In collaboration with colleagues at the Rosenstiel School of Marine and Atmospheric Science, VIMS researchers have found that the dinoflagellate Pfiesteria shumwayae most likely lacks the molecular machinery to express a fish-killing toxin. Their findings, recently reported in the Proceedings of the National Academy of Sciences, raise important questions about whether a Pfiesteria toxin exists.
Previous reports have held secretion of one or more toxin(s) by P. shumwayae and P. piscicida responsible for fish-lesion outbreaks, fish-kill events, and a suite of human ailments in East Coast estuaries. However, a Pfiesteria toxin has never been identified or isolated.
Dinoflagellates are natural and often beneficial components of aquatic ecosystems. However, a few dinoflagellate species can cause or contribute to "harmful algal blooms," or HABs. Most toxins produced by HAB dinoflagellates, including all toxins known to debilitate or kill fish (ichthyotoxins), are either polyketides or non-ribosomal peptides.
Polyketides are produced by the PKS family of enzymes. Reasoning that Pfiesteria would likely contain a PKS gene if it produces an ichthyotoxin, genetic researchers at Rosenstiel and VIMS tested the Pfiesteria genome for the presence of PKS genes—without success. "PCR primers that amplify the polyketide synthase (PKS) genes that produce polyketides in dinoflagellates do not yield a product with P. shumwayae DNA," says Dr. Kimberly Reece, a VIMS molecular geneticist and co-author of the PNAS report.
Tests with NPRS primers likewise failed to amplify the genes that dinoflagellates use to produce non-ribosomal peptides. However, these primers did amplify a gene fragment from P. shumwayae DNA that appears similar to the dinoflagellate genes that synthesize polyketides or fatty acids. "This gene may be involved in fatty acid biosynthesis, rather than production of a polyketide toxin," says Reece. "We need to further characterize it and examine its activity to definitely determine its function; particularly in light of the lack of other evidence for exotoxin production by P. shumwayae."
While the researchers' work does not entirely eliminate the possibility that Pfiesteria might produce polyketide toxins, it strongly suggests that any polyketides the dinoflagellate might produce are either non-toxic, not secreted into the water, or not released when a dinoflagellate cell ruptures.