You could try this:

http://www.scirp.org/journal/PaperInformation.aspx?PaperID=41446#.VImduWNqOgA

I used a similar technique for my Arthrospira strain few years ago (I couldnt find the exact protocol that i used). Good luck :)

Thank you Gayathri.

The paper title "Isolation, Identification and Germplasm Preservation of Different Native Spirulina Species from Western Mexico" suggested 5% DMSO, showed survival less than 1% after 3 months of cryopreservation. 

I used very high cell density (< 2.5x10^5), 5% glycerol and slow freezing. When I needed to use the cell pellet, I did rapid thawing at +40C...this prevents ice crystals from forming. My samples were moderately viable upto may be 3 months.

Unfortunately, long term storage at -80C does not work very well for Arthrospira.