Evolution in toxic blooms
”(F)or the plankton environment, whose allelochemistry has been much studied and discussed, the lack of contact between competing organisms combined with the dispersion of allelochemicals through the water essentially invalidates evolutionary analogies with terrestrial systems or aquatic systems that have a fixed spatial structure. This difficulty has been passed over by students of plankton allelochemistry. As a result, plankton allelochemistry has been consistently viewed in the wrong evolutionary context.” -William Lewis, 1986
Toxic algal blooms present a major challenge to our current understanding of the evolution of so-called “public goods” traits. In a wide array of taxa, including dinoflagellates, diatoms, cyanobacteria, and haptophytes, populations of unicellular (or sometimes colonial) algae can grow to extremely high densities, killing prey, competitors, and predators en route to an ecosystem-disruptive bloom. However, the many population-level beneficial effects of extracellular toxins--including reduced competitive and grazing pressures--would appear to be highly susceptible to cheating. Cheaters are mutants that do not invest in the toxin themselves, but can still benefit from these population-level effects of toxin production by neighboring cells. Many of the species that show this behavior exist in an apparently well-mixed, free-swimming state, and evidence is fast accumulating that these populations are genetically and functionally diverse. These conditions would appear to stack the deck against the sort of broad cooperation we seem to observe in toxic algal blooms. How, then, is algal toxicity maintained by natural selection?
My work focuses on the toxic, unicellular, mixotrophic haptophyte Prymnesium parvum (see below for a few pictures). P. parvum is an invasive pest in inland waterways in several U.S. states, including Texas, where it has caused an estimated $13 million in damage through fish kills, water fouling, and remedial action. I have isolated several genetically separate strains from one late bloom population, including two coexisting strains that differ markedly in their toxicity, growth parameters, and competitive ability. I’m currently pursuing the above question as it applies to P. parvum using theoretical and empirical tools.