1977 broda
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8/11/2019 1977 Broda
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I
eitschrift
fur
Allg. Mikrobiologie 1 7
6
I
1977
I
491-493
Institut fiir Physikalische Chemie,
Universitit Wien)
Two
kinds
of lithotrophs missing in nature
E.
BRODA
Eingegangen a m 14.9.1976
Two groups of lithotrophic bacteria, the existence of which may be expected on evolutionary
and thermodynamical grounds, have not yet been detected: (A) photosynthetic, anaerobic, am-
monia bacteria, analogous to coloured sulphur bacteria, and
B)
chemosynthetic bacteria that
oxidize ammonia to nitrogen with
0
or nitrate as oxidant.
The versatility of the prokaryotes in their energy metabolism has long astonished
microbiologists. The bacteria have developed processes, i.e., enzymes, for the utili-
zation of a wide range indeed of exergonic reactions. Attention is now drawn to further
processes in energy metabolism which on the basis of considerations on the evolution
of the bioenergetic processes
BRODA
975a) may be expected to have existed, or
to exist, but which have not yet been found. Two kinds of lithotrophic bacteria
with such mechanisms will now be predicted. Lithotrophs are bacteria that use in-
organic reductants in their energy metabolism FROMAOEOTnd SENEZ 960); all
autotrophs must belithotrophs, though the reverse need not be true. The two bac-
teria here predicted would generate dinitrogen (N,).
The nitrifying bacteria make adenosine triphosphate, ATP, through oxidative phos-
phorylation coupled to the aerobic oxidation of ammonia, a highly exergonic process.
Thus, in nitrification Nitrosomonas produces nitrite, and Nitrobacter makes nitrate.
The redox reactions are :
NH + 1.5
0
= H,O + NO; +
2
H + ; AG; = 5 kcal
NO;
+
0.5
0
=NO;;
AG;
= 18kcal
(1)
2)
The negativity of the free enthalpy change, AG;, is th e precondition for the produc-
tion of ATP and, consequently, for the endergonic reduction of
CO,
to biomass. The
reduction occurs, as in plants, through t h e
CALVIN
cycle; the reductant, NADH, is
obtained by reverse electron flow forced by ATP.
Clearly, the nitrificants, one main group of the chemolithotrophic bacteria, could
evolve only after the biosphere began to contain,
as
a consequence of the photo-
synthetic activity of the blue-green algae, free oxygen BRODA975a b). The tran-
sition to the oxidizing biosphere took place about 2 giga-years (Gy) ago
RUTTEN
971)
Similarly the free oxygen made possible th e rise of the second important class of
chemolithotrophs, the colourless (white) sulphur bacteria. These thiobacilli make
ATP on the basis of reactions
of
the overall types:
HS-
+
0 5
0 +
H+ = H,O S; AG; = 51 kcal
S
+
1.5
0
+
H,O
=
S O 2
+
2
H + ;
AG;
=
139 kcal
3)
4)
The thiobacilli presumably descended from coloured, photosynthetic, sulphur bac-
teria, i.e., the photolithotrophs gave, after the advent of O,, rise t o the chemoli-
thotrophs. In other words, oxidative phosphorylation evolved from photosynthetic
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492 E.
BRODA
phosphorylation. This is indicated by the conversion hypothesis for the origin of
respiration from photosynthesis (BRODA
175a).
The basic processes in the energy metabolism of the photosynthetic sulphur bac-
teria are
2
HS- + 2 Hf
+
CO, = (CH20)+ H 2 0
+ 2
S; AG; = 11 kcal
5 )
0.5
HS-
+
CO,
+
H,O
= (CH,O)
+
0.5
H+
+
0.5
SO:-; AG;
=
18
kcal
Gj
(CH,O) indicates unit quantity of biomass, no t formaldehyde. Reactions
(5)
and
6),
which are endergonic, are energized by light,
Le . ,
electrons are promoted photochemi-
cally. A separation into exergonic and endergonic partial reactions would, in con-
trast t o the position with the chemolithotrophs, not be meaningful with the photo-
lithotrophs because
CO,
is indispensable as terminal (extracellular) electron acceptor.
I n reactions (1) to 4) this role is played by 0 . Incidentally, for the processes 4) and
(6) he term sulphurication might be introduced, in analogy to nitrification (processes
Who, then, were the ancestors of the nitri ficants? Can they, in parallel to the
evolution
of
the sulphur bacteria, have descended from photosynthetic ammonia
bacteria ? Such (coloured)bacteria are not known. But apparently no search has ever
been made for them. They may exist
or
else they may have existed, but
died
out.
