Effects of two species of macroalgae--Ulva pertusa and Gracilaria lemaneiformis--on growth of Heterosigma akashiwo (Raphidophyceae)

The effects of fresh thalli, culture filtrate, water-soluble extract and dry powder of two species of macroalgae, Ulva pertusa (Chlorophyta) and Gracilaria lemaneiformis (Rhodophyta), on the growth of a bloom-forming microalga, Heterosigma akashiwo, were studied in co-culture under controlled laboratory conditions. Both fresh thalli and culture filtrate of U. pertusa and G. lemaneiformis, particularly in the form of fresh thalli, significantly inhibited microalgal growth; indeed, the microalga was completely killed during the course of the experiment. A clear concentration-dependent relationship was observed between the initial concentration of fresh thalli (either U. pertusa or G. lemaneiformis) and its inhibitory effect on H. akashiwo. Simultaneous nutrient assays showed that nitrate and phosphate were almost exhausted in G. lemaneiformis fresh thalli co-culture but remained well above nutrient limitation for microalgal growth in U. pertusa co-culture, in which the microalgal cells were completely killed. However, daily f/2 medium repletion would obviously alleviate the growth inhibition in G. lemaneiformis co-culture. Since the present study was carried out under controlled conditions, fluctuations in environmental factors (i.e., light, temperature, carbon limitation, bacterial presence and pH) were limited during the experiment. We thus concluded that allelopathy was the most likely explanation for microalgal growth inhibition in U. pertusa co-culture, while the combined roles of allelopathy and nutrient limitation were responsible for growth inhibition in G. lemaneiformis co-culture. Similarly, macroalgal water-soluble extracts and dry powders affected the co-cultured H. akashiwo greatly, with more obvious effects observed in water-soluble extract co-cultures. A dose-dependent relationship was also observed over the course of the experiment. It can be concluded that macroalgal thalli contain some bioactive compounds. The results of the present study suggest that U. pertusa and G. lemaneiformis, especially in the form of fresh thalli, effectively inhibit the growth of H. akashiwo and could thus be potential candidates for use in the control and mitigation of H. akashiwo blooms.