Chlorobium chlorochromatii sp. nov., a symbiotic green sulfur bacterium isolated from the phototrophic consortium “Chlorochromatium aggregatum”

Abstract

A symbiotic green sulfur bacterium, strain CaD, was isolated from an enrichment culture of the phototrophic consortium "Chlorochromatium aggregatum". The capability of the epibiont to grow in pure culture indicates that it is not obligately symbiotic. Cells are Gram-negative, nonmotile, rod-shaped and contain chlorosomes. Strain CaD is obligately anaerobic and photolithoautotrophic, using sulfide as electron donor. Acetate and peptone are photoassimilated in the presence of sulfide and hydrogencarbonate. Photosynthetic pigments contain bacteriochlorophylls a and c, and gamma-carotene and OH-gamma-carotene glucoside laurate as the dominant carotenoids. In cells from pure cultures, chlorosomes are equally distributed along the inner face of the cytoplasmic membrane. In contrast, the distribution of the chlorosomes in symbiotic epibiont cells is uneven, with chlorosomes being entirely absent at the site of attachment to the central bacterium. The symbiotic epibiont cells display a conspicuous additional layered structure at the attachment site. The G + C content of genomic DNA of strain CaD is 46.7 mol%. On the basis of 16S rRNA sequence comparison, the strain is distantly related to Chlorobium species within the green sulfur bacteria phylum (<or=94.6% sequence homology). The novel isolate is therefore described as a novel species within the genus Chlorobium, Chlorobium chlorochromatii.

Metabolic analysis of Chlorobium chlorochromatii CaD3 reveals clues of the symbiosis in ‘Chlorochromatium aggregatum’

“…Chl. chlorochromatii CaD3 is not the exception; it cannot assimilate 2-oxoglutarate when free-living (Vogl et al, 2006). This limitation could be overcome if there is a transporter protein for 2-oxoglutarate that only expresses in the symbiotic state.…”

“…aggregatum' shows chemotactic behavior towards 2-oxoglutarate (Frö stl . Several lines of evidence suggest that the b-proteobacterium and not the chlorobia, assimilates this metabolite from the medium (Frö stl and Overmann et al, 1998;Glaeser and Overmann, 2003b;Vogl et al, 2006). However, it is not known whether the central bacterium transfers organic carbon to the epibiont (Mü ller and Overmann, 2011).…”

“…Their major photosynthetic pigment is bacteriochlorophyll c, and sulfide is used as electron donor. CO 2 is fixed via the reverse tricarboxylic acid cycle (rTCA) (Vogl et al, 2006). In natural conditions they are exclusively found in symbiosis although they can be cultured without their symbiont, which is not the case for the obligate-symbiotic b-proteobacterium (Mü ller and Overmann, 2011).…”

Molecular Characterization of the Nonphotosynthetic Partner Bacterium in the Consortium “ Chlorochromatium aggregatum ”

“…In the associated state, epibionts grow photoautotrophically like their free-living green sulfur bacterial relatives (17). Recently, the epibiont of the phototrophic consortium "Chlorochromatium aggregatum" could be isolated in pure culture, and its physiology was characterized in detail (47). Also, the genome sequence of the epibiont has just been completed (see http://genome.jgi-psf.org/finished_microbes/chlag /chlag.download.html).…”

Central Role of the Cell in Microbial Ecology

“…The notion that cultivation attempts fail because exotic compounds serve as exclusive carbon sources for growth is probably not correct. There are many success stories where former "unculturable" microorganisms (in some cases known for decades) have been cultivated in the laboratory using common nutrients (25,204,226); to rely exclusively on exotic substrates (140) is therefore more likely the exception than the norm. However, we should note that various signal molecules, as discussed later in this review, seem to play an important role during cultivation and that isolation procedures likely disrupt this signaling.…”