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 mu M FeCl2 flowed up through a sample well into an aerated reservoir, thereby creating an suboxic-oxic interface similar to the p hysicochemical conditions that exist in natural iron seeps. When micro bial mat material from the Marselisborg iron seep that contained up to 10(9) bacterial cells per cm(3) (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 sampl e well was oxidized at rates of up to 1,200 nmol of Fe2+ oxidized per h per cm(3) 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 sig nificantly inhibited oxidation. These results suggest that at least 50 % and up to 80% of the Fe oxidation in the chamber were actively media ted by the microbes in the mat. It also appeared that Fe stimulated th e growth of the community since chambers fed with FeCl2 accumulated ma sses of either filamentous or particulate growth, both in the sample w ell and attached to the walls of the chamber. Control chambers that di d 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 prote in than chambers not supplemented with FeCl2. Leptothrix ochracea and, to a lesser extent, Gallionella spp. were responsible for the filamen tous growth, and the sheaths and stalks, respectively, of these two or ganisms harbored large numbers of Fe-encrusted, nonappendaged unicellu lar bacteria. In chambers where particulate growth predominated, the u nicellular bacteria alone appeared to be the primary agents of iron ox idation. These results provide the first clear evidence that the ''iro n bacteria'' commonly found associated with neutral-ps iron seeps are responsible for most of the iron oxidation and that the presence of fe rrous iron appears to stimulate the growth of these organisms.