Reusing the microalgae culture medium is a significant factor in economy of algal biomass production. However, reusability depends on the microalgae harvesting method and the post-harvesting conditions of the medium. In the pH adjustment-sedimentation method, which is an economical method for microalgae harvesting, the salinity of the environment increases after biomass separation. In this study, the effect of increasing salinity on nitrate assimilation in reused culture medium was investigated. The results showed that most of the nitrate is consumed in the first (fresh) culture, so that in fresh medium with high salinity (EC: 4230 S) 93% of nitrate and in fresh medium with normal salinity (EC: 1623 S) 66% of nitrate is consumed. However, in reused cultures (second and third), increasing salinity due to the harvesting method significantly reduces nitrate assimilation so that in the medium with high salinity, nitrate assimilation approaches to zero. Thus, for suitable cell growth in reused culture media, re-nutrition of the constituent elements of culture is necessary

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