They make up one-tenth of our body's dry weight, they are our oldest ancestors, and we would not exist without their presence today. Microbes, or microscopic organisms, are gaining respect among scientists, not least because of their amazing ability to inhabit every conceivable ecological niche. Yet, say researchers, this world is just beginning to be explored.
For many years the sheer numbers of microbes in the biosphere were vastly underestimated because many of them are difficult or impossible to grow in the laboratory. They include bacteria, algae, fungi, and protozoa. Some authorities include viruses among microscopic organisms, while others argue that viruses, which do not have true cells, should be categorized differently. The development of new genetic techniques for identifying microscopic organisms is opening up a whole new microbial world to us. Only a few years ago, scientists discovered an unknown marine bacterium that is now believed to make up fully a quarter of the biomass of all living things in the world's oceans and, some scientists think, play a key role in the global cycling of carbon and oxygen.
It is not much of a reach to say that microbes are the building blocks of life. For 2 billion years, they were the only form of life on our planet. As they evolved, they created the basic processes of fermentation, photosynthesis, oxygen breathing, and fixation of nitrogen in the atmosphere into proteins. Our bodies' mitochondria, the miniature power plants in the cytoplasm of our cells that provide energy for biological processes, are descendants of early microbes.
In a single bit of soil there may be 10 billion microscopic organisms belonging to thousands of different species. Many are as yet unidentified. Though most people think of microbes as the enemy -- causing sickness, spoiling food -- they are, in fact, indispensable. They carry out photosynthesis on a huge aggregate scale, providing up to half the oxygen in Earth's atmosphere. In addition, they decompose dead animals and plants that would otherwise be an enormous burden on the ecosystem.
As life forms capable of exploiting a wide range of niches, they cannot be beat. Microscopic species inhabit dry, icy valleys in Antarctica, feed off heavy metals in the heat of deep-sea thermal vents, and can be found in blocks of rock a mile below Earth's surface.
The emerging scientific view, then, is that we owe microbes something beyond the status of ancient, lower forms of life. As scientists Lynn Margulis and Dorian Sagan put it: "Most of life's history has been microbial."