Effect of light intensity on physiological changes, carbon allocation and neutral lipid accumulation in oleaginous microalgae

Determination of photoautotrophic growth and inhibition kinetics by the Monod and the Aiba models and bioenergetics of local microalgae strain

The usage of fossil fuels results in a high amount of greenhouse gas (GHG) emissions and renewable green energy requirements entail saving ecological balance. Therefore, microalgae cultivation is widespread as a suitable raw material to produce renewable and sustainable fuel. Mathematical models are useful tools for the estimation of different conditions of a system. In this study, mathematical models were developed for monitoring the cultivation of local species of microalgae based on the chlorophyll-a and biomass concentration. Coefficients that were calculated from the Monod kinetic model were μmax; 0.03 day^-1, K_S, C_i; 0.53 mM with an R² value of 0.93 and from the Aiba inhibition kinetic model was μmax and K_S, C_i 1.48 day^-1 and 0.08 mM with an R² value of 0,73. According to the literature, there was no model was developed for the determination of kinetic coefficients based on chlorophyll-a production due to the inorganic carbon consumption. While both growth and inhibition models were developed for the inorganic carbon consumption, chlorophyll-a concentration was used for the growth model and biomass concentration was used for the inhibition model which caused and directly affected by the decrease of light penetration. The maximum biomass and chlorophyll-a concentrations were found as 1.2 g/L and 27.8 mg/L respectively with 10.24 mg/L. day^-1 nitrogen and 1.19 mg/L.day^-1 phosphorus uptake rate.

Improved Productivity of Astaxanthin from Photosensitive Haematococcus pluvialis Using Phototaxis Technology

Haematococcus pluvialis is a microalgae actively studied for the production of natural astaxanthin, which is a powerful antioxidant for human application. However, it is economically disadvantageous for commercialization owing to the low productivity of astaxanthin. This study reports an effective screening strategy using the negative phototaxis of the H. pluvialis to attain the mutants having high astaxanthin production. A polydimethylsiloxane (PDMS)-based microfluidic device irradiated with a specific light was developed to efficiently figure out the phototactic response of H. pluvialis. The partial photosynthesis deficient (PP) mutant (negative control) showed a 0.78-fold decreased cellular response to blue light compared to the wild type, demonstrating the positive relationship between the photosynthetic efficiency and the phototaxis. Based on this relationship, the Haematococcus mutants showing photosensitivity to blue light were selected from the 10,000 random mutant libraries. The M1 strain attained from the phototaxis-based screening showed 1.17-fold improved growth rate and 1.26-fold increases in astaxanthin production (55.12 ± 4.12 mg g⁻¹) in the 100 L photo-bioreactor compared to the wild type. This study provides an effective selection tool for industrial application of the H. pluvialis with improved astaxanthin productivity.

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.

The Effects of Photobioreactor Type on Biomass and Lipid Production of the Green Microalga Monoraphidium pusillum in Laboratory Scale

Mass production of microorganisms, algae among them, for new bioactive compounds and renewable innovative products is a current issue in biotechnology. The greatest challenge of basic research on this topic is to find the best solution for both physiology and scalability. In this study, the main goal was to highlight the contradictions of physiological and technological optimization in the same, relatively small, laboratory scale. The green alga Monoraphidium pusillum (Printz) Komárková-Legnorová was cultured in a conventional Erlenmeyer flask (as air bubbled in a tank-type photobioreactor) and in a hybrid (fermenter type + helical tubular type) photobioreactor of the same volume (2.8 L). Higher cell numbers from 1.7–2.3-fold, 2–2.8-fold higher dry masses, and 1.9–2.6-fold higher total lipid contents (mg·L−1) were measured in the tank reactor than in the hybrid reactor. Cultures in the conventional tank reactor were characterized with better nutrient utilization (42.8–77.7% higher phosphate uptake) and more diverse lipid composition than in the hybrid reactor. The study highlights that well-scalable arrangements and settings could be not optimal (or unsuitable in some cases) from a physiological point of view. The results suggest certain developmental directions for complex, well-scalable devices and highlight the importance of testing the gained physiological optima on these systems.

Microalgae-Derived Health Supplements to Therapeutic Shifts: Redox-Based Study Opportunities with AIE-Based Technologies

Reactive oxygen species (ROS) are highly reactive molecules, serve the normal signaling in different cell types. Targeting ROS as the chemical signals, different stress based strategies have been developed to synthesis different anti-inflammatory molecules in microalgae. These molecules could be utilized as health supplements in human. To provoke the ROS-mediated defence systems, their connotation with the associated conditions must be well understood, therefore, proper tools for studying ROS in natural state are essential. The in vivo detection of ROS with phosphorescent probes offers promising opportunities to study these molecules in a non-invasive manner. Most of the common problems in the traditional fluorescent probes are lower photostability, excitation intensity, slow responsiveness, and the microenvironment that challenge their performance. Some ROS-specific aggregationinduced emission luminogens (AIEgens) with pronounced spatial and temporal resolution have recently demonstrated high selectivity, rapid responsiveness, and efficacies to resolve the aggregation-caused quenching issues. The nanocomposites of some AIE-photosensitizers can also improve the ROS-mediated photodynamic therapy. These AIEgens could be used to induce bioactive components in microalgae through altering the ROS signaling, therefore are more auspicious for biomedical research. This study reviews the prospects of AIEgen-based technologies to understand the ROS mediated bio-physiological processes in microalgae for better healthcare benefits.

