2.3 Functional groups and cell morphology
The algal sample was centrifuged and stored in a refrigerator for freeze-drying. Then, the functional groups were analyzed by ATR-IR (Nicolet iS5 FTIR, USA) [16]. The centrifuged cells were fixed with ethanol and freeze-dried with acetone. The samples were stored for SEM (Zeiss-Supra55) analysis [2]. For TEM (Tecnai G2 F20S-TWIN + AZtec X-Max 80T) observation, the samples were first fixed with glutaraldehyde and then stored at 4 oC. Finally, ultrathin (60–80 nm) was cut by a diamond knife for TEM analysis [26].