What iron-salts will dissolve in water?

[CYP450]

So I've been doing biochem research on bacterial biodegradation of exotic organic compounds. Based on a lot of research I've done, a lot of biodegradation pathways involve step-by-step mono-oxygenation reactions catalyzed by Cytochrome P450 enzymes (which contain a ferrous heme group) and redox partners (usually iron-sulfur proteins).

I've been incubating bacteria in liquid media with samples of solid organic waste generated by our university's undergraduate organic chemistry lab as a sole carbon source, but there was no degradation. Then I looked at the chemical profiles of the media I was using and found there was no iron in them.

I tried adding tiny amounts of Ferrous Sulfate (FeSO₄) to the liquid solution but it quickly precipitates; even a few sand-sized grains per mL and these white cloudy chunks forms and it looks like tiny specs of rusty iron fall to the bottom. I tried adjusting the pH of the solution to prevent precipitation but it didn't work.

Is anyone aware either of an iron-salt that will dissolve in water? And if not, how can I get ferrous sulfate to evenly dissolve in water without clouding or precipitation?

 

[Mike H]

So, there are two things one should consider up front.

If the organic waste you're trying to degrade is truly solid and not porous, you might be dealing with a matter of kinetics. The bacteria can - presumably - only eat away at what is presented at the surface. If you were to disperse the solid waste, you'd increase the surface area accessible to the bacteria and their enzymological assault.

The organic waste is stated to be solid. How certain are you that it is not mixing into some extent? This can non-trivially change the composition of the liquid media. I know something like ferric chloride can tolerate some polar organic solvents pretty well, but I doubt it does as well with non-polar organics. If you feel the need to supplement with iron, I would add it before adding the organic waste so the bacteria have time to take it up. Personally, when I prepare defined bacterial growth media, I use ferric chloride - no problems with solubility in water in my experience growing cells in this manner for over a decade now.

 

[Yanick]

Ferrous iron will very quickly oxidize to Ferric near neutral pH. I've seen papers using Ferric Chloride and I have used FeSO₄NH₄ + Citrate when supplementing minimal media with Iron.

 

[MrSid]

Ferrous iron will very quickly oxidize to Ferric near neutral pH. I've seen papers using Ferric Chloride and I have used FeSO₄NH₄ + Citrate when supplementing minimal media with Iron.

In this media one is using the citrate as a complexing agent that keeps the Fe in solution and doesn't cause the required phosphate nutrient to precipitate out insoluble iron phosphates.

The solid waste may be depleting other nutrients than iron from your media.

Have you some idea of the possible composition of the waste? Are you confident it has the carbon content to sustain growth?

Some bio-remediations of recalcitrant hydrocarbons (aerobic) benefit by co-metabolizing an emulsified fat or oil (it allows induction of enzymes needed to attack the C-H and C-C bonds). It would make your experiment harder to blank and you may have to devise a way to collect all of the CO2 evolved (i.e BOD setups and titration of the CO2).