Hi :D!

Is it the first time you de-freeze Chlorella preserved at -70°C? In my experience, I had preserved Chlorella and Chlamydomonas strains in liquid nitrogen and I didn't have problems. I used DMSO as cryoprotectant. 

I think you didn't freeze properly cells may be got damaged during the process. 

In my experience the glycerol procedure works well for bacteria, but algae are a bit more finicky. Best to change the freezing method/preservative combo. The slow growth rate is probably just from a loss of viable cell population leading to a longer apparent lag, and the yellowing of the culture is likely from faster growth of a competing low level contaminant that was better preserved in the freezing process.

Dear Mr. Klinger

Thanks a lot for your opportune answer. The culture is clean now, yellowed color isn´t contamination in this particular case. 

Mr. Prasad.

Thank you in advance. Have you some protocol about manage of preserved green algae culture? Some aspects to consider ?

Best regards.

G. Gutiérrez.