Characterisation of cryoinjury in Euglena gracilis using flow-cytometry and cryomicroscopy

Flow-cytometry and cryomicroscopy elucidated that the unicellular algal protist Euglena gracilis was undamaged by cryoprotectant added at 0 degrees C, and super-cooling in the absence of ice. Cryoinjuries were however induced by: osmotic shock resulting from excessive cryodehydration, intracellular ice, and fracturing of the frozen medium on thawing. Suboptimal cooling at -0.3 degrees C min(-1) to -60 degrees C and osmotic shock invariably resulted in damage to the organism's pellicle and osmoregulatory system causing, a significant (P > 0.005) increase in cell size. Cell damage was not repairable and led to death. The responses of E gracilis to cryopreservation as visualised by flow-cytometry and cryomicroscopy assisted the development of an improved storage protocol. This comprised: cryoprotection with methanol [10%(v/v)] at 0 degrees C, cooling at 0.5 degrees Cmin(-1) to -60 degrees C, isothermal hold for 30min, and direct immersion in liquid nitrogen. Highest post-thaw viability (> 60%) was obtained using two-step thawing, which involved initial slow warming to - 130 degrees C followed by relatively rapid warming (similar to 90 degrees C min(-1)) to ambient temperature (ca. 25 degrees C.) (c) 2005 Elsevier Inc. All rights reserved.