Bacteria are prokaryotes, as are the archaea.    This means that they are small organisms, with no nucleus.  They are very tiny -- and very tough.  Although bacteria reproduce by binary fission, and so change slowly, over the millenia they have adapted to almost every possible environment on earth.  They are in the soil, in the air, in the water.  They are found in the digestive tracts of animals and people, in our mouths, on our skin.  Sometimes they cause disease, but most bacteria are not harmful to us.  

    One very important group of bacteria is the cyanobacteria.  They are found as the earliest bacterial fossils, and today still look very like their ancient ancestors.  Cyanobacteria release oxygen into our water and atmosphere.  They gradually added oxygen to  the atmosphere on earth.   As a result, complex life forms were able to evolve.  

     Bacteria make major contributions to the global ecosystem.  

Cyanobacteria produce oxygen by  photosynthesis, in ponds and streams and in the cells of plants, where they have become the chloroplasts in the eukaryotic cells.  

Some bacteria become decomposers, breaking down dead leaves, twigs, and other organic material so that the molecules can be recycled by other life forms.

Some bacteria are nitrogen fixing.  The bodies of our life forms are built up almost entirely of hydrogen, carbon, oxygen and nitrogen.  These are plentiful elements with small atoms, but they have to be put together.  The plants capture carbon from the air and build it into sugars.  Nitrogen, however, is difficult to capture in a useable form, and it is nitrogen fixing bacteria that can do this.  Leguminous plants, such as peas and beans, have special nodules in their roots where nitrogen fixing bacteria can live,  These plants are often used in crop rotation, because the leave the soil enriched with nitrogen in a form that plants can use.

     Bacteria have mechanisms for surviving difficult times.  Some kinds of bacteria can form spores, which are hard little packets that enclose their DNA.  When the conditions that they are living in make life very difficult, the bacteria enclose their DNA in a spore instead of just dying.  The spores are very tough.  When conditions improve, the bacterium goes back to being actively alive.  No one knows how long spores can live.  Spores recovered from ice or even amber may have survived for millions of years.  Scientists are continuing to work on this question.  

     Each domain consists of groups of life forms that are related to each other and have characteristics in common.  We are looking at these domains now because they are the most fundamental divisions of life forms.  These domains describe the characteristics of every living creature that we know about.  Learning about them is an important part of this unit.

Choose the domain that you would like to read about next.