Bacteria and Archaea and the Cycles of Elements in the Environment (page 2)
(This chapter has 4 pages)
© Kenneth Todar, PhD
The Carbon Cycle
Carbon is the backbone of all organic molecules and is the most
element in cellular (organic) material. In its most oxidized form, CO2,
it can be viewed as an "inorganic" molecule (no C - H bond). Autotrophs,
include plants, algae, photosynthetic bacteria, lithotrophs, and
use CO2 as a sole source of carbon for growth, which reduces
the molecule to organic cell material (CH2O). Heterotrophs
require organic carbon for growth, and ultimately convert it back to CO2.
Thus, a relationship between autotrophs and heterotrophs is
wherein autotrophs fix carbon needed by heterotrophs, and heterotrophs
produce CO2 used by the autotrophs.
CO2 + H2O-----------------> CH2O
CH2O + O2-----------------> CO2
Since CO2 is the most prevalent greenhouse gas in the
it isn't good if these two equations to get out of balance (i.e.
predominating over autotrophy, as when rain forests are destroyed and
Autotrophs are referred to as primary producers at the
of the food chain" because they convert carbon to a form required by
Among procaryotes, the cyanobacteria, the lithotrophs and the
are a formidable biomass of autotrophs that account for a corresponding
amount of CO2 fixation in the global carbon cycle.
The lithotrophic bacteria and archaea that oxidize reduced N and S
and play important roles in the natural cycles of N and S (discussed
are virtually all autotrophs. The prevalence of these organisms in
environments (marine sediments, thermal vents, hot springs,
etc. may indicate an unappreciated role of these procaryotes as primary
producers of organic carbon on earth.
The methanogens play a dual role in the carbon cycle. These
are inhabitants of virtually all anaerobic environments in nature where
CO2 and H2 (hydrogen gas) occur. They use CO2
in their metabolism in two distinct ways. About 5 percent of CO2
taken up is reduced to cell material during autotrophic growth; the
95 percent is reduced to CH4 (methane gas) during a unique
of generating cellular energy. Hence, methane accumulates in rocks as
fuel ("natural gas"), in the rumen of cows and guts of termites, in
swamps, landfills and sewage digesters. Since CH4 is the
prevalent of the greenhouse gases, it is best to discourage processes
lead to its accumulation in the atmosphere.
CO2 + H2 -----------------> CH2O
material) + CH4 methanogenesis
Under aerobic conditions, methane and its derivatives (methanol,
etc.) can be oxidized as energy sources by bacteria called
this is a version of decomposition or biodegradation during the carbon
cycle which is discussed below.
Biodegradation is the process in the carbon cycle for which microbes
get most credit (or blame). Biodegradation is the decomposition
of organic material (CH2O) back to CO2 + H2O
and H2. In soil habitats, the fungi play a significant role
in biodegradation, but the procaryotes are equally important. The
decomposition scenario involves the initial degradation of biopolymers
(cellulose, lignin, proteins, polysaccharides) by extracellular
followed by oxidation (fermentation or respiration) of the monomeric
The ultimate end products are CO2, H2O and H2,
perhaps some NH3 (ammonia) and sulfide (H2S),
on how one views the overall process. These products are scarfed up by
lithotrophs and autotrophs for recycling. Procaryotes which play an
role in biodegradation in nature include the actinomycetes, clostridia,
bacilli, arthrobacters and pseudomonads.
Overall Process of Biodegradation
polymers (e.g. cellulose)-----------------> monomers (e.g.
monomers-----------------> fatty acids (e.g. lactic acid, acetic
propionic acid) + CO2 + H2 fermentation
monomers + O2 -----------------> CO2 + H2O
The importance of microbes in biodegradation is embodied in the
that "there is no known natural compound that cannot be degraded by
microorganism." The proof of the adage is that we aren't up to our ears
in whatever it is that couldn't be degraded in the last 3.5 billion
Actually, we are up to our ears in cellulose and lignin, which is
than concrete, and some places are getting up to their ears in teflon,
plastic, styrofoam, insecticides, pesticides and poisons that are
slowly by microbes, or not at all.
The figure above mostly ignores the role of
methanogenesis in the carbon cycle. Since methanogens have the
to remove CO2 from the atmosphere, converting it to cell
and CH4, these procaryotes not only influence the carbon
but their metabolism also affects the concentration of major greenhouse
gases in earth's atmosphere.
Figure 1. The Carbon Cycle.
Organic matter (CH2O) derived from photosynthesis (plants,
and cyanobacteria) provides nutrition for heterotrophs (e.g.
associated bacteria), which convert it back to CO2. Organic
wastes, as well as dead organic matter in the soil and water, are
broken down to CO2 by microbial processes of biodegradation.
Recently, I asked a
colleague, Professor Paul
Weimer of the University of Wisconsin Department of Bacteriology,
mathanogenesis, which utilizes CO2 while producing CH4
was better or worse on the greenhouse effect and global warming. This is his response. "Worse. During methanogenesis involving CO2
reduction, the stoichiometry is 4H2 + CO2 -->
+ 2 H2O, so one mole of a greenhouse gas is exchanged for
But methane is about 15 times more potent than is CO2 in
of heat absorption capability on a per-molecule basis, so the net
is a functional increase in heat absorption by the atmosphere. Remember
also that in most natural environments, around two-thirds of the
is produced by aceticlastic methanogenesis (CH3COOH
CH4 + CO2) - an even less welcome situation, as
products are greenhouse gases.
Even though methane
concentrations in the atmosphere
are two orders of magnitude below those of CO2, methane is
to account for about 15% of the anthropogenic climate forcing, compared
to about 60% from CO2. Most of the rest of the contribution
is from nitrous oxide (N2O, a respiratory denitrification
that has something like 300 times the heat absorbing capacity as CO2)
and the old chlorofluorocarbons (CFCs), even stronger heat absorbers
but more famous and dangerous as stratospheric ozone-depleters."
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