Cryopreservation of microalgae and cyanobacteria.

Most culturable cyanobacteria and soil microalgae can be cryopreserved with relatively high viability. Furthermore, many freshwater and marine eukaryotic algae can also be cryopreserved, but typically with lower post-thaw viability levels. However, to date, most dinoflagellates, cryptophytes, synurophytes, and raphidophytes cannot be successfully cryopreserved. Marine diatoms can be cryopreserved, and often have high viability, although freshwater diatoms have thus far proven more problematic. Large numbers of strains have been examined, most notably at the four major protistan collections: Culture Collection of Algae and Protozoa (CCAP) (UK), The Provasoli-Guillard National Center for Culture of Marine Phytoplankton (CCMP) (USA), Sammlung von Algenku Huren Göttingen (SAG) (Germany), and The Culture Collection of Algae at the University of Texas at Austin (UTEX) (USA), and it has been observed that chlorarachniophytes, eustigmatophytes, pelagophytes, phaeothamniophytes, and ulvophytes also have very high success rates, comparable with the other green algae and cyanobacteria. It has been noted that virtually all algae with a large cell size, as well as most filamentous strains, cannot as yet be cryopreserved. There are no known fundamental reasons why large and more complex algae cannot be successfully cryopreserved. Thus, it is anticipated that further research on the basic mechanisms of freezing damage and the empirical development of improved protocols will continue to expand the number and diversity of algal taxa that can be successfully cryopreserved.