Archaea are also single-celled organisms that lack nuclei. In the past, the differences between bacteria and archaea were not recognised and archaea were classified with bacteria as part of the kingdom Monera. However, in 1990 the microbiologist Carl Woese proposed the three-domain systemthat divided living things into bacteria, archaea and eukaryotes.[33] Archaea differ from bacteria in both their genetics and biochemistry. For example, while bacterial cell membranes are made fromphosphoglycerides with ester bonds, archaean membranes are made of ether lipids.[34]Archaea were originally described in extreme environments, such as hot springs, but have since been found in all types of habitats.[35] Only now are scientists beginning to realize how common archaea are in the environment, with crenarchaeota being the most common form of life in the ocean, dominating ecosystems below 150 m in depth.[36][37] These organisms are also common in soil and play a vital role in ammonia oxidation.
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