We now turn to water as the medium that supports life. All organisms require
water, and a large fraction of them make their home in rivers, lakes, and the oceans.
Life started in the ocean and occupied dry land only later. Moreover, biological processes
have a profound influence on the chemistry of natural waters, and indeed of the entire
globe. Were it not for the evolution of photosynthetic organisms, first in the ocean, and
then on land, the atmosphere would be devoid of oxygen. The profound influence of
oxygen on the chemistry of the atmosphere was considered at length in Part II. O2 is
also the dominant actor in the chemistry and biochemistry of the hydrosphere. The
limited availability of O2 in water sets the boundary between aerobic and anaerobic life,
with crucial consequences for water quality and the health of ecosystems. In this chapter
we consider
• Redox energy and dissolved oxygen
• Biological redox and the reduction potential
• Linkage of redox with acid/base chemistry
• Earth’s redox evolution
• Biological CO2 pump
• Overfertilization of surface waters: eutrophication
• Redox and metal pollution
• Ocean fertilization with iron