Many microbes cooperatively secrete extracellular products that favorably modify their environment. Consistent with social evolution theory, structured habitats play a role in maintaining these traits in microbial model systems, by localizing the benefits and separating strains that invest in these products from ‘cheater’ strains that benefit without paying the cost. It is thus surprising that many unicellular, well-mixed microalgal populations invest in extracellular toxins that confer ecological benefits upon the entire population, for example, by eliminating nutrient competitors (allelopathy). Here we test the hypotheses that microalgal exotoxins are (1) exploitable public goods that benefit all cells, regardless of investment, or (2) non-exploitable private goods involved in cell-level functions. We test these hypotheses with high-toxicity (TOX+) and low-toxicity (TOX-) strains of the damaging, mixotrophic microalga Prymnesium parvum and two common competitors: green algae and diatoms. TOX+ actually benefits from dense populations of competing green algae, which can also be prey for P. parvum, yielding a relative fitness advantage over coexisting TOX-. However, with non-prey competitors (diatoms), TOX- increases in frequency over TOX+, despite benefiting from the exclusion of diatoms by TOX+. An evolutionary unstable, ecologically devastating public good may emerge from traits selected at lower levels expressed in novel environments.

Evolution 2012 summary dataThese are summary data from laboratory experiments described in the paper in Driscoll et al. (2012). Most data were collected as simple cell counts with a hemacytometer. A few are based on automated cell size measurements using ImageJ. D4 and D5 are short for Dunaliella started at 10^4 cells mL^-1 and 10^5 cells mL^-1, respectively. (So 'TOX1 and D4' means TOX1 initiated at 10^3 with Dunaliella at 10^4.) Corresponding figures are shown somewhere relatively obvious on each worksheet in blue.