Allelopathic effect of Pyropia haitanensis (Rhodophyta) on the bloom-forming Skeletonema costatum (Bacillariophyta)

Blooms caused by microalgae have become a serious threat to public health, fisheries, ecosystems and economies around the world. Recent studies have indicated that some macroalgae have the potential to be used to control bloom-forming microalgae. Here, we report the ability of Pyropia haitanensis, a commercial macroalga cultivated in southeastern China, to inhibit the growth of the common bloom-forming diatom Skeletonema costatum. The results of initial experiments using fresh thalli of P. haitanensis at various temperatures clearly indicated that this macroalga had a high inhibition effect on the growth of S. costatum at 21 °C. Therefore, further treatments with fresh thalli, culture filtrate, dry powder and water-soluble extract were used to determine the dose-dependent inhibition effect on the growth of S. costatum at 21 °C by P. haitanensis. Fresh thalli were potent and significantly inhibited the growth of S. costatum at all concentrations, but the microalga was alive at a very low density. Treatments with culture filtrate, dry powder and water-soluble extract greatly affected the growth of S. costatum at different concentrations, and no living microalgal cells were found after treatment with the water-soluble extract. The present study was carried out under controlled laboratory conditions excluding fluctuations in light and temperature. Therefore, our results, together with the well-known nutrient removal capacity of macroalgae, allows us to conclude that P. haitanensis contains bioactive compounds and allelopathy is the most likely reason for the negative effect on S. costatum.