Effect of high-temperature stress on microalgae at the end of the logarithmic phase for the efficient production of lipid

The potential of Pavlovophyceae species as a source of valuable carotenoids and polyunsaturated fatty acids for human consumption

Microalgae are a group of microorganisms, mostly photoautotrophs with high CO2 fixation capacity, that have gained increased attention in the last decades due to their ability to produce a wide range of valuable metabolites, such as carotenoids and polyunsaturated fatty acids, for application in food/feed, pharmaceutical, and cosmeceutical industries. Their increasing relevance has highlighted the importance of identifying and culturing new bioactive-rich microalgae species, as well as of a thorough understanding of the growth conditions to optimize the biomass production and master the biochemical composition according to the desired application. Thus, this review intends to describe the main cell processes behind the production of carotenoids and polyunsaturated fatty acids, in order to understand the possible main triggers responsible for the accumulation of those biocompounds. Their economic value and the biological relevance for human consumption are also summarized. In addition, an extensive review of the impact of culture conditions on microalgae growth performance and their biochemical composition is presented, focusing mainly on the studies involving Pavlovophyceae species. A complementary description of the biochemical composition of these microalgae is also presented, highlighting their potential applications as a promising bioresource of compounds for large-scale production and human and animal consumption.

Population and functional changes in a multispecies co-culture of marine microalgae and cyanobacteria under a combination of different salinity and temperature levels

Changes in the temperature or salinity of ocean waters can affect marine organisms at multiple trophic levels. Both environmental variables could have an impact on marine microalgae populations. Therefore, the effect of the combination of three levels of temperature (20, 24 and 28 °C), and three levels of salinity (33, 36, and 39 PSU) were evaluated on the growth of a multispecies community of five common species of phytoplankton: (one cyanobacteria, Synechococcus sp., and four microalgae, Chaetoceros gracilis, Amphidinium carterae, Pleurochrysis roscoffensis and Rhodomonas baltica). The co-culture was monitored by flow cytometry under controlled conditions in a 96 h study. The effect of both variables on dissolved oxygen concentrations was measured using the SDR SensorDish Reader system. The results demonstrated that Synechococcus sp., C. gracilis, and A. carterae displayed a high growth at the temperature of 28 °C combined with the lowest salinity assayed. However, salinity increases negatively affected the growth of P. roscoffensis and R. baltica. Decreased salinity combined with decreased temperature exhibited a higher net O2 production. The interaction of two environmental factors related to global change such as temperature and salinity can cause structural (community growth) and functional (net oxygen production) changes in a phytoplanktonic community.

Utilization of lipidic food waste as low-cost nutrients for enhancing the potentiality of biofuel production from engineered diatom under temperature variations

The scarcity of natural fossil fuels presents a promising opportunity for the development of renewable microalgae-based biofuels. However, the current microalgae cultivation is unable to effectively address the high costs of the production of biofuels. To tackle this challenge, this study focused on recruiting engineered Phaeodactylum tricornutum (FabG-OE) to enhance biomass accumulation and lipid production by employing food waste hydrolysate under temperature variations. The biomass and lipid accumulations of FabG-OE were improved effectively in mixed culture medium and food waste hydrolysate at a volume ratio (v/v) of 80:20 at 30 °C. It was found that oxidative stress might contribute to the overexpression of lipogenic genes, thereby leading to lipogenesis at 30 °C. Upscaling cultivation of FabG-OE at 30 °C using a semi-continuous strategy and batch strategy was conducted to achieve 0.73 and 0.77 g/L/d of biomass containing 0.35 and 0.38 g/L/d of lipid, respectively. In summary, these findings provide valuable insights for advancing microalgae-based biofuel production.

