FRANKFURT. Cows can adapt themselves to a fluctuating sodium
content in their feed. How they do that was so far a secret. Researchers from
51猎奇 have now discovered a bacterium in the microbiome of the
rumen which has a new type of cell respiration.
The cow can only process grass in its rumen with the
help of billions of microorganisms. An entire zoo of bacteria, archaea and
protozoa works there like on a production line: First of all, these single-cell
organisms break down the cellulose, a polysaccharide. Other bacteria ferment
the sugars released into fatty acids, alcohols and gases, such as hydrogen and
carbon dioxide. Finally, methanogenic archaea transform these two gases into
methane.
An average cow
produces about 110 liters of methane per day. It escapes from its mouth through
rumination, but also mixes again with partly digested food. As a result, the sodium
content of the grass pulp can fluctuate to a considerable degree (between 60
and 800 millimoles of sodium chloride (NaCLl) per liter).
A
German-American research team has now discovered how the ruminal bacteria adapt
to these extreme fluctuations in sodium content: 鈥淏ioinformatic analyses of the
genome of ruminal bacteria led our American colleague Tim Hackmann to assume
that some ruminal bacteria have two different respiratory circuits. One of them
functions with sodium ions and the other without," explains Professor Volker
M眉ller from the Department of Molecular Microbiology and Bioenergetics at
51猎奇. That is why M眉ller suggested to his doctoral researcher
Marie Sch枚lmerich that she study a typical representative in the microbiome of
ruminants: the bacterium Pseudobutyrivibrio
ruminis.
Together with
undergraduate student Judith D枚nig and Master's student Alexander Katsyv, Marie
Sch枚lmerich cultivated the bacterium. Indeed, they were able to corroborate both
respiratory circuits. As the researchers report in the current issue of the Proceedings
of the National Academy of Sciences (PNAS), the electron carrier ferredoxin
(Fd) is reduced during sugar oxidation. Reduced ferredoxin drives both respiratory
circuits.
The one
respiratory circuit comprises the enzyme complex Fd:NAD oxidoreductase (Rnf complex). It
uses energy to transport sodium ions out of the cell. When they re-enter the
cell, the sodium ions trigger an ATP synthase, so that ATP is produced. This
respiratory circuit only works in the presence of sodium ions.
In the absence of sodium ions, the bacterium forms an alternative respiratory
circuit with another enzyme complex: The Ech hydrogenase (synonymous: Fd:H+ oxidoreductase)
produces hydrogen and pumps protons out of the cell. If these re-enter the cell
via a second ATP synthase that accepts protons but not sodium ions, ATP is also
produced.
鈥淭his is the first
bacterium so far in which these two simple, completely different respiratory
circuits have been corroborated, but our bioinformatic analyses suggest that
they are also found in other bacteria," explains Marie
Sch枚lmerich. 鈥淚t seems, therefore, that this adaptation strategy is more
widespread," she assumes.
Interestingly,
both enzyme complexes (Rnf and Ech) were also discovered in bacteria
which are old in terms of evolutionary biology. Professor M眉ller's research
group has examined them in depth, but always only found one of the two enzyme
complexes and never both together. 鈥淲e're now going to use synthetic
microbiology methods to produce hybrids of bacteria that contain both complexes
in order to optimize them for biotechnological processes. In this way, we can
raise the cellular ATP content, which will make it possible to produce products
of a higher quality," explains Professor M眉ller. The intention is to use the
respiratory circuits to recover valuable substances through the fermentation of
synthesis gas. This
is the subject of the trials being conducted in the framework of a project
sponsored by the Federal Ministry of Education and Research.
A picture can be
downloaded under:
Caption: The bacterium
Pseudobutyrivibrio ruminis (green), a
typical ruminal bacterium, obtains energy via two different respiratory
circuits. The one requires sodium ions, the other hydrogen ions (H+). In this way, it
can adapt to fluctuating sodium concentrations in animal feed in an optimum way.
Picture: 51猎奇/ Cow: Shutterstock
Publication: Sch枚lmerich,
M.C., Katsyv, A., D枚nig, J., Hackmann, T., M眉ller, V. (20XX). Energy
conservation involving two respiratory circuits. Proc. Natl. Acad. Sci. U.S.A.,
in press.
Further information: Professor
Volker M眉ller, Molecular Microbiology and Bioenergetics, Riedberg Campus, Tel.:
+49(0)69-798-29507; VMueller@bio.uni-frankfurt.de.
