Background: Colonial and multicellular volvocine green algae have been extensively studied recently in various fields of the biological sciences. However, only one species (Pandorina morum) has been cryopreserved in public culture collections.

Results: Here, we investigated conditions for cryopreservation of the multicellular volvocine alga Gonium pectorale using vegetative colonies or cells and zygotes. Rates of vegetative cell survival in a G. pectorale strain after two-step cooling and freezing in liquid nitrogen were compared between different concentrations (3% and 6%) of the cryoprotectant N,N-dimethylformamide (DMF) and two types of tubes (0.2-mL polymerase chain reaction tubes and 2-mL cryotubes) used for cryopreservation. Among the four conditions investigated, the highest rate of survival [2.7 ± 3.6% (0.54-10%) by the most probable number (MPN) method] was obtained when 2.0-mL cryotubes containing 1.0 mL of culture samples with 6% DMF were subjected to cryogenic treatment. Using these optimized cryopreservation conditions, survival rates after freezing in liquid nitrogen were examined for twelve other strains of G. pectorale and twelve strains of five other Gonium species. We obtained ≥ 0.1% MPN survival in nine of the twelve G. pectorale strains tested. However, < 0.1% MPN survival was detected in eleven of twelve strains of five other Gonium species. In total, ten cryopreserved strains of G. pectorale were newly established in the Microbial Culture Collection at the National Institute for Environmental Studies. Although the cryopreservation of zygotes of volvocine algae has not been previously reported, high rates (approximately 60%) of G. pectorale zygote germination were observed after thawing zygotes that had been cryopreserved with 5% or 10% methanol as the cryoprotectant during two-step cooling and freezing in liquid nitrogen.

Conclusions: The present study demonstrated that cryopreservation of G. pectorale is possible with 6% DMF as a cryoprotectant and 1.0-mL culture samples in 2.0-mL cryotubes subjected to two-step cooling in a programmable freezer.

The authors declare that they have no competing interests.