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PhD defense Suzann Vollrath
Microbial Fe(II) oxidation at circumneutral pH: Reaction kinetics, mineral products, and distribution of neutrophilic iron oxidizers in wetland soils

Multiple studies have shown that neutrophilic Fe(II) oxidizers can conserve energy from Fe(II) oxidation, however, it is still unclear how they can compete against the fast abiotic reaction at neutral pH, or to which extent these bacteria increase the overall Fe(II) oxidation rate. Similar to acidophilic Fe(II) oxidizers, neutrophilic oxidizers use Fe(II) as electron source. In contrast to acidophilic Fe(II) oxidizers, however, they are challenged by the fast kinetics of the abiotic reaction, because Fe(II) oxidation is significantly faster at neutral pH compared to oxidation under acidic conditions. The competition with abiotic Fe(II) oxidation may be attenuated by environmental factors other than pH. For example, temperature or oxygen concentration may potentially favor microbial Fe(II) oxidation, but the dependence of microbial Fe(II) oxidation rates on these factors has not yet been systematically investigated. This thesis aims at filling this gap by studying the effect of temperature and oxygen concentration on the kinetics and products of microbial Fe(II) oxidation. While the experimental work was conducted in the laboratory under controlled conditions, with a single strain of iron oxidizing bacteria, an attempt was also made to better characterize the relationship between physico-chemical conditions and the occurrence of Fe(II) oxidizers in sediments from a freshwater marsh.

The highest abundance of Gallionella-like Fe(II) oxidizers was observed in the upper 5-12cm of sediments collected in the fresh water part of the Scheldt estuary in Belgium. In spring, the largest diversity and cell densities of Fe(II) oxidizers were found when coincidently also the highest concentrations of extractable Fe(III) were detected. The diversity of the Fe(II) oxidizers was smaller and no Fe(III) was detected in samples collected during summer and fall. The change in diversity and possibly the activity of the bacteria may be related to changes in temperature and oxygen concentration. However, no simple relationship between geochemistry and occurrence of Fe(II) oxidizers could be deduced from the results.

The laboratory experiments with Leptothrix cholodnii Appels showed that microbial and abiotic Fe(II) oxidation proceed in two phases with different kinetics. During the initial phase, microbial Fe(II) oxidation rates exceeded those of the abiotic reaction. In this phase, the oxygen dependency of the microbial Fe(II) oxidation followed Michaelis-Menten kinetics and the temperature dependency was characterized by an optimum temperature around 30-37°C. During the second phase, the accumulated iron oxides catalyze the reaction so that abiotic Fe(II) oxidation was dominant also in experiments with Leptothrix. Hence, microbial and abiotic rates were similar and showed similar dependencies on temperature and oxygen concentration. Characterizing the oxides from microbial and abiotic experiments revealed that both contained lepidocrocite and ferrihydrite. However oxides produced in the presence of Leptothrix were smaller and better ordered compared to abiotic oxides. In conclusion, oxygen and temperature can influence the role of neutrophilic iron oxidizers in Fe redox cycling by affecting the rates of microbial and abiotic Fe(II) concentration.

Promotors: Prof. dr. P. Van Cappellen and Prof. dr. H.J. Laanbroek
Copromotor: Dr. T. Behrends