The Toughness of Life
Another notable feature of life on Earth is that of its toughness and
tenacity. In order to consider this
issue, we should briefly return to the discussion of prokaryotes and
eukaryotes. Some of the key properties
used to distinguish the prokaryotes from their more complex eukaryotic cohorts
are shown in Table 1. The eukaryotes are defined by the presence of a nucleus
and nuclear membrane (eu = true; karyon = nucleus), and in general are
characterized by complex structures, complex behavioral features, and simple
metabolism. Their metabolism is
primarily via oxygen-based respiration of organic carbon, and the sizeable
energy yields from these processes are used to support their complex structural
and behavioral investments. Basically,
plants make organic carbon via photosynthesis, and animals eat the plants (and
other animals), leading to the kind of complex communities we easily recognize
under the general heading of trophic levels or predator-prey cycles. The very existence of complex structures
(both intracellular organelles, and multicellular tissues and organs) renders
the eukaryotes sensitive to environmental extremes often easily tolerated by
their structurally simple prokaryotic relatives (e.g., above 50 C, it is
unusual to find functional eukaryotes).
Figure
5. Properties of the prokaryotes and eukaryotes. The small anucleate organisms known as
prokaryotes share some properties that allow us to group them into functional
domains that are quite different from their eukaryotic counterparts.
On the other hand, the prokaryotes are the environmental “tough guys,”
tolerant to many environmental extremes of pH, temperature, salinity,
radiation, and dryness. I refer to these
organisms as the sundials of the living world - tough, simple, effective, and
nearly indestructible. Some of the
fundamental properties that distinguish them from the eukaryotes are shown in
Figure 5. First, they are small -- they have optimized their surface to volume
ratio so as to maximize chemistry. On
the average, for the same amount of biomass, a prokaryote may have 10-100 times
more surface area. Thus, for a human whose
body mass may include a few percent (by mass) bacteria (as gut symbionts), the
bacteria comprise somewhere between 24 and 76% of the effective surface
area! In environments like lakes and
oceans, where bacterial biomass is thought to be approximately 50% of the
total, the bacteria comprise 91 to 99% of the active surface area, and in
anoxic environments, where the biomass is primarily prokaryotic, the active
surface areas are virtually entirely prokaryotic. In essence, if you want to know about environmental chemistry,
you must look to the prokaryotes!
Contributed by: Dr. Kenneth Nealson
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