The existence of archaea in the oceans has been known for a long time. However, they were only discovered in the North Sea in the beginning of the 21st century. Archaea resemble normal bacteria, but are very different internally. They can live under extreme conditions: there are archaea known to live in boiling water! Archaea play an important role in the nitrogen cycle in the North Sea. These organisms are capable of changing ammonia into nitrite. One liter of seawater by Texel contains 5 to 45 million archaea. They are found particularly in the winter, when there is too little light for algae to grow.