Download Introduction: Anaerobic and Aerobic Environments

452
BIOCHEMICAL SOCIETY TRANSACTIONS
MICROBIAL CATABOLISM: ITS ROLE IN THE
CARBON CYCLE AND ENVIRONMENTAL
SIGNIFICANCE : a Colloquium organized by
P. A. Williams (Bangor)
Introduction: Anaerobic and Aerobic Environments
W. CHARLES EVANS
Department of Biochemistry and Soil Science, University ColIege of North Wales,
Bangor, Gwynedd LL57 2UW, Wales, U.K.
Life on Earth depends on a dynamic balance between living organisms possessing a
diversity of biochemical competence, and the environment. This ‘throughput’ of
chemicals from inanimate materials (rocks, soils, natural waters and air, with the aid of
solar radiation) to their participation in biochemical processes and their subsequent
mineralization again, has evolved over a geological time-scale and under a variety of
physical conditions.
The constituents ofliving organisms must be susceptible to degradation or they would
eventually accumulate on the surface of the earth and thus lock the chemicals in an unavailable form. Micro-organisms are the scavengers par excellence in maintaining the
carbon cycle, producing eventually COz and methane from all naturally occurring
organic compounds.
Man is introducing increasingly large amounts of synthetic organic chemicals into the
environment, particularly as herbicides, pesticides, detergents and the unwanted byproducts of industry. Some of these molecules are recalcitrant to microbial attack and
therefore accumulate in the biosphere; their effects (apart from any direct toxicity to
man) on normal food-chains are not easy to predict, but have been disastrous in some
instances.
Biodegradability is clearly obligatory for soluble, potentially toxic, organic chemicals if they are to be used by man for whatever purpose, and are likely to be disseminated
in the environment.
Microbial catabolism is a wide topic; it seems appropriate to focus attention on some
of the fundamental biochemical aspects of the central ‘carbon’ cycle. Guiding principles may emerge that have a bearing on biodegradability in Nature.
Carbon cycIe
Scheme 1 illustrates the biological carbon cycle. The principal geochemical reservoirs
of carbon have been quoted as follows (Doetsch & Cook, 1973):
Atmosphere = 2.3 x 10i8g(as COz)
Biosphere
= 2.6 x lO”g (inorganic and organic C)
Lithosphere = 1.2 x 1Oz2g(sedimentary rocks and carbonates)
= 3.6 x loz1g (fossil fuels: coal, petroleum and kerogen)
= 1.3 x
1Oz2g(igneous and metamorphic rocks)
The smallest segment with respect to the amount of this element is the biosphere.
Carbon enters the biosphere primarily by photosynthesis (terrestrial plants, aquatic
algae and especially the unicellular marine diatoms and dinoflagellates), i.e. by the
reduction of COz; the contribution of photosynthetic and chemolithotrophic bacteria is
thought to be relatively small in comparison. Estimates of COz fixation vary widely;
possibly 5 x 1016-15x 10l6gof carbon enters the biosphere annually by photosynthesis
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562nd MEETING, BANGOR
Carbonates
Volcanoes,
magma,
Igneous rock
deposits
Preclplratlon
Scheme 1. Carbon cycle
e)
(i.e. 8-23 %ofthe total carbon in the atmosphere). O2is also released from water in this
process, and it is apparent that cycles of C and Ot must be inter-related.
Transformations of carbon in the biosphere may be represented as follows :
Heterotrophs
Animals
Micro-organisms
Auto trophs
Micro-organisms
Plants
Organic
compounds
Photosynthesis
H20
O2
Mineralization
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BIOCHEMICAL SOCIETY TRANSACTIONS
As is well known, three general methods exist by which organic compounds are
catabol ized .
(a) Aerobic respiration. Here, the oxidation stages occur at the expense of O2 as the
terminal electron acceptor; O2is also a molecular reactant of microbial oxygenase action.
The energy produced comes mainly from oxidative phosphorylation, although the
complexity of the electron-transport system varies among different micro-organisms. The
availability of O2is often a limiting factor in mineralization.
(b) Anaerobic respiration. Some prokaryotes are able to utilize certain inorganic electron acceptors, by a process confusingly called anaerobic respiration. Thus NO3- is
reduced to NH3, N20 or N2, SO4’- is reduced to S2- and COz to methane. Since these
inorganic electron acceptors have higher redox potentials than oxygen, less ATP will
usually be produced compared with aerobic respiration. Although pathways of substrate
degradation are usually identical in aerobic and anaerobic respiration, this is not the
case for aromatic compounds.
(c)Fermentation. Here, no external electron acceptor is required. Instead, the C source
is metabolized by a series of reactions that release energy by substrate-level phosphorylation. There is a wide range of fermentation end-products; anaerobic growth is much less
efficient energetically than aerobic respiration.
In practice all naturally occurring carbon compounds are susceptible to attack in the
biosphere by one or other of the above methods; that great reservoir, soil humus, has a
definite, if slow, turn-over rate (Jenkinson, 1968). Elemental C, whether amorphous
(carbon black) or crystalline (graphite, diamonds), however, is apparently not metabolized by living organisms.
