Comparison of heat resistance and application potential of two lipid-rich Isochrysis galbana strains

Isochrysis galbana is considered to be a promising candidate for biodiesel feedstock for its fast growth and high lipid content. Outdoor mass cultivation of this species often suffers from heat stress, especially at noon in summer, thus screening of heat-resistant strains is essential. The heat resistance and application potential of two I. galbana strains IOAC683S and IOAC724S were compared for their growth characteristics, Hsp90 gene expression, photoprotective mechanism, fatty acid profile and lipid productivity at 25 °C, 30 °C and 35 °C. Cell density, total chlorophyll and biomass of strain IOAC683S increased at 25 °C and 30 °C, while decreased at 35 °C. In contrast, all these indexes of strain IOAC724S increased gradually in all three temperatures during 144 h culture period. The HSP90 mRNA expression levels of strain IOAC683S increased significantly after cultured at 35 °C for 24 h (P < 0.05), but that of strain IOAC724S did not increase significantly at 72 h at 35 °C compared with 25 °C (P > 0.05). These results indicated that strain IOAC724S was more resistant to 35 °C than strain IOAC683S. It was found that, the lower chlorophyll content per cell in IOAC724S could reduce the light energy absorption, and higher non-photochemical energy quenching (NPQ) and alternative electron flow with PSII (AEF-II) could efficiently dissipate excess light energy, therefore protect it against photodamage at high temperature of 35 °C. In addition, strain IOAC724S has higher lipid productivity than strain IOAC683S, giving this unicellular alga potential for outdoor commercial cultivation for biofuel production.