要旨
Sulfur bacteria of the family Beggiatoaceae are of special interest with respect
to the phosphorus cycle, because they can store large amounts of polyphosphate
and are proposed to influence phosphorus sequestration in marine
sediments (e.g. Schulz and Schulz, 2005). The aim of this thesis was to study
different aspects of polyphosphate storage in members of the family Beggiatoaceae
on a physiological and genomic level with a special focus on the heterotrophic
freshwater strain Beggiatoa alba B15LD. In addition, polyphosphate-
related enzymes, which are encoded in different members of the Beggiatoaceae,
were identified and possible pathways of polyphosphate utilization
were discussed, including its exploitation as an energy source.
In the second part, the structure and elemental composition of polyphosphate
inclusions in Beggiatoa alba were analyzed. Studies using various techniques
revealed that these inclusions are not acidic and are associated with sodium
cations. This is the first time that a co-occurrence of polyphosphate with Na+
was observed in Beggiatoaceae and among bacteria in general.
In addition, the factors controlling the storage and degradation of polyphosphate
in Beggiatoa alba were studied under laboratory conditions. Comparison
of the triggers for polyphosphate synthesis and degradation to those effective
in other members of the family Beggiatoaceae, which are lithoautotrophic
and originate from freshwater, marine, and hypersaline environments,
revealed that only Beggiatoa alba stored polyphosphate at nitrogen limitation.
Under these conditions, polyphosphate was possibly stored as an energy reserve,
since growth was inhibited and excess energy was available through
the oxidation of acetate. In E. coli, it was shown that polyphosphate was
stored at nitrogen limitation to induce the degradation of ribosomal proteins to
use them as an intracellular amino acid pool. The produced guanosine tetraphosphate
(ppGpp) serves as an important signaling molecule during this
process. Elevated ppGpp concentrations were also measured in nitrogenlimited cultures of Beggiatoa alba, suggesting that this mechanism is also present
in this species.
Polyphosphate degradation in Beggiatoa alba was induced by different stress
factors, such as high and low pH (Havemeyer, 2010), elevated temperatures,
and high ammonium concentrations, which did not affect polyphosphate storage
in the marine strain Beggiatoa sp. 35Flor. In contrast, the marine strain
degraded polyphosphate in response to high sulfide concentrations and anoxia
(Brock and Schulz-Vogt, 2011). While sulfide concentrations in freshwater
sediments are much lower than in marine sediments, pH changes are
more likely to occur, since freshwater is, in contrast to seawater, not carbonate
buffered. Hence, polyphosphate degradation in Beggiatoaceae seems to
be related to habitat-specific environmental factors.
Finally, polyphosphate storage in environmental samples of filamentous Beggiatoaceae
from Lake Grevelingen, The Netherlands, and Aarhus Bay, Denmark,
were investigated. Fluctuations in redox conditions together with high
sulfide concentrations are a prerequisite for polyphosphate degradation and
phosphate release in a marine Beggiatoa strain (Brock and Schulz-Vogt,
2011). Although both sampling sites have fluctuations in redox conditions, no
polyphosphate storage was observed and therefore a potential influence on
benthic phosphorus cycling cannot be assumed. The facts that sulfide concentrations
were not very high and fluctuations of redox conditions occurred
seasonally and not more frequently might explain the difference. Hence, sulfur
bacteria do not in general influence benthic phosphorus cycling. Instead, it
seems that only special conditions, as found at sites of recent phosphorus
sequestration, have the potential to induce massive accumulation of polyphosphate
and rapid phosphate release by sulfur bacteria.
