Interactions between the brown tide-causing microalga Aureococcus anophagefferens and the small diatom Minutocellus polymorphus under laboratory culture

Brown tides caused by Aureococcus anophagefferens (Pelagophyta) have broken out frequently and have caused serious ecological problems in the coastal waters of the Bohai Sea, China, since 2009. During the brown tides caused by A. anophagefferens, the small marine diatom Minutocellus polymorphus has often become dominant, co-existing with A. anophagefferens. To investigate the bloom formation mechanisms and co-existence factors involved, bi-algal culture and growth experiments with culture filtrates were performed under laboratory-controlled conditions. In our experiments, A. anophagefferens and M. polymorphus growth was allelopathically suppressed by the other species, depending on initial cell densities and growth stages; however, the species did not outcompete each other during the observed time frame. The results also suggested that the relevant allelochemicals might be a mixture containing some temperature-sensitive compounds. Growth simulation performed via the Lotka-Volterra model indicated that the initial cell density of these two species was critical in determining their growth and competition in bi-algal culture. Our results showed that a similar allelopathic effect was exerted by A. anophagefferens on M. polymorphus and vice versa. This effect could be important in allowing the observed co-existence of these two species in natural waters during brown tides.