Spectral characteristics and fluorescence of lycopene from a purple-colored Microbacterium albipurpureum ET2

A recently described novel species of Microbacterium albipurpureum ET2, isolated from the aerial roots of a leafless orchid, revealed strong plant growth-promoting activities. It also showed unique pigmentation, changing from white-yellowish to bright purple. In this study, using genome mining, the strain was found to possess a non-mevalonate biosynthetic pathway and the genes responsible for С40 carotenoid production. Several biosynthetic gene clusters (BGC) were predicted by antiSMASH, including a terpene synthesis-encoding one, consisting of seven genes with structural and sequence similarity to known BGCs. The core crtEBI genes for lycopene biosynthesis are clustered together in the genome. Biosynthesis of lycopene was confirmed by mass-spectrometry in both purple and yellowish cultures. Complementary spectroscopic techniques, such as Raman, UV-Vis, and fluorescence spectroscopy, also revealed the presence of lycopene and its precursor, phytoene. Fluorescence from S2 sub-energy levels, with lifetimes of 0.8 ns, 3.2 ns, and 9.0 ns, determined by time-correlated single photon counting, was detected. Narrowing Raman peak shifts by FWHM reach 15-22% for the purple-colored culture compared to yellowish one, suggesting aggregation of lycopene molecules.