A Review on Synchronous Microalgal Lipid Enhancement and Wastewater Treatment

Microalgae are unicellular photosynthetic eukaryotes that can treat wastewater and provide us with biofuel. Microalgae cultivation utilizing wastewater is a promising approach for synchronous wastewater treatment and biofuel production. However, previous studies suggest that high microalgae biomass production reduces lipid production and vice versa. For cost-effective biofuel production from microalgae, synchronous lipid and biomass enhancement utilizing wastewater is necessary. Therefore, this study brings forth a comprehensive review of synchronous microalgal lipid and biomass enhancement strategies for biofuel production and wastewater treatment. The review emphasizes the appropriate synergy of the microalgae species, culture media, and synchronous lipid and biomass enhancement conditions as a sustainable, efficient solution.

Influence of Light Conditions on Microalgae Growth and Content of Lipids, Carotenoids, and Fatty Acid Composition

Microalgae are a valuable natural resource for a variety of value-added products. The growth of microalgae is determined by the impact of many factors, but, from the point of view of the implementation of autotrophic growth, light is of primary importance. This work presents an overview of the influence of light conditions on the growth of microalgae, the content of lipids, carotenoids, and the composition of fatty acids in their biomass, taking into account parameters such as the intensity, duration of lighting, and use of rays of different spectral composition. The optimal light intensity for the growth of microalgae lies in the following range: 26-400 µmol photons m-2 s-1. An increase in light intensity leads to an activation of lipid synthesis. For maximum lipid productivity, various microalgae species and strains need lighting of different intensities: from 60 to 700 µmol photons m-2 s-1. Strong light preferentially increases the triacylglyceride content. The intensity of lighting has a regulating effect on the synthesis of fatty acids, carotenoids, including β-carotene, lutein and astaxanthin. In intense lighting conditions, saturated fatty acids usually accumulate, as well as monounsaturated ones, and the number of polyunsaturated fatty acids decreases. Red as well as blue LED lighting improves the biomass productivity of microalgae of various taxonomic groups. Changing the duration of the photoperiod, the use of pulsed light can stimulate microalgae growth, the production of lipids, and carotenoids. The simultaneous use of light and other stresses contributes to a stronger effect on the productivity of algae.

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.

Effect of residence time in continuous photobioreactor on mass and energy balance of microalgal protein production

There is increasing interest in new protein sources for the food and feed industry and for the agricultural sector, and microalgae are considered a good alternative, having a high protein content and a well-balanced amino acid profile. However, protein production from microalgae presents several unsolved issues, as the biomass composition changes markedly as a function of cultivation operating conditions. Continuous systems, however, may be properly set to boost the accumulation of protein in the biomass, ensuring stable production. Here, two microalgae and two cyanobacterial species were cultivated in continuous operating photobioreactors (PBR) under nonlimiting nutrient conditions, to study the effects of light intensity and residence time on both biomass and protein productivity at steady state. Although light strongly affected biomass growth inside the PBR, the overall protein pool did not vary in response to irradiance. On the other hand, shorter residence times resulted in protein accumulation of up to 68 % in cyanobacteria, in contrast with green algae, where a minor influence of residence time on biomass composition was observed. Energy balance showed that light conversion to protein decreased with light intensity. Protein content was also related to energy costs for cell maintenance. In conclusion, it is shown that residence time is the key variable to increase protein content and yield of protein production, but its effect depends on the specific species.

Trophic Transition Enhanced Biomass and Lipid Production of the Unicellular Green Alga Scenedesmus acuminatus

Microalgal heterotrophic cultivation is an emerging technology that can enable producing high cell-density algal cell cultures, which can be coupled with photoautotrophic cultivation for valuable chemicals such as lipids manufacturing. However, how the heterotrophically grown algal cells respond to the lipid-inducing conditions has not been fully elucidated so far. In this study, when the heterotrophically grown Scenedesmus acuminatus cells were subjected to the high light (HL) and nitrogen-limited (NL) conditions, both the biomass and lipid productivity were enhanced as compared to that of the photoautotrophically grown counterparts. The chlorophyll a fluorometry analysis showed that the Fv/Fm and Y(II) of the heterotrophically grown cells subjected to the HL and NL conditions was recovered to the maximum value of 0.75 and 0.43, respectively, much higher than those of the photoautotrophically grown cells under the same stress conditions. Transcriptomic analysis revealed that heterotrophically grown cells fully expressed the genes coding for the photosystems proteins, including the key photoprotective proteins D1, PsbS, light-harvesting-complex (LHC) I and LHC II. Meanwhile, downregulation of the carotenoid biosynthesis and upregulation of the glycolysis/gluconeogenesis, tricarboxylic acid (TCA) cycle and oxidative phosphorylation pathways were observed when the heterotrophically grown cells were subjected to the HL and N-limited conditions for lipid production. It was deduced that regulation of these pathways not only enhanced the light utilization but also provided the reducing power and ATP by which the biomass accumulation was significantly elevated. Besides, upregulation of the acetyl-CoA carboxylase/biotin carboxylase, digalactosyl diacylglycerol synthase and diacylglycerol acyltransferase 2 encoding genes may be attributable to the enhanced lipid production. Understanding the cellular responses during the trophic transition process could guide improvement of the strength of trophic transition enhancing microalgal biomass and lipid production.

