Monera and Protista are kingdoms of single-celled organisms. Living in ponds, oceans, soil, and practically everywhere on Earth, they play a crucial role in nature.

Some simple life forms have chlorophyll. They can make their own food. This process, which is also used by all green plants, is called photosynthesis. The organism uses sunlight and carbon dioxide to make a simple sugar plus oxygen.

Life forms that can't do this generally feed on other organisms. This way, they indirectly benefit from the sun's energy.

Most life survives in a fairly narrow range of living conditions. Temperatures generally must be above freezing and below 170°F (77°C). The surrounding environment can't be too salty, nor do most life forms tolerate strong acids or bases. These are chemicals like chlorine bleach and vinegar (acids) or ammonia (base).

But life turns up in all sorts of places. Bacteria have adapted to the frigid cold of the polar regions.

At the other extreme, scientists have found single-celled organisms in Yellowstone National Park's hot springs and also near lava flows from volcanoes. Microscopic organisms have even been found in rocks and caves far below the ground.

Some simple life forms provide food and nutrients for other organisms. Others help break down other material. Bacteria are used this way at sewage treatment plants.

Simple life forms may even help other organisms stay healthy. Normal bacteria and other organisms inside your intestines keep you well. Even the tiniest life forms do important work in nature.