Environmental Engineering Reference
In-Depth Information
Appendix: A Primer on Biologically
Mediated Redox Reactions
in Ecosystems
Stuart E.G. Findlay and David L. Strayer
Cary Institute of Ecosystem Studies, Millbrook, NY
Most of you know that plants capture energy by oxygenic photosynthesis and release
energy contained in organic matter through aerobic respiration according to the following
reactions:
CO
2
H
2
O
light
!
CH
2
O
O
2
ð
oxygenic photosynthesis
Þ
1
1
1
CH
2
O
O
2
!
CO
2
H
2
O
energy
ð
aerobic respiration
Þ
1
1
1
These familiar equations are examples of
redox reactions
, which involve a transfer of
electrons. Many organisms (especially prokaryotic microbes) use less familiar redox reactions
to obtain the energy they need for life. These reactions are key parts of both energy flow and
biogeochemical cycling in ecosystems; an understanding of ecosystems requires familiarity
with these reactions. Because some of you may be unfamiliar with these reactions and their
occurrence in ecosystems, we offer a brief primer to biologically mediated redox reactions
in ecosystems. Additional information on these reactions is available in topics by
Stumm
and Morgan (1995)
,
Fenchel et al. (1998)
,
Maier et al. (2000)
,
Wetzel (2001)
,and
Reddy and
DeLaune (2008)
, as well as in Chapter 3 on energetics and Chapters 5-7 on biogeochemical
cycles.
Redox reactions involve the movement of electrons from one element or compound (the
electron donor) to another (the electron acceptor). Such reactions will either yield energy
that may be used by organisms or require some energy input to proceed. An easy way to
think about these reactions is to use a physical analogy for the chemical potential energy
that may be released and used by organisms (
Figure A.1
). For an element capable of
releasing an electron and yielding energy in the process we can think of that electron as
an object at the top of the hill capable of doing work as it rolls downhill. There must be
an electron acceptor capable of “catching” the electron to complete the process. If the com-
bined process of donating and receiving the electron yields energy, then this energy can
potentially be captured and used by organisms. This energy is the Gibbs free energy
(represented as
G
), and if this term is negative, the reaction yields energy.
Δ
