Role of Microscopic Life Forms in the Evolution of the Earth's Atmosphere

Eugene S. Takle
© 2002

Microorganisms likely played a major role in the evolution of the atmosphere before the rise in atmospheric oxygen content. Photosynthesis creates atmospheric oxygen and organic matter from carbon dioxide and water. Land plants use up most of the oxygen created by photosynthesis during the decomposition phase of their life cycles. Some marine organisms, however, end their life cycles by losing some carbon to the deep ocean (e.g., skeletons of marine life that fall to the ocean floor) in a process sometimes referred to as "biological pumping" (see Unit on Carbon Cycle). With some carbon "leaking" out of the carbon cycle, some oxygen then is left in the atmosphere. The net result is that marine photosynthesis contributes to the rise in atmospheric oxygen. Marine blue-green algae (cyanobacteria) can live aerobically or anaerobically and are believed to have been the organisms that created much of the earth's early atmospheric oxygen.

Microorganisms known as methanogenic bacteria also were the likely source of early concentrations of methane in the earth's atmosphere. Production of methane by this source and absence of the currently dominating methane destruction mechanism (hydroxyl radical) in the anoxic early earth atmosphere would have allowed atmospheric methane concentrations much higher than those of today. This would have provided an effective greenhouse gas environment to raise the earth's surface temperature in spite of a rather dim sun, compared to current levels.

The bottom line is that microbial organisms played a dominant role in the evolution of the early atmosphere of the earth.