Abstract:Using the experimental ecological method，the study investigated the effects of 3-hydroxybutyric acid on photosystem Ⅱ（PSⅡ）photochemical activity and pigment synthesis in Haematococcus pluvialis.The results revealed that 3-hydroxybutyric acid caused stress-induced damage to the PSⅡphotoreaction centers of the H.pluvialis.PSⅡphotochemical parameters，including maximum light energy conversion efficiency（Fv/Fm），effective light energy conversion efficiency（Yield），and apparent electron transfer rate（ETR），were all reduced to varying degrees，with the most significant effects observed in the groups treated with 3-hydroxybutyric acid at 0.02 and 0.10 μg/L.Furthermore，3-hydroxybutyric acid led to fluctuating changes in chlorophyll a content in H.pluvialis and eventually decreased it to a lower level.Additionally，3-hydroxybutyric acid promoted the synthesis and accumulation of astaxanthin in H.pluvialis，with the most potent effect observed at a concentration of 0.01 μg/L，resulting in the highest astaxanthin yield of 7.57 mg/L.Overall，3-hydroxybutyric acid affected the allocation of resources in H.pluvialis between photosynthesis and stress resistance synthesis.The findings provided a basis for using 3-hydroxybutyric acid as an exogenous inducer to promote astaxanthin production in H.pluvialis.