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This study employed a statistical methodology to investigate the optimization of conversion conditions and evaluate the reciprocal interaction of reaction factors related to the process of red-algae Gracilaria verrucosa conversion to sugars (glucose, galactose), levulinic acid and 5-hydroxymethylfurfural (5-HMF) by acidic hydrolysis. Overall, the conditions optimized for glucose formation included a higher catalyst concentration than did those for galactose, and these conditions for galactose were similar to those for 5-HMF. Levulinic acid production, meanwhile, was optimized at a higher reaction temperature, a higher catalyst concentration, and a longer reaction time than was glucose, galactose or 5-HMF production. By this approach, the optimal yields (and reaction conditions) for glucose, galactose, levulinic acid, and 5-HMF were as follows: glucose 5.29 g/L (8.46 wt%) (reaction temperature 160 °C, catalyst concentration 1.92%, reaction time 20 min), galactose 18.38 g/L (29.4 wt%) (160 °C, 1.03%, 20 min), levulinic acid 14.65 g/L (18.64 wt%) (180.9 °C, 2.85%, 50 min), and 5-HMF 3.74 g/L (5.98 wt%) (160.5 °C, 1%, 20 min).

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.

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a b s t r a c tA two-stage culture strategy was used for maximum biomass production under nutrient-sufficient conditions, followed by cultivation under low-salt stress, to cause the accumulation of oil in the biomass. Controlled conditions of nitrate, salt concentration, and time to exposure to stress were optimized for oil production with four species of microalgae, Isochrysis galbana, Nannochloropsis oculata, Dunaliella salina, and Dunaliella tertiolecta. Using conditions with addition of nitrate to 24.0 mg/L, I. galbana and N. oculata showed higher biomass productions than D. salina and D. tertiolecta. The oil contents of the microalgae increased from 24.0% to 47.0% in I. galbana with 10 psu for 2 days, from 17.0% to 29.0% in N. oculata with 0 psu for 3 days, from 22.0% to 43.0% of D. salina with 10 psu for 1 day, and from 23.0% to 40.0% (w/w) in D. tertiolecta with 0 psu for 2 days as the second stage culture with low-salt stress. Thus, I. galbana could be a suitable candidate microalga for oil production.