|MadSci Network: General Biology|
You want to know about populations of organisms of one cell or more. That includes every known life form. Your question hints at something, and you are correct about what you hint, unicellular organisms, organisms composed of a single cell, out number multicellular organisms by such a huge proportion that in terms of numbers it's essentially correct to say that life on Earth is predominately unicellular. You ask if there are more numbers of organisms on land or in the oceans. First there is a big difference in the amount of land and water covering the surface of the Earth. The oceans cover nearly 2/3s of the planets surface, so oceans are twice the area of the land. Even though most of the open oceans are nutrient poor and life is spread very thinly, the populations of planktonic organisms living in the oceans are huge. There are countless unicellular organisms in virtually every gallon of sea water. Of course lots of organisms live on land too, but don't bother counting insects and other animals, or plants, there aren't enough of those to matter much in the comparison. Every cubic inch of soil, a volume about the size of a sugar cube, is estimated to contain between 9 and 21 million organisms. Enough soil to fill a four inch diameter flower pot, about big enough for one small plant, would harbor 12 billion organisms. Based on size and volume, I would guess there are more organisms in the oceans, but scientists have made some recent discoveries that reveal even more organisms in an even more unlikely location. Deep within the Earth's crustal sediments, in the rocks beneath the land and oceans live countless microorganisms, mostly a group of prokaryotes called Archaebacteria. Such organisms live in deep, hot places, and scientists were actually surprised to find them, and in such numbers. Many of the Archaebacteria are extremophiles, literally lovers of extreme environments, and others are found in very hot, acidic springs or very salty water at the Earth's surface. In the geothermally heated oil fields of the North Sea, the production fluids from 3500 meters deep contain any where from 100,000 to 100 million organisms per milliliter, a cubic centimeter of fluid! Boiling sulfurous mud baths in Iceland contain up to 1 billion organisms per milliliter. So where are the most organisms? Probably not either on land or in the oceans, but the largest population of organisms lives in the crustal sediments deep beneath our feet, a region that covers the whole Earth. And until very recently this abundant life was virtually unknown. This is so new to science that it isn't in the textbooks yet. And that's why some scientists take time to answer questions, so we can tell you about things that are new to science.
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