**5. Conclusions**

We have discovered that many AAP produce siderophores or metallophores as diffusible secondary metabolites. Production could be related to acquisition of metal(loid)s including magnesium, vanadium, manganese, iron, cobalt, nickel, copper, zinc, selenium and tellurium, or to provide resistance to toxic metal(loid)s in environments with elevated concentrations. A correlation existed between site of isolation and production of siderophores, whereas tested freshwater AAP produced siderophores, and AAP that required NaCl predominantly did not. Furthermore, there could be connection between phylogeny of isolates and their ability to form siderophores, but as with many phenotypes to genotype comparisons, it did not appear as a strictly followed rule. With such considerations, siderophore production cannot be recommended as a taxonomic marker for AAP identification, as variable production types occurred. However, a potential application exists to use this phenotypic feature during taxonomic differentiation between species. Future work will hopefully identify the siderophores, and potential metallophores, produced by each AAP, and determine the total list of metal cations that can be targeted.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/microorganisms9050959/s1, Figure S1: Determination of siderophore size, Table S1: Siderophores examples listed from largest to smallest.

**Author Contributions:** Conceptualization, S.B.K., E.H. and V.Y.; methodology, S.B.K., E.H. and V.Y.; software, S.B.K.; validation, S.B.K., V.Y.; formal analysis, S.B.K.; investigation, S.B.K., E.H.; resources, V.Y.; data curation, S.B.K.; writing—original draft preparation, S.B.K., E.H.; writing—review and editing, S.B.K., V.Y.; visualization, S.B.K.; supervision, V.Y.; project administration, V.Y.; funding acquisition, V.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by an NSERC Discovery Grant and a University of Manitoba GETS grant, both held by V. Yurkov.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data is contained within the article.

**Conflicts of Interest:** The authors declare no conflict of interest.
