energy of sunlight to extract from water molecules the hydrogen they needed
for self-construction, leaving oxygen as a by-product. A primitive form of pho-
tosynthesis probably began with the first appearance of blue-green algae or its
predecessor, a photosynthetic bacteria called green sulfur bacteria.These organ-
isms were best suited to an oxygen-poor environment. Oxygen was kept to a
minimum by reacting with both dissolved metals in seawater and reduced gases
emitted from submarine hydrothermal vents.
The first green-plant photosynthesizers called proalgae were probably
intermediate between bacteria and blue-green algae.They could switch from
fermentation, a primitive form of metabolism, to photosynthesis and back
again, depending on their environment. Since sunlight penetrates seawater to
a maximum effective depth of only a few hundred feet, the proalgae were
confined to shallow water.Around 2.8 billion years ago, microorganisms called
cyanobacteria began to use sunlight as their main energy source to drive the
chemical reactions needed for sustained growth.
The development of photosynthesis was possibly the single most impor-
tant step in the evolution of life; it gave a primitive form of blue-green algae
a practically unlimited source of energy. Photosynthesis involved the absorp-
tion of sunlight by chlorophyll (in plants) to split water molecules into hydro-
gen and oxygen. The hydrogen combined with carbon dioxide, abundant in
the early ocean and atmosphere, to form simple sugars and proteins, thereby
liberating oxygen in the process (Fig. 22). The growth of photosynthetic
organisms was phenomenal.The population explosion would have gotten out
of hand except that oxygen, generated as a waste product of photosynthesis,
was poisonous to these organisms. If not for the development of special
enzymes to help organisms cope with and later use oxygen for their metabo-
lism, life would have certainly been in jeopardy.
Photosynthesis also dramatically increased the oxygen content of the
ocean and atmosphere. The oxygen level jumped significantly between 2.2
and 2 billion years ago. During that time, the ocean’s high concentration of
dissolved iron, which depleted free oxygen to form iron oxide, similar to the
rusting of metal, was deposited onto the seafloor, creating the world’s great
iron ore reserves. Around this time, Earth experienced its first major period of
glaciation.The cold ocean waters caused the iron to settle out of suspension.
To create and maintain an oxygen-rich atmosphere, carbon dioxide used
in the photosynthetic process had to be buried in the geologic column (rock
strata) in the form of carbonate rocks faster than the consumption of oxygen
by the oxidation of carbon, metals, and reduced volcanic gases. About 2 bil-
lion years ago, oxygen began replacing carbon dioxide in the ocean and
atmosphere. Therefore,organisms had to either develop a means of shielding
their nuclei and other critical sites from oxygen or use chemical pathways that
oxidized by removing hydrogen instead of adding oxygen. These innovations
ARCHEAN ALGAE