Toxins and Toxicity from the Cosmopolitan, Bloom-Forming Dinoflagellate Karlodinium micrum

Abstract

Karlodinium micrum (Leadbeater and Dodge) Taylor was first described in the United States during an investigation into the cause of repeated fish kills at an estuarine aquaculture facility located on a tributary of the Chesapeake Bay, MD, USA. As part of that investigation we described toxins in this species for the first time. Named karlotoxins (KmTx), these compounds possess hemolytic, cytotoxic, and anti-fungal properties. However, the primary harmful effect associated with blooms of this organism is ichthyotoxicity. Karlotoxins are lethal to fish through damage to gill epithelia. This research focused on two of these toxins, KmTx 1 (1338 Da.) and KmTx 2 (1344 Da.) These two toxins have been the main toxins, in terms of amount and potency, in all US isolates tested to date. Using a range of mammalian cell types, the mode of KmTx 2 cytotoxicity was shown to be non-selective permeabilization of cell membranes to a range of small ions and molecules resulting in cell death through osmotic lysis. Membrane sterol composition appears to play a role in the sensitivity of different species to KmTx's membrane disrupting effects. This sterol specificity also appears to be responsible for the apparent immunity of K. micrum from its own toxins. We have described various karlotoxins in K. micrum isolates and bloom samples from US east coast estuaries from Maryland to Florida. Among US east coast isolates, a geographic strain variation appears to exist in that KmTx 1 has only been found in Maryland isolates while KmTx 2 has been found in all other isolates tested from North Carolina, South Carolina, and Florida. Recently, a KmTx 2-like compound (1342 Da.) has been isolated from bloom samples from Western Australia, being the first confirmation of karlotoxin production outside of the United States. This work confirms the association between blooms of K. micrum and fish kills that has been observed in temperate estuaries around the world for over half a century. It also lays the foundation for future studies to determine the ecological function of toxin production in this species and the consequences of this production both on K. micrum's environment and ours.