Isochrysis sp. IOAC724S, a newly isolated, lipid-enriched, marine microalga for lipid production, and optimized cultivation conditions

• Oleaginous microalga IOAC724S is new isolated and considered as Isochrysis sp.
• Isochrysis sp. IOAC724S had 1.0 d−1 growth rate and 55.6% DCW lipid content in MAX.
• IOAC724S grew well at high temperature (35–40 °C).
• A well known strategy of a growth stage followed by a nutrient starvation phase produced the highest lipid yield.

An oleaginous, unicellular, marine microalga termed IOAC724S was isolated from the South China Sea. Morphology and genetic analyses indicated it belongs to the genus Isochrysis. Gas chromatography (GC) results showed that more than 10 types of fatty acids existed in Isochrysis sp. IOAC724S and that 90% of them were suitable for lipid production. The culture conditions suitable for cell growth were progressively optimized through photosynthetic and respiratory analyses. The optimal culture conditions were: photon flux 200–500 μmol m−2 s−1, temperature 35 °C during daytime and 24 °C at night, pH value between 7 and 8, NaNO3 160 g m−3 and NaH2PO4·2H2O 80 g m−3 for starting culture. When microalgal cultures were exposed to these optimal conditions, the specific growth rate reached to 0.26 d−1 on average and 1.0 d−1 in MAX. Lipid production was optimized through nutrient starvation processes, including nitrate or phosphate deprivation and simultaneous nitrate and phosphate deprivation. The highest lipid mass fraction of dry cell weight (about 55.6%) was obtained after the stationary phase algal culture was transferred into phosphate-free medium for 3 days. GC data demonstrated that the enhancement of lipid accumulation in algal cells maintained under nutrient starvation came mainly from an increase of C16:0 and C18:1 fatty acids; however, the lipids with a chain length appropriate for fuel use (C14 to C18) were unchanged at 90% mass fraction of the dry cell weight. Based on these good characteristics, Isochrysis sp. IOAC724S appeared to be a strong candidate for lipid production.