Which group of bacteria is known to be some of the oldest, and has been discovered in harsh environments like marshes, salty places, and hot springs? 

Archaebacteria are some of the oldest bacteria, and they have been discovered in harsh environments such as marshes, salty places, and hot springs. They use an autotrophic method of metabolism to obtain nutrients.

Archaebacteria, also known as archaea, are a group of prokaryotic microorganisms that are genetically distinct from bacteria and eukaryotes. They are some of the oldest known life forms and have been found in extreme environments such as hot springs, acidic or alkaline lakes, and deep-sea hydrothermal vents. Archaebacteria can use a variety of metabolic pathways, including autotrophic metabolism, to obtain nutrients. Autotrophic metabolism involves using inorganic compounds such as carbon dioxide or methane as a carbon source and converting them into organic compounds through processes like photosynthesis or chemosynthesis. Cyanobacteria are also autotrophic bacteria, but they are not necessarily found in extreme environments and are not considered to be among the oldest bacteria. Eubacteria, or true bacteria, are a diverse group of bacteria that can be found in a wide range of environments and have various metabolic pathways.