Effect of Light Intensity and Wavelength on Biomass Growth and Protein and Amino Acid Composition of Dunaliella salina

Dunaliella salina is a halotolerant, photoautotrophic marine microalga and one of the richest sources of natural carotenoids but also shows potential as a novel food source with high protein quality. This study sought to optimise the production of biomass, protein and amino acids from D. salina, alongside carotenoids using a two-stage cultivation approach based on the use of light of different intensities and quality, i.e., white, red and blue LED light. In stage 1, four white LED light intensities were tested. In stage 2, the same four light intensities from either blue or red LEDs were applied once exponential growth ceased and cells reached the stationary phase under white LED light in stage 1. Remarkably, both biomass concentration and biomass productivity showed a 1.3–1.7-fold increase in stage 2, without medium replenishment, while protein concentration and protein productivity showed an ~1.1-fold increase. The amino acid content and amino acid index remained unchanged from stage 1 to stage 2, and minimum difference was found across different light intensities. Overall, D. salina delivered so-called high protein quality, with an essential amino acid index (EAAI) of 0.99, and red light, which has previously been shown to increase carotenoid production, boosted further biomass production over and above white light, at all light intensities tested.

Stepwise-incremental physicochemical factors induced acclimation and tolerance in oleaginous microalgae to crucial outdoor stresses and improved properties as biodiesel feedstocks

Stress-tolerant oleaginous microalgae are promising for economical outdoor cultivation and biofuel production. This study aimed to induce acclimation and adaptive evolution of oleaginous Scenedesmus sp. SPP to tolerate crucial outdoor stresses by stepwise increasing of physicochemical factors: salinity, light intensity and temperature. The acclimatized strains showed better growth and accumulated 20-30% higher contents of lipids and chlorophylls. The adaptive-evolved strain showed greater tolerance to culture stresses by giving > 2-fold higher biomass under nitrogen rich and accumulating > 1.5-fold higher lipid content under nitrogen starvation compared to the parental strain. Moreover, stepwise increasing of multi-stresses successfully induced the multi-tolerance of the adaptive-evolved strain and gave the highest lipid content of 44.1 ± 1.5%. The extracted lipids from acclimatized/evolved strains show improved prospect fuel properties in terms of high cetane number and oxidative stability. These results show the effectiveness of stepwise-incremental physicochemical factors to intensify potential of microalgae for outdoor cultivation and as biodiesel feedstocks.

Enhancement of euryhaline Asterarcys quadricellulare biomass production for improving biogas generation through anaerobic co-digestion with carbon rich substrate

The microalga was isolated from Muktsar, the southwestern zone of Indian Punjab and identified as Asterarcys quadricellulare BGLR5 (MF661929) by 18S rRNA sequence analysis. The optimization of various cultural factors by the Plackett-Burman and central composite (CCD) designs helped in discerning the significant cultural factors for the increased production of biomass and other functional components (chlorophyll, carbohydrate, lipid and protein). The optimal cultural conditions as per the model were pH 9.9, 81 μmol m^-2 s^-1 light intensity, 22 °C temperature, growth period of 25 days, NaNO₃ 12 mM, 15 mM NH₄Cl, and 7 mM K₂HPO₄. In comparison to the basal condition biomass (0.886 g L^-1), a 0.42-fold increase in biomass yield was attained. Further, the highest yield of biogas (P: 361.81 mL g^-1 VS) with enhanced biogas production rate (R _m: 8.19 mL g^-1 day^-1) was achieved in co-digesting paddy straw with Asterarcys quadricellulare biomass in 1:1 ratio compared to their digestion individually. Further, the co-digestion resulted in the positive synergistic effect which increased the observed biogas yield compared to the estimated yield by 11-58% depending upon the amount of algal biomass and paddy straw used. Hence, the present study signifies that the biomass of Asterarcys quadricellulare BGLR5 can be utilized as a co-substrate with paddy straw to enhance the biogas yield.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02792-x.

Single-phase and two-phase cultivations using different light regimes to improve production of valuable substances in the anoxygenic photosynthetic bacterium Rhodopseudomonas faecalis PA2

This study aimed to improve biomass, carotenoid, bacteriochlorophyll, protein, lipid, and carbohydrate contents of Rhodopseudomonas faecalis PA2 using different light regimes. Light intensity (4000, 6000, 8000, and 10,000 lx), together with photoperiod (24:0, 16:8, 12:12, and 8:16 h light/dark), was assigned as single-phase (SP) cultivation while two-phase (TP) cultivation used two light intensities (using 4000 lx as the first phase), together with the control of phase shift (3, 6, and 9 days) and photoperiod. Biomass, carotenoid, and bacteriochlorophyll contents were maximized by SP cultivation; light at 8000 lx with light-dark cycle of 24:0 was optimal for pigments synthesis. In contrast, TP was useful to enhance storage compounds; protein, lipid, and carbohydrate productivities were significantly increased by 121.69%, 101.69%, and 92.44%, respectively, in TP when compared with SP. This indicates that the novel light strategy proposed in this study was able to manipulate the production of valuable substances in this strain.

Comparative responses of cell growth and related extracellular polymeric substances in Tetraselmis sp. to nonylphenol, bisphenol A and 17α-ethinylestradiol

