Fluorescence-based bioassays with dose–response curve and relative potency in measuring algicidal virulence of Bacillus sp. B1 exudates against Heterosigma akashiwo

Abstract

This study demonstrated a Bacillus strain, B1, which was isolated from Pearl river estuary, China, and extracted extracellular algicidal compounds (EACs), exhibited algicidal effects against H. akashiwo upon fertilization experiments (>90% growth inhibition when exposed to 10% volume concentration of EACs within 96 h). Here we evaluated the feasibility of fluorescence-based bioassays (auto or stained fluorescence detected from on PAM fluorometry and flow cytometry) in quantifying algicidal potency of EACs released from Bacillus sp. B1 on H. akashiwo. Esterase activity and maximum photosystem II quantum yield (Fv/Fm) inhibition were used as sensitive endpoints in the bioassays. Logarithmic dose–response curve (DRC) based on three-parameters log-logistic model was applied to derived effective EACs concentrations (ECy, y being typically 10%, 50% or 90% of maximal effect) and relative potency (RP) was used to compare esterase activity and Fv/Fm inhibition sensitivities in dose-dependent manner. Esterase activity inhibition was more sensitive when exposed to low-dose EACs (RP10, fv/fm = 0.57 ± 0.01 < 1), conversely, Fv/Fm inhibition was accepted as a sensitive parameter when H. akashiwo exposed to higher-EACs doses. The fluorescence-based bioassays with dose–response curve and relative potency will help to assess bacterial virulence against H. akashiwo and its physiological mechanistic studies, and may be applicable for further insights into the role and influence of bacteria producing bioactive compounds in harmful algae blooms and shaping marine ecosystems.