Anaerobic and aerobic environments exist in the biosphere, and biochemical degradative reactions occur in both; their relative magnitude is a matter for conjecture (Zobell,
1964). Although aerobic processes are of major importance in microbial catabolism,
O2is also necessary to complete that part of the carbon cycle initiated under anaerobic
conditions, e.g.
CH4
+
Organic
c
Methane-utilizing bacteria
These aerobic pathways will receive prime consideration at this Symposium. Perhaps
therefore it is opportune to draw attention to the anaerobic metabolism of aromatic
substrates, even if only as a counterpoise.
Certain of the photosynthetic bacteria Athiorhodaceae (e.g. Rhodopseudomonas
palustris) photometabolize many aromatic compounds under anaerobic conditions by
the reductive pathway shown in Scheme 2 for benzoate (Dutton & Evans, 1969; Guyer
& Hegeman, 1969; P. J. Whittle & W. C. Evans, unpublished work). Thioesterification
apparently allows the aromatic ring to be reduced to cyclohexanoyl-CoA; this intermediate then undergoes a B-oxidation sequence culminating in ring cleavage with the
production of pimelate-diCoA ester, suitable for energy-yielding reactions by its subsequent metabolism.
Pseudomonas sp. (PNl) and a Moraxe//a sp. isolated from soil by Taylor et a/. (1970)
and Williams & Evans (1975) respectively, metabolize benzoate anaerobically by nitrate
respiration. The latter organism employs a reductive pathway similar to that shown in
Scheme 2 except that decarboxylation occurs and adipate is the ring-fission product.
A ‘consortium’ of bacteria occur in anaerobic environments, e.g. rumen and intestinal
contents, sewage sludge from anaerobic digesters etc., that, in the absence of 02,NO3‘
or
produce methane and C 0 2 from benzoate according to the equation:
4CsHSC02H+ 18H20 -+ 15CH4+ 13C02
(Clark & Fina, 1952; Fina & Fiskin, 1960; Roberts, 1962; Nottingham & Hungate,
1969; Ferry, 1974). M. Balba & W. C. Evans (unpublished work) have recently detected
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562nd MEETING, BANGOR
COA-SH
“60
OC-S-COA
HzO
L
-7-’
455
@
OC-S-COA
@
OC-S-COA
7-+
NAD+
2WHz)
FAD
J
OC--S--CoA
I
c
OC-S-COA
I
OC-S-COA
Metabolic pool
Scheme 2. Anaerobic photometabolicpathway of benzoate b y Rhoabpseudomonaspalustris
cyclohex-l-enecarboxylate,cyclohexanecarboxylate and adipate in the fermentation
liquor metabolizing benzoate. Propionate and acetate were also identified, as reported
previously by Roberts (1962) and Ferry (1 974) respectively. o-Chlorobenzoate inhibits
benzoate utilization without affecting methane production from acetate (Ferry, 1974).
Therefore it seems likely that the methanogenic fermentation of benzoate occurs by the
co-operation of bacteria in the consortium that accomplish the reductive pathway of
metabolism, affording aliphatic acids and eventually acetate for the methane bacteria to
convert this to methane and COs.
Clark, F. M.& Fina, L.R. (1952) Arch. Biochem. Biophys. 36, 26-32
Doetsch, R.N. & Cook, T. M.(1973) in Bacteria and their Ecobiology, pp. 294-295, University
Park Press, Baltimore
Dutton, P. L. & Evans, W. C. (1969) Biochem. J. 113, 525-536
Ferry, J. G. (1974) Ph.D. Thesis, University of Illinois
Fina, L. R. & Fiskin, A. M. (1960) Arch. Biochem. Biophys. 91, 163-165
Guyer, M. & Hegeman, G. (1969) J. Eacleriol. 99, 906-907
Jenkinson, D. S. (1968) Biochem. J. 109, 2~
Nottingham, P. M.& Hungate, R. E. (1969) J. Bacteriol. 98, 1170-1172
Roberts, F. F. (1962) M.Sc. Thesis, Kansas State University
Taylor, B. F., Campbell, W.L. & Chinoy, I. (1970) J. Bacferiof.102, 430-436
Williams, R.J. & Evans, W. C. (1975) Biochem. J. 148, 1-10
Zobell, C. (1964) in Proc. Ruablfs’ Research ConJ, Rutgers, Tie State Univ., New Brunswick,
NJ, U.S.A.:PrinciplesandApplicationsin Aquatic Microbiology (Heukelekian, H. & Dondero,
N. C., eds.), pp. 337-339, John Wiley and Sons,New York, London and Sydney
Microbial Catabolism and the Carbon Cycle
STANLEY DAGLEY
Department of Biochemistry, University of Minnesota, St. Paul, MN 55108, U.S.A.
It was possible for living forms to evolve in all their diversity when they became capable
of harnessing energy released by the aerobic oxidation of biochemicals. These compounds
were replenished by the reactions of photosynthesis that were initiated on this planet by
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