pi-vitamins supplemented medium), second normal growth medium (control). Both media contained a low amount of nitrogen which was consumed during batch growth in three days. The batch experiments continued for another 4 days with the absence of soluble nitrogen in the medium. When the mixed microalgal culture was incubated in the Pi-vitamin supplemented medium, the lipid, and starch content of the culture increased within the first 3 days to 102.0 ± 5.2 mg/L (12.7 ± 0.6% of DW) and 31.7 ± 1.6 mg/L (4.0 ± 0.2% of DW), respectively. On the last day of the experiment, the lipid, and starch content in Pi-vitamin medium increased to 663.1 ± 32.5 mg/L (33.4 ± 1.6% of DW) and 127.5 ± 5.2 mg/L (6.4 ± 0.3% of DW). However, the lipid and starch content in the control process, reached to 334.7 ± 16.4 mg/L (20.1 ± 1.0% of DW) and 94.3 ± 4.6 mg/L (5.7 ± 0.3% of DW), respectively. The high Pi-vitamin medium induced storing lipid formation clearly while the starch formation was not affected. The lipid contents reported here are among the high reported in the literature, note that already under full growth conditions significant lipid levels occurred in the algal enrichment culture. The high lipid productivity of the reported mixed microalgae culture provides an efficient route for efficient algal biodiesel production.