Melatonin: A Multifunctional Molecule That Triggers Defense Responses against High Light and Nitrogen Starvation Stress in Haematococcus pluvialis

Melatonin (MLT), a ubiquitously distributed small molecule, functions in plant responses to various biotic and abiotic stresses. However, the interactions between melatonin and other important molecules in Haematococcus pluvialis response stresses are largely unknown. In the present study, exogenous melatonin improved H. pluvialis resistance to nitrogen starvation and high light. We concluded that exogenous melatonin treatment prevented the reactive oxygen species (ROS) burst and limited cell damage induced by abiotic stress through activation of antioxidant enzymes and antioxidants. Astaxanthin, a major antioxidant in H. pluvialis cells, exhibited a 2.25-fold increase in content after treatment with melatonin. The maximal astaxanthin content was 32.4 mg g^-1. The functional roles of the nitric oxide (NO)-mediated mitogen activated protein kinase (MAPK) signaling pathway and cyclic adenosine monophosphate (cAMP) signaling pathway induced by melatonin were also evaluated. The results clearly indicate that cAMP signaling pathways are positively associated with microalgal astaxanthin biosynthesis. Additionally, the NO-dependent MAPK signaling cascade is activated in response to astaxanthin accumulation induced by melatonin, confirming that MAPK is a target of NO action in physiological processes. This work is the first to use H. pluvialis as in vivo model and documents the influence of melatonin on the physiological response to abiotic stress in this microalgae.