Irene Wagner-Döbler

Microbial Transformations of Mercury: Potentials, Challenges, and Achievements in Controlling Mercury Toxicity in the Environment

Publisher Summary This chapter reviews achievements in mercury microbiology research during the past 35 years and identifies areas in which more information is needed to complete our understanding of how microbes in the diverse ecological niches that exist on earth interact with mercury. Mercury is a potent toxic substance, the toxicity of which is elicited at very low concentrations. Although all chemical forms of mercury are toxic, public health concerns are focused on methylmercury (MeHg). The major routes of human exposure to this toxic element are through the consumption of contaminated fish, where mercury is mostly present in its methylated form. This is the result of the bioaccumulation and biomagnification of MeHg in the aquatic food chain. MeHg is a neurotoxin that causes pathologies ranging from mild numbness of the extremities to blindness, loss of balance, and death. Because MeHg is more toxic than other forms of mercury, and mercury is mostly deposited in the environment in its ionic form, the biogeochemical cycling of mercury in the environment plays a key role in modulating mercury toxicity. Furthermore, microbial transformations play critical roles in the mercury geochemical cycle, and understanding the mechanisms of these transformations is essential for controlling mercury transport and accumulation in the biosphere.

Is autoinducer-2 a universal signal for interspecies communication: a comparative genomic and phylogenetic analysis of the synthesis and signal transduction pathways

BackgroundQuorum sensing is a process of bacterial cell-to-cell communication involving the production and detection of extracellular signaling molecules called autoinducers. Recently, it has been proposed that autoinducer-2 (AI-2), a furanosyl borate diester derived from the recycling of S-adenosyl-homocysteine (SAH) to homocysteine, serves as a universal signal for interspecies communication.ResultsIn this study, 138 completed genomes were examined for the genes involved in the synthesis and detection of AI-2. Except for some symbionts and parasites, all organisms have a pathway to recycle SAH, either using a two-step enzymatic conversion by the Pfs and LuxS enzymes or a one-step conversion using SAH-hydrolase (SahH). 51 organisms including most Gamma-, Beta-, and Epsilonproteobacteria, and Firmicutes possess the Pfs-LuxS pathway, while Archaea, Eukarya, Alphaproteobacteria, Actinobacteria and Cyanobacteria prefer the SahH pathway. In all 138 organisms, only the three Vibrio strains had strong, bidirectional matches to the periplasmic AI-2 binding protein LuxP and the central signal relay protein LuxU. The initial two-component sensor kinase protein LuxQ, and the terminal response regulator luxO are found in most Proteobacteria, as well as in some Firmicutes, often in several copies.ConclusionsThe genomic analysis indicates that the LuxS enzyme required for AI-2 synthesis is widespread in bacteria, while the periplasmic binding protein LuxP is only present in Vibrio strains. Thus, other organisms may either use components different from the AI-2 signal transduction system of Vibrio strains to sense the signal of AI-2, or they do not have such a quorum sensing system at all.

Aerobic Anoxygenic Photosynthesis in Roseobacter Clade Bacteria from Diverse Marine Habitats

ABSTRACT The marine Roseobacter clade comprises several genera of marine bacteria related to the uncultured SAR83 cluster, the second most abundant marine picoplankton lineage. Cultivated representatives of this clade are physiologically heterogeneous, and only some have the capability for aerobic anoxygenic photosynthesis, a process of potentially great ecological importance in the worlds oceans. In an attempt to correlate phylogeny with ecology, we investigated the diversity of Roseobacter clade strains from various marine habitats (water samples, biofilms, laminariae, diatoms, and dinoflagellate cultures) by using the 16S rRNA gene as a phylogenetic marker gene. The potential for aerobic anoxygenic photosynthesis was determined on the genetic level by PCR amplification and sequencing of the pufLM genes of the bacterial photosynthesis reaction center and on the physiological level by detection of bacteriochlorophyll (Bchl) a. A collection of ca. 1,000 marine isolates was screened for members of the marine Roseobacter clade by 16S rRNA gene-directed multiplex PCR and sequencing. The 42 Roseobacter clade isolates found tended to form habitat-specific subclusters. The pufLM genes were detected in two groups of strains from dinoflagellate cultures but in none of the other Roseobacter clade isolates. Strains within the first group (the DFL-12 cluster) also synthesized Bchl a. Strains within the second group (the DFL-35 cluster) formed a new species of Roseovarius and did not produce Bchl a under the conditions investigated here, thus demonstrating the importance of genetic methods for screening of cultivation-dependent metabolic traits. The pufL genes of the dinoflagellate isolates were phylogenetically closely related to pufL genes from Betaproteobacteria, confirming similar previous observations which have been interpreted as indications of gene transfer events.

The complete genome sequence of the algal symbiont Dinoroseobacter shibae: a hitchhiker's guide to life in the sea.

