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Nitrogen fixing cyanobacteria: their diversity, ecology and utilisation with special reference to rice cultivation

Authors:

SA Kulasooriya ,

LK
About SA

National Institute of Fundamental Studies, Hantana Road, Kandy.

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DN Magana-Arachchi

LK
About DN

National Institute of Fundamental Studies, Hantana Road, Kandy.

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Abstract

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. Among the early life forms of the primitive Earth, cyanobacteria were the first to perform oxygenic photosynthesis. This process slowly changed the original reducing atmosphere of the Earth to an oxidising one, triggering off a dramatic evolution of global biodiversity. Under the original reducing atmosphere N2 fixation was widespread among the organisms that inhabited the Earth. As this atmosphere gradually became oxygenated, some including certain cyanobacteria developed mechanisms to protect their nitrogenase enzyme from damage by O2. Among the filamentous cyanobacteria a common adaptation is the formation of heterocysts, which are specialised for N2 fixation, but many unicellular species and some non-heterocystous filamentous forms have also developed mechanisms for aerobic N2 fixation. Recent findings have revealed the diversity and importance of novel diazotrophic unicellular cyanobacteria and their associations in the oligotrophic deep oceans. The discovery of the occurrence of unculturable marine diazotrophic cyanobacteria that possessed an unknown photo-fermentative metabolism, which was later found to be a symbiotic association is a case in point. Such findings indicate that there could be many new organisms and systems yet to be discovered and open up new vistas for future research. They also lend support to the novel hypothesis of symbiogenetic evolution. These reports have elevated the role of unicellular N2 fixing cyanobacteria and their symbiotic systems in the deep oceans.

The pioneering habitation of the early Earth by the ancestors of present day cyanobacteria is reflected in their ubiquitous global distribution from barren tropical deserts to freezing environments of glaziers. Together with certain Proteobacteria and Archaea they are common inhabitants of extreme environments that are generally inhospitable to most organisms. The role of N2 fixing cyanobacteria are exemplified by the significant contribution they make to the nitrogen budget of the oceans, which occupy more than 80 % of the Earth’s surface. Cyanobacteria are also found in freshwater and terrestrial habitats spanning across all latitudes and longitudes of the world and contribute significantly to the carbon and nitrogen cycles of wetlands, freshwater and brackish ecosystems as free-living, symbiotic, epiphytic, benthic and periphyton organisms. Certain N2 fixing cyanobacteria form blooms in lentic water bodies and some of them produce cyanotoxins. N2 fixing cyanobacteria therefore play both positive and negative roles in nature.

Cyanobacteria form symbiotic associations with all other major groups of organisms and contribute to the nutrition of their hosts. Certain cyanobacteria and their symbiotic systems like Azolla are used as biofertilizers, particularly in rice production. This paper is a review of literature on N2 fixing cyanobacteria, their diversity, their roles in nature and their utilisation.

How to Cite: Kulasooriya, S. and Magana-Arachchi, D., 2016. Nitrogen fixing cyanobacteria: their diversity, ecology and utilisation with special reference to rice cultivation. Journal of the National Science Foundation of Sri Lanka, 44(2), pp.111–128. DOI: http://doi.org/10.4038/jnsfsr.v44i2.7992
Published on 30 Jun 2016.
Peer Reviewed

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