Estuarine ecosystems near mega-cities are sinks of anthropogenic endocrine disrupting chemicals (EDCs). As the most important primary producer, indigenous microalgae and their secreted extracellular polymeric substances (EPSs) might interact with EDCs and contribute to their fate and risk. Tetraselmis sp. is a representative model of estuarine microalga, for which EDC toxicity and its effects on EPS synthesis have rarely been studied. Through microalgal isolation, algal cell growth tests, EDC removal and the characterization of related EPS profiles, the present work intends to clarify the comparative responses of Tetraselmis sp. to nonylphenol (NP), bisphenol A (BPA) and 17α-ethinylestradiol (EE₂). The results showed that the half inhibitory concentration on cell growth was 0.190-0.313 mg/dm³ for NP, which was one order of magnitude lower than the comparable values for BPA and EE₂ at 2.072-3.254 mg/dm³. Regarding chlorophyll, NP induced its degradation, EE₂ led to its decreased production, and BPA had no obvious effect. Under EDC stress, only the concentrations of colloidal polysaccharides and proteins responded dose-dependently to EE₂. Except for the colloidal fraction in the EE₂ treatment group, the increase in neutral monosaccharides, especially glucose and galactose, was a common response to EDCs. Compared to the recalcitrant BPA, NP underwent abiotic degradation in alga-free water, and EE₂ could be biodegraded in water containing this microalga. The chemical-specific responses of cell growth, chlorophyll and related EPS profiles were driven by the different fates of EDCs, and the underlying mechanism was further discussed. The results obtained in the present work are of critical importance for understanding the fate and effects of different EDCs mediated by microalgae and their related EPSs.

Microalgae culture quality indicators: a review

Microalgae are photosynthetic microorganisms that have generated increasing interest in recent years due to their potential applications. Their biological capacity to grow faster than higher plants and their ability to convert solar energy into biomass and other bioactive molecules, has led to the development of various culture systems in order to produce different high-value products with commercial interest. The industrialization of the microalgae cultivation process requires the introduction of standardized quality parameters. In order to obtain bioactive compounds with high added value at a commercial level, it is necessary to sustainably produce biomass at a large scale. Such a process would imply specific stress conditions, such as variation in temperature, light or pH. These environmental conditions would make it more difficult to maintain the viability of the culture and protect the yield and condition of the target molecules. The physiological and biochemical impact of these stress factors on the microalgae biomass can be potentially measured by the presence and activity of various biochemical indicators called biomarkers. This review presents an overview of the main techniques that exist for assessing the "quality" of microalgae cultures through quantification of cell viability and vitality by monitoring specific markers indicative of the status of the culture.

Perspective Design of Algae Photobioreactor for Greenhouses—A Comparative Study

The continued growth and evolving lifestyles of the human population require the urgent development of sustainable production in all its aspects. Microalgae have the potential of the sustainable production of various commodities; however, the energetic requirements of algae cultivation still largely contribute to the overall negative balance of many operation plants. Here, we evaluate energetic efficiency of biomass and lipids production by Chlorella pyrenoidosa in multi-tubular, helical-tubular, and flat-panel airlift pilot scale photobioreactors, placed in an indoor environment of greenhouse laboratory in Central Europe. Our results show that the main energy consumption was related to the maintenance of constant light intensity in the flat-panel photobioreactor and the culture circulation in the helical-tubular photobioreactor. The specific power input ranged between 0.79 W L−1 in the multi-tubular photobioreactor and 6.8 W L−1 in the flat-panel photobioreactor. The construction of multi-tubular photobioreactor allowed for the lowest energy requirements but also predetermined the highest temperature sensitivity and led to a significant reduction of Chlorella productivity in extraordinary warm summers 2018 and 2019. To meet the requirements of sustainable yearlong microalgal production in the context of global change, further development towards hybrid microalgal cultivation systems, combining the advantages of open and closed systems, can be expected.

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.

A brief period of darkness induces changes in fatty acid biosynthesis towards accumulation of saturated fatty acids in Chlorella vulgaris UMT-M1 at stationary growth phase

OBJECTIVE

The effects of a brief (3 days) and prolonged (6 days) period of incubation in darkness and light on the biomass content, lipid content and fatty acid profile in Chlorella vulgaris UMT-M1 were determined.

RESULTS

Three days of incubation in darkness increased saturated fatty acid (SFA) content from 34.0 to 41.4% but decreased monounsaturated fatty acid (MUFA) content from 36.7 to 29.8%. Palmitic acid (C16:0) content was increased from 23.2 to 28.9%, whereas oleic acid (C18:1) content was reduced from 35.4 to 28.8%. Total oil content was slightly decreased from 20.4 to 18.7% after 3 days of darkness, without a significant reduction in biomass compared to 3 days of incubation in light. Biomass and oil content was highest in cultures incubated for 6 days in light, however the stimulatory and inhibitory effects of darkness (or light) on SFA and MUFA content was no longer present at 6 days of incubation.

CONCLUSIONS

Findings from this study suggests that fatty acid composition in C. vulgaris could be modulated to favor either C16:0 or C18:1 by a brief period of either darkness or light incubation, prior to harvesting.

Requirement of the exopolyphosphatase gene for cellular acclimation to phosphorus starvation in a cyanobacterium, Synechocystis sp. PCC 6803

Polyphosphate, which is ubiquitous in cells in nature, is involved in a myriad of cellular functions, and has been recently focused on its metabolism related with microbial acclimation to phosphorus-source fluctuation. In view of the ecological importance of cyanobacteria as the primary producers, this study investigated the responsibility of polyphosphate metabolism for cellular acclimation to phosphorus starvation in a cyanobacterium, Synechocystis sp. PCC 6803, with the use of a disruptant (Δppx) as to the gene of exopolyphosphatase that is responsible for polyphosphate degradation. Δppx was similar to the wild type in the cellular content of polyphosphate to show no defect in cell growth under phosphorus-replete conditions. However, under phosphorus-starved conditions, Δppx cells were defective in a phosphorus-starvation dependent decrease of polyphosphate to show deleterious phenotypes as to their survival and the stabilization of the photosystem complexes. These results demonstrated some crucial role of exopolyphosphatase to degrade polyP in the acclimation of cyanobacterial cells to phosphorus-starved conditions. Besides, it was found that ppx expression is induced in Synechocystis cells in response to phosphorus starvation through the action of the two-component system, SphS and SphR, in the phosphate regulon. The information will be a foundation for a fuller understanding of the process of cyanobacterial acclimation to phosphorus fluctuation.

