Environmental Engineering Reference
In-Depth Information
3
P
HOTOSYNTHESIS
Bioenergetics of Primary Production
The Sun appears to be poured down, and in all directions
indeed it is diffused, yet it is not effused. For this diffusion
is extension.
Marcus Aurelius (121-180), Meditations
The origins of photosynthesis remain speculative. If
the early Archean (4-3.5 Ga bp) atmosphere had no ox-
ygen, then the first phototrophic prokaryotes had to as-
similate carbon in ways akin to those of the still extant
anoxygenic bacteria. These were eventually marginalized
by high levels of atmospheric O
2
produced by water-
splitting photosynthesis (Peschek 1999). The sequencing
of genes involved in photosynthesis and phylogenetic
analyses indicates that green nonsulfur bacteria (heliobac-
teria) are the last common ancestors of all photosynthetic
lineages (Xiong et al. 2000). But if the Archean atmo-
sphere already had more than a trace of oxygen, then
the earliest phototrophs might have been more like
today's cyanobacteria. Many strains of these prokaryotes
can perform anoxygenic photosynthesis in hypoxic or
anoxic sulphide-rich environments (hot springs, ocean
sediments) and then shift to O
2
-releasing photosynthesis
in aerobic niches (Whitton and Potts 2000).
The shift to water-cleaving photosynthesis freed the
bacteria from their dependence on limited amounts of
And what an extension! All complex life on this planet,
all its incredible diversity, all our hopes and worries, are
but transmutations of the Sun's light, and photosynthesis
is the agent of this miracle. Absorption of sunlight and
the subsequent sequence of photochemical and thermo-
chemical reactions in the chloroplasts of photosynthesiz-
ing bacteria and green plants are the most important
energy conversions on the Earth. Plants provide (directly
or after being eaten by animals) all our foods; their
immediate harvests (as wood and crop residues) or the
extraction of their fossilized remains (as coals and hydro-
carbons) supply all our fuels. All the richness of hetero-
trophic life and all the intricacies of human civilizations
are thus energized by photosynthesis (with help from pri-
mary electricity).
