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      Close Interspecies Interactions between Prokaryotes from Sulfureous Environments

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          Abstract

          Green sulfur bacteria are obligate photolithoautotrophs that require highly reducing conditions for growth and can utilize only a very limited number of carbon substrates. These bacteria thus inhabit a very narrow ecologic niche. However, several green sulfur bacteria have overcome the limits of immobility by entering into a symbiosis with motile Betaproteobacteria in a type of multicellular association termed phototrophic consortia. One of these consortia, “ Chlorochromatium aggregatum,” has recently been established as the first culturable model system to elucidate the molecular basis of this symbiotic interaction. It consists of 12–20 green sulfur bacteria epibionts surrounding a central, chemoheterotrophic betaproteobacterium in a highly ordered fashion. Recent genomic, transcriptomic, and proteomic studies of “ C. aggregatum” and its epibiont provide insights into the molecular basis and the origin of the stable association between the two very distantly related bacteria. While numerous genes of central metabolic pathways are upregulated during the specific symbiosis and hence involved in the interaction, only a limited number of unique putative symbiosis genes have been detected in the epibiont. Green sulfur bacteria therefore are preadapted to a symbiotic lifestyle. The metabolic coupling between the bacterial partners appears to involve amino acids and highly specific ultrastructures at the contact sites between the cells. Similarly, the interaction in the equally well studied archaeal consortia consisting of Nanoarchaeum equitans and its host Ignicoccus hospitalis is based on the transfer of amino acids while lacking the highly specialized contact sites observed in phototrophic consortia.

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          Most cited references61

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          Genome streamlining in a cosmopolitan oceanic bacterium.

          The SAR11 clade consists of very small, heterotrophic marine alpha-proteobacteria that are found throughout the oceans, where they account for about 25% of all microbial cells. Pelagibacter ubique, the first cultured member of this clade, has the smallest genome and encodes the smallest number of predicted open reading frames known for a free-living microorganism. In contrast to parasitic bacteria and archaea with small genomes, P. ubique has complete biosynthetic pathways for all 20 amino acids and all but a few cofactors. P. ubique has no pseudogenes, introns, transposons, extrachromosomal elements, or inteins; few paralogs; and the shortest intergenic spacers yet observed for any cell.
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            Computational improvements reveal great bacterial diversity and high metal toxicity in soil.

            The complexity of soil bacterial communities has thus far confounded effective measurement. However, with improved analytical methods, we show that the abundance distribution and total diversity can be deciphered. Reanalysis of reassociation kinetics for bacterial community DNA from pristine and metal-polluted soils showed that a power law best described the abundance distributions. More than one million distinct genomes occurred in the pristine soil, exceeding previous estimates by two orders of magnitude. Metal pollution reduced diversity more than 99.9%, revealing the highly toxic effect of metal contamination, especially for rare taxa.
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              Deletional bias and the evolution of bacterial genomes.

              Although bacteria increase their DNA content through horizontal transfer and gene duplication, their genomes remain small and, in particular, lack nonfunctional sequences. This pattern is most readily explained by a pervasive bias towards higher numbers of deletions than insertions. When selection is not strong enough to maintain them, genes are lost in large deletions or inactivated and subsequently eroded. Gene inactivation and loss are particularly apparent in obligate parasites and symbionts, in which dramatic reductions in genome size can result not from selection to lose DNA, but from decreased selection to maintain gene functionality. Here we discuss the evidence showing that deletional bias is a major force that shapes bacterial genomes.
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                Author and article information

                Journal
                Front Microbiol
                Front. Microbio.
                Frontiers in Microbiology
                Frontiers Research Foundation
                1664-302X
                26 April 2011
                05 July 2011
                2011
                : 2
                : 146
                Affiliations
                [1] 1simpleBereich Mikrobiologie, Department Biologie I, Ludwig-Maximilians-Universität München Planegg-Martinsried, Germany
                Author notes

                Edited by: Thomas E. Hanson, University of Delaware, USA

                Reviewed by: Jennifer Macalady, Pennsylvania State University, USA; James T. Hollibaugh, University of Georgia, USA

                *Correspondence; Jörg Overmann, Leibniz-Institut, Deutsche Sammlung von Mikroorganismen und Zellkulturen, Inhoffenstraße 7B, 38124 Braunschweig, Germany. e-mail: joerg.overmann@ 123456dsmz.de

                Present address: Johannes Müller and Jörg Overmann, Leibniz-Institut, Deutsche Sammlung von Mikroorganismen und Zellkulturen Braunschweig and Technical University of Braunschweig, Inhoffenstraße 7B, 38124 Braunschweig, Germany.

                This article was submitted to Frontiers in Microbial Physiology and Metabolism, a specialty of Frontiers in Microbiology.

                Article
                10.3389/fmicb.2011.00146
                3132602
                21779277
                fafe3237-0041-474d-83b0-5ece4fb985f0
                Copyright © 2011 Müller and Overmann.

                This is an open-access article subject to a non-exclusive license between the authors and Frontiers Media SA, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and other Frontiers conditions are complied with.

                History
                : 24 February 2011
                : 20 June 2011
                Page count
                Figures: 4, Tables: 1, Equations: 0, References: 88, Pages: 11, Words: 10003
                Categories
                Microbiology
                Review Article

                Microbiology & Virology
                symbiosis,nanoarchaeum equitans,green sulfur bacteria,chlorochromatium aggregatum,ignicoccus hospitalis,phototrophic consortia

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