Evaluation of Chlorella vulgaris and Scenedesmus obliquus growth on pretreated organic solid waste digestate

In this research Scenedesmus obliquus and Chlorella vulgaris growth was tested on digestate sludge obtained from the anaerobic co-digestion treatment of the organic fraction of municipal solid waste (OFMSW) together with waste activated sludge (WAS). Digestate was diluted 1:10 and tested in three batch experimental conditions: with no pre-treatments (noPT), after centrifugation (AC) and after filtration (AUF), in order to evaluate microalgae limiting growth factors. The best growth was obtained by C. vulgaris on digestate AC compared to S. obliquus, reaching 479 ± 31 cell million ml^-1 and 131 ± 12 cell million ml^-1 respectively. Ammonia removal evaluated in C. vulgaris and S. obliquus cultures was 99.2% ± 0.3 and 98.146% ± 0.008 in AC condition, respectively. Considering that AUF showed similar microalgae growth values, the digestate pretreatment for microalgae growth, could be limited to centrifugation.

Temperature-Dependent Lipid Accumulation in the Polar Marine Microalga Chlamydomonas malina RCC2488

The exploration of cold-adapted microalgae offers a wide range of biotechnological applications that can be used for human, animal, and environmental benefits in colder climates. Previously, when the polar marine microalga Chlamydomonas malina RCC2488 was cultivated under both nitrogen replete and depleted conditions at 8°C, it accumulated lipids and carbohydrates (up to 32 and 49%, respectively), while protein synthesis decreased (up to 15%). We hypothesized that the cultivation temperature had a more significant impact on lipid accumulation than the nitrogen availability in C. malina. Lipid accumulation was tested at three different temperatures, 4, 8, and 15°C, under nitrogen replete and depleted conditions. At 4°C under the nitrogen replete condition C. malina had the maximal biomass productivity (701.6 mg L-1 day-1). At this condition, protein content was higher than lipids and carbohydrates. The lipid fraction was mainly composed of polyunsaturated fatty acids (PUFA) in the polar lipid portion, achieving the highest PUFA productivity (122.5 mg L-1 day-1). At this temperature, under nitrogen deficiency, the accumulation of carbohydrates and neutral lipids was stimulated. At 8 and 15°C, under both nitrogen replete and depleted conditions, the lipid and carbohydrate content were higher than at 4°C, and the nitrogen stress condition did not affect the algal biochemical composition. These results suggest that C. malina is a polar marine microalga with a favorable growth temperature at 4°C and is stressed at temperatures ≥8°C, which directs the metabolism to the synthesis of lipids and carbohydrates. Nevertheless, C. malina RCC2488 is a microalga suitable for PUFA production at low temperatures with biomass productivities comparable with mesophilic strains.

Flashing light emitting diodes (LEDs) induce proteins, polyunsaturated fatty acids and pigments in three microalgae

As the periodic emission of light pulses by light emitting diodes (LEDs) is known to stimulate growth or induce high value biocompounds in microalgae, this flashing light regime was tested on growth and biochemical composition of the microalgae Nannochloropsis gaditana, Koliella antarctica and Tetraselmis chui. At low flashing light frequencies (e.g., 5 and 50 Hz, Duty cycle = 0.05), a strain-dependent growth inhibition and an accumulation of protein, polyunsaturated fatty acids, chlorophyll or carotenoids (lutein, β-carotene, violaxanthin and neoxanthin) was observed. In addition, a 4-day application of low-frequency flashing light to concentrated cultures increased productivities of eicosapentaenoic acid (EPA) and specific carotenoids up to three-fold compared to continuous or high frequency flashing light (500 Hz, Duty cycle = 0.05). Therefore, applying low-frequency flashing light as finishing step in industrial production can increase protein, polyunsaturated fatty acids or pigment contents in biomass, leading to high-value algal products.

Microalgae starch: A promising raw material for the bioethanol production

Ethanol is currently the most successful biofuel and can be produced from microalgal biomass (third-generation). Ethanol from microalgal biomass has advantages because it does not use arable land and reduces environmental impacts through the sequestration of CO₂ from the atmosphere. In this way, micro and macroalgal starch, which is structurally similar to that from higher plants can be considered a promise raw material for the production of bioethanol. Thus, strategies can be used to intensify the carbohydrate concentration in the microalgal biomass enabling the production of third-generation bioethanol. The microalgae biomass can be destined to biorefineries so that the residual biomass generated from the extraction processes is used for the production of high value-added products. Therefore, the process will have an impact on reducing the production costs and the generation of waste. In this context, this review aims to bring concepts and perspectives on the production of third-generation bioethanol, demonstrating the microalgal biomass potential as a carbon source to produce bioethanol and supply part of the world energy demand. The main factors that influence the microalgal cultivation and fermentation process, as well as the processes of transformation of biomass into the easily fermentable substrate are also discussed.

