Microorganisms the staff of life

SIGNS OF LIFE: First in a three-day series

When life began on Earth, microorganisms led the way.

Bacteria were the sole inhabitants of this planet for more than 2 billion years, until photosynthesizing cyanobacteria, also known as blue-green algae, created the oxygen-rich atmosphere that allowed plants and animals to emerge.

Even today, humans, animals and plants are utterly reliant on bacteria and other microbes for the recycling of key nutrients and the degrading of organic matter. Without microbes, all higher life forms would die.

A single gram of fertile soil contains about 100 million bacteria. One thousand laid end to end would span the head of a pin.

On Earth, bacteria live wherever there is liquid water. They have been found in nuclear reactors, in ice-covered polar lakes, inside rocks from dry Antarctic valleys, in cores bored more than a mile below Virginia, and at the bottom of the 35,839-foot-deep Marianas Trench in the western Pacific.

In the past five years, microbes have been found below the sea floor at more than 20 sites in the Pacific. Some scientists, including Cornell University’s Thomas Gold, author of “The Deep Hot Biosphere,” believe life started in fluid-filled cracks deep inside the Earth’s crust, then migrated up into the oceans and, finally, to the surface. Microbes tolerate astonishing extremes of temperature, acidity, alkalinity, salinity and pressure. They account for at least half the mass of all living material on Earth.

Microorganisms display a staggering variety of mechanisms for obtaining energy and food. While humans and other animals need oxygen to breathe, and depend on sunlight for food from plants, some bacteria require neither.

Plants use the energy from sunlight to convert carbon dioxide into food through photosynthesis. In the process, they split water molecules, releasing oxygen.

At dark, sea-bottom hydrothermal vents , some bacteria act like green plants, converting carbon dioxide into organic matter. They rely on chemical energy from sulfur compounds emitted from the vents rather than sunlight.