Effects of organic carbon source and pH on growth, astaxanthin accumulation and endogenous phytohormone secretion of Haematococcus pluvialis

Endogenous phytohormones play a vital role in the growth and secondary metabolism performance of microalgae and their secretion is significantly affected by external environmental conditions. In this study, the combined influences of three carbon sources (glucose, sodium pyruvate, and sodium acetate) and three pH levels (6, 8 and 10) on growth, astaxanthin accumulation, and contents of four endogenous phytohormones (indole-3-acetic acid, IAA; zeatin riboside, ZR; gibberellic acid 3, GA(3); abscisic acid, ABA) of Haematococcus pluvialis were investigated. The results showed that carbon source and pH level had significant isolated and interactive effects on growth, astaxanthin accumulation, and endogenous phytohormone secretion. The cell density, specific growth rate and cell dry weight reached the highest levels in the glucose treatment at pH 8. The maximum astaxanthin concentration and productivity occurred in the glucose treatment with pH 10. The contents of IAA, ZR and GA(3) significantly increased at the green stage (vegetative cell stage), indicating their potential involvement in cell proliferation. ABA contents, in contrast, significantly increased at the red stage (red cyst cell stage), probably contributing to astaxanthin accumulation. This experiment provided valuable insights for optimizing the mixotrophic cultivation conditions of H. pluvialis and successfully developing biological regulators.