Phylum Chloroflexi (Green non-sulfur Bacteria)

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tree

Taxonomy

  • Phylum Chloroflexi
    • Class Chloroflexi
      • Order Chloroflexales
        • Family Chloroflexaceae
          • Genus Chloroflexus
          • Genus Chloronema
          • Genus Chlorothrix
          • Genus Heliothrix
          • Genus Roseiflexus
          • Genus Kouleothrix
        • Family Oscillochloridaceae
          • Genus Oscillochloris
      • Order Herpetosiphonales
        • Family Herpetosiphonaceae
          • Genus Herptosiphon
    • Class Anaerolineae
      • Order Anaerolinaeles
        • Family Anaerolinaeceae
          • Genus Anaerolinea
      • Order Caldilineales
        • Family Caldilineae
          • Genus Bellilinea
          • Genus Caldilinea
          • Genus Leptolinea
          • Genus Levilinea
          • Genus Longiliea

General characteristics of the Chloroflexi

Diversity

Just over a dozen genera, with only one or sometimes two species in each. Thermomicrobium roseum is sometimes included in this group (and so is included in the tree above), or it may constitute a phylum of its own.

Metabolism

The familiar members of this phylum fall into two general metabolic types: the thermophilic phototrophs (Chloroflexales) and the heterotrophs, some of which are thermophiles (Anerolineae) and some mesophiles (Herpetosiphon).

The phototrophic Chloroflexi carry out anoxygenic photosynthesis; they have a single type of photosystem and carry out cyclic photophosphorylation. They grow best photochemotrophically, but most can fix carbon, but do not use either the Calvin cycle or reverse TCA cycle. Instead they use an unusual pathway found otherwise only in a few Archaea, the hydroxypropionate pathway. Reducing power to reduce CO2 to glyoxylate is from NADPH derived by reverse electron transport from the oxidation of sulfide or hydrogen. Most can also grow by aerobic respiration (heterotrophically).

Morphology

Members of this phylum are flexible unbranched filaments, usually thin (<2μm) with a uniform diameter, and motile by gliding. Septa are present, but not usually visible. Phototrophic species are green or orange en masse, the color depending of which photopigments are produced. Most contain membranous “chlorosomes” resembling individual thylakoids directly beneath the cell membrane; these contain very high concentrations of accessory photopigments (carotinoids and bacteriochlorophyl c), whereas the reaction centers and bacteriochlorophyl a are primarily found in the cytoplasmic membrane. These chlorosomes resemble those of the Chlorobi (see below), but phototrophy in Chloroflexi otherwise resembles that of typical purple bacterial photosynthesis. Phototrophic species grown aerobically become etiolated, and resemble the non-phototrophic species.

The presence of an outer membrane makes this organism formally “Gram-negative”, but they generally seem to lack the lipopolysaccharide (LPS) typical of the Gram-negative envelop. Some are said to have thin sheaths, visible at the ends of filaments, but these may represent cell envelop material left over after breakage of the filaments at dead cells rather than true sheaths.

Habitat

Phototrophic members of this phylum are common and conspicuous in moderate pH hot springs at temperatures from 35°C up to 70°C or more, but are most abundant at about 60°C. Mesophilic heterotrophs are commonly abundant in wastewater sludges. They are also commonly seen in sediments, soil, and freshwater.

Example species

Chloroflexus aurantiacus

C.aurantiacus
Chloroflexus aurantiacus : photomicrograph from the Joint Genome Institute of the United States Department of Energy

C. aurantiacus is the most well-studied phototrophic member of this phylum, most of which have not been grown in pure culture. It was originally isolated from photosynthetic mats surrounding hot springs in Yellowstone National Park. It seems to be the primary producer of the distinctive orange mats found in these hot springs (but see below, the discussion of Roseiflexus), and is also abundant in an orange layer directly beneath the light green cyanobacterial mats at somewhat higher temperatures. Filaments are thin (0.5-1.2μm) and glide slowly, ca. 1μm per minute.

Chloroflexus contains chlorosomes and bacteriochlorophyl c, whereas other Chloroflexi do not. These are traits otherwise found only in the green sulfur Bacteria (Chlorobi), and it is likely that the genes encoding these were acquired by Chloroflexus by horizontal transfer.

Yellowstone Chloroflxus mat
Chloroflexus mat, Yellwostone National Park ; James W. Brown

Although C. aurantiacus is capable of carbon fixation, in most mats they are found in association with cyanobacteria. In these communities, it is the cyanobacteria that are the primary producers, and the Chloroflexi grow photoheterotrophically from organic carbon produced by the cyanobacteria. They are not capable of fixing nitrogen, but acquire their sulfur from sulfate.

Roseiflexus castenholzii

R.castenholzii
Roseiflexus castenholzii : unatributed from: http://genome.jgi-psf.org/finished_microbes/ros_r/ros_r.home.html

Roseiflexus very closely resembles Chloroflexus except that it lacks chlorosomes and the associated accessory bacteriochlorophyl c. Although Chloroflexus is usually thought to be the primary member of this phylum in hot springs mats, and is readily cultivated from these mats, both molecular phylogenetic analysis (fluorescent in situ hybridization and rRNA-based surveys) and spectral analysis of photopigments suggests that Roseiflexus is far more abundant.

Herpetosiphon aurantiacus

H. aurantiacus
Herpetosiphon aurantiacus : from Hans Reichenbach “The Prokaryotes”

Herpetosiphon is a mesophilic heterotrophic (non-phototrophic) member of this phylum, isolated from the polysacharide matrix of the eukaryotic alga Chara from Birch lake in Minnesota. Herpetosiphon and morphologically similar organisms are commonly seen in soils and sediments, and especially activated sludge in wastewater treatment, but have rarely been cultivated. Although they are always present in activated sludges, their overgrowth causes the bacterial flocs they reside in to retain bulk water, and not form compact sludge. In some cases, these flocs trap gas bubbles and float, producing a thick foam. Microbiologists at wastewater treatment facilities therefore monitor these organisms in the aerobic digestion process, guided by morphological keys to their categorization (rather than cultivation).

Anaerolinea thermophila

A. thermophila
Anaerolinea thermophila : from Sekiguchi IJSEM 53:1843

A. thermophila was isolated from an industrial thermophilic anaerobic digester (treating fried soybead curd wastewater). It represents a second branch of the Chloroflexi that is poorly understood, comprised of thermophilic heterotrophic filaments. A. thermophila grows at 45-65°C (55°C optimum) and is an obligate anaerobe; other members of this group are facultatively aerobic. Protons are the terminal electron acceptor, generating hydrogen. Growth is strongly inhibited by the accumulation of hydrogen, and so growth is promoted by co-cultivation with hydrogen-consuming methanogens. Unlike others members of this phylum, these are nonmotile.