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