The cell structure and growth rate of three species of marine microalgae (Chlorella vulgaris, Tetraselmis chuii and Isochrysis sp.) before and after cryopreservation

Three species of marine phytoplankton Chlorella vulgaris, Tetraselmis chuii and Isochrysis sp. were cultured using Conway media in 20 °C with the light intensity of 90 μmol m-2s-1 and a photoperiod of 12:12 h L/D cycle. Cells were harvested in stages at logarithmic and stationary phases. Equilibration time for cyroprotectant exposure was 10 to 20 minutes before the samples was placed in the programmable freezer for a controlled rate of cooling at -1 °C /min. The temperature was reduced uniformly from 200C until it reaches -40 °C where the samples were maintained at this temperature for 30 minutes before direct immersion into liquid nitrogen at -196 °C. Samples were stored in liquid nitrogen for one week. Percentage of viable cells was relatively higher in the stationary phase for the three species that is 95.13 % for Chlorella vulgaris, 79.09 % for Isochrysis sp. and 69.36 % for Tetraselmis chuii. The growth rate of viable cells for Chlorella vulgaris a both logarithmic and stationary phases and Tetraselmis chuii at logarithmic phase showed insignificant differences (p > 0.05) before and after cryopreservation. Tetraselmis chuii of stationary phase and Isochrysis sp. of both phases gave significant differences (p < 0.05) for growth rate before and after cryopreservation due to photo-oxidation. The cell structure after cryopreservation showed disorganization and abnormality in the ultra-structure but this did not seem to affect the growth rate of the viable cells.

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