Astaxanthin accumulation in the green microalga Haematococcus pluvialis: Effect of initial phosphate concentration and stepwise/continuous light stress

Nutrient composition and light stress significantly affect the productivity of astaxanthin in Haemotococcus pluvialis. Hence, the present study aimed to investigate the effect of initial phosphate concentration and two distinct light regimes on astaxanthin accumulation in H. pluvialis. In the green stage, microalgae were cultivated in different initial phosphate concentrations under 2000 lx and a 12:12 h photoperiod. To initiate astaxanthin accumulation, an increased light intensity of 5000 lx was provided using two methods; (i) stepwise light stress, where a 12:12 h photoperiod was provided for 14 days, followed by 14 days of continuous illumination, and (ii) continuous illumination for 28 days. Phosphate limitation and continuous light stress were favourable to enhance cellular astaxanthin accumulation, which reached 7% by weight. The highest astaxanthin concentration of 27.0 ± 1.9 mg/L and lowest specific light energy consumption of 32.9 ± 2.3 kW h/g astaxanthin were reported in cultures grown in 41 mg/L phosphate under continuous light stress.

Anaerobic Co-Digestion Effluent as Substrate for Chlorella vulgaris and Scenedesmus obliquus Cultivation

Anaerobic digestate supernatant can be used as a nutrient source for microalgae cultivation, thus integrating phytoremediation processes with high value products storage in microalgae biomass. Microalgae are able to use nitrogen and phosphorous from digestate, but high nutrient concentration can cause growth inhibition. In this study, two microalgae strains (C. vulgaris and S. obliquus) were cultivated on the anaerobic co-digestion supernatant (obtained from the organic fraction of municipal solid waste (OFMSW) and waste activated sludge (WAS)) in a preliminary Petri plate screening at different dilutions (1:10 and 1:5) using a synthetic medium (ISO) and tap water (TW). Direct Nile red screening was applied on colonies to preliminarily identify hydrophobic compound storage and then a batch test was performed (without air insufflation). Results show that C. vulgaris was able to grow on digestate supernatant 1:5 diluted, while Nile red screening allowed the preliminary detection of hydrophobic compound storage in colonies. The analysis carried out at the end of the test on ammonia, phosphate, nitrate and sulphate showed a removal percentage of 47.5 ± 0.8%, 65.0 ± 6.0%, 95.0 ± 3.0% and 99.5 ± 0.1%, respectively.

Comprehensive Utilization of Marine Microalgae for Enhanced Co-Production of Multiple Compounds

Marine microalgae are regarded as potential feedstock because of their multiple valuable compounds, including lipids, pigments, carbohydrates, and proteins. Some of these compounds exhibit attractive bioactivities, such as carotenoids, ω-3 polyunsaturated fatty acids, polysaccharides, and peptides. However, the production cost of bioactive compounds is quite high, due to the low contents in marine microalgae. Comprehensive utilization of marine microalgae for multiple compounds production instead of the sole product can be an efficient way to increase the economic feasibility of bioactive compounds production and improve the production efficiency. This paper discusses the metabolic network of marine microalgal compounds, and indicates their interaction in biosynthesis pathways. Furthermore, potential applications of co-production of multiple compounds under various cultivation conditions by shifting metabolic flux are discussed, and cultivation strategies based on environmental and/or nutrient conditions are proposed to improve the co-production. Moreover, biorefinery techniques for the integral use of microalgal biomass are summarized. These techniques include the co-extraction of multiple bioactive compounds from marine microalgae by conventional methods, super/subcritical fluids, and ionic liquids, as well as direct utilization and biochemical or thermochemical conversion of microalgal residues. Overall, this review sheds light on the potential of the comprehensive utilization of marine microalgae for improving bioeconomy in practical industrial application.

Response of Scenedesmus sp. to microwave treatment: Enhancement of lipid, exopolysaccharide and biomass production

The present study focuses on the use of microwave irradiation to improve the production of lipid, exopolysaccharide and biomass in the microalgae Scenedesmus sp. Microwave treatment conditions such as microwave power, duty cycle % and time was optimized to increase the biomass and lipid content of Scenedesmus sp. Microwave power 100 W, duty cycle 40 %, and 2 min treatment time led to a substantial improvement in the biomass and lipid content. Due to the simultaneous improvement in both the biomass and lipid content, the total lipid production was improved from 0.76 (microwave untreated) to 1.42 g/L (microwave treated) (12 days grown cells). In addition, with biomass and lipid content, microwave treatment also enhanced the production of Exopolysaccharides (EPS) up to 2.3-folds. Furthermore, biodiesel properties were improved to some extent after the microwave treatment. Microwave irradiation is a promising physical treatment method for microalgae to improve total lipid production.

Chlorella vulgaris growth in different biodigested vinasse concentrations: biomass, pigments and final composition

Vinasse, an effluent generated during sugar and alcohol production, has great potential for soil and water pollution; however, it can be treated, used in biomass production and reused in sugarcane plantations. Thus, this work uses different types of biodigested vinasse to produce more biomass. The effect is the removal of ammonia nitrogen quickly and the end of the exponential growth phase of microalgae at different levels from the sixth day of cultivation. Among the concentrations used, the use of 50% biodigested vinasse showed the highest biomass concentration (255 mg L^-1) after 10 days of growth, coinciding with the end of ammoniacal nitrogen availability and stabilization of effluent color removal. The addition of biodigested vinasse also provides an increase in Chlorophyll a (5.33 mg L^-1) and b (4.66 mg L^-1) levels, obtained on the sixth day with 40% of vinasse, as well as protein (40.50%) with 50% effluent. Therefore, with the obtained results we noticed the variation of the biomass composition according to the vinasse concentration and increase of the pigment concentration in the presence of the effluent with higher nutrient concentration. Thus, the higher concentration of vinasse was more productive of the cultivation of Chlorella vulgaris.

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.

