Characterization of Sphingomicrobium aquimarinum sp. nov. and Sphingomicrobium maritimum sp. nov. highlights astaxanthin-producing bacteria in the family Sphingomonadaceae.

Bacteria are considered as better models for industrial production of astaxanthin (AXT) because of their fast growth and convenience for the downstream extraction. Nevertheless, few bacterial species of native AXT producers are explored to date. In this study, bacterial strains XHP0235 (=GDMCC 1.3093 = MCCC 1K07532 = JCM 35574) and XHP0239 (=GDMCC 1.3086 = MCCC 1K07530 = JCM 35575) were isolated from coastal seawater of China. Polyphasic taxonomy suggested that strains XHP0235 and XHP0239 were closely related to Sphingomicrobium aestuariivivum AH-M8, S. arenosum CAU 1457, and S. astaxanthinifaciens CC-AMO-30B, and should be recognized as two novel species of the genus Sphingomicrobium, for which the names Sphingomicrobium aquimarinum sp. nov. and Sphingomicrobium maritimum sp. nov. are proposed, respectively. Comparative genomic analysis revealed the complete AXT pathway (genes crtB, crtI, crtY, crtZ, and crtW) present in strains XHP0235, XHP0239 and the three relative strains, and the carotenoid-targeted metabolome analysis confirmed the productions of AXT and other carotenoids. Inspired by these findings, the AXT pathway was further explored in the family Sphingomonadaceae, and 45 (18.8 %) out of the 240 type strains was identified to harbor all the five genes for AXT synthesis, of which the gene arrangement can be classified as seven types. The flanking gene contents were extremely variable with a concerted evolutionary history of the AXT genes. However, the AXT pathway is still predictable if an isolate is closely related to a known AXT-pathway-positive strain (evolutionary distance <0.14). This study significantly enlarged the diversity of AXT-producing bacteria with highlighting such species in the family Sphingomonadaceae.