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Abstract:
The family Beggiatoaceae comprises large, colorless sulfur bacteria, which are best
known for their chemolithotrophic metabolism, in particular the oxidation of reduced
sulfur compounds with oxygen or nitrate. This thesis contributes to a more
comprehensive understanding of the physiology and ecology of these organisms
with several studies on different aspects of their dissimilatory metabolism. Even
though the importance of inorganic sulfur substrates as electron donors for the
Beggiatoaceae has long been recognized, it was not possible to derive a general
model of sulfur compound oxidation in this family, owing to the fact that most of
its members can currently not be cultured. Such a model has now been developed
by integrating information from six Beggiatoaceae draft genomes with available
literature data (Section 2). This model proposes common metabolic pathways of
sulfur compound oxidation and evaluates whether the involved enzymes are likely
to be of ancestral origin for the family.
In Section 3 the sulfur metabolism of the Beggiatoaceae is explored from a different
perspective. Besides oxidizing stored elemental sulfur further to sulfate,
members of this family can use sulfur as a terminal electron acceptor under anoxic
conditions. So far, sulfur respiration in the Beggiatoaceae has only been discussed
in the context of energy acquisition, but the here presented data suggest that this
reaction could also be employed to dispose of stored sulfur when sulfide is oxidized
at high rates. If strongly sulfidic conditions and high sulfide oxidation rates persist,
sulfur can accumulate in such an excessive manner that the cell integrity can
eventually not be maintained.
Reduced sulfur compounds are surely the most important electron donors for
chemolithoautotrophically growing Beggiatoaceae, but the traditional focus on this
topic has left other possible inorganic electron donors largely unexplored. Hence, a
major part of this thesis is dedicated to investigating the capacity of Beggiatoaceae to use molecular hydrogen as an electron donor. Physiological experiments have
shown that a chemolithoautotrophic Beggiatoa strain oxidizes hydrogen at high
rates and under various conditions, indicating that hydrogen could play an important
role in the metabolism of the Beggiatoaceae (Section 4.1). The physiological
studies on hydrogen oxidation have been complemented by screening all available
Beggiatoaceae draft genomes and several cultured members of the family for
hydrogenase-encoding genes (Section 4.2). [NiFe]-hydrogenase genes from four
phylogentically and functionally distinct clades have been identified repeatedly,
illustrating that the capacity for hydrogen oxidation in the Beggiatoaceae is likely
both, widespread and versatile. The possible influence of hydrogen oxidation on
the metabolic plasticity of the Beggiatoaceae is discussed and environmental settings
are pointed out, in which hydrogen oxidation could be important for members
of the family.
In recent years, it became evident that molecular hydrogen can indeed be an important
electron donor for sulfur bacteria of very different phylogenetic origin and
lifestyle. The general discussion of this thesis therefore presents a comparison of
how much energy members of the family Beggiatoaceae—and sulfur bacteria in
general—could gain from the oxidation of reduced sulfur compounds and molecular
hydrogen (Section 5). This comparison includes both, thermodynamic and
biochemical considerations.
