Abstract
The Journal of the National Science Foundation of Sri Lanka publishes the results of research in all aspects of Science and Technology. The journal also has a website at http://www.nsf.gov.lk/. 2021 Impact Factor: 0.682The JNSF provides immediate open access to its content on the principle that making research freely available to the public supports a greater global exchange of knowledge.Cover :Leatherback (a), green (b - adult & f - hatchling), hawksbill (c) and olive ridley (d) turtles who nest in Sri Lankan beaches, green turtle crawl marks (e) and by-catch of sea turtles in fisheries (c & d)Photo credits: SJ Perera (a & c), WPN Perera (b & e), K Ekaratne (d & f)
Highlights
NITROGEN FIXATION IN CYANOBACTERIABiological nitrogen fixation (BNF) is the conversion of inert dinitrogen (N2) into a combined form by living organisms through catalytic process of the enzyme nitrogenase
Cyanobacteria are unique in their ability to conduct the two incompatible processes of O2 evolving photosynthesis and O2 sensitive N2 fixation within non-compartmentalised prokaryotic cells
Originating as one of the earliest photo-autotrophic organisms of the primitive Earth, nitrogen fixing cyanobacteria have eventually established themselves as the only oxygenic photosynthetic prokaryotic group that could sustain the oxygen sensitive nitrogenase activity in them under aerobic conditions
Summary
Biological nitrogen fixation (BNF) is the conversion of inert dinitrogen (N2) into a combined form by living organisms through catalytic process of the enzyme nitrogenase. From phylogenetic studies it has been postulated that all the nitrogenases have been derived from a common prokaryotic ancestral group that existed prior to the oxygenation of the Earth’s atmosphere (BermanFrank et al, 2003). This anoxic atmosphere was quite different from the present day atmosphere and had a predominance of CH4, CO2, N2 and NH3. The slow process of oxygenation over several millions of years, primarily due to the oxygenic photosynthesis of cyanobacteria, resulted in the partial pressure of atmospheric oxygen to increase from 0.0004 % to > 0.03 % This oxygenation was inimical to N fixation and all the organisms had to adopt various methods to protect their nitrogenase enzyme from damage by O2 (Gallon, 1981). Such evolutionary processes of adaptation would have been slow and gradual over a long period of time because the anaerobic atmosphere went through a long journey of micro-aerobic conditions to reach oxygen concentrations inimical to the nitrogenase enzyme. Gallon et al (1991) and Gallon (1992) have described these mechanisms as behavioural adaptations, physical barriers, physiological and biochemical strategies and structural changes, and treated the cyanobacteria as a special group and these have been discussed by Kulasooriya (2008)
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