Social evolution of toxic metal bioremediation in Pseudomonas aeruginosa

Abstract

Bacteria are often iron-limited, and hence produce extracellular iron-scavenging siderophores. A crucial feature of siderophore production is that it can be an altruistic behaviour (individually costly but benefitting neighbouring cells), thus siderophore producers can be invaded by non-producing social 'cheats'. Recent studies have shown that siderophores can also bind other heavy metals (such as Cu and Zn), but in this case siderophore chelation actually reduces metal uptake by bacteria. These complexes reduce heavy metal toxicity, hence siderophore production may contribute to toxic metal bioremediation. Here, we show that siderophore production in the context of bioremediation is also an altruistic trait and can be exploited by cheating phenotypes in the opportunistic pathogen Pseudomonas aeruginosa. Specifically, we show that in toxic copper concentrations (i) siderophore non-producers evolve de novo and reach high frequencies, and (ii) producing strains are fitter than isogenic non-producing strains in monoculture, and vice versa in co-culture. Moreover, we show that the evolutionary effect copper has on reducing siderophore production is greater than the reduction observed under iron-limited conditions. We discuss the relevance of these results to the evolution of siderophore production in natural communities and heavy metal bioremediation.

Keywords: Pseudomonas; bacterial communities; bioremediation; cooperation; public goods.

Figure 1.

(a) After 100 generations, de novo siderophore non-producing mutants reach far higher frequencies in highly toxic copper populations than all other populations (Kruskal–Wallis and Kruskal mc: H₃ = 29.4762, p < 0.001. (b) Per capita pyoverdine production is reduced after evolving in highly toxic copper for approximately 100 bacterial generations, compared with other metal treatments (one-way ANOVA and Tukey HSD, F_3,44 = 45.95, p < 0.001). (c) The reduction in per capita pyoverdine production instigated by high copper toxicity (relative to high iron control) is stronger than the reduction caused by iron limitation (relative to standard KB broth; Welch's t-test, t_12.254 = 4.2841, p < 0.005). When y-axis = 1, treatment has no effect of reducing pyoverdine relative to control. Error bars represent standard error.

Figure 2.

The relative fitness of mutants compared with wild-type (W) depends on an interaction between metal species (copper or iron) and growth conditions (monoculture, grey; co-culture, black; two-way interaction between metal and growth conditions: F_1,88 = 30.583, p < 0.001), and an interaction between metal species and metal concentration (high = 6.17 mM, low = 624 μM; two-way interaction between metal species and concentration, F_1,88 = 11.612, p < 0.001). In co-culture, W > 1 (i.e. mutants invade) for highly toxic copper concentrations (one-sample t-test on relative fitness, alternative = 1, t₁₁ = 7.3307, p < 0.001).