When it is not known if a strain can survive cryopreservation, then it may be desirable to test several different protocols simultaneously.
Past experience at UTEX indicates that cultures survive best when frozen in their normal medium with a low concentration of either DMSO (typically 8-12% v/v) or methanol (typically 5% v/v). Strains can also be tested under different conditions such as different concentrations or different kinds of cryoprotective agents (CPAs), different rates of cooling, different culture media, different concentration of algae, etc.
A copy of the UTEX cryopreservation protocol is available here which utilizes a controlled rate freezing canister.
We have cryopreserved ~2/3 of the strains at UTEX using this procedure with various CPAs and concentrations.