CHLOROSULFATAE

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KINGDOM CHLOROSULFATAE

INTRODUCTION TO THE KINGDOM CHLOROSULFATAE AND ITS SINGLE PHYLUM, CHLOROBACTERIA

Chlorosulfatae (klo-ro-sul-FAT-e) is derived from the name, Green Sulfur Bacteria with a Greek (chloro - χλοερός) and Latin root (sulfur meaning brimstone). This is a standardization of the name for the group called Chlorobi (Garrity and Holt 2001), after Chlorobium, a common genus in the kingdom.

These photosynthetic green sulfur bacteria are anaerobes and are often found at the interface between anoxic layers of water and an upper oxygen bearing layer that allows sufficient light penetration to drive photosynthesis. The organisms range from straight or curved rods (Figure A) to spheres with prosthecae to branching filaments. All utilize a modification of the Krebs Cycle for the fixation of carbon dioxide. They have specialized photosynthetic membranes called chlorosomes (see Figure B).

A. Chlorobium viewed with a light microscope.

B. A TEM micrograph of Chlorobium showing the chlorosomes just beneath the cell membrane.

Images taken from:
A&B: http://biology.kenyon.edu/Microbial_Biorealm/bacteria/bacteroidete_chlorob_group/Chlorobium/chlorobium.htm

SYNOPTIC DESCRIPTION OF THE CHLOROBACTERIA

The following description comes mainly from Margulis and Schwartz (1998), Barnes (1984), Brock et al. (1994), and Tudge (2000).

I. SYNONYMS: Anaerobic phototrophic bacteria, anoxygenic green sulfur bacteria.

II. PHYLUM CHARACTERISTICS:

A. Structure

Cell Form: Rods.

Cell Wall: Gram-.

Motility: Non-motile.

B. Physiology

O₂ Tolerance: Most are obligate anaerobes.

Substrates: Photoheterotrophs; they use light energy to generate ATP, and they use H₂, H₂S, or reduced sulfur compounds to reduce CO₂ to a form of organic carbon. They generally have the ability to take reduced sulfur species (sulfide, thiosulfide or sulfite) and oxidize them to sulfate or elemental sulfur.

Products: Food (or reduced, organic carbon); sulfate or elemental sulfur; they may also fix nitrogen.

C. Other: They contain bacteriochlorophylls c and d or e on thylakoids or photosynthetic membranes called chlorosomes.

D. Ecology: Green or brown in appearance. Found in anaerobic aquatic environments which contain reduced compounds and sufficient sunlight to allow photosynthesis. Typically, they occur in anoxic regions of clear lakes, sulfur springs, and anoxic sediments in salt marshes.

SYSTEMATICS OF THE CHLOROBACTERIA

The Green Sulfur Bacteria clearly are different from the other anoxygenic phototrophs and should be separated despite their superficial similarities. To that end I follow the system of Margulis and Schwartz (1998) who separate the Chlorobines and raise it to phylum status (Chlorobia B-8). However, they lump the rest of the rest of the Anoxyphotobacteria together with the Nitroxybacteria, Pseudomonadobacteria, Omnibacteria (except the order Chlamydines which they now call the the phylum Pirellulae [B-11]), Chemobacteria, and Myxobacteria into a new phylum called Proteobacteria based on 16S RNA. Bergey's Manual of Systematic Bacteriology, volume 3, section 18 (Holt 1989a) lumps all of the Anoxygenic Phototrophic Bacteria with 2 subgroups: Purple Bacteria and Green Bacteria. Volume 4 of Bergey's Manual (Holt 1989b) classify this group in the Class Anoxyphotobacteria within the Division (phylum) Gracilicutes. Bergey's Manual of Systematic Bacteriology, 2nd edition (Garrity et al. 2001) treats the taxa that I include in the Chlorosulfatae as Phylum B XI (Chlorobi).

HIERARCHICAL CLASSIFICATION OF THE CHLOROBACTERIA

I follow the system of Margulis and Schwartz (1998) and Garrity et al. (2001 and 2003) who recognize the unique nature of the green sulfur bacteria.

CLASS CHLOROBI

ORDER CHLOROBIALES

Chlorobium, Prosthecochloris, Ancalochloris, Pelodictyon, Clathrochloris, Chloroherpiton.

This page is maintained by Jack R. Holt. Last revised: 04/22/2008.