Thanyakorn Nilsakha

Two Gram-stain-negative, aerobic, non-motile, non-gliding, rod-shaped bacterial strains, designated as TBRC 19031^T and TBRC 19032, were isolated from water samples collected from the Mekong River, Thailand. Strain TBRC 19031^T was obtained from Chiang Saen in the upstream section near the borders with China and Myanmar, while TBRC 19032 originated from Khong Chiam, in the downstream section where the river exits Thailand. Colonies of both strains were circular, smooth and deep yellow on Reasoner’s 2A agar and did not produce flexirubin-type pigments. Phylogenetic analysis with 16S rRNA gene sequences placed both strains within the genus Flavobacterium, showing the highest sequence similarity to Flavobacterium cheonhonense ARSA-15^T (98.29% for TBRC 19031^T and 98.22% for TBRC 19032). However, whole-genome comparisons between the strains and F. cheonhonense ARSA-15^T revealed average nt identity (89.39% and 89.29%), average aa identity (92.84% and 92.95%) and digital DNA–DNA hybridization (35.00% and 34.70%). The predominant fatty acids were iso-C_15:1, iso-C_15:0 and iso-C_15:0 3-OH, and menaquinone MK-6 was the major respiratory quinone. The major polar lipids of both strains included phosphatidylethanolamine, steryl ester and diacylglycerol. The genome sizes were 3.02 and 3.04 Mbp, with G+C contents of 38.3% and 38.2% for TBRC 19031^T and TBRC 19032, respectively. Comparative genomic analyses revealed the absence of genes involved in sulphate reduction and denitrification pathways and the presence of a gene encoding phosphatidylinositol synthase, distinguishing them from other Flavobacterium within the clade. Ecological profiling using public metagenomic datasets showed that both strains were associated with lotic freshwater environments. This study not only introduces Flavobacterium mekongense sp. nov. as a new species but also provides broader insights into the ecology, metabolism and environmental distribution of freshwater Flavobacterium. The genomic features identified here offer promising leads for future studies in microbial ecology, comparative genomics and functional gene mining in aquatic ecosystems. The type strain is TBRC 19031^T (TBRC 19031^T=NBRC 117006^T).

Four newly discovered Gram-stain-negative bacteria, designated as BL00010^T, BL00058, D8-11^T and BL00200, were isolated from water samples collected at three hydrological monitoring stations (namely Chiang Saen, Chiang Khan and Nong Khai) located along the Mekong River in Thailand. An investigation encompassing phenotypic, chemotaxonomic and genomic traits was conducted. The results of phylogenetic analysis based on 16S rRNA gene sequences indicated that all four isolates represented members of the genus Rhodoferax. These isolates were closely related to Rhodoferax bucti KCTC 62564^T with a similarity of 99.59%. The major fatty acids of the four novel isolates included C_16:0 and C_16:1ω7c and/or C_16 : 1ω6c, whereas the major respiratory quinone was identified as ubiquinone Q-8. In addition, phosphatidylethanolamine was identified as a major polar lipid in these bacteria. The genomes of BL00010^T, BL00058, D8-11^T and BL00200 were similar in size (3.88–4.01 Mbp) and DNA G+C contents (59.5, 59.3, 59.5 and 59.3 mol%, respectively). In contrast to R. bucti KCTC 62564^T and Rhodoferax aquaticus KCTC 32394^T, the newly discovered species possessed several genes involved in nitrite and nitrile metabolism, which may be related to their unique adaptation to nitrile-rich environments. From the results of the pairwise analysis of average nucleotide identity of the whole genome and digital DNA–DNA hybridisation, it was evident that BL00010^T and D8-11^T represented two novel species, for which we propose the nomenclature Rhodoferax potami sp. nov., with the type strain BL00010^T (TBRC 17198^T = NBRC 116413^T), and Rhodoferax mekongensis sp. nov., with the type strain D8-11^T (TBRC 17307^T = NBRC 116415^T).