e.8 2. 70
Zeitschrift für Allg. Mikrobiologie
1977
491-493
(Institut für Physikalische Chemie,
Universität Wien)
Two kinds of lithotrophs missing in nature
E. BRODA
(Eingegangen am 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 O2 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 utilization of a wide range indeed of exergonic reactions. Attention is now drawn to furt her
processes in energy metabolism which on the basis of considerations on the evolution
of the bioenergetic processes (BRODA 1975a) 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 inorganic reductants in their energy metabolism (FROMAGEOT and 8ENEZ 1960); all
autotrophs must be lithotrophs, though the reverse need not be true. The two hacteria here predicted would generate dinitrogen (N2 ).
The nitrifying bacteria make adenosine triphosphate, ATP, through oxidative phosphorylation 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:
NHt
+ 1.5 O2 = H 20 + N0 2 + 2 H +; ßG~ = N02 + 0.5 O2 = NO;-; ßG~ = - 18 kcal
65 kcal
(1)
(2)
The negativity of the free enthalpy change, ßG~, is the precondition for the production of ATP and, consequently, for the endergonic reduction of 00 2 to biomass. The
reduction occurs, as in plants, through the OALVIN cycle; the reductant, NADH, is
obtained by reverse electron flow forced by ATP.
Olearly, the nitrificants, one main group of the "chemolithotrophic" bacteria, could
evolve only after the biosphere began to contain, as a consequence of the photosynthetic activity of the blue-green algae, free oxygen (BRODA 1975a, b). The transition to the oxidizing biosphere took place about 2 giga-years (Gy) ago (RUTTEN 1971)
8imilarly the free oxygen made possible the 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:
+ 0.5 O2 + H + = H 0 + 8; ßG~ = 8 + 1.5 O2 + H 20 = 80 4 + 2 H+; ßG~ = H8-
2
2
51 kcal
139 kcal
(3)
(4)
The thiobacilli presumably descended from coloured, photosynthetic, sulphur bacteria, i .e., the "photolithotrophs" gave, after the advent of 02' rise to the chemolithotrophs. In other words, oxidative phosphorylation evolved from photosynthetic
492
E.
BRODA
phosphorylation. This is indicated by the "conver sion hypothesis" for t he origin of
respiration from photosynthesis (BRODA 1975 a) .
The basic processes in t he en ergy metabolism of the photosynthetic sulphur bacteria are
+ 2 H + + CO = (CH 0) + H 0 + 2 S; .6..G~ = 11 kcal
+ CO + H 0 = (CH 0) + 0.5 H + + 0.5 SO~-; .6..G ~ = 18 kcal
2 HS0.5 HS-
2
2
2
2
2
2
(5)
(6)
(CH 2 0) indicates unit quantity of biomass, not formaldehyde. Reactions (5) and (6),
which are endergonic, ar e energized by light,i.e., electrons are promoted photochemically, Aseparation into exergonic and endergonic partial reactions would, in cont rast to the position with the chemolithotrophs, not be meaningful with the photolithotrophs because CO 2 is indispensable as terminal (extracellular) electron acceptor.
In reactions (1) to (4) this role is played by 02. Incidentally, for the processes (4) and
(6) the term "sulphurication" might be introduced, in analogy to nitrification (processes
1 + 2).
Who, then, were the ancestors of the nitrificants? Can they, in parallel to the
evolution of the sulphur bacteria, have descended from photosynthetic ammonia
bacteria? Such (coloured) bacteria ar e 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 NHt to reduce CO 2 , the fundamental
feature of su ch hypothetical bacteria, would from t he point of view of en ergetics not
be too difficult:
1.3 NHt
+ CO
2
=
(CH 2 0)
+ 0.65 N + H 0 + 1.3 H+;
2
2
.6..G~
=
12 kcal
(7)
This would involve a direct biotic oxidation of NHt, i.e., of NH 3 , to N 2 • Such a
reaction is unknown.