Stresses as First-Line Tools for Enhancing Lipid and Carotenoid Production in Microalgae

Microalgae can produce high-value-added products such as lipids and carotenoids using light or sugars, and their biosynthesis mechanism can be triggered by various stress conditions. Under nutrient deprivation or environmental stresses, microalgal cells accumulate lipids as an energy-rich carbon storage battery and generate additional amounts of carotenoids to alleviate the oxidative damage induced by stress conditions. Though stressful conditions are unfavorable for biomass accumulation and can induce oxidative damage, stress-based strategies are widely used in this field due to their effectiveness and economy. For the overproduction of different target products, it is required and meaningful to deeply understand the effects and mechanisms of various stress conditions so as to provide guidance on choosing the appropriate stress conditions. Moreover, the underlying molecular mechanisms under stress conditions can be clarified by omics technologies, which exhibit enormous potential in guiding rational genetic engineering for improving lipid and carotenoid biosynthesis.

Isoprostanoid Profiling of Marine Microalgae

Algae result from a complex evolutionary history that shapes their metabolic network. For example, these organisms can synthesize different polyunsaturated fatty acids, such as those found in land plants and oily fish. Due to the presence of numerous double-bonds, such molecules can be oxidized nonenzymatically, and this results in the biosynthesis of high-value bioactive metabolites named isoprostanoids. So far, there have been only a few studies reporting isoprostanoid productions in algae. To fill this gap, the current investigation aimed at profiling isoprostanoids by liquid chromatography -mass spectrometry/mass spectrometry (LC-MS/MS) in four marine microalgae. A good correlation was observed between the most abundant polyunsaturated fatty acids (PUFAs) produced by the investigated microalgal species and their isoprostanoid profiles. No significant variations in the content of oxidized derivatives were observed for Rhodomonas salina and Chaetoceros gracilis under copper stress, whereas increases in the production of C18-, C20- and C22-derived isoprostanoids were monitored in Tisochrysis lutea and Phaeodactylum tricornutum. In the presence of hydrogen peroxide, no significant changes were observed for C. gracilis and for T. lutea, while variations were monitored for the other two algae. This study paves the way to further studying the physiological roles of isoprostanoids in marine microalgae and exploring these organisms as bioresources for isoprostanoid production.

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.

Potential of Chlorella vulgaris and Nannochloropsis salina for nutrient and organic matter removal from municipal wastewater reverse osmosis concentrate

Municipal wastewater reverse osmosis concentrate (ROC) poses health and environmental risks on its disposal as it contains nutrients and harmful organic compounds at elevated concentrations. This study compared a freshwater microalga Chlorella vulgaris and a marine microalga Nannochloropsis salina in suspended and alginate-immobilised cultures for batch and semi-continuous treatment of the ROC. The immobilised algae gave comparable nutrient removal rates to the suspended cells, demonstrating immobilisation had no apparent negative impact on the photosynthetic activity of microalgae. Semi-continuous algal treatment illustrated that the microalgae could remove significant amounts of nutrients (> 50% and > 80% for TN and TP, respectively), predominantly through algal uptake (> 90%), within a short period (48 h) and generate 335-360 mg DCW L^-1 d^-1 of algal biomass. The treatment also removed a significant amount of organic matter (12.7-13.3 mg DOC L^-1 d^-1), primarily (> 65%) through the biotic pathway.

Photochemistry and proteomics of ginger (Zingiber officinale Roscoe) under drought and shading

Drought has become an increasingly serious ecological problem that limits crop production. However, little is known about the response of ginger (Zingiber officinale Roscoe) to drought and shading, especially with respect to photosynthetic electron transport. Here, differential proteomics was used to study the response of ginger to four experimental treatments: control, drought, 50% shading, and the combination of 50% shading and drought. Proteomic analysis suggested that ginger increased cyclic electron flow under drought stress by enhancing the expression of proteins related to photosystem I and cytochrome b6f. Shading significantly increased the expression of proteins related to the light harvesting complex, even under drought stress. In addition, shading increased the expression of proteins related to the oxygen evolution complex, plastocyanin, and ferredoxin-NADP+ reductase (FNR), thereby enhancing the efficiency of photosynthetic electron utilization. The shading and drought combination treatment appeared to enhance ginger's drought tolerance by reducing the expression of FNR and enhancing cyclic electron flow. Photosynthetic and fluorescence parameters showed that drought stress caused non-stomatal limitation of photosynthesis in ginger leaves. Drought stress also significantly reduced the quantum efficiency of photosystem II (Fv/Fm), the non-cyclic electron transfer efficiency of photosystem II (ϕPSII), and photochemical quenching (qP), while simultaneously increasing nonphotochemical quenching (NPQ). The addition of shading improved photosynthetic efficiency under drought. These results provide important baseline information on the photosynthetic mechanisms by which ginger responds to drought and shading. In addition, they provide a theoretical basis for the study of shade cultivation during the arid season.

Biphasic optimization approach for maximization of lipid production by the microalga Chlorella pyrenoidosa

The aim of the study was to identify the optimum cultivation conditions for the microalgal growth and lipid production of the oleaginous microalga Chlorella pyrenoidosa Chick (IPPAS C2). Moreover, an appropriate NO₃^- concentration in the cultivation medium for maximized lipid accumulation was determined. The experimental design involved a biphasic cultivation strategy with an initial biomass accumulating phase under optimized light (400 μmol/m² per s), temperature (25 °C), and elevated CO₂ concentration in the air mixture (3%), followed by a mid-elevated CO₂ concentration (0.5%) for lipid induction. The highest lipid yields of 172.47 ± 18.1 and 179.65 ± 25.4 mg/L per day were detected for NO₃^- concentrations of 100 and 150 mg/L. The optimization approach presented here led not only to the maximization of lipid yield but also to the development of a biphasic cultivation strategy easily applicable to the cultivation process without the necessity for algal cell harvesting between the first and second cultivation phases.