Mixotrophy, a more promising culture mode: Multi-faceted elaboration of carbon and energy metabolism mechanisms to optimize microalgae culture

Some mixotrophic microalgae appear to exceed the sum of photoautotrophy and heterotrophy in terms of biomass production. This paper mainly reviews the carbon and energy metabolism of microalgae to reveal the synergistic mechanisms of the mixotrophic mode from multiple aspects. It explains the shortcomings of photoautotrophic and heterotrophic growth, highlighting that the mixotrophic mode is not simply the sum of photoautotrophy and heterotrophy. Specifically, microalgae in mixotrophic mode can be divided into separate parts of photoautotrophic and heterotrophic cultures, and the synergistic parts of photoautotrophic culture enhance aerobic respiration and heterotrophic culture enhance the Calvin cycle. Additionally, this review argues that current deficiencies in mixotrophic culture can be improved by uncovering the synergistic mechanism of the mixotrophic mode, aiming to increase biomass growth and improve quality. This approach will enable the full utilization of advantagesin various fields, and provide research directions for future microalgal culture.

Physiological insights into enhanced lipid accumulation and temperature tolerance by Tetraselmis suecica ultraviolet mutants

High outdoor temperatures significantly inhibit the growth and lipid production of the industrially promising marine microalga Tetraselmis suecica, which is viewed as a potential feedstock for high-value bioproducts and biofuels. To overcome this limitation, T. suecica was subjected to ultraviolet irradiation to generate mutants capable of being productive at higher temperatures. The top two high-lipid mutants UV-25 and UV-31 isolated at 25 °C and 31 °C, respectively, were compared to the wild type (WT) to delineate physiological alterations and shed light on the mutants' increased biomass and lipid productivity. At 25 °C, UV-25 and UV-31 exhibited lipid productivity of 36.12 and 31.33 mg/L day, which were 1.4- and 1.2-fold higher than WT, respectively. This increase in lipid biosynthesis correlated well with increased carotenoid content in UV-25 (2.2-fold) and UV-31 (3.6-fold), indicating an improved capacity to quench reactive oxygen species. At 31 °C, the growth and lipid accumulation of UV-31 remained high, signifying adaptation to higher temperatures. This is attributed to a well-coordinated modulation of the mutant's cellular metabolism through an increase in galactose and phosphatidylglycerol levels, as well as in protein, all of which contributed to its performance at elevated temperatures. The study successfully established a UV mutagenesis strategy for producing superior- performing microalgae strains with industrially desired traits, paving the way for future outdoor cultivation deployment.

Predicting biomass and hydrocarbon productivities and colony size in continuous cultures of Botryococcus braunii showa

Mathematical models were developed to predict biomass and hydrocarbon productivities and colony size (ouputs) of Botryococcus braunii showa cultures based on light intensity, temperature and dilution rate (inputs). These models predicted the following maximum values: biomass productivity, 1.3 g L^-1 d^-1; hydrocarbon productivity, 1.5 mg L^-1 d^-1; colony size, 320 µm under different culture conditions respectively. These values were confirmed experimentally. Additionally, the combination of inputs that simultaneously maximize all the possible outputs combinations were determined. The prediction for biomass-hydrocarbon-colony size were 1 g L^-1 d^-1, 12.05 mg L^-1 d^-1 and 156.8 µm respectively; biomass productivity-hydrocarbon productivities: 1 g L^-1 d^-1 and 13.94 mg L^-1 d^-1 respectively; biomass productivity-colony size: 1 g L^-1 d^-1 and 172.8 µm respectively; hydrocarbon productivity-colony size: 9 mg L^-1 d^-1 and 240 µm respectively. All these predictions were validated experimentally. These models might be very useful to implement a Botryococcus braunii showa large scale production.

Bacterial-algal coupling system for high strength mariculture wastewater treatment: Effect of temperature on nutrient recovery and microalgae cultivation

In the present study, bacterial-algal coupling system, an integration process of acidogenic fermentation and microalgae cultivation was used for high strength mariculture wastewater (HSMW) treatment, resource recovery and low-cost biomass production. The effect of temperature on Chlorella vulgaris (C. vulgaris) cultivation was investigated with culture medium of acidogenic liquid. The results showed that acidogenic liquid could be used as culture medium for C. vulgaris and higher biomass was obtained compared to control. The acidogenic liquid obtained at initial pH of 8 was the most suitable culture medium for C. vulgaris growth due to befitting C/N and considerable volatile fatty acids. Moreover, the optimum temperature for C. vulgaris cultivation was 25 °C and the removal efficiency of chemical oxygen demand (COD) and NH₄⁺-N from acidogenic liquid could reach 94.4% and 68.8%, respectively. The outcome could create an innovative value chain with environmental sustainability and economic feasibility in aquaculture industry.

