Loss of waterborne brevetoxins from exposure to phytoplankton competitors

We tested whether interactions among phytoplankton competitors affect toxin dynamics involving the red tide dinoflagellate Karenia brevis, whose brevetoxins incapacitate and kill coastal wildlife. The addition of a live diatom, Skeletonema costatum, led to decreased concentrations of brevetoxin B (PbTx-2) associated with K. brevis cells in co-culturing experiments and with two of three natural bloom samples containing K. brevis. Similar decreases in PbTx-2 concentration, but not PbTx-3 concentration, occurred when a mixture of brevetoxins (without live K. brevis cells) was exposed to S. costatum, indicating that S. costatum metabolizes waterborne PbTx-2. Liquid chromatography–mass spectrometry (LC–MS) and ELISA analyses indicated that PbTx-2 is probably not transformed into other brevetoxins or into known brevetoxin metabolites, and instead is biotransformed by a previously unrecognized mechanism. Four different S. costatum strains from around the world caused similar loss of PbTx-2, suggesting that evolutionary experience with K. brevis is not a pre-requisite for the ability to metabolize PbTx-2. Additionally, phytoplankton-associated bacteria were found to play no role in the loss of PbTx-2, as bacteria-free S. costatum strains metabolized PbTx-2. Finally, loss of waterborne PbTx-2 caused by exposure to a dinoflagellate, a cryptophyte, and two additional diatom species indicates that this phenomenon is widespread among phytoplankton. Our results unexpectedly suggest that competing phytoplankton species present during K. brevis blooms, and possibly other red tides, could mediate bloom toxicity and therefore ecosystem-level consequences of red tides.