Post rapid freezing growth of Antarctic strain of Heterococcus sp. monitored by cell viability and chlorophyll fluorescence

Abstract The soil microalgae of the genus Heterococcus are found in cold environments and have been reported for the terrestrial ecosystems of several Sub-Antarctic and Antarctic Islands. This study focused on resistance of Heterococcus sp. to sub-zero temperature. Heterococcus sp. was isolated from soil samples from James Ross Island, Antarctica. Culture of Heterococcus sp. grown in liquid medium were used to study ribitol effects at sub-zero temperatures on the species resistance to rapid freezing (RF, immersion of a sample into liquid nitrogen) and consequent cultivation on agar. Before the experiment, Heterococcus sp. was cultured in liquid medium for 11 months and then treated in ribitol concentrations of 32 or 50 mM for 2 h. Then, 1 ml samples were frozen to −196 °C in liquid nitrogen (day 0) and inoculated on BBM agar after thawing. Number of living and dead cells was evaluated and the cell viability (P ν ) was calculated repeatedly using the optical microscopy approach. The addition of ribitol caused a noticable increase in P ν on days 9, 12, 14 (with a P ν of 25–45% in ribitol-treated samples compared to 10% in the untreated control). In the following period (d 16–19), the positive effect of ribitol on P ν was less pronounced but still statistically significant. To evaluate the negative effects of RF on chlorophyll fluorescence parameters, the potential yield of photochemical reactions in PS II (F V /F M ), and the effective quantum yield of photochemical reactions in PS II (Ф PSII ) were measured immediately before and after RF. Consequently, F V /F M and Ф PSII of agar inoculates were measured repeatedly for 30 d cultivation in 3 d interval. Both the 32 and the 50 mM addition of ribitol caused earlier detection of the parameters (d 16) compared to the control measurements (d 23) as well as reaching the maximum values of the chlorophyll fluorescence parameters earlier (d 23 in ribitol-treated samples compared to d 25 in control samples). Heterococcus sp. proved to be a species resistant to rapid freezing. The ability may help the species to survive in harsh Antarctic environments typified by rapid fluctuations in temperature that may bring a rapid freezing of the alga.