Giving an answer would require some supplementary details: what is the requested use, for your preserved algae and diatoms ? E.g. DNA analyses ? Microscopic slides ? SEM use? Any other use? Depending on the further use your consider, preservation methods and procedures should certainly be different, since adapted.

Thankyou Madam,   I am using algae for textile dye bioremediation. I have green algae cultures ( Tetraselmis sp.  and Chlorella sp), and diatoms- (Odontella aurita, Chaetoceros calcitrans and Navicula sp.). Please help me with your suggestions.

Does this mean that you are attempting long-term preservation of diatom and algae LIVING CULTURES?

Yes Madam, I need to preserve live cultures.

Prof. John Day from the Scottish Association of Marine Sciences has done a lot of work on cryopreservation of algae cultures as part of his work in the CCAP algae collection. I would recommend to take a look at his work http://www.sams.ac.uk/john-day

Thankyou María Huete-Ortega Madam for your help.

Thankyou so much for your advice Sir. Please also give me suggestion for long term preservation

Here is a reprint.  I was a student of Greta Fryxell's. We mostly found diatoms.  Good luck.    Botanica Marina. Volume 36, Issue 6, Pages 489–498

Thankyou madam, for your advice. I will go through the issue you suggested.

Thanks and have a ball.  I hope you cite this paper. It was my thesis from undergrad.  My advisor, Greta Fryxell, is very old now.  I am sure you are familiar with her work. 

Janani  this chemical used for green algae especially good preserve at long time use  glutaraldehyde or Lugol’s Iodine.  I am used above the chemical. Good result for me.

Thankyou so much for your valuable answer. Can you please suggest, the concentration of Glutaraldehyde/Lugol's Iodine or suggest me suitable journals on the procedure you follow.

Janani. this following article. how to collection and preservation of diatom sample. preparation of lugol's solution.

Thankyou so much, Sir. The referance you sent me really useful. I will follow your steps in preserving my algae.