INVESTIGATION OF AN IRON-OXIDIZING MICROBIAL MAT COMMUNITY LOCATED NEAR AARHUS, DENMARK - FIELD STUDIES

We investigated the microbial community that developed at an iron seep where anoxic groundwater containing up to 250 mu M Fe2+ flowed out of a rock wall and dense, mat-like aggregations of ferric hydroxides for med at the oxic-anoxic interface. In situ analysis with oxygen microel ectrodes revealed that the oxygen concentrations in the mat were rarel y more than 50% of air saturation and that the oxygen penetration dept h was quite variable, ranging from <0.05 cm to several centimeters. Th e bulk pH of the mat ranged from 7.1 to 7.6. There appeared to be a co rrelation between the flow rates at different subsites of the mat and the morphotypes of the microorganisms and Fe oxides that developed. In subsites with low how rates (<2 ml/s), the iron-encrusted sheaths of Leptothrix ochracea predominated. Miniature cores revealed that the to p few millimeters of the mat consisted primarily of L. ochracea sheath s, only about 7% of which contained filaments of cells. Deeper in the mat, large particulate oxides developed, which were often heavily colo nized by unicellular bacteria that were made visible by staining with acridine orange. Direct cell counts revealed that the number of bacter ia increased from approximately 10(8) to 10(9) cells per cm(3) and the total iron concentration increased from approximately 0.5 to 3 mmol/c m(3) with depth in the mat. Primarily because of the growth of L. ochr acea, the mat could accrete at rates of up to 3.1 mm/day at these subs ites. The iron-encrusted stalks of Gallionella spp. prevailed in local ized zones of the same low-flow-rate subsites, usually close to where the source water emanated from the wall. These latter zones had the lo west O-2 concentrations (<10% of the ambient concentration), confirmin g the microaerobic nature of Gallionella spp. In subsites with high fl ow rates (>6 ml/s) particulate Fe oxides were dominant; direct counts revealed that up to 10(9) cells of primarily unicellular bacteria per cm(3) were associated with these particulate oxides. These zones exhib ited little vertical stratification in either the number of cells or i ron concentration. Finally, mat samples incubated anaerobically in the presence of acetate or succinate exhibited significant potential for iron reduction, suggesting the possibility that a localized iron cycle could occur within the mat community.