Classification of Nostoc-like cyanobacteria isolated from paddy soil into Aliinostoc, Aulosira, and Desmonostoc

Accurate identification of cyanobacterial strains is an essential step for subsequent research to be performed on these organisms. The classification of cyanobacteria in Nostocaceae remains a significant challenge due to the lack of reference data for type species and robust morphological characters for each genus. This study aims to classify 38 new isolated <i>Nostoc</i>-like strains at the genus level. The relationship between phylogenetic classification and morphological characteristics at the genus level was also investigated. The 16S rRNA gene sequences served as primary data for phylogenetic classification, supporting the designation of 18 isolates into the <i>Aliinostoc</i>, 7 isolates into the <i>Aulosira</i>, and 13 isolates into the <i>Desmonostoc</i>. Subsequently, we used these isolates as living materials to discover the most distinct features at each genus level of <i>Aliinostoc</i>, <i>Aulosira</i>, and <i>Desmonostoc</i>. As a result, the morphological characteristics of the three genera became distinguishable when grown in the BG11₀ medium. There, the mature vegetative cells of all isolated strains in the <i>Aliinostoc</i> were gray or brown, the strains in the <i>Aulosira</i> exhibited basal heterocysts at the beginning of cultivation, and the <i>Desmonostoc</i> strains showed the appearance of akinetes in the life cycle as an alternative reproduction. All isolated strains exhibited heterocysts, indicating their ability to fix nitrogen and potentially improve nutrient availability in paddy soil, especially in nitrogen-deficient conditions. This study provides a dataset of 16S rRNA gene sequences and morphological characteristics of <i>Nostoc</i> morphotypes, contributing to cyanobacterial taxonomy.