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 e cient 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.
