Novel insights into saline stress on photosynthetic activity and astaxanthin production of Haematococcus pluvialis

Saline treatment is a low-cost, simple, and effective method to stimulate astaxanthin accumulation in Haematococcus pluvialis, and is proposed to be applied in the second stage of a 2-stage culture since it does not necessitate changing the medium. To understand the effect of salinity on the astaxanthin production of H. pluvialis, the photosynthetic activity and the biocomponents production in 1- and 2-stage cultures in different salinities were investigated. Except for astaxanthin synthesis, which increased at low salinities of 2 and 5-g/L NaCl, most biocomponent yields decreased in 1-stage cultures as salinity increased. At a salinity of 5-g/L NaCl, the 2-stage culture further increased astaxanthin production to 18.41±0.24 mg/L, which was more than 2.0 times that of the control. Saline treatment led to an overall decrease in photosynthetic performance indices of H. pluvialis, and had an impact on five sites of the electron transport chain: the energy connection between antenna and reaction center of photosystem II (PS II), oxygen evolving complex activity on the donor side, the electron transfer from plastoquinone A (Q_A) to plastoquinone B (Q_B) and from plastoquinone (PQ) to receptor side of photosystem I (PS I), and the pool size of the end electron acceptors in PS I acceptor side. The excitation imbalance between PS I and PS II caused by the variance in the electron transfer chain necessitated the synthesis of antioxidants like astaxanthin in order to ensure cell viability. The accumulation of astaxanthin was found to be closely correlated with the stabilized or enhanced the maximum relative electron transfer rate (rETR_max) and the PS II actual quantum yield (QY_SS) as well as the increased fluorescence yield at J-step (V_J). This work offers the novel insight of how saline stress controls H. pluvialis photosynthetic activity and astaxanthin synthesis.