Investigation of an Iron-Oxidizing Microbial Mat Community Located near Aarhus, Denmark: Laboratory Studies

We constructed a small flow chamber in which suboxic medium containing 60 to 120 μM FeCl₂ flowed up through a sample well into an aerated reservoir, thereby creating an suboxic-oxic interface similar to the physicochemical conditions that exist in natural iron seeps. When microbial mat material from the Marselisborg iron seep that contained up to 10⁹ bacterial cells per cm³ (D. Emerson and N. P. Revsbech, Appl. Environ. Microbiol. 60:4022-4031, 1994) was placed in the sample well of the chamber, essentially all of the Fe²⁺ flowing through the sample well was oxidized at rates of up to 1,200 nmol of Fe²⁺ oxidized per h per cm³ of mat material. The oxidation rates of samples of the mat that were pasteurized prior to inoculation were only about 20 to 50% of the oxidation rates of unpasteurized samples. Sodium azide also significantly inhibited oxidation. These results suggest that at least 50% and up to 80% of the Fe oxidation in the chamber were actively mediated by the microbes in the mat. It also appeared that Fe stimulated the growth of the community since chambers fed with FeCl₂ accumulated masses of either filamentous or particulate growth, both in the sample well and attached to the walls of the chamber. Control chambers that did not receive FeCl₂ showed no sign of such growth. Furthermore, after 4 to 5 days the chambers fed with FeCl₂ contained 35 to 75% more protein than chambers not supplemented with FeCl₂. Leptothrix ochracea and, to a lesser extent, Gallionella spp. were responsible for the filamentous growth, and the sheaths and stalks, respectively, of these two organisms harbored large numbers of Fe-encrusted, nonappendaged unicellular bacteria. In chambers where particulate growth predominated, the unicellular bacteria alone appeared to be the primary agents of iron oxidation. These results provide the first clear evidence that the “iron bacteria” commonly found associated with neutral-pH iron seeps are responsible for most of the iron oxidation and that the presence of ferrous iron appears to stimulate the growth of these organisms.