Dinitrogen fixation by Cyanobacteria

Abstract

Nitrogen is the most abundant element in the Earth’s atmosphere in the form of di-nitrogen gas; it is not assimilable by plants or any other organism as such. Biological Nitrogen Fixation (BNF) is carried out by a specialized group of prokaryotes, like Cyanobacteria. These microorganisms fix the atmospheric nitrogen (N₂) to ammonia (NH₃) using a complex of enzymes called Nitrogenase. According to one estimate, total biologically fixed nitrogen approaches approx. 2 × 10² Mt annually, and the average nitrogen fixation is estimated to be approx. 90 kg N ha^-1Y^-1. Cyanobacteria occur in almost every conceivable habitat, including soil, on rocks, in freshwater, and saltwater, and these are cosmopolitan prokaryotes. Cyanobacteria have been divided into five 'subsections' or orders according to Bergey's Manual. Of these five subsections, three are for non-heterocystous types and two for heterocystous types. They have an unmatched range of symbiotic hosts (e.g., protists, fungi, sponges, and angiosperms) and are not restricted to roots only but may enter a symbiotic association with any plant part and need not be housed intracellularly within the host plants. All plant-associated cyanobionts can differentiate specialized nitrogen-fixing cells known as heterocysts. Heterocyst polar nodules may function as nitrogen storage products and share the same composition as cyanophycin granules, which are copolymers of aspartate and arginine. Heterocysts are distinguished from ordinary vegetative cells by their considerably larger and rounder form, decreased pigmentation, thicker cell envelopes, and typically noticeable cyanophycin granules at poles. Heterocysts' extra envelope layers shield the enzyme Nitrogenase from oxygen's harmful rays. There are about 20 genes organized as large contiguous genomic regions in a heterocyst, which are required to be expressed as upregulated in a coordinated fashion to synthesize heterocyst-specific envelopes. NtcA, as a transcription activator, has been identified to switch on the genes involved in heterocyst differentiation as well as in Nitrogenase synthesis. HetR, a serine-type protease also known as a specific master regulator, works in association with NtcA. Transcriptional regulation by NtcA is modulated per C:N balance of the cells, with 2-oxoglutarate serving as the actual effector molecule. The C/N ratio, thus, is crucial to setting early steps of heterocyst differentiation. The nif genes, which are activated by the NtcA protein, undergo programmed rearrangements during nitrogen fixation. Anabaena PCC 7120 has been shown to contain a stretch of 17 genes that encode the Nitrogenase complex. Two major clusters have been identified among these 17 genes: (a) nifB–fdxN–nifS–nifU gene cluster (of which the nifB, nifS, and nifU encode the complex FeMo cofactor assembly and fdxN encodes ferredoxin) and (b) nifH-nifD-nifK gene cluster, wherein nifH encodes Dinitrogenase reductase and nifD and nifK encode, respectively alpha and beta subunits of Dinitrogenase. The Nitrogenase enzyme complex is a multimeric complex of protein enzymes. Numerous filamentous and unicellular heterocystous and non-heterocystous cyanobacteria species, were able to fix molecular nitrogen both temporally and spatially when they synthesize the enzyme Nitrogenase complex. However, it protects the Nitrogenase enzyme complex from oxidative deactivation. There must be temporal shifts for oxygenic photosynthesis during the day and anoxic Nitrogenase activity during dark periods. The assimilatory form of N₂ for photosynthetic organisms is ammonium, which becomes incorporated into carbon skeletons metabolically. It is the most reduced inorganic form of nitrogen, which is available for assimilation. Fixed nitrogen as NH⁴⁺ is energetically a favoured form. NH^4+, either as direct uptake from the medium or as metabolically produced from alternative sources of nitrogen, is incorporated into carbon skeletons via the sequential activity of two enzymes, Glutamine Synthetase (GS) and Glutamate Synthase (Glutamine Oxoglutarate Aminotransferase-GOGAT), of a cycle commonly known as the GS-GOGAT pathway.