Effects of Two Potential Allelochemicals on the Photosystem II of Nitzschia closterium and Monostroma nitidum

In aquaculture, high-density seaweed farming brings higher economic benefits but also increases outbreaks of diatom felt. The effective control of diatom felt in high-density seaweed farming has always been a research hotspot.This study selected two potential allelochemicals 2-hydroxycinnamic acid and quinic acid to explore their effectson a diatom Nitzschia closterium and an economic seaweed Monostroma nitidum. The results showed that2-hydroxycinnamic acid had better inhibitory effects than quinic acid on the growth, pigment content and photosynthetic efficiency of N. closterium. Their half-maximal inhibitory concentrations at 120 h (IC_{50–120 h}) were0.9000 and 1.278 mM, respectively. Additionally, these allelochemicals had limited inhibitory effects on thegrowth, pigment content and photosynthetic efficiency of M. nitidum before 24 h. To further explore the allelopathic effect of these chemicals, this study focused on the photosystem II energy fluxes of N. closterium. It wasfound that 3 mM 2-hydroxycinnamic acid could destroy the whole photosynthetic system by devastating the PSIIreaction centre (RC) before 24 h; however, the same concentration of quinic acid could only down-regulate theelectron transport efficiency by changing the effective antenna size of an active RC and downregulating the PSIIreaction centre density. These experimental results are expected to provide a new strategy to control diatom feltblooms on the high-density seaweed farming areas.