What is the order of halophiles?

Halophiles are a group of extremely halophilic microorganisms.

All halophiles are able to grow in the presence of high concentrations of salt (up to at least 50% w/v NaCl).

Is there an order of halophiles? As you might expect, the halophiles with the highest salt concentration in their growth media are most closely related to each other. They can be separated into two distinct clusters. The first cluster contains organisms that are able to grow at very high salt concentrations, such as up to 65% NaCl.

The second cluster consists of organisms that can grow at much lower salt concentrations, and typically have less than 35% NaCl in their growth media. These organisms are called moderately halophilic.

The table below shows the approximate salt concentration that organisms in each group can grow at. The names of the groups within the moderately halophilic group indicate an order based on their ability to grow at low salt concentrations. Organisms in the moderately halophilic group can grow at up to 55% NaCl, whereas those in the moderately halophilic group can grow at around only 10% NaCl. Thus, the order is defined as follows: 0-5% NaCl (Moderately Halophilic) > 5-10% NaCl (Moderately Halophilic) > 10-20% NaCl (Moderately Halophilic) > 20-35% NaCl (Moderately Halophilic) > 35-55% NaCl (Moderately Halophilic) > 55-65% NaCl (Extremely Halophilic). These names are a bit confusing because they imply that organisms in the 0-5% NaCl group have greater salt requirements than organisms in the 5-10% NaCl group. This is incorrect.

For instance, Halobacterium marismortui has a salt concentration requirement of 25% w/v NaCl to grow and can grow at very high salt concentrations. It also has a salt concentration requirement of 5% w/v NaCl to grow and can grow at moderate salt concentrations.

This is because H. Marismortui has evolved to grow at moderately high salt concentrations.

There is an exception to this rule.