Perspective of nitrate assimilation and bioremediation in Spirulina platensis (a non-nitrogen fixing cyanobacterium): An overview

Abstract

Cyanobacterium, Spirulina platensis, has been used for many centuries as a food product and also has important applications in industry and environmental remediation. Apart from being a very good source of essential nutrients such as provitamins, minerals, polyunsaturated fatty acids, S. platensis is also characterized by its high protein content. The nitrate assimilation genes of S. platensis is organised in a systemic operon with the structure: nirA (nitrite reductase)-permease gene(s)-nar6 (nitrate reductase). Genomic localisation in S. platensis is different from the operons found in other cyanobacteria due to the presence of both types of nitrate transporters (nrtP and ABC types). Both nitrate uptake and transcription of the nitrate assimilatory genes in S. platensis are regulated. However, the mechanism of regulation of nitrate assimilatory genes is different from other non-nitrogen fixing cyanobacterial species. In the last decade, it has also been exploited for its capacity to decontaminate water and environmental pollution caused by hazardous materials. Role of S. platensis in removal of toxic metal ions from the polluted effluent have taken more importance in this area because of theirsmall size have a high surface area-to-volume ratio and therefore provide a large contact area for metal binding. The biosorption processes have been studied extensively using S. platensis microbial biomass as biosorbent for heavy metal ions removal because of adherent advantage of mass cultivation. All of these desirable physiological characteristics and applications have made S. platensis as a model organism to understand nitrogen metabolism. This review deals with recent advances in the characterization of nitrate assimilation and bioremediation potential of S. platensis.