Helms, Gert

[["dc.bibliographiccitation.firstpage","74"],["dc.bibliographiccitation.issue","1"],["dc.bibliographiccitation.journal","Journal of Molecular Evolution"],["dc.bibliographiccitation.lastpage","84"],["dc.bibliographiccitation.volume","55"],["dc.contributor.author","Bhattacharya, Debashish"],["dc.contributor.author","Friedl, Thomas"],["dc.contributor.author","Helms, Gert"],["dc.date.accessioned","2018-11-07T10:22:13Z"],["dc.date.available","2018-11-07T10:22:13Z"],["dc.date.issued","2002"],["dc.description.abstract","One family within the Euascomycetes (Ascomycota), the lichen-forming Physciaceae, is particularly rich in nuclear ribosomal [r]DNA group I introns. We used phylogenetic analyses of group I introns and lichen-fungal host cells to address four questions about group I intron evolution in lichens, and generally in all eukaryotes: 1) Is intron spread