Opacimonas immobilis sp. nov., a novel non-flagellated Alteromonadaceae species with the smallest genome isolated from the phycosphere of the diatom Actinocyclus curvatulus.

A bacterial strain LMIT016, characterized as Gram-stain-negative, oval shaped, aerobic and non-motile, was isolated from the phycosphere of the diatom Actinocyclus curvatulus. Both the phylogenetic tree constructed from 16S rRNA gene sequences and the phylogenomic analysis using GTDB-Tk revealed that strain LMIT016 was closely related to O. viscosa LMIT007 (96.08% of 16S rRNA gene sequence identity). Strain LMIT016 and related type species in the family exhibited average nucleotide identity values ranging from 76.37 to 79.96%, with digital DNA-DNA hybridization values below 30.0%, confirming that they represent different species. Notably, strain LMIT016 possesses the smallest genome (2.53 Mbp) among Alteromonadaceae family, completely lacking genes related to flagellar assembly and chemotaxis. However, comparative genomic analysis revealed that it kept adaptive functions for phycosphere lifestyle, such as biofilm formation, carbohydrate metabolism, and vitamin and carotenoid synthesis. Further, strain LMIT016 exhibited optimal growth under conditions of 1.0-5.0% NaCl, pH 6-10, and temperatures between 20 and 35 ℃. The major respiratory quinone was ubiquinone-8. The major fatty acids (> 10%) were identified as Cω7c/Cω6c (32.4%), C (17.3%) and Cω7c/Cω6c (31.0%), while the major polar lipids comprised phosphatidylethanolamine, aminophospholipid and phosphatidylglycerol. These findings, combined with the phylogenetic and genotypic features, confirmed that strain LMIT016 represented a novel species in the genus Opacimonas, for which the name Opacimonas immobilis sp. nov. was proposed, with type strain LMIT016 (= KCTC 92596 = MCCC 1K08254). The identification of this strain enriches the understanding of microbial diversity within the genus Opacimonas and highlights the specialized bacterial evolution for inhabiting the phycosphere.