in which fCH2Og represents the reduced carbon of glucose and its polymers. Living organisms are
thermodynamically unstable and require a continual source of free energy merely to survive. This they
obtain either from sunlight or from chemical free energy sources such as the carbohydrate products
of the above reaction. The metabolic activities of organisms are largely centered around the stepwise
breakdown of these foodstu®s in a way that transfers smaller increments of free energy to intermediates
(mainly ADP) that are able to e±ciently convey it to sites of synthesis or mechanical movement where
it can be utilized. The overall e®ect can be summarized as the transfer of electrons from the reduced
carbon of fCH2Og to an electron acceptor such as O2 (this is the reverse of photosynthesis) or to SO2¡
3
or NO¡
3 , accompanied by the release of CO2 to the environment.
It is largely through these processes that the major bio-active elements carbon, nitrogen, sulfur, iron
and manganese (but not phosphorus) are moved through their geochemical cycles. From the global
geochemical standpoint, the O2 produced by photosynthetic organisms maintains an electron de¯ciency
in the atmosphere and in waters in contact with the atmosphere; respiration and reactions with the
reduced components of the lithosphere (including volcanic gases, substances introduced at sites of sea°oor
spreading, and substances leached into natural waters) act to maintain a non-equilibrium, more or less
steady state between global electron sources and electron sinks.
At the same time there are some other natural redox processes that are not directly connected with
the biosphere. Direct absorption of light by Fe3+ in surface waters, for example, can result in its photochemical
reduction to Fe2+ even in the presence of oxygen, which would normally tend to drive the
reaction the other way. The vents at geothermally active oceanic ridges emit considerable amounts of
reduced compounds which react with oxidants such as O2, SO2¡
3 and Fe3+(all originally of biologic origin)
which initiate processes which alter the composition and oxidation state of the oceans.
In order to understand these processes we will begin by reviewing the fundamental ideas of oxidation
and reduction and electrochemical potentials. We will extend these concepts somewhat beyond what you
may recall from earlier chemistry courses, and discuss electron activity, the electron free energy scale, and
pE. This will provide the basis for the major subject of this chapter which deals with the redox state of
the aquatic environment and its relation to the geobiochemistry of some of the major element. In doing
so, two general principles should be kept in mind:
² Redox reactions tend to be kinetically inhibited, and are often extremely slow. As a consequence
of this, redox equilibrium is rarely achieved, either in natural waters or on the Earth as a whole.
² This kinetic sluggishness, upon which life itself depends, can be overcome by suitable enzymes.
Thus the major redox processes of the geochemical cycle are mediated by organisms.