Psychrosphaera algicola sp. nov. and Paraglaciecola algarum sp. nov., and reclassification of Pseudoalteromonas elyakovii, Pseudoalteromonas flavipulchra, and Pseudoalteromonas profundi as later heterotypic synonyms of P. distincta, P. maricaloris, and P. gelatinilytica.

Two Gram-negative, obligately aerobic, rod-shaped bacteria, strains G1-22^T and G1-23^T, were isolated from the phycosphere of a marine brown alga. Both strains exhibited catalase- and oxidase-positive activities. Strain G1-22^T displayed optimal growth at 25 °C, pH 8.0, and 2.0-3.0% (w/v) NaCl, while strain G1-23^T exhibited optimal growth at 25 °C, pH 8.0, and 4.0% NaCl. Ubiquinone-8 was identified as the sole isoprenoid quinone in both strains. As major fatty acids (> 5%), strain G1-22^T contained C_16 : 0, summed feature 3 (C_16 : 1 ω7c and/or C_16 : 1 ω6c), summed feature 8 (C_18 : 1 ω7c and/or C_18 : 1 ω6c), C_12 : 1 3-OH, and C_10 : 0 3-OH, while strain G1-23^T contained C_16 : 0, summed feature 3 (C_16 : 1 ω7c and/or C_16 : 1 ω6c), and C_14 : 0. Phosphatidylethanolamine, phosphatidylglycerol, and diphosphatidylglycerol were major polar lipids in both strains. Strains G1-22^T and G1-23^T had DNA G+C contents of 40.2 and 38.9 mol%, respectively. Phylogenetic analyses based on 16S rRNA and genome sequences revealed that strains G1-22^T and G1-23^T formed distinct phylogenetic lineages within the genera Psychrosphaera and Paraglaciecola, respectively. Strain G1-22^T showed closest relatedness to Psychrosphaera ytuae MTZ26^T with 97.8% 16S rRNA gene sequence similarity, 70.2% average nucleotide identity (ANI), and a 21.5% digital DNA-DNA hybridization (dDDH) value, while strain G1-23^T was most closely related to Paraglaciecola aquimarina KCTC 32108^T with 95.6% 16S rRNA gene sequence similarity, 74.6% ANI, and a 20.1% dDDH value. Based on phenotypic and molecular characteristics, strains G1-22^T and G1-23^T are proposed to represent two novel species, namely Psychrosphaera algicola sp. nov. (type strain G1-22^T=KACC 22486^T=JCM 34971^T) and Paraglaciecola algarum sp. nov. (type strain G1-23^T=KACC 22490^T=JCM 34972^T), respectively. Additionally, based on the comparison of whole genome sequences, it is proposed that Pseudoalteromonas elyakovii, Pseudoalteromonas flavipulchra, and Pseudoalteromonas profundi are reclassified as later heterotypic synonyms of Pseudoalteromonas distincta, Pseudoalteromonas maricaloris, and Pseudoalteromonas gelatinilytica, respectively.