Agriculture Reference
In-Depth Information
Table 5.3
Comparison of Heterotrophic and Autotrophic Microorganisms
Heterotrophic microorganisms
Autotrophic microorganisms
Require C in organic compounds to feed on
and provide C for cell growth; include the
majority of species of bacteria and all fungi.
Those that feed on dead organic matter are
called saprophytes, whereas those feeding
on living tissue are called parasites. Several
parasitic bacteria and fungi are pathogens of
grapevines (see “Soil Interaction in Fungal
and Bacterial Diseases,” this chapter).
Synthesize their cell substance from the
C of CO
2
and include the remaining
bacteria and most algae. Energy for this
synthesis comes from sunlight (for the
photosynthetic bacteria, some Archaea, and
algae), or is chemical energy derived from
the oxidation of inorganic compounds.
Nitrifying bacteria are the most notable of
the second type, one group of which gains
energy from the oxidation of NH
4
+
ions
and a second from the oxidation of NO
2
−
(see box 3.2, chapter 3).
Note.
C = carbon; CO
2
= carbon dioxide; NH
4
+
= ammonium; NO
2
−
= nitrite.
Second, soil microorganisms show a general response to drastic changes
such as a rapid change from air dryness to wetness or from a frozen to thawed
state. When soil becomes air-dry or frozen, a substantial fraction of the microbial
population dies and the remainder becomes quiescent. On rewetting or thawing,
however, the bodies of the dead organisms provide an ideal substrate for the survi-
vors. The surviving organisms and their progeny experience a flush of activity that
slowly subsides as the readily decomposable material is consumed.
Third, groups of microorganisms differ in their need for O
2
. Box 1.3 in chap-
ter 1 introduced the distinction between aerobic and anaerobic conditions, but
microbial metabolism is more complex in that we can recognize the following
groups:
• AerobicorganismsthatgrowonlyinthepresenceoffreeO
2
• Facultativeanaerobes,whichnormallyuseO
2
, but can adapt to O
2
-free
conditions by using nitrate (NO
3
−
), or other inorganic compounds such as
ferric iron (Fe
3+
), as electron acceptors in respiration, or C compounds as
in fermentation
• Obligateanaerobes,whichgrowonlyintheabsenceofO
2
because free O
2
is toxic to them
Most fungi are aerobic, with the notable exception of yeast, which is a faculta-
tive anaerobe that ferments simple sugars predominantly to alcohol. Fungi and
actinomycetes are more abundant in the litter layer because their ability to decom-
pose lignin gives them a competitive advantage over bacteria (figure 5.3).
Fourth, soil microorganisms produce enzymes that act inside or outside their
cells to digest substrates, but there are also enzymes that exist in soil independently
