Cyanobacterial Diversity: A Potential Source of Bioactive Compounds

Cyanobacteria are a group of oxygenic photosynthetic prokaryotes that grow and multiply at the simple expense of light, H2O, CO and inorganic nutrients [1]. They are widely distributed in nature, ranging from the Antarctic regions to hot springs, deserts to usar soils and to salt lakes and brine water [2]. Their ability to grow over a wide range of ecological situations such as light, temperature, salinity, alkalinity and pollution makes them model photosynthetic prokaryotes, for the study of various metabolites synthesized by these organisms under such situations [3-5].


Introduction
Cyanobacteria are a group of oxygenic photosynthetic prokaryotes that grow and multiply at the simple expense of light, H 2 O, CO and inorganic nutrients [1]. They are widely distributed in nature, ranging from the Antarctic regions to hot springs, deserts to usar soils and to salt lakes and brine water [2]. Their ability to grow over a wide range of ecological situations such as light, temperature, salinity, alkalinity and pollution makes them model photosynthetic prokaryotes, for the study of various metabolites synthesized by these organisms under such situations [3][4][5].
The application potential of cyanobacteria in biotechnology is enormous [6] and includes photo-production of NH4+ and hydrogen [7], reclamation of deserts [8], waste water treatment [9], biofertilization of rice agriculture [10], functioning as a bioinsecticide for grazing mosquito larvae [11], and providing a rich source of important pharmaceuticals [12]. The full realization of cyanobacterial potential would possible only when their total knowledge at the molecular level is made available to the industry and modern civilization. Chemoheterotrophic microorganisms namely bacteria and fungi are the traditional source of antibiotics and bioactive compounds. In the late 20th century microalgae have been exploited as a source of biologically active compounds [12]. The biologically active compounds isolated from microalgae are known to show antibacterial, antiviral, antifungal, enzyme inhibitors, immunostimulants, cytotoxic and anti-plasmodium activities [13,14]. The antimicrobial compounds isolated from microalgae are chemically characterized as polyketides, amides, alkaloids, and peptides [13]. In addition, microalgae are also a good source of other important compounds such as vitamins, amino acids, fatty acids, siderophores, simple hydrates and other compounds that are essential to support the growth of other microorganisms [15].

Advances in Biotechnology & Microbiology
Cyanobacterial alkaloids isolated from a variety of species show structural diversity and variety of biological actions such as antifungal activity, cytotoxicity, sodium channel modulation, and inhibition of proteases [20,21]. The major sources of these alkaloids are the filamentous genera such as Lyngbya, Oscillatoria, and Symploca [22,23]. Moreover, cyanobacteria are known for the production of toxins associated with water blooms. About 40 genera of cyanobacterial species that are responsible for the production of freshwater and marine cyanobacterial toxins have been reported [23]. These toxins, namely cyanotoxins, can be grouped according to their toxicity in vertebrates as hepatotoxins, neurotoxins, irritants and dermatotoxins, and general cytotoxins [24]. They fall into three broad groups of chemical structures: cyclic peptides, alkaloids and lipopolysaccharides [25,26]. The best known cyanotoxins are microcystins and nodularins, which are potent inhibitors of protein phosphatases.
Together with those toxins, during the last few decades, hundreds of cyanobacterial secondary metabolites were reported [17,27,28,29]. These metabolites possess different chemical structures such as fatty acids, phenolics, terpenoids, N-glycosides, lipopeptides, linear and cyclic peptides and alkaloids. In addition, they exhibit a diverse spectrum of biological activities including antibacterial algicidal, antifungal, antiviral, anticancer, cytotoxic, and enzyme inhibiting activities. Most of the potential bioactive compounds are synthesized from the machinery of non-ribosomal peptide synthetase or mixed with polyketide synthase [30]. Apart from the above bio-synthetic pathway some bioactive compounds such as microviridins and cyanobactins used ribosomal pathways [31]. Therefore, cyanobacteria are considered as a source of potential pharmaceutical substances including antifungal compounds.

Conclusion
Multidrug resistant phenomenon is widespread among the human pathogens. So, it is a demand of the hour to search and develop new drugs from the new source. Now a day cyanobacteria are considered as good candidates to find novel bioactive compounds/drugs. However, all the experiments were performed in vitro; so the in vivo activities of such compounds are questionable. Secondly, the natural products isolated from cyanobacteria are relatively large and not each functional subunit plays vital role in showing the bioactivity, so their commercial production could be possible only after optimizing their chemical structure with organic synthesis methods. In addition, most of the studies confined to characterize the effects of the individual bioactive compound, rather than the effect of various compounds in different combination. This idea will provide a new insight into the synthetic biology.