Effects of cold-dark storage on growth of Cylindrotheca closterium and its sensitivity to copper

Cylindrotheca closterium cells were maintained at low temperature (4 ± 1 °C) and dark conditions up to 21 weeks to assess the effect on survival and physiological status. From a control culture under standard conditions, three densities were prepared: (A) 2 × 104, (B) 10 × 104, and (C) 25 × 104 cells ml−1. Weekly, inoculums of each stored density were exposed to continuous light and at 20 ± 1 °C. Sensitivity to copper for microalgal cultures was evaluated in order to assess possible changes in cells sensitivity due to storage. Concurrently, assays with a control culture were carried out in order to assess the sensitivity of C. closterium to copper and to be able to generate a standard sensitivity control chart with a mean value of EC50-72 h ± 2SD (standard deviation). Density-C presented higher cell yield values, between 40% and 80% relative to control culture. Cell density showed to be important feature that may be taken into account in cell storage experiments. There was an increase in sensitivity of cells submitted to storage; however results always kept in the range established as standard sensitivity with no statistically significant difference with regards to control culture. EC50-72 h mean value for the control culture was 29 ± 10 μg Cu l−1, while for densities-A, B and C were 22 ± 7; 23 ± 9 and 23 ± 8 μg Cu l−1, respectively. In spite of drastic changes in the environmental conditions due to storage, it is concluded that C. closterium cells stored during 5 months remained metabolically active and with no significant change in its sensitivity.