Iron acquisition strategies in pseudomonads: mechanisms, ecology, and evolution

Abstract

Iron is important for bacterial growth and survival, as it is a common co-factor in essential enzymes. Although iron is very abundant in the earth crust, its bioavailability is low in most habitats because ferric iron is largely insoluble under aerobic conditions and at neutral pH. Consequently, bacteria have evolved a plethora of mechanisms to solubilize and acquire iron from environmental and host stocks. In this review, I focus on Pseudomonas spp. and first present the main iron uptake mechanisms of this taxa, which involve the direct uptake of ferrous iron via importers, the production of iron-chelating siderophores, the exploitation of siderophores produced by other microbial species, and the use of iron-chelating compounds produced by plants and animals. In the second part of this review, I elaborate on how these mechanisms affect interactions between bacteria in microbial communities, and between bacteria and their hosts. This is important because Pseudomonas spp. live in diverse communities and certain iron-uptake strategies might have evolved not only to acquire this essential nutrient, but also to gain relative advantages over competitors in the race for iron. Thus, an integrative understanding of the mechanisms of iron acquisition and the eco-evolutionary dynamics they drive at the community level might prove most useful to understand why Pseudomonas spp., in particular, and many other bacterial species, in general, have evolved such diverse iron uptake repertoires.

Keywords: Citrate; Diversifying selection; Ferrous iron importer; Heme; Siderophore exploitation and competition; Siderophores.

Fig. 1

Schematic of iron acquisition systems in Pseudomonas spp. All iron transporters are shown as cylinders for reasons of simplicity. The molecular complexity of the various uptake systems is covered elsewhere (Cornelis and Dingemans ; Schalk and Cunrath 2016). a Ferrous iron permeases (like FeoB and EfeU) can directly take up Fe²⁺ without the need of a carrier. In environments, where ferric (Fe³⁺, red circle) iron prevails, a reduction step to Fe²⁺ (blue circle), for example through phenazines, is required. b Siderophores (three-quarter circles) are secondary metabolites that are secreted in the environment to scavenge iron. Most Pseudomonas spp. produce pyoverdine as their primary siderophore with strong iron affinity (Fig. 2), and a variety of secondary siderophores with lower iron affinity (Fig. 3). The ferri-siderophore complexes are recognized and internalized via outer membrane-embedded TonB-dependent transporters. c Transporters for the uptake of heterologous ferri-siderophores (e.g., enterobactin, desferrioxamines) produced by other bacterial species. d Transporters for the uptake of heterologous fungal ferri-siderophores (e.g. ferrichrome). e Transporters for the uptake of ferri-citrate (green triangle), a metabolite and iron chelator exuded from plant roots. f Transporters for the uptake of the iron-containing heme group (blue square). The Phu-system and the HxuA system (on the left) can directly take up the heme group. The Has-system (on the right) relies on the secretion of a hemophore protein (purple hexagon) for heme scavenging

Fig. 2

Pyoverdine and its structural diversity among Pseudomonas spp. The primary siderophore pyoverdine consists of a conserved chromophore (green), a strain-specific peptide backbone (black) with a variable number of amino acids, and a variable set of side chains (blue). The functional groups marked in red are involved in iron chelation. The black box shows examples of peptide backbone variation found among P. aeruginosa strains (Schalk et al. 2020) and among co-isolated natural strains (Rehm et al. 2022). The blue box shows a list of side chain variants found among co-isolated natural strains (Rehm et al. 2022)

Fig. 3

Currently known secondary siderophores in Pseudomonas spp. (Cornelis 2010). a Secondary siderophores for which structural variants have been described: pyochelin (2) vs. enantio-pyochelin (3), ornicorrugatin (4) vs. corrugatin (5), quinolobactin (6) vs. thio-quinolobactin (7). Some of these variants—pyochelin vs. enantio-pyochelin—are known to confer specificity with regard to the uptake of the iron-loaded siderophores. b Secondary siderophores for which a single structural variant exists: achromobactin (8), PDTC (9), yersiniabactin (10), pseudomonine (11)

Fig. 4

Siderophores as public goods in clonal populations. The depicted scenario shows a case where there is a single particulate source of iron (light red dots in grey circle) (a). All clonal bacteria secrete siderophores (dark red three-quarter circles) into the environment (b). The siderophores scavenge iron from the particulate source, creating a public pool of ferri-siderophores (c). All group members have the opportunity to take up ferri-siderophores from this public pool for their own use (d)

Fig. 5

Cheating and how it could select for pyoverdine diversity. Left panel: Cooperative iron scavenging (a), as explained in Fig. 4, can select for mutants that no longer contribute to pyoverdine production (dark grey cells), but keep the transporter to access the public pool of ferri-pyoverdines created by producers (b). Middle panel: cheating could select for producer mutants (blue cells) that make a structurally different pyoverdine variant (blue three-quarter circle) (c). In the initial phase, this producer mutant makes a pyoverdine that no one can use, and it thus still relies on the original ferri-pyoverdine (red three-quarter circle) for iron scavenging. In a next step, the transporter mutates in the producer mutant to allow selective uptake of the novel pyoverdine variant (d). Right panel: pyoverdine and strain diversity can be maintained in the population when the cheater outcompetes the original cooperator (e), the new producer mutant is resistant to cheating (f) yet loses in competition against the original pyoverdine producer (g)

Fig. 6

Evolutionary scenarios in inter-species competition for iron. Left panel: In a scenario where two bacterial species compete for the same limited stock of iron (a), the outcome of competition depends on the relative iron binding affinities (K_D) of the two competing siderophores (orange vs. blue three-quarter circles) and the relative amounts of siderophores produced (b). Competition could thus select for higher siderophore production levels and for siderophores with increased binding affinities. Middle panel: under conditions of increased iron availability, where species do not compete for the same iron stocks (c), bacteria might reduce investment into their expensive primary siderophores (colored three-quarter circles) and shift to the production and use of their less expensive secondary siderophores (grey three-quarter circles) (d). This scenario could explain the evolutionary maintenance of low-iron affinity secondary siderophores. Right panel: A scenario is shown where competing species use their unique primary siderophore (colored three-quarter circles) to compete for the same limited stock of iron (e). This scenario could select for the acquisition of transporters from competitors (probably via horizontal gene transfer) to pirate on each other’s ferri-siderophores (f)