Salt-stress and plant hormone-like responses for selective reactions of esterified xanthophylls in the aerial microalga Coelastrella sp KGU-Y002

Esterification of xanthophylls is important for improving their bioactive potency. The effects of metal cations on esterified xanthophyll biosynthesis and on the accumulation and composition of salt stress-induced carotenoids were investigated in the aerial microalga Coelastrella sp. KGU-Y002 (Chlorophyta, Scenedesmaceae). Under nitrogen-deficient and high-light conditions, salt-supplemented cultures showed enhanced synthesis of esterified astaxanthin, adonixanthin, and zeaxanthin. In particular, KCl supplementation caused a remarkable increase in esterification, such that 73 % of xanthophylls were esterified. After 1 day of KCl treatment, the concentration of endogenous abscisic acid had increased to 200-fold its initial concentration. In algal cells treated with abscisic acid in nitrogen-deficient and high-light conditions, the reactive oxygen species concentration was almost the same as that in the KCl-supplemented culture. In these algal cells, abscisic acid enhanced esterification without affecting total carotenoid accumulation. These findings suggested that xanthophyll esterification is regulated during the response to salt stress by abscisic acid, which has plant hormone-like bioactivity in this aerial microalga. Further studies have been designed to examine large-scale production of esterified xanthophylls by this aerial microalga under salt-stress conditions, and to assess the bioactivity of esterified xanthophylls extracted from algal cells.