Environmental Engineering Reference
In-Depth Information
Other anaerobes use nitrates, nitrites, sulfates, and fer-
ric iron as electron acceptors. Pseudomonas and Clostri-
dium are common bacteria that reduce nitrate to nitrite.
Thiopneute bacteria, including red Desulfovibrio com-
mon in muds and in estuarine brines, reduce sulfates to
H
2
S or S while incompletely oxidizing lactate and ace-
tate. These dissimilatory reductions (less exergonic than
anaerobic glycolysis, more exergonic than fermentations)
are critical for the functioning of global biospheric cycles.
Methanogens perform the final task of biomass degrada-
tion in those environments where oxygen, nitrate, sul-
fate, and ferric iron have been depleted. They use CO
2
as the final electron acceptor to produce CH
4
(Ferry
1993). Anaerobic fermentation proceeds naturally in
marine and freshwater sediments, marshes, bogs, flooded
soils, gastrointestinal tracts, and geothermal habitats.
Aerobic respiration is a way of life for 8 of 16 phyla of
Monera (bacteria), ranging from myxobacteria to nitro-
gen fixers, for nearly all fungi, and for the whole king-
dom of Animalia. Energy costs of this prevalent mode
of metabolism are examined in this chapter. Evolutionary
steps toward the origin of aerobic respiration are not dif-
ficult to postulate, but details and timings are elusive.
The preplanetary matter was clearly anoxic, and the
Earth's secondary atmosphere had only a limited amount
of the gas formed by photolysis of water vapor by UV
radiation. Eventually photosynthesis (see chapter 3)
increased the partial pressure of oxygen to the point
where some prokaryotes could use aerobic respiration to
generate energy in the form of ATP more efficiently than
by fermentation.
Finding a generally accepted division between plants
and animals has been difficult because both phycologists
and protozoologists claim taxonomic dominion over
euglenids and trichomonads. Many eukaryotic protoctists
can be classified in both groups. Many organisms have
found it advantageous to evolve toward mobile proto-
zoan existence, first by osmotrophy (absorbing dissolved
nutrients through cell surfaces), then by phagotrophy (as
active consumers, even predators). Evolution of hetero-
trophs remains to be clarified for the late Proterozoic
eon, but it is fairly clear after 530 Ma bp. Metazoa
evolved between 1100 and 600 Ma ago, and the fossil
record from about 530 Ma ago documents a spectacular
emergence of diversified skeletonized fauna in a span of
just a few million years (Erwin, Valentine, and Jablonski
1997). Sponges are the most primitive surviving animal
phylum; other simple phyla include Ctenophora (comb
jellies), Cnidaria (jellyfish and sea anemones), and Pla-
thelminthes (flat worms).
More complex animals are classified by the different
fate of the initial opening of the primitive digestive tract
in an embryo: Arthropoda, Annelida (earthworms), and
Mollusca (snails, clams, squids) belong to protostomes;
Echinodermata (star fish, sea urchins) and Vertebrata
(fish to mammals) to deuterostomes. Fish-like animals
from the early Cambrian (Shu et al. 1999) put vertebrates
among the organisms of the Cambrian eruption of new
life forms. Transition between fish and amphibians (tetra-
pods) took place during the Late Devonian, about 365
Ma bp (Janvier 1996). Some 310 Ma bp mammal-like
reptiles split from bird-like reptiles, and molecular clocks
confirm that modern orders of mammals go back to the
Cretaceous period, more than 100 Ma bp, and that they
diversified before the extinction of dinosaurs (Kumar and
Hedges 1998).
4.1 Metabolic Capabilities
Lives of Metazoa require highly differentiated mouth
and digestive organs, well developed circulation, efficient
