Explosive mechanism of red jets Heterosigma akashiwo and Chaetoceros mulleri under different culture conditions

Heterosigma akashiwo and Chaetoceros mulleri are prominent red tide species in the South China Sea, yet their outbreak mechanisms remain poorly understood. This study investigated their growth under varying nitrogen (N) to phosphorus (P) ratios, nitrogen concentrations, salinities, and light intensities. Results showed that optimal growth light for both species is 4000 lx, with H. akashiwo better adapted to high light intensities. Under these conditions, biomass increased with phosphorus concentration. H. akashiwo thrived at an N : P ratio of 9 : 1, while C. mulleri preferred 18 : 1. Both species exhibited optimal growth at 37.5 mg per L nitrogen, with C. mulleri absorbing nitrogen more rapidly. Optimal salinity for both was 20 psu, but C. mulleri demonstrated greater salt stress tolerance. A specific relationship between light and N : P ratio was noted for H. akashiwo, with maximum light inhibition occurring at N : P = 18 : 1 and minimal inhibition at N : P = 144 : 1. Significant biomass variations for H. akashiwo were observed under different light intensities and phosphorus levels.