Passive Blacksmiths?

Iron-reducing bacteria reduce Fe(III) to provide energy. The majority of previous literature has found that the bacteria which utilise iron-rich compounds are both motile, and use conductive nanowires to provide conduit for electron transfer. This paper focuses on Geobacter which has been shown incabale of producing soluble electron shuttles to reduce Fe(III), unlike other iron reducing species. However, previous studies have shown that minerals containing Fe(III) can be found closely associated with cell surfaces, but the mechanism for this aggregation is still unknown. The key question this paper tries to answer is: How do Geobacter species, which require a direct contact with iron-rich compounds, proliferate as planktonic cells.

The paper produced many images by Cryo-TEM and FISH which show large aggregates of Fe(III) growing in multiple places around the outermembrane of Geobacter sp. which are very clear. Luef et al. (2013) grew these bacteria in a solution which contain soluble iron. So any aggregation around the cell surfaces of these planktonic bacteria is deduced to be passive, rather than aggregated through motility to iron rich compounds. Although the paper clearly showed an accumulation of iron around these motile cells, it is still unclear whether iron naturally aggregates when in solution. Also this experiment had no control in the laboratory experiments.

All in all, this paper was fascinating. The images they collected are clear and informative, however the way they lead you through the paper is slightly confusing. They also left a few questions unanswered in the discussion, such as the aggregation of iron around planktonic cells in the lab experiments completely disappeared after 6 days of growth. Was this due to experimental procedures? or could it be that the percentage of reduced iron (which they reported to be 63.2% after 6 days) had reached a critical threshold where it was no longer energy efficient for the Geobacter to extract iron when it is at a low concentration? And as such switches metabolism? This would be an interesting follow up experiment!

Birgit Luef, Sirine C Fakra, Roseann Csencsits, Kelly C Wrighton, Kenneth H Williams, Michael J Wilkins, Kenneth H Downing, Philip E Long, Luis R Comolli and Jillian F Banfield (2013) Iron-reducing bacteria accumulate ferric oxyhydroxide nanoparticle aggregates that may support planktonic growth