EURYARCHAEOTA

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PHYLUM HALOBACTERIA

INTRODUCTION TO THE PHYLUM HALOBACTERIA

The halophilic bacteria are unicellular cocci or rods (Figure A) that inhabit environments where the solute concentrations can be much higher than the concentrations in sea water. For example, the Great Salt Lake (Utah) has a solute concentration that is about 10X the concentration of seawater in the same relative proportions. Other saline sites like the Dead Sea (Israel and Jordan) have chloride concentrations that are >10X that of sea water, but the dominant cation is magnesium. Thus, the relative proportions of ions can vary quite a lot. Still, the hypersaline environments, though simple, can be quite productive, and the extreme halophils can form the productive basis of that ecosystem. All of them are chemoorganotrophs and most are aerobic. Halobacteria have special lipids in the membranes to prevent them from falling apart at high salt concentrations. They employ K⁺ as an inorganic compatible solute inside the cell.

*A. Halobacterium* taken with an SEM.**	*B. Halobacterium* colors water in this salt evaporation pond near the Great Salt Lake.**
Image taken from: A&B: http://biology.kenyon.edu/Microbial_Biorealm/archaea/halobacterium/halobacterium.html

SYNOPTIC DESCRIPTION OF THE HALOBACTERIA

The following description of the Halobacteria came from Brock et al. (1994), Margulis and Schwartz (1998), Garrity et al. (2001 and 2003), and Black (2002).

I. SYNONYMS: Halophilic bacteria.

II. PHYLUM CHARACTERISTICS:

A. Structure

Cell Form: Variable; cocci or rods.

Cell Wall: Archaebacterial; gram-.

Motility: Non-motile cocci, or rods with polar flagella.

B. Physiology

O₂ Tolerance: Aerobes or facultative anaerobes.

Substrates: Chemoheterotrophs using amino acids (in absence of O₂); photophosphorylation in presence of light and O₂. All require large amounts of Na⁺ and K⁺ for growth. In anaerobic conditions, elemental sulfur, thiosulfate or nitrate can serve as electron acceptors (see products).

Products: CO₂ in aerobic conditions. In anaerobic conditions, they reduce elemental Sulfur (to S^-2), or NO₃ (to NO₂ or NH₄⁺).

C. Other: Special lipids in the membranes; high concentrations of K ions in the cell.

D. Ecology: Require NaCl concentration of 2mol/l; found in salt lakes, brines and salt-preserved products.

SYSTEMATICS OF THE HALOBACTERIA

Of all the higher taxa in the Archaea, the extreme halophiles are most likely to be a natural group. Garrity et al. (2001) place halophiles together as a class called Halobacteria within their phylum Euryarchaeota. Margulis and Schwartz (1998) lump the halobacteria with the methanobacteria into a taxon called Euryarcheota which I treat as a kingdom.

HIERARCHICAL CLASSIFICATION OF THE HALOBACTERIA

I depart from Margulis and Schwartz (1998) in separating the halophiles from the methanogens. In that regard my system is more like that of Garrity et al. (2001 and 2003). However, I set the group apart as a phylum within the kingdom Euryarcheota. Also, I believe that the simplicity of this system is a reflection of our knowledge of the group and not of the real diversity within it.

CLASS HALOBACTEREI

ORDER HALOBACTERIALES

Halobacterium, Haloarcula, Halobaculum, Halococcus, Haloferax, Halogeometricum, Halorhabdus, Halorubrum, Haloterrigena, Natrialba, Natrinema, Natronobacterium, Natronomonas, Natronorubrum.

This page is maintained by Jack R. Holt. Last revised: 02/07/2008.