What Is the Largest Group of Organisms?

The largest group of organisms on earth are bacteria. Bacteria makeup about 90% of all living things on Earth and inhabit every environment imaginable, from soil to water to inside animals like humans. With over 5 million varieties, there are more types of bacteria than any other type of organism. This makes them an important part of many ecological systems because they help break down organic matter and recycle nutrients. One example is that some bacteria can turn nitrogen into ammonia, which fertilizes plants; without these microorganisms, the world would be a very different place!

Bacteria in the oceans and in freshwater habitats make up over half of Earth’s total biomass. Most bacteria live in water, soil, and plants- for example, about 1000 bacterial cells can be found per cubic centimeter of ocean water!

Bacteria are also important members of many food chains because they provide energy to other organisms. The total amount of bacteria on Earth is enormous- if the earth’s entire biomass was represented as a 70 kg human, then the total weight of all bacteria would be 10,000 people!

Bacteria are so tiny that they cannot be seen with the naked eye. However, their populations can be measured using plate counts. To do this, a few drops of bacteria are placed on a sterile agar plate and allowed to grow. Once enough cells have grown, the number of colonies in that sample is counted. This measurement can then be used to calculate the concentration or amount of bacteria in that area- for example, one cubic centimeter contains about 10 million bacterial cells!

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