The photochemical promotion of electrons from
NH:
t o reduce CO,, the fundamental
feature of such hypothetical bacteria, would from the point of view
of
energetics not
be too difficult
1.3 NHZ + CO, = (CH,O) + 0.65 N, + H,O + 1.3 H + ; AG; = 12 kcal (7)
This would involve a direct biotic oxidation of
NH2,
i.e. of NH,, to N,. Such a
reaction is unknown.
In contrast to the anaerobic and endergonic reaction
(7) ,
an aerobic and exergonic
oxidation of NH, to
N,
could, like th at to NO, or NO;, occur only after the appe-
arance of
0
in the biosphere:
8)
1 2) .
NH?
0.75
0 = 0.5 N, 1.5 H,O + H+ ; AG; =
5
kcal
(The exergonicity of NH, oxidation by 0 is,
of
course, also evident from the fact
that NH, is considered as a commercial fuel). Chemolithotrophs capable of reaction
(8) would compete with the nitrificants, responsible for reactions (1) and
(2).
They
would likewise be colourless,
i .e . ,
white. But, like reaction
(7),
reaction (8) has never
been observed.
.
In reaction
8), 0
could be replaced as an oxidant by
NO,
or NO;:
NHZ + NO, =N, + 2 H,O; AG; = 86 kcal
9)
The resulting reaction, here written down only for the stoichiometrically simpler
case
of NO;,
could also be considered as a variant of denitrification, i . e . , of nitrate
or nitrite dissimilation, or, in the terms of
EGAMI
TAKAHASHTt at . 1963), of nitrate
or nitrite respiration.
Thus the missing photolithotrophs and chemolithotrophs would both produce
N, from
NH,. So
far only
NO;
or NO, are known as important biotic sources
of
N,.
This is set free in denitrification:
NO,
0.75
(CH,O)
H+
= 0.5N,
+
0.75
CO,
+
1.25 H,O; AG; = -95 kcal
The only exception is the production, of uncertain quantitative importance, of
N,O
from NH, by some aerobic chemoorganotrophs
YOSITIDA
nd ALEXANDER
970)
;
the N20 further yields, abiotically,
N, JOHNSTON972).
Apart from this N O by-
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Two missing lithotrophs
493
pass, the biotic pathway from NH, to N,, reversing the fixation of atmospheric N,,
must take the detour via nitrification.
This
s,
or
was, not true if the missing litho-
trophs here put forward exist, or existed.
Acknowledgement
I like to thank Dr. G. A. PESCHEKor discussions.
Addi t ion in proof
An extensive survey of the role of
N,O
in the atmosphere has now been given by
HUN
and JUNQE 1977).
Refe rences
BRODA,.,
1975a.
The Evolution of the Bioenergetic Processes. Pergamon Press Oxford.
BRODA,
.,
1975b.
The history of inorganic nitrogen in the biosphere. J. Mol. Evol.,
T,87-100.
FROMAQEOT,
. and SENEZ,
. C., 1960.
Aerobic and anaerobic reactions of inorganic substances.
In: Comparative Biochemistry, Vol. 1, 347-409
(M. FLORKIN
nd
H. S. MASON,
Editors). Aca-
demic Press New York.
HAHN,J. and
JUNQE., 1977.
Atmospherous nitrous oxide:
a
critical review.
Z.
Naturforsch.,
828. 190-214.
JOHNSTON
.,
1972.
Newly recognized vital nitrogen cycle. Proc. net. Aced. Sci. Wash.,
69,
2369-2372.
RUTTEN, . G., 1971. The Origin of Life by Natural Causes. Elsevier Amsterdam.
TAKAHASHI,., TANIQUCHI,
.
and EQAMI, F.,
1963.
Inorganic nitrogen compounds: Distribution
and metabolism. In : Comparative Biochemistry,
Vol. 5,92-202
M.
ITLORKIN
nd H.
S.MASON
Editors). Academic Press New York.
YOSHIDA,. and
ALEXA NDER, ., 1970.
Nitrous oxide formation by
N i t r o s omoms
europea
and
heterotrophic organisms. Soil Science Amer. Proc.,
34
880-882.
Mailing address: Prof. Dr. E. BRODA
Institute of Physical Chemistry,
University
Wiihringer StraDe 42
A-1090
Wien, Austria,
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