Evidence for inhibition of bacterial luminescence by allelochemicals from Fibrocapsa japonica (Raphidophyceae), and the role of light and microalgal growth rate

The marine microalga Fibrocapsa japon- ica Toriumi and Takano (Raphidophyceae) produces haemolysins, neurotoxins and reactive oxygen spe- cies (ROS). To quantify potential effects of such bioactive compounds on surrounding organisms the marine bacterium Vibrio fischeri was exposed to F. japonica culture samples. Inhibition of V. fischeri 's natural luminescence, indicative of impaired metab- olism, was related to the number of F. japonica cells added. The effect was fast, within 15 min. It was caused by one, possibly several, excreted substances that were less active after heating. Freezing of culture supernatant partly inactivated these substances, but ROS-scavenging enzymes had no effect. Light enhanced the V. fischeri luminescence inhibition in two ways. The direct effect of light on the action of F. japonica luminescence inhibiter(s) could be de- scribed by a saturation curve with maximum effect above 20 mmol photons m � 2 s � 1 . Light also had an indirect effect: biomass production, dependent on light availability, was closely related to the amount of inhibiting compound(s) produced by the alga. Algal growth rate, rather than its cell density, determined the bacterial luminescence inhibition per F. japonica cell, resulting in a 5-fold stronger inhibition at maximum growth rates compared to cells that barely grew during the stationary growth phase. The bioas- say with F. japonica and V. fischeri has allowed quantification of the negative effects on bacteria in the microalgal microenvironment. The results pre- sented here suggest that at favourable growth condi- tions F. japonica releases bioactive compounds that improve its competitive abilities.