Molecular Characterization of Fischerella uthpalarensis, the first subsection V cyanobiont from a tropical Azolla species containing dual nitrogenases

There have been theories presented on Azolla cyanobionts, known for voracious nitrogen fixation, vertical transmission of cyanobiont and helping transform a greenhouse planet to an icehouse one ~49 Mya. One such theory encapsulates the existence of two cyanobionts, named Major and Minor. We show here the identity of a possible minor cyanobiont of Azolla named Fischerella uthpalarensis. A likely cyanobiont with straight or curved filaments that were truly branched was isolated. Seven gene fragments, namely 16s rDNA (Forward and Reverse), RNA Polymerase, ITS1 region (Forward and Reverse), nifD and GroEL genes were utilized to identify the isolated cyanobiont. The best match based on BLASTn search tool was found in the RNA Polymerase beta subunit (rpoC) gene fragment, that showed 99.54% identity with 55% coverage to Fischerella muscicola. Phylogenetic inferences with the rpoC genetic locus and the GroEL protein sequence suggest a likely Fischerella genus identity. Furthermore, VnfDG and VnfN fragments too were amplified using PCR and sequenced to demonstrate that this cyanobiont has alternate nitrogenase genes, on top of the molybdenum counterpart, providing an advantage in lifestyle. We encountered a higher level of genomic-level synonymous substitutions, which was not reflected in protein sequences, namely VnfDG and VnfN gene products, which may be due to codon heterogeneity. We also propose for F. uthpalarensis atypicality in codon usage due to the likely acquisition of the V-nitrogenase operon from a presumed recent horizontal gene transfer (HGT) event. The cyanobiont from this study showcases a higher preference for AT over GC at the VnfDG composite locus again hinting at a symbiotic lifestyle.