Assessment of Synechococcus elongatus PCC 7942 as an option for sustainable wastewater treatment

Industrial wastewaters are recognized as a valuable resource, however, their disposal without proper treatment can result in environmental deterioration. The associated environmental/operational cost of wastewater treatment necessitates upgrade of applied processes towards the goals of sustainability and mitigation of climate change. The implementation of cyanobacteria-based processes can contribute to these goals via resources recovery, production of high-value products, carbon fixation and green-energy production. The present study evaluates the cyanobacterium Synechococcus elongatus PCC 7942 (S7942) as a biological component for novel and sustainable alternatives to typical biological nutrient removal processes. Valuable results regarding cultivation temperature boundaries, applied disinfection techniques and analytical methods, as well as regarding relations between parameters expressing S7942 biomass concentration are presented. The results show that at typical industrial wastewater temperatures, S7942 efficiently grew and removed nitrates from treated snack-industry's wastewater. Moreover, in cultures with treated and relatively saline dairy wastewater, its growth rate slightly decreased, but nevertheless nitrates removal rate remained efficiently high. A comparison between typical denitrification processes and the proposed nutrient removal process indicated that a S7942-based system may constitute an alternative or a supplementary to denitrification process. Thus, Synechococcus elongatus PCC 7942 proved to be a potent candidate towards sustainable industrial wastewater treatment applications.

Elucidating temperature on mixotrophic cultivation of a Chlorella vulgaris strain: Different carbon source application and enzyme activity revelation

With depletion of fossil fuel, microalgae is considered as a promising substitute due to high growth rate, efficient cost and high biofuels content. This study investigated the effect of temperature on mixotrophic cultivation of Chlorella vulgaris. In addition, the combination carbon source of inorganic (HCO₃^- or CO₃^2-) and organic (glucose or acetate) for microalgae cultivation was evaluated to obtain the optimum carbon source for mixotrophic cultivation. The results showed that the optimum temperature of microalgae cultivation was at the range of 15-20 °C. The activity of Rubisco was obviously inhibited at the temperature of 30 °C, however, citrate synthase was not susceptible to the increasing temperature. COD removal efficiency was all higher than 64.0%. Low temperature was benefit for protein formation, and the lipid accumulation occurred at high temperature. The results provide a fresh perspective between enzyme activity and temperature variation for product accumulation of microalgae.

Characterization of Chlorella sorokiniana and Chlorella vulgaris fatty acid components under a wide range of light intensity and growth temperature for their use as biological resources

This study aims to characterize the potential of three strains of microalgal species (Chlorella sorokiniana KNUA114 and KNUA122; C. vulgaris KNUA104) for use as feedstock, based on their fatty acid compositions. Each strain was molecularly identified using four marker genes (ITS, SSU, rbcL, and tufA) and phylogenetically characterized. C. sorokiniana and C. vulgaris collected from Ulleung Island, South Korea, were homologous with other known species groups. Samples' fatty acid components were measured using GC/MS analysis in growth temperatures of 10 °C, 25 °C, and 35 °C. The growth rate of C. sorokiniana strains was higher than that of C. vulgaris under high-temperature conditions, confirming the potential industrial applicability of the former as feedstock material. Additionally, saturated fatty acid contents and productivities increased as biological resources of the C. sorokiniana strains were higher than those of C. vulgaris under high light intensity and temperature conditions. These results suggest that the fatty acid components of C. sorokiniana strains may potentially be used as biological resources (e.g., feedstock).

Multifaceted applications of isolated microalgae Chlamydomonas sp. TRC-1 in wastewater remediation, lipid production and bioelectricity generation

Green microalga, Chlamydomonas sp. TRC-1 (C. TRC-1), isolated from the outlet of effluent treatment plant of textile dyeing mill, was investigated for its competence towards bioremediation. Algal biomass obtained after remediation (ABAR) was implied for bioelectricity and biofuel production. C. TRC-1 could completely decolorize the effluent in 7 days. Significant reduction in pollution-indicating parameters was observed. Chronoamperometric studies were carried out using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Maximum current density, power and power density of 3.6 A m^-2, 4.13 × 10^-4 W and 1.83 W m^-2, respectively were generated in ABAR. EIS studies showed a decrease in resistance of ABAR, supporting better electron transfer as compared to algal biomass before remediation (ABBR). Its candidature for biofuel production was assessed by estimating the total lipid content. Results revealed enhancement in lipid content from 46.85% (ABBR) to 79.1% (ABAR). Current study advocates versatile potential of isolated C. TRC-1 for bioremediation of wastewater, bioelectricity production and biofuel generation.

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.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

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

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

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

Temperature and light intensity are important environmental factors influencing microalgae for biodiesel production. The aim of present work was to study the effects of temperature (15 °C, 25 °C, and 35 °C) and its combination with high light intensity (HL, 400 μmol photon m^-2 s^-1) on lipid production of Monoraphidium dybowskii Y2 which was isolated from desert. The results demonstrated that algal growth was only inhibited at 15 °C. Promoted lipid content and decreased Fv/Fm were observed in 15 °C and 35 °C. Cellular carbohydrate, protein conversion and membrane lipid (MGDG, DGDG and SQDG) remodeling contributes for lipid accumulation. Stress combined temperatures with HL are benefit for lipid production, especially desired neutral lipid productivity all exceed 40 mg L^-1 d^-1. Fatty acids compositions of C16:0 and C18:1 were further promoted under 15 °C or 35 °C combined with HL. Thus, M. dybowskii Y2 will well-adapted to outdoors cultivation for biodiesel production.

Molasses wastewater treatment and lipid production at low temperature conditions by a microalgal mutant Scenedesmus sp. Z-4

BACKGROUND

Simultaneous wastewater treatment and lipid production by oleaginous microalgae show great potential to alleviate energy shortage and environmental pollution, because they exhibit tremendous advantages over traditional activated sludge. Currently, most research on wastewater treatment by microalgal are carried out at optimized temperature conditions (25-35 °C), but no information about simultaneous wastewater treatment and lipid production by microalgae at low temperatures has been reported. Microalgal growth and metabolism will be inhibited at low temperature conditions, and satisfactory wastewater treatment performance will be not obtained. Therefore, it is critical to domesticate and screen superior microalgal strains with low temperature adaptability, which is of great importance for wastewater treatment and biodiesel production.

RESULTS

In this work, simultaneous wastewater treatment and lipid production were achieved by a microalgal mutant Scenedesmus sp. Z-4 at the low temperature conditions (4, 10, and 15 °C). The results showed that algal growth was inhibited at 4, 10, and 15 °C compared to that at the optimal temperature of 25 °C. However, decreased temperature had no significant effect on the total cellular lipid content of algae. Importantly, lipid productivity at 10 °C was compromised by more net energy output relevant to biodiesel production, which demonstrated that the low temperature of 10 °C was favorable to wastewater treatment and energy recovery by Scenedesmus sp. Z-4. When molasses wastewater with optimal COD concentration of 8000 mg L^-1, initial inoculation ratio of 15%, and C/N ratio of 15 was used to cultivate microalgae, the maximum removal rate of COD, TN, and TP at 10 °C reached 87.2, 90.5, and 88.6%, respectively. In addition, lipid content of 28.9% and lipid productivity of 94.4 mg L^-1 day^-1 were obtained.

CONCLUSIONS

Scenedesmus sp. Z-4 had good adaptability to low temperature conditions, and showed great potential to realize simultaneous wastewater treatment and lipid production at low temperatures. The proposed approach in the study was simple compared to other wastewater treatment methods, and this potential novel process was still efficient to remove COD, N, and P at low temperatures. Thus, it had a vital significance for the wastewater treatment in low temperature regions.