An Exploration of Common Greenhouse Gas Emissions by the Cyanobiont of the Azolla-Nostoc Symbiosis and Clues as to Nod Factors in Cyanobacteria.

Dilantha Gunawardana

doi:10.3390/plants8120587

Abstract

Azolla is a genus of aquatic ferns that engages in a unique symbiosis with a cyanobiont that is resistant to cultivation. Azolla spp. are earmarked as a possible candidate to mitigate greenhouse gases, in particular, carbon dioxide. That opinion is underlined here in this paper to show the broader impact of Azolla spp. on greenhouse gas mitigation by revealing the enzyme catalogue in the Nostoc cyanobiont to be a poor contributor to climate change. First, regarding carbon assimilation, it was inferred that the carboxylation activity of the Rubisco enzyme of Azolla plants is able to quench carbon dioxide on par with other C3 plants and fellow aquatic free-floating macrophytes, with the cyanobiont contributing on average ~18% of the carboxylation load. Additionally, the author demonstrates here, using bioinformatics and past literature, that the Nostoc cyanobiont of Azolla does not contain nitric oxide reductase, a key enzyme that emanates nitrous oxide. In fact, all Nostoc species, both symbiotic and nonsymbiotic, are deficient in nitric oxide reductases. Furthermore, the Azolla cyanobiont is negative for methanogenic enzymes that use coenzyme conjugates to emit methane. With the absence of nitrous oxide and methane release, and the potential ability to convert ambient nitrous oxide into nitrogen gas, it is safe to say that the Azolla cyanobiont has a myriad of features that are poor contributors to climate change, which on top of carbon dioxide quenching by the Calvin cycle in Azolla plants, makes it an efficient holistic candidate to be developed as a force for climate change mitigation, especially in irrigated urea-fed rice fields. The author also shows that Nostoc cyanobionts are theoretically capable of Nod factor synthesis, similar to Rhizobia and some Frankia species, which is a new horizon to explore in the future.

Highlights

Three gases—carbon dioxide, methane (~34 times greater global warming potential than carbon dioxide), and nitrous oxide (a ~298-fold higher global warming potential than carbon dioxide)—are the chief contributors to global warming and, climate change [1,2]
It is known that in heterocysts, where nitrogen fixation occurs, there are strong respiration rates to maintain a low stream of oxygen at the active site of the nitrogenase enzyme, which means that heightened carbon dioxide emissions are a common occurrence from Nostoc filaments, especially in the dark
Climate change, unless the emanated carbon dioxide is quenched by climate change,(Calvin unless the emanated carbon dioxide quenched by photosynthesis (Calvin cycle) taking place in the taking place in the

Summary

Introduction

Three gases—carbon dioxide, methane (~34 times greater global warming potential than carbon dioxide), and nitrous oxide (a ~298-fold higher global warming potential than carbon dioxide)—are the chief contributors to global warming and, climate change [1,2]. The cultivation of crops or agriculture is one of the main causes of climate change, because it leads to the emission of all three of these gases. Farming of ruminants and rice cultivation, which promote anaerobic decomposition, are both emitters of methane. The use of synthetic nitrogen fertilizers potentiates the denitrification of nitrogen compounds, which leads to the emission of nitrous oxide as a by-product. It is necessary to reduce the effect of agriculture on climate change by promoting the use of biological alternatives to nitrogen fertilizers (biofertilizers), i.e., bacterial/cyanobacterial organisms that possess the remarkable ability to fix nitrogen gas and to furnish the nitrogen requirements of plants [3]

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