Karlodinium veneficum, or K. veneficum, is a dinoflagellate – a single-celled organism, somewhere between an animal and a plant – found in both marine and estuarine ecosystems. This organism can produce harmful algal blooms (HABs) in abundance as well as a lethal toxin that has been responsible for fish-kills around the world, including on Maryland’s Eastern Shore in the 1990s. How, exactly, this happened was unknown, until recently.
From a 60-year-old K. veneficum culture kept at the U.K.’s Plymouth Laboratory, a research team led by the University of Maryland Center for Environmental Science (UMCES) discovered the cause of the Chesapeake Bay’s fish-kill, which was originally blamed on another phytoplankton, Pfiesteria piscicida. UMCES researchers determined that Pfiesteria was not, in fact, the cause of the Bay’s fish-kill; rather, the karlotoxins found in K. veneficum were to blame.
In a paper recently published in Nature Scientific Reports, using surface plasma resonance and a suite of bioassays, the authors report the functions of membrane pores formed by these karlotoxins. Moreover, the formation is a highly targeted mechanism involving the cell membrane and sterol-specificity; cholesterol, in this case.
"What is so unique about this toxin is its mode of action, how it works," said Allen Place, an UMCES Professor and expert in biochemistry, serving as the primary investigator on the project. "To consume other phytoplankton, it generates holes in their cell walls, creating a porous membrane through which other ions can flow; this incapacitates and eventually kills its prey, which it then eats."
In the early 1990s, fish, menhaden specifically, swam through a bloom of K. veneficum in the Chesapeake Bay at which point the toxin was transferred to their gills, incapacitating the affected fish. As a result, Pfiesteria started growing and eating the dead and decaying menhaden.
"It was a new organism – a plant that eats other plants and produces a toxin that, as it turns out, could have useful clinical applications in cancer research," said Mark T. Hamann, co-author, professor of biomedical science at the Medical University of South Carolina and UMCES Adjunct Professor. "We have yet to complete the major hurdle of generating enough metabolites to determine the potential for the treatment of cancer, but this is definitely a goal associated with this discovery." The membrane of lung cancer cells, specifically, are a potential target for the control of cancer utilizing karlotoxins and could be used in the development of antibody-drug conjugates, or ADCs, which target and kill cancer cells while leaving healthy cells intact.
The team’s analysis proves karlotoxins generate pores in membranes with desmethyl sterols – like cholesterol, phytosterols and ergosterol – that weaken membranes, disrupt motor function, cause cells to swell, and eventually, burst, all for the sake of capturing and devouring susceptible organisms. They use the toxin as a defense mechanism, too.
"Think of a venomous snake that stores venom inside special glands – the venom can’t harm the snake, in other words, it can’t self-intoxicate due to the make-up of its anatomy, but it can certainly harm creatures it comes into contact with," Place said. "The same holds true for K. veneficum, and it’s not unique to Chesapeake Bay; this has a world-wide distribution. Moreover, it’s sensitive to environmental inputs such as global warming, so the higher the temperature, the more abundant this organism becomes."
Since the initial description of K. veneficum (fka, Gymnodinium veneficum) in 1956 by British scientist, Dorothy Ballentine, toxic and nontoxic strains have been observed. This, however, is the first report of the functional properties of the membrane pores formed by karlotoxins and is consistent with the initial 1957 observations of Ballantine and B.C. Abbott. This organism is now recognized as a harmful algal species with extensive ecosystem impacts.
Although the toxin has proven deadly to fish, shellfish, small sharks and even laboratory mice, it’s important to note that the presence of cholesterol deactivates the threat, similar to how vinegar neutralizes lye. There is little physical impact to people. The impact to the environment, however, could potentially be significant.
"Relative to climate change, organisms like this that act as both plant and animal will become more abundant due to higher temperatures and elevated nutrients in the water as a result of pollutant run-off," Place said. "HABs like karlotoxins will be the result and they can definitely impact our ecosystem and human health."
The research paper, entitled, "Sterolysin from a 1950s culture of Karlodinium veneficum (aka Gymnodinium, Ballantine) forms lethal sterol dependent membrane pores," was published in Nature Scientific Reports (DOI: 10.1038/s41598-024-68669-0) on August 3, 2024.
This work was a collaboration between scientists at the University of Maryland Center for Environmental Science, Morgan State University (Baltimore, MD), Medical University of South Carolina, Rush University Medical Center (Chicago, IL) and the University of Central Oklahoma.
For additional information, send an email to Katie Ross (kross@umces.edu).