Dinoroseobacter shibae DFL12T, a member of the globally important marine Roseobacter clade, comprises symbionts of cosmopolitan marine microalgae, including toxic dinoflagellates. Its annotated 4 417 868 bp genome sequence revealed a possible advantage of this symbiosis for the algal host. D. shibae DFL12T is able to synthesize the vitamins B1 and B12 for which its host is auxotrophic. Two pathways for the de novo synthesis of vitamin B12 are present, one requiring oxygen and the other an oxygen-independent pathway. The de novo synthesis of vitamin B12 was confirmed to be functional, and D. shibae DFL12T was shown to provide the growth-limiting vitamins B1 and B12 to its dinoflagellate host. The Roseobacter clade has been considered to comprise obligate aerobic bacteria. However, D. shibae DFL12T is able to grow anaerobically using the alternative electron acceptors nitrate and dimethylsulfoxide; it has the arginine deiminase survival fermentation pathway and a complex oxygen-dependent Fnr (fumarate and nitrate reduction) regulon. Many of these traits are shared with other members of the Roseobacter clade. D. shibae DFL12T has five plasmids, showing examples for vertical recruitment of chromosomal genes (thiC) and horizontal gene transfer (cox genes, gene cluster of 47 kb) possibly by conjugation (vir gene cluster). The long-range (80%) synteny between two sister plasmids provides insights into the emergence of novel plasmids. D. shibae DFL12T shows the most complex viral defense system of all Rhodobacterales sequenced to date.

Discovery of complex mixtures of novel long-chain quorum sensing signals in free-living and host-associated marine alphaproteobacteria

More than 100 bacterial isolates from various marine habitats were screened for AHL production by using gfp reporter constructs based on the lasR system of Pseudomonas aeruginosa and the luxR system of Vibrio fischeri. Of the 67 Alphaproteobacteria tested, most of which belonged into the so‐called Roseobacter clade, 39 induced fluorescence in either one or both sensor strains up to 103‐fold compared to controls. Acylated homoserine lactones were identified by GC‐MS analysis and shown to have chain lengths of C8, C10, C13–C16, and C18. One or two double bonds were often present, while a keto or hydroxyl group occurred only rarely in the side chain. Most strains produced several different AHLs. C18‐en‐HSL and C18‐dien‐HSL were produced by Dinoroseobacter shibae, an aerobic anoxygenic phototrophic bacterium isolated from dinoflagellates, and are among the longest AHLs found to date. Z7‐C14‐en‐HSL, which has previously been detected in Rhodobacter sphaeroides, was produced by Roseovarius tolerans and Jannaschia helgolandensis. This signal molecule was synthesised and shown to induce a similar response to the culture supernatant in the respective sensor strain. The widespread occurrence of quorum‐sensing compounds in marine Alphaproteobacteria, both free‐living strains and those associated to eukaryotic algae, points to a great importance of this signalling mechanism for the adaptation of the organisms to their widely different ecological niches.

Biofilm Community Structure in Polluted Rivers: Abundance of Dominant Phylogenetic Groups over a Complete Annual Cycle

ABSTRACT The seasonal dynamics of river biofilm communities in two German rivers, the Elbe and one of its tributaries, the Spittelwasser, were investigated for the first time by using fluorescence in situ hybridization and a standardized biofilm sampling procedure. We show the importance of members of the β subclass of the classProteobacteria, which formed the largest single group in the massively polluted Spittelwasser at all times. Clear seasonal peaks of abundance were observed for the planctomycetes and theCytophaga-Flavobacterium cluster.

Species Diversity Improves the Efficiency of Mercury-Reducing Biofilms under Changing Environmental Conditions

ABSTRACT Six mercury-resistant environmental proteobacterial isolates and one genetically modified mercury-resistant Pseudomonas putida strain were analyzed for physiological traits of adaptive relevance in an environment of packed-bed bioreactors designed for the decontamination of mercury-polluted chlor-alkali wastewater. The strains displayed characteristic differences in each trait (i.e., biofilm formation capability, growth rate in mercury contaminated wastewaters, and mercury reduction efficiency). Subsequently, they were immobilized either as a monoculture or as a mixed culture on porous carrier material in packed-bed bioreactors through which different batches of filter-sterilized industrial chlor-alkali wastewater were pumped. In monospecies bioreactors, the mercury retention efficiency was sensitive to rapidly increasing mercury concentrations in the wastewater. Mixed culture biofilms displayed a high mercury retention efficiency that was not affected by rapid increases in mercury or continuously high mercury concentrations. The dynamic in the community composition of the mixed culture bioreactors was determined by ribosomal intergenic spacer polymorphism analysis. Mercury-mediated selective pressure decreased the number of prevalent strains. Microbial diversity was completely restored after easing of the selective pressure. Microbial diversity provides a reservoir of strains with complementary ecological niches that results in a superior bioreactor performance under changing environmental conditions.

Autoinducer-2-regulated genes in Streptococcus mutans UA159 and global metabolic effect of the luxS mutation.

