LHCF15 facilitates the absorption of longer wavelength light and promotes growth of Phaeodactylum tricornutum under red light

Abstract

Diatoms are important primary producers on the Earth and are widely distributed, partially because of their successful adaptation to habitats characterized by sharp fluctuations in light intensity and light quality. Monochromatic red light reportedly induces red shifts of absorption and fluorescence spectra in diatoms; however, the detailed mechanism remains unclear. In this study, LHCF15, one of light-harvesting protein of the pennate diatom Phaeodactylum tricornutum, was knocked out using CRISPR-Cas9. The f15-KO strain showed different responses to monochromatic red light. Quantitatively PCR and western blot results suggested that in the wild type, LHCF15 expression was negligible under blue and control light; however, remarkably upregulated expression and accumulation of LHCF15 in large quantities was observed under monochromatic red light, accompanied by an evident red shift of the fluorescence spectrum. These phenomena were absent in the f15-KO strain but were restored in the complemented strain. Compared the amino acid sequences with other LHCf and LHCr, the His was replaced by Asn73 in the helix B of LHCF15, which may be the main cause of the red shift after the accumulation of LHCF15. Growth results showed that the knockout of LHCF15 had no significant effect on growth under blue and control light but significantly hindered growth under monochromatic red light (with a peak at 630 nm). When cultured under longer wavelength light with a peak at 682 nm, growth of f15-KO strains completely ceased; however, the wild type still could grow at low rate. Our results convincingly demonstrate that LHCF15 protein plays a key role in the adaptation of diatoms to longer wavelength light conditions.