Microbes often produce more EPS when they are nitrogen-limited. You might try varying the nitrogen content of your medium. I know Nostoc is an N2-fixer, but if you haven't optimized growth conditions, nitrogen content might be one thing to look at.

ya thank greg cote but i have tried this but doesn't work  can u tel me some other method   plss  its urgent  . 

Dear Barath, you could also try induce salinity stress. It improved the yield of EPS when I was carrying out some small project and it is suggested by many authors as a very important factor for the synthesis of EPS in many microorganisms. Also, introducing increased quantities of adequate carbon source enhances production of EPS. 

I agree with Lukasz, EPS produced to combat dehydration so if you create osmotic stress by increasing carbon source or salinity then EPS should increase. Please note I do not have an experience with Nostoc.  high carbon, high EPS works well for bacteria

In Synechocystis EPS were mainly synthetized in "old cultures", it is true that the nitrate concentration was decreased in "old cultures". In Anabaena, the combined nitrogen starvation leads to the formatiojn  of heterocyste and to the assimilation of molecular nitrogen. However, it is also possible that "old cultures" of Anabaena under low light intensity should help to the formation of EPS.

The answer is very simple. Grow the cyanobacteria in dark without nitrogen source but add carbohydrate like mannitol in excess of 2% as it will divert from photosynthetic mode of energy production to heterotrophic mode.

Dear Ghosh

This paper can help you : https://link.springer.com/referenceworkentry/10.1007/978-3-319-03751-6_51-1

The kinds of medium is very effective try to find industrial medium for high production