Autoinducer 2 (AI-2) is the only species-nonspecific autoinducer known in bacteria and is produced by both gram-negative and gram-positive organisms. Consequently, it is proposed to function as a universal quorum-sensing signal for interaction between bacterial species. AI-2 is produced as the by-product of a metabolic transformation carried out by the LuxS enzyme. To separate the metabolic function of the LuxS enzyme from the signaling role of AI-2, we carried out a global transcriptome analysis of a luxS null mutant culture of Streptococcus mutans UA159, an important cariogenic bacterium and a crucial component of the dental plaque biofilm community, in comparison to a luxS null mutant culture supplemented with chemically pure 4,5-dihydroxy-2,3-pentanedione, the precursor of AI-2. The data revealed fundamental changes in gene expression affecting 585 genes (30% of the genome) which could not be restored by the signal molecule AI-2 and are therefore not caused by quorum sensing but by lack of the transformation carried out by the LuxS enzyme in the activated methyl cycle. All functional classes of enzymes were affected, including genes known to be important for biofilm formation, bacteriocin synthesis, competence, and acid tolerance. At the same time, 59 genes were identified whose transcription clearly responded to the addition of AI-2. Some of them were related to protein synthesis, stress, and cell division. Three membrane transport proteins were upregulated which are not related to any of the known AI-2 transporters. Three transcription factors were identified whose transcription was stimulated repeatedly by AI-2 addition during growth. Finally, a global regulatory protein, the delta subunit of the RNA polymerase (rpoE), was induced 147-fold by AI-2, representing the largest differential gene expression observed. The data show that many phenotypes related to the luxS mutation cannot be ascribed to quorum sensing and have identified for the first time regulatory proteins potentially mediating AI-2-based signaling in gram-positive bacteria.

Pilot plant for bioremediation of mercury-containing industrial wastewater

Mercury is an extremely toxic pollutant that is currently being emitted mainly by low level industrial sources. It is distributed globally through the atmosphere, from where it precipitates onto the surface of the Earth, enters aquatic organisms, accumulates in fish and finally affects the health of human populations. Microbes have evolved a mechanism for mercury detoxification [mercury resistance operon (mer)] based on intracellular reduction of Hg2+ to non-toxic Hg0 by the mercuric reductase enzyme and subsequent diffusional loss of Hg0 from the cell. It was shown that Hg0 produced by microbial detoxification can be retained quantitatively in packed bed bioreactors, in which biofilms of mercury-resistant bacteria are grown on porous carrier material. This review describes operation of this system on a technical, fully automated, scale, and its operation at a chloralkali electrolysis factory. It was shown to work with high efficiency under fluctuating mercury concentrations and to be robust against transiently toxic conditions. The gradient of mercury concentration in the technical scale system exerted a strong selective pressure on the microbial community, which resulted in a succession of mercury-resistant strains at high mercury concentrations and an increase in phylogenetic and functional diversity at low mercury concentrations. Clean-up of mercury-containing wastewater by mercury-resistant microbes is a simple, environmentally friendly and cost-effective alternative to current treatment technologies.

Diversity and Seasonal Variability of β-Proteobacteria in Biofilms of Polluted Rivers: Analysis by Temperature Gradient Gel Electrophoresis and Cloning

ABSTRACT The β-subgroup of the Proteobacteria has been shown to be important in aquatic habitats and was investigated in depth here by molecular 16S rRNA techniques in biofilms of the Elbe River and its polluted tributary, the Spittelwasser River. The bacterial 16S rRNA genes were cloned from each site, screened for β-proteobacterial clones and sequenced. River biofilm clones from both rivers grouped into 9 clusters (RBFs). RBFs 1, 2, and 3 fell into the recently described βI cluster of cosmopolitan freshwater bacteria, where they represented new species related to Rhodoferax, Aquaspirillum, and Hydrogenophaga. RBFs 4 to 7 affiliated with Aquabacterium commune, Ideonella dechloratans, and Sphaerotilus natans, respectively. The two remaining RBFs were uncultivated clusters, one of them being distantly related to Gallionella ferruginea. Seasonal changes in the relative intensity of the β-proteobacterial 16S rRNA genes of biofilms harvested monthly for 1 year were determined by specific amplification and separation by temperature gradient gel electrophoresis (TGGE). Bands were identified by comparison of clones to community fingerprints by TGGE. Eight of 13 identified bands were shared by both habitats but showed different relative abundance and seasonal variability in the two rivers, probably caused by differences in temperature and pollutants. The data indicate new not-yet-cultivated clusters of river biofilm organisms, some of them probably distributed globally. They confirm the importance of certain known freshwater genera in river biofilms. The high phylogenetic resolution obtained by clone library analysis combined with the high temporal resolution obtained by TGGE suggest that the observed microdiversity in the river biofilm clone libraries might be caused by phylogenetically closely related microbial populations which are adapted to ecological parameters.