Critical processes and variables in microalgae biomass production coupled with bioremediation of nutrients and CO2 from livestock farms: A review

Development of renewable and clean energy as well as bio-based fine chemicals technologies are the keys to overcome the problems such as fossil depletion, global warming, and environment pollution. To date, cultivation of microalgae using wastewater is regarded as a promising approach for simultaneous nutrients bioremediation and biofuels production due to their high photosynthesis efficiency and environmental benefits. However, the efficiency of nutrients removal and biomass production strongly depends on wastewater properties and microalgae species. Moreover, the high production cost is still the largest limitation to the commercialization of microalgae biofuels. In this review paper, the state-of-the-art algae species employed in livestock farm wastes have been summarized. Further, microalgae cultivation systems and impact factors in livestock wastewater to microalgae growth have been thoroughly discussed. In addition, technologies reported for microalgal biomass harvesting and CO₂ mass transfer enhancement in the coupling process were presented and discussed. Finally, this article discusses the potential benefits and challenges of coupling nutrient bioremediation, CO₂ capture, and microalgal production. Possible engineering measures for cost-effective nutrients removal, carbon fixation, microalgal biofuels and bioproducts production are also proposed.

Biological nutrient recovery from human urine by enriching mixed microalgal consortium for biodiesel production

Utilization of waste resources is necessary to harness the long-term sustainability of algal technology. The study focused on the use of human urine as the basic nutrient source for culturing native microalgal consortium and further optimized the process parameters using response surface methodology. A full factorial, central composite rotatable design (CCRD) with three variables: urine concentration (1-10% vol of urine/vol of distil water [%v/v]), pH (6.5-9) and light intensity (50-350 μmolphotonsm^-2sec^-1) was used to evaluate the microalgal biomass and lipid content. Results indicated that at 95% confidence limits, the selected factors influence the biomass and lipid productivity. The maximum biomass productivity of 211.63 ± 1.40 mg l^-1 d^-1 was obtained under optimized conditions with 6.50% v/v of urine, pH of 7.69 and at light intensity of 205.40 μmolphotonsm^-2sec^-1. The lipid content was found to increase from 18.96 ± 1.30% in control media to 26.27 ± 1.94% under optimal conditions. The interactive effect of variables over the microalgal biomass and lipid content has also been elucidated. The data obtained were comparable to the BG11 media (control). Optimized diluted urine media in the presence of ammonium ions and under limited nitrate showed better lipid yields. Significant lipid biomolecules were detected in the algal oil extracts obtained from the diluted urine media characterized by Fourier transform infrared spectroscopy (FTIR) and Nuclear magnetic resonance (NMR). Gas chromatography-mass spectrometry (GCMS) revealed the presence of several monounsaturated and polyunsaturated fatty acids in the transesterified algal oil. Such studies would aid in technically realizing the field scale cultivation of microalgae for biofuels.

A review of high value-added molecules production by microalgae in light of the classification

This work reviews applications of high added value molecules produced from microalgae. Older forms of valorization - health food and quality feed, polyunsaturated fatty acids, pigments, carbohydrates - are currently penetrating their markets. They are driven by desirable properties: texturer and dye for food industry, antioxidant for cosmetics and the appetite of the general public for biosourced compounds. Most recent developments, such as peptides, vitamins, polyphenols, phytosterols and phytohormones, are struggling to meet their market and reach economical competitiveness. Still they are pushed forward by the very powerful driver that is pharmaceutical industry. In addition this work also proposes to link microalgae phyla and related potential applications. This is done through highlighting of which bioactive compounds can be found in which phyla. While some seem to be restricted to aquaculture, Cyanobacteria, Chlorophyta and Rhodophyta show great promises.

Chlorella vulgaris cultivation in airlift photobioreactor with transparent draft tube: effect of hydrodynamics, light and carbon dioxide on biochemical profile particularly ω-6/ω-3 fatty acid ratio

Chlorella vulgaris is used for food and feed applications due to its nutraceutical, antioxidant and anticancer properties. An airlift photobioreactor comprising transparent draft tube was used for C. vulgaris cultivation. The effect of reactor parameters like hydrodynamics (0.3-1.5 vvm), light intensity (85-400 μmol m^-2 s^-1), photoperiod (12-24 h) and gas-phase carbon dioxide (CO₂) concentration (5-15% v/v) were evaluated on microalgae and associated bacterial growth, biochemical profile; with special emphasis on ω-3, ω-6 fatty acids, and vitamin B₁₂. The optimal growth of C. vulgaris without CO₂ supplementation was observed at 1.2 vvm, which was associated with higher algal productivity, chlorophyll, vitamin B₁₂ content, and bacterial load along with 72% of nitrate removal. The higher light intensity (400 μmol m^-2 s^-1) and photoperiod (24:0) increased biomass productivity and ω-3 fatty acid content (in lipid) up to 2-3 fold. The elevated levels of gas-phase CO₂ concentration (15% v/v) enhanced EPA content up to 7% and biomass productivity up to 171 mg L^-1 day^-1. However, the increase in CO₂ concentration lowered vitamin B₁₂ content (up to 30%) and bacterial load (2-3 log). Also, all the cultivation conditions favoured desirable ω-6/ω-3 ratio(in the range of 1-2).