Microorganisms (generally from a fraction of a micron to several hundred microns in size - a micron is a thousandth of a millimeter) pretty much run the planet Earth. Bacteria predate multicellular life forms on this planet by several billion years. They eventually evolved, through complex fusions and symbiotic relationships (some would say entrapment) into the basic eucaryotic cell, one in which there is a nucleus containing the genetic material DNA. At first these new life forms were single-celled (these are now often referred to at protists), but then multicellular life developed, apparently through protist's cells remaining connected after cell division. Even so bacteria and protists did not disappear, but still make up most of the individual organisms on Earth. Most people are aware of the importance of the various microscopic organisms that cause disease- typhoid, the flu, malaria, West Nile virus, the common cold, etc. - but few understand that our very existence on this planet depends on the health of the microscopic communities in soil, sea and freshwater. The health of our soil, and thus the health of our crops, depends on the multitude of microorganisms that are incorporated within its invisible interior. One of the great difficulties in studying soil organisms is that their undisturbed habitat is generally unavailable to us. It is easier, but still difficult, to study the microscopic fauna and flora in water.
Of course there are several of factors and organisms that make our life possible, such as the sun (without which the energy would not be present for life, at least as we know it, to even exist - except perhaps for some bacteria deep under the surface), water (without which life would also be impossible), the insects, birds, bats, and others that pollinate our food crops and other vascular plants and help control pests (from our perspective), and the photosynthetic (oxygen-producing) vascular plants that make up to forests, both temperate and tropical. However, beside the sun's energy and the presence of free water, the microscopic community is the next most important factor that allows us to live, and as such it should not be taken lightly. Our bodies are literally covered and infested with millions of minute organisms, most of which are beneficial or benign. The mitochondria that supply our cells (and most living things, including those that photosynthesize) with chemical-based energy are nothing more or less than captured bacteria, and the chloroplasts that utilize sunlight to turn carbon dioxide into sugars in photosynthetic eucaryotic cells (cells of some protists and most vascular plants) are similarly captured cyanobacteria.
Microscopic algae (including diatoms) are an important component of the latter group of organisms - photosynthetic protists - and in addition are the basis for much of the vital food chain that eventually leads to us. Oceanic algae, including seaweeds and diatoms, are said to produce anywhere from 30-70% of our planet's oxygen. Even at the lower figure the amount is significant. They also are responsible for about 30% of the earth's primary productivity. Such calculations are of course somewhat uncertain, but in any case the loss of marine algae would be very detrimental to our existence. The sun is the ultimate source of our energy and water is vital to life, but oxygen-breathing life forms would not exist but for photosynthesizing bacteria, algae and vascular plants. The microscopic phytoplankton of the oceans are thus vital to our survival!
Freshwater forms also contribute, but freshwater makes up only 3% of the water on the planet. Marine diatoms may be the most important of all of the photosynthetic plankton, yet most people have hardly or never heard of them!
Green, brown and red algae, plus diatoms, make up what we call "algae." Most are microscopic, except in mass, but a few, like the huge kelp, are sizable. Of these "algae," diatoms are unique in producing shells made from hydrated silicon dioxide (the same chemical formula as opal!), which they extract from the surrounding water. Other than diatoms only glass-sponges, silicoflagellates and radiolarians (which used to be placed with amoebae) make silicon dioxide shells or skeletons. Many such structures (the bodies or shells of calcareous sponges, corals, mollusks and the bones of vertebrates) are made from calcium carbonate. Arthropods use chitin (as do fungi) and vascular plants use cellulose (both organic) for structural materials. Because silicon dioxide is not porous, diatoms have tiny holes that open to the outside of the cell which allow them to respire properly.
And then of course there are still the bacteria, most of which do not cause disease. Bacteria come basically in two flavors, Archebacteria and Eubacteria. The DNA of these organisms indicate that despite their similarity they are not closely related. In fact there appear to be three domains in life - Archebacteria, Eubacteria and the Eucaryota, with the several kingdoms falling under these. For simplification it may be easier to think of all of life falling under six kingdoms- Archebacteria, Eubacteria, Plantae, Fungi, Animalia and Protista (or Protoctista), but anything we devise is only a model and may not totally reflect reality. Some biologists envision a multi-kingdom system under the three domains, which may actually be a more accurate picture. However, nature is not arranged in nice neat catagories for our convenience.
A living filamentous green alga, Spyrogyra sp., from The Gila National Forest of New Mexico. This is a pond scum forming species.
A desmid (microscopic green alga) in the genus Closterium from the Chiricahua Mountains of Arizona.
Arachnoidiscus ornatus, a fossil marine centric diatom from Santa Monica, California.
A few diatoms, such as Didymosphenia geminata (otherwise known as rock snot for the visible manifestations of its colonies on rocks in freshwater habitats), are invaders that take over other diatom habitats, and some, such as species of Pseudo-nitzschia cause shellfish poisoning through the production of domoic acid. However, for the most part diatoms are extremely beneficial, in fact essential to our very existence. They are also quite beautiful and varied in their structure, making them a favorite of amateur and professional phycologists (people who study algae, also known as algologists) alike.
Didymosphenia geminata, an invasive freshwater diatom species.
Cymbella sp., a strange boomerang-shaped fossil diatom from Ireland.
A fossil marine diatom Trigonium dobreeanum from the Eocene of Oamaru, New Zealand.
Trigonium arcticum, a large marine diatom from McMurdo Sound, Antarctica.
A strange seed-shaped diatom in the genus Surirella, fossil from Ireland.
Bacteriastrum furcatum, a strange marine form from the Caribbean.
It is a great pity that we generally notice microorganisms only if they give us diseases. The lack of our general understanding of the importance of the micro-world may lead to our ultimate demise. We need to especially protect the phytoplankton of the oceans because their presence is vital to all life, including our own. It remains to be seen if we are capable of doing so and living up to our Latin designation Homo sapiens - "man the wise."
References: Two references to many microorganisms that are also fascinating reads include:
Canter-Lund, Hilda, and John WG Lund. 1995. Freshwater Algae: Their Microscopic World Explored. Biopress Ltd.
Dyer, Betsey Dexter. 2003. A field Guide to Bacteria. Comstock Pub. Assoc. (Cornell Univ. Press.
A useful book on the classification and phylogeny of life on the planet is:
Tudge, Colin. 2000. The Variety of Life. Oxford University Press.
There are many more books and many sites on the Internet (such as Micscape at http://www.microscopy-uk.org.uk/...) for those who would like to explore further. I have only touched the surface, not mentioning hosts of other microorganisms, such as ciliates, rotifers, gastrotrichs, and many others.
As usual all photographs are by me. Photos taken originally at 200-400 X, but resizing of photos makes this a bit difficult to apply directly to the photos as they appear here.