In contrast to the anaerobic and endergonic reaction (7) , an aerobic and exergonic
oxidation of NH 3 to N 2 could, like that to NO;- 01' NOs, occur only after the appearance of O2 in the biosphere:
NHt
+ 0.75 O
2
= 0.5 N 2
+ 1.5 H 0 + H +;
2
.6..G~ =
75 k cal
-
(8)
(The exergonicity of NH 3 oxidation by O2 is, of course, also evident from the fact
t hat NH 3 is considered as a commercial fuel). Chemolit hotrophs capable of reaction
(8) would compete with the nitrificants, r esponsible for reactions (1) and (2) . They
would likewise be colourless, i .e., white. But, like reaction (7), r eaction (8) has never
been observed.
In r eaction (8), O2 cOlud be r eplaced as an oxidant by NO;- or N Os:
NHt
+ NO;- =
N2
+ 2 H 0;
2
.6..G~ =
-
86 kcal
(9)
The resulting reaction, here written down only for t he stoichiometrically simpler
ca se of NO;-, could also be considered as a variant of denitrification, i.e., of nitrate
or nitrite dissimilation, or, in the terms of EGAMI (TAKAHASHI et al. 1963), of "nitrate
or nitrite r espiration".
Thus the missing photolithotrophs and chemolithotrophs would both produce
N 2 from NH 3 • So far only NO;- or NOs are known as important biotic sources of
N 2 • This is set free in denitrification:
NO;
+ 0.75 (CH 0) + H + =
2
0.5 N 2
+ 0.75 CO + 1.25 H 0;
2
2
.6..G~
=
- 95 k cal
The only exception is the production, of uncertain quantitative importan ce, of N 2 0
from NH 3 by some aerobic chemoorganotrophs (YOSHIDA and ALEXANDER 1970) ;
the N 2 0 further yields, abiotically, N 2 (JOHNSTON 1972). Apart from this "N2 0 by-
Two missing lithotrophs
493
pass", the biotic pathway from NH 3 to N 2 , reversing the fixation of atmospheric N 2 ,
must take the detour via nitrification. This is, or was, not true if the "missing lithotrophs" here put forward exist, 01' existed.
Acknowledgement
I like to thank Dr. G. A. PESCHEK for discussions.
Addition in proof
An extensive survey of the role of N 2 ü in the atmosphere has now been given by
HAHN and JUNGE (1977).
References
BRODA, E., 1975a. The Evolution of the Bioenergetie Processes. Pergamon Press Oxford.
BRODA, E., 1975b. The history of inorganie nitrogen in the biosphere. J. Mol. Evol., 7,87 -100.
FROMAGEOT, C. and SENEZ, J. C., 1960. Aerobic and anaerobic reaetions of inorganie substanees.
In: Comparative Bioehemistry, Vol. I, 347 -409 (M. FLORKIN and H . S. MASON, Editors). Aeademie Press New Y ork.
HAHN, J. and J UNGE, C., 1977. Atmospherous nitrous oxide: a eritical review. Z. Naturforsch.,
32a, 190-214.
JOHNSTON, H., 1972. Newly reeognized vital nitrogen eycle. Proe. nato Aead. Sei. Wash., 69,
2369-2372.
RUTTEN, M. G., 1971. The Origin of Life by Natural Causes. Elsevier Amsterdam.
TAKAHASID, H. , TA1<-:IGUCID, S. and EGAl\:!I, F., 1963. Inorganie nitrogen eompounds: Distribution
and metabolism. In: Comparative Bioehemistry, Vol. 5, 92-202 (M. FLORKIN and H. S. MASON,
Editors). Aeademie Press New York.
YOSIDDA, T. and ALEXANDER, M., 1970. Nitrous oxide formation by Nib'osomonas eumpea anel
heterotrophie organisms. Soil Seienee Amer. Proe., 34,880-882.
Mailing address: Prof. Dr. E. BRODA
Institute of Physieal Chemistry,
University
Währinger Straße 42
A-I090 Wien, Austria