**5. Conclusions**

Aqueous suspensions of GR(SO4 <sup>2</sup>−) were obtained by mixing a solution of Fe(II) and Fe(III) salts with a NaOH solution and ageing for 1 week in anoxic conditions. Bacterial cells (dead or still alive and active) and/or associated organic species, for instance, extracellular polymeric substances, prevented the transformation of GR(SO4 <sup>2</sup>−) to magnetite in anoxic conditions. Similar effects, albeit less important, were observed with acetate ions added to the system. It is proposed that the adsorption of organic species on the lateral sides of the GR(SO4 <sup>2</sup>−) crystals hinders the release of SO4 2− ions into solution, a process required for the transformation of GR(SO4 <sup>2</sup>−) to magnetite. After further ageing (up to two months) in the presence of bacterial cells, magnetite was not observed either.

GR(Cl−) was observed as a minor component together with GR(SO4 <sup>2</sup>−) in the absence of bacteria or acetate. Acetate ions decreased the amount of obtained GR(Cl−) whereas bacteria prevented completely its formation. This suggests that monovalent organic anionic species compete with Cl− during the precipitation reaction and prevent the formation of GR(Cl−), consequently favoring that of the more stable GR(SO4 <sup>2</sup>−).

The corrosion product layer formed on carbon steel in a natural marine environment appeared to be enriched in GR(SO4 <sup>2</sup>−) with respect to the layer formed in artificial seawater. This may be a consequence of the interaction between GR(SO4 <sup>2</sup>−), and, more generally, GR compounds, with bacterial cells and associated organic matter.

**Author Contributions:** Conceptualization, P.R., M.J., R.S., I.L. and S.S.; methodology, P.R., M.J., R.S., I.L. and S.S.; validation, P.R., M.J., R.S., I.L. and S.S.; formal analysis, J.D., J.V., P.R., M.J., I.L. and S.S.; investigation, J.D., J.V., P.R. and M.J.; data curation, P.R., M.J., I.L. and S.S.; writing—original draft preparation, P.R.; writing—review and editing, P.R., M.J., I.L., R.S. and S.S.; visualization, P.R.; supervision, P.R., M.J., I.L., R.S. and S.S.; project administration, P.R., M.J., R.S., I.L. and S.S.; funding acquisition, P.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

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