The brown tide algae, Aureococcus anophagefferens and Aureoumbra lagunensis (Pelagophyceae), allelopathically inhibit the growth of competing microalgae during harmful algal blooms

Abstract Allelopathy is an ecological strategy that can facilitate the occurrence of algal blooms. Here, we describe the ability of the harmful pelagophytes, Aureoumbra lagunensis isolated from coastal waters of Cuba, Florida, and Texas and Aureococcus anophagefferens (CCMP1785, CCMP1984) isolated from New York, U.S.A., to inhibit the growth of potentially co‐occurring phytoplankton. The allelopathic effects of brown tide algae were broad and strong with both whole cells and culture filtrate causing 10–96% reductions in cell abundance relative to controls in 10 phytoplankton strains originating from nine species and eight classes. The inhibition of target phytoplankton was dose‐dependent with Aureococcu s and Aureoumbra cell densities exceeding 2.5 × 10 5 cell mL −1 causing significant reductions in abundance of competing phytoplankton. Recently isolated strains of Aureoumbra (2012–2013) were more allelopathically potent than a strain isolated 20 yr ago. Passage of culture filtrate through hydrophobic resins nearly eliminated the effects of allelochemicals on eukaryotic, but not prokaryotic phytoplankton. Allelochemicals were heat‐stable, but degraded at 20°C over 2 weeks. Filtrate of each pelagophyte was capable of reciprocally inhibiting the growth of the other pelagophyte indicating each alga synthesizes a unique set of allelochemicals. Filtrate from brown tide cultures and bloom events were capable of significantly reducing densities of natural assemblages of eukaryotic and prokaryotic phytoplankton from bloom‐prone waters in New York and Florida, respectively. Collectively, these findings suggest that allelopathic inhibition of competing phytoplankton is an important process that facilitates intensification and persistence of harmful brown tide blooms caused by Aureococcus and Aureoumbra .