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Microalgae are known as renewable, potential, and sustainable feedstocks for biofuel production. The present work investigated the efficient valorization of green microalgae Chlorella sp. to produce sugars and 5-hydroxymethylfurfural (5-HMF) using thermochemical conversion with a metal-salt (ferric sulfate) as catalyst using a statistical approach and two-step conversion. A statistical approach with a Box-Behnken design was introduced to optimize the conversion for producing sugars. As a result of optimization, 86.46% sugar yield (68.32% glucose yield) was achieved under the condition of 5% biomass and 0.6 g-catalyst/g-biomass at 155 °C and 40 min. Two-step thermochemical conversion was introduced to produce 5-HMF from microalgae. In the first step, sugars were produced from the above optimum condition; in the second step, sugar hydrolysates were converted into 5-HMF by thermochemical conversion without an additional catalyst. In two-step conversion, the maximum 5-HMF yield (37.23%) was achieved at 170 °C and 60 min from the sugar hydrolysate of microalgae obtained from the first-step thermochemical conversion with ferric sulfate. In conclusion, the microalgae as biomass and ferric sulfate as catalyst have availability and the potential to produce biosugars and platform chemicals., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

The optimal conditions for high biomass and lipid production from Nannochloris atomus were evaluated. The parameters used in this study were light emitting diode (LED) wavelength mixing ratio, the photoperiod, salinity tolerance, and single and combined stresses. Biomass production was monitored in the first phase using red LED (625 nm), followed by lipid production by green LED (520 nm) in the second phase. The optimal conditions were obtained using a single red LED with light:dark durations of 20:4 h and two days of exposure in combined stresses of 1.06 M NaCl and green LED. Under these conditions, 68.6 % (w/w) lipid content were obtained. Compared to the non-stress control, the lipid content was increased by 31.9 %. Linolenic acid (C18:3) the omega-3 fatty acid was produced up to 52.4 % in 1.06 M NaCl as a single stress., (Copyright © 2020. Published by Elsevier B.V.)

Scenedesmus obliquus, a green microalga of the class Chlorophyceae, has been used to produce biofuels. However, limited research has been reported on platform chemicals that use microalgae as biomass to replace fossil sources. This paper reports on the investigation of levulinic acid (LA) production from lipid-extracted S. obliquus with an acid-catalyzed thermochemical conversion using a statistical experimental approach. For the reaction factors, the highest effect on LA yield resulted from catalyst concentration. The optimized LA yield of 45.63 wt% (70.7 mol%) was achieved with 5 wt% lipid-extracted microalgae and reaction factors of 0.85 M HCl as a catalyst at 180 °C for 10 min. Also, the LA yield as a function of the combined severity factor followed a sigmoid curve. High LA yield resulted from combined severity factors greater than 3.4. These results indicate that the production of platform chemicals may be possible using microalgae feedstocks and thermochemical conversion., Competing Interests: Conflict of interest The authors declare that there was no conflict of interest in this work., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

In this study, Pavlova lutheri, Chlorella vulgaris, and Porphyridium cruentum were cultured using modified F/2 media in a 1 L flask culture. Various nitrate concentrations were tested to determine an optimal nitrate concentration for algal growth. Subsequently, the effect of light emitted at a specific wavelength on biomass and lipid production by three microalgae was evaluated using various wavelengths of light-emitting diodes (LED). Biomass production by P. lutheri, C. vulgaris, and P. cruentum were the highest with blue, red, and green LED wavelength with 1.09 g dcw/L, 1.23 g dcw/L, and 1.28 g dcw/L on day 14, respectively. Biomass production was highest at the complementary LED wavelength to the color of microalgae. Lipid production by P. lutheri, C. vulgaris, and P. cruentum were the highest with yellow, green, and red LEDs' wavelength, respectively. Eicosapentaenoic acid production by P. lutheri, C. vulgaris, and P. cruentum was 10.35%, 10.14%, and 14.61%, and those of docosahexaenoic acid were 6.09%, 8.95%, and 11.29%, respectively.

Integrations of two-phase culture for cell growth and lipid accumulation using mixed LED and green LED wavelengths were evaluated with the microalgae, Phaeodactylum tricornutum, Isochrysis galbana, Nannochloropsis salina, and Nannochloropsis oceanica. Among the single and mixed LED wavelengths, mixed LED produced higher biomass of the four microalgae, reaching 1.03 g DCW/L I. galbana, followed by 0.95 g DCW/L P. tricornutum, 0.85 g DCW/L N. salina, and 0.62 g DCW/L N. oceanica than single LED or fluorescent lights at day 10. Binary combination of blue and red LEDs could produce the high biomass and photosynthetic pigments in the four microalgae. The highest lipid accumulation during second phase with the exposure to green LED wavelengths was 56.0% for P. tricornutum, 55.2% for I. galbana, 53.0% for N. salina, and 51.0% for N. oceanica. The major fatty acid in the four microalgae was palmitic acid (C16:0) accounting for 38.3-47.3% (w/w) of the total fatty acid content.

Blue and red light-emitting diodes (LEDs) were used to study the effects of wavelength mixing ratios, photoperiod regimes, and green wavelength stress on Nannochloropsis salina, Isochrysis galbana, and Phaeodactylum tricornutum cell biomass and lipid production. The maximum specific growth rates of I. galbana and P. tricornutum were obtained under a 50:50 mixing ratio of blue and red wavelength LEDs; that of N. salina was obtained under red LED. Maximum cell biomass for N. salina and P. tricornutum was 0.75 and 1.07 g dcw/L, respectively, obtained under a 24:0 h light/dark cycle. However, the maximum I. galbana biomass was 0.89 g dcw/L under an 18:6 h light/dark cycle. The maximum lipid contents for N. salina, I. galbana, and P. tricornutum were 49.4, 63.3 and 62.0% (w/w), respectively, after exposure to green LED. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) were obtained 1% in P. tricornutum and 2% in I. galbana., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

The effects of light-emitting diode (LED) wavelength, light intensity, nitrate concentration, and time of exposure to different LED wavelength stresses in a two-phase culture on lipid production were evaluated in the microalga, Picochlorum atomus. The biomass produced by red LED light was higher than that produced by purple, blue, green, or yellow LED and fluorescent lights from first phase of two-phase culture. The highest lipid production of P. atomus was 50.3% (w/w) with green LED light at 2days of second phase as light stress. Fatty acid analysis of the microalgae showed that palmitic acid (C16:0) and linolenic acid (C18:3) accounted for 84-88% (w/w) of total fatty acids from P. atomus. The two-phase culture of P. atomus is suitable for biofuel production due to higher lipid productivity and favorable fatty acid composition., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Conditions of light-emitting diode (LED) wavelength, light intensity, nitrate concentration, and time of exposure to green LED light stress in a two-phase culture were optimized for lipid production with three species of microalgae. Among the three microalgae, Nannochloropsis oculata showed the highest specific growth rate (μmax) of 0.037h(-1) and the lowest saturation constant (Ks) of 1.32mg/L. The highest lipid contents of the three microalgae in the second phase under green LED light stress were 52.0% (w/w) for Nannochloropsis salina at 2days, 53.0% (w/w) for Nannochloropsis oceanica at 2days, and 56.0% for N. oculata at 2days. Fatty acid analysis of the microalgae showed that 85-87% (w/w) of total fatty acids from N. salina, N. oceanica, and N. oculata consisted of palmitic acid (C16:0) and oleic acid (C18:1)., (Copyright © 2016. Published by Elsevier Ltd.)

Microalgae are renewable, eco-friendly, and sustainable bioresources that have the potential to substitute fuels and chemicals originated fossil resources. The conversion of Chlorella sp. to obtain 5-hydroxymethylfurfural (5-HMF) and levulinic acid (LA) via thermochemical reaction with a metal-sulfate catalyst (Al 2 (SO 4) 3) is introduced here. The statistical approach is introduced to optimize the conversion conditions. By optimization of the 5-HMF yield, about 23.46% of 5-HMF was obtained under 5% biomass amount and 0.75 g-catalyst/g-biomass at 165 °C and 30 min; the highest LA yield of 32.89% was obtained under 2.5% biomass and 1.0 g-catalyst/g-biomass at 205 °C and 60 min. Furthermore, under such harsh temperature condition and high catalyst amount, the maximum 5-HMF yield of 31.46% was obtained at 0 min, which indicates the time required to attain the set reaction temperature. In the evaluation of the catalytic performance via relationship between combined severity factor and 5-HMF/LA, the yields of 5-HMF and LA were demonstrated to be reasonably satisfied for non-linear sigmoidal equations with high regression. Hence, the application of microalgae and metal-salt catalyst has high potential and accessibility in the production of platform chemicals. • Efficient hydrothermal conversion of C hlorella sp. into 5-HMF and LA by metal sulfate catalyst. • 23.46% 5-HMF was obtained under 5% biomass, 0.75 g-catalyst/g-biomass at 165 °C and 30 min. • 32.89% LA was obtained under 2.5% biomass, 1.0 g-catalyst/g-biomass at 205 °C and 60 min. [ABSTRACT FROM AUTHOR]

Microalgae are potential alternative resources that can replace fossil resources for the production of biofuels and chemicals. Levulinic acid (LA) and formic acid (FA) were produced from defatted Scenedesmus obliquus (a green microalga) via sulfuric acid-catalyzed thermochemical conversion using Box-Behnken design. Following optimization, 39.93% LA and 25.82% FA yields were obtained with 5 wt% biomass concentration and 0.8 M H 2 SO 4 as the catalyst at 180 °C for 40 min. Furthermore, LA and FA yields matched well with a non-linear sigmoidal equation with the combined severity factor (CSF). High LA and FA yields were achieved at a CSF > 10.4. The results demonstrated the potential of microalgae as a biomass and Brönsted acid as a catalyst for producing platform chemicals. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Nitrate depletion, salt, and LED wavelengths of second-phase were used for stresses. • Green LED wavelength stress induced the production of microalgal lipid. • The three microalgae, P. tricornutum have high nitrate uptake for cell growth. • The synthesis of fatty acids in microalgae led to form UFAs by stresses. Abstract The effects of wavelengths of light-emitting diode (LED), nitrate concentration, and salt concentration were evaluated for the two-phase culture of the microalgal species Phaeodactylum tricornutum , Dunaliella tertiolecta , and Isochrysis galbana on cell growth and lipid production. Blue LEDs produced the highest biomass of P. tricornutum at a nitrate concentration of 8 mg/L, reaching 0.97 g dcw/L with a specific growth rate (μ) of 0.047 h−1, followed by I. galbana with 0.79 g dcw/L and μ = 0.040 h−1 and D. tertiolecta with 0.55 g dcw/L and μ = 0.028 h−1. Of the three microalgae, P. tricornutum had the highest specific growth rate of μ max = 0.070 h−1 and lowest saturation constant of K s = 4.18 mg/L, resulting in fast cell growth. The highest lipid production was obtained under green LED wavelength stress on day 14, reaching 60.6% (w/w) of the dry cell weight among the three microalgae. The main fatty acids produced by the three microalgae were myristic acid (C14:0), palmitic acid (C16:0), oleic acid (C18:1), and arachidic acid (C20:0), which comprised 72.68%–84.16% (w/w) of the total fatty acids content under three stresses. [ABSTRACT FROM AUTHOR]