By keeping pH really low (pH 1.6-1.8), jarosite precipitates can be largely avoided. Counting can be greatly facilitated by fluorescent staining of cells with ethidium bromide - this does of course require access to a fluorescence microscope but has been used successfully for quantification in complex matrices - see e.g.:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC203732/pdf/aem00121-0052.pdf  

good luck!

You can please follow the link below to obtain certain interesting informations:-

http://article.sapub.org/10.5923.j.microbiology.20120204.03.html

Further you can download pdf of the file attached herewith.

In acidic environments, the light actually catalises iron(III) reduction, not oxidation (check paper). This phenomenon allows the recycling of the iron during the day in natural environments.

The chemical iron oxidation at low pH, is about 105–106 slower than the oxidation by biological action in the same conditions. In my experience, that may take months before you can see any substantial oxidation in complete absence of bateria (filter-sterilised stocks), compared to few days when culturing.

Article Photoreduction fuels biogeochemical cycling of iron in Spain...

Xabier Sir,

Thanks, Nice to know

It has also been observed that ferrous salts are deteriorated when exposed to light. The phenomenon which you have described occurs in nature where many other factors also influence the process. But in lab studies oxidization occurs rather slowly and the answer which we want to know is that how to know the growth of acidithiobaciluus ferrooxidans where chances of oxidation are still there?

Well, that's the reason of having negative controls. You can also track of the oxidation by measuring the iron redox state by the ferrozine method

Anyway, photooxidation of iron is not happening at the low pH you will grow At. ferrooxidans. There will be some limited oxidation due to oxygen but very limited