Metal eating bacteria

[Mouse]

I was wondering do iron eating bacteria eat the metal iron itself or just the compounds.

[CharonY]

For the most part they do not eat the metal as in using it as source of food, but rather they use it the same way we use oxygen. In essence electrons are dumped from the bacteria to the metal (in order to create a proton gradient that in turn allows the generation of energy).

The reduced iron is more soluble and is susceptible to re-oxidation reactions. However, as almost any other organisms bacteria also require minute amount of iron as micronutrient. They can dissolve iron either again by direct reduction or, more commonly, using siderophores to solubilize it. That happens on a very slow scale, though.

[Mouse]

Ok so it's the Fe by itself or Fe+ ions?

[CharonY]

Generally iron is present as a an iron(hydr)oxide as e.g. hematite, goethite or transiently also as more bioaccesible ferrihydrite. Those are generally the most common sources.

[Mouse]

What if it's the pure iron left over from like say the waste products of iron reducing bacteria like Shewanella oneidensis?

[Greippi]

On the topic of metal "eating" bacteria, many bacteria need metals for things other than creation of a proton gradient (they're often used as enzyme cofactors). For example, molybdenate is essential for many nitrogen fixing bacteria - Azotobacter vinelandii which takes up Mo as well as iron. These bacteria produce catechol siderophores which complex the Mo or Fe which can then be taken up by the bacteria. In this case, Fe3+ is complexed in to a water-soluble form which can be taken up.

 

That may or may not be useful to you - but it's an example of how iron can be taken up in many different ways,

[Mouse]

so it's Fe ions not the pure metal?

[Greippi]

so it's Fe ions not the pure metal?

 

In that example, yes.

(I assume by 'pure metal' you mean non-ionic iron. Not elementally pure ion - because in this case the iron is elementally pure - until it is purposefully complexed.)

 

I would guess that the reason why bacteria rarely take up a non-ionic form is because it's just not useful. It can't accept electrons, and therefore can't form complexes with anything else in the 'stable' form. It would require energy to break that apart, which just isn't practical if iron ions could be taken up instead.

[Mouse]

Then what about iron reducing bacteria? Like Shewanella oneidensis? They are iron reducing those that meant that their waste product is Fe2 or Fe+2?

[Greippi]

Shewanella oneidensis is relatively unique in that it can reduce Fe3+ using formate as an electron donor. How it uptakes Fe3+ from the environment I'm not sure, I have a feeling it uptakes both soluble and insoluble (from goethite) forums. There is a lot of literature on the genomics and biochemistry of this, but little I could find after a brief search on actual uptake!

[Mouse]

So the formula is??

 

Fe+3 +???

[Greippi]

Fe(III) or Fe3+ (don't know how to do superscript on here).

 

Ruebush et al. Applied and Environmental Microbiology, April 2006, p. 2925-2935, Vol. 72, No. 4

[Mouse]

Ok sorry so If Shewanella oneidensis breathes Fe[iII] then it comes out as Fe[iI]? But it has to have some other substances right?

Edited April 21, 2010 by Mouse

[Greippi]

Breathes? Most likely it has some sort of transport protein in its membrane.

Maybe it re-oxidises the Fe(II), maybe it transports it out, I don't know. Hopefully there's someone else here who knows better!

[CharonY]

As mentioned above, the bulk of bacterially induced changes in metals is respiration. Breathing is not too wrong as the mechanisms are very similar to other forms of respiration. Keep in mind that most metals are not present in significant amounts as either solubilized ions (i.e. free Fe3+) or in elemental form. The majority will be in a stable oxide form as e.g. Goethite (FeO(OH)) in case of iron.

The respiration is just a means to power the proton pump. So the iron in the iron oxide will get reduced to Fe(II). What happens then depends on the environment. If oxygen is present it will get reoxidized and depending on pH and other parameters different iron oxides may form.

 

Now about iron uptake. The amount is relatively low in terms of iron turnover as only relatively low amounts are required. However, in many environments iron can be limiting. The reason is the low solubility of iron oxides. However, Shewanella often does not face these restrictions. The reason is that it is also often found in anoxic zones where reduced iron (e.g. by respiration) does not readily reoxidize. Hence, they can easily take up the more soluble Fe(II). Correspondingly, they do possess Fe(II) transporters but (to my knowledge) no elaborate siderophore uptake systems.

Most aerobic bacteria have no access to Fe(II). Hence they mostly produce siderophores, which are a diverse class of iron chelators that bind Fe(III) and solubilize them. The alternative strategy is to reduce them (assimilatory iron reduction) and then take up the more soluble Fe(II).

 

Just remember to distinguish between iron uptake and dissimilatory iron reduction (which does not result directly in iron uptake but is used for energy production).

[Mouse]

I see thanks a lot... so if they breath iron[iII] like say from hemetite. then what do they "eat".

 

And if will they survive in a cave on Mars? If so which part of the food chain will they be? Producers?

[CharonY]

All organisms need to consume certain elements to survive and grow. Metal reducers, for instance often utilize acetate as carbon source as well as electron donor, but it really depends on the bug and the environment they live in. But they require all the other nutrients as all other organisms. Dissimilatory metal reduction is just a way to gain energy. Biomass has to come from somewhere else.

[Mouse]

Thanks a lot. So that means they might eat other organisms for acetate or is there another way to synthesize their own?

[CharonY]

Acetate is the endproduct of a number of oxidation processes from a variety of carbon sources. They essentially use what others cannot.

[Mouse]

I see... sorry if this sounds whimical. I am just curious....

 

I believe something like this could be going on on Mars. But you can't make acetate in CO2 rather even if you are near a hot spring or volcano?