Ferrous iron oxidation by denitrifying bacteria in profundal sediments of a deep lake (Lake Constance)

Profundal sediments of Lake Constance were sampled at 60-130 in water depth and studied with respect to the population sizes and depth distribution of mixotrophic and lithotrophic iron-oxidizing, denitrifying bacteria. The se diment cores showed a distinct stratification with oxygen and nitrate being consumed within the first centimeter of the sediment. The sediment contain ed large amounts of acid-extractable iron (0.8-1.4% of the dry mass). The p roportion of ferrous iron in the solid phase increased dramatically from 16 -35% in the oxidized surface layer to 79-97% below 4-cm depth, whereas the total iron content increased only slightly with depth. The concentration of dissolved ferrous iron in the pore water ranged between 10 and 80 muM, whi ch was about three orders of magnitude lower than the total iron content of the solid phase. The redox profile of the pore water iron followed that of the solid phase. Most probable numbers (MPNs) of acetate-oxidizing, denitr ifying bacteria in the different cores ranged from 1.0 x 10(6) to 2.1 x 10( 8) cells (ml sediment)(-1): highest numbers were found in the upper sedimen t layer. In most cores, bacteria capable of iron oxidation made up a signif icant part (1-58%) of the total cultivable denitrifying population. Between 1.0 x 10(4) and 5.8 x 10(5) cells ml(-1) oxidized iron mixotrophically: th e numbers of lithotrophic iron-oxidizing denitrifiers were about one order of magnitude lower. MPNs of mixotrophic iron oxidizers and aerobic iron oxi dizers among the denitrifying populations in the upper sediment layers were in the same range. Generally, the MPNs of the different iron-oxidizing pop ulations did not change significantly with depth. C, 2001 Federation of Eur opean Microbiological Societies. Published by Elsevier Science B.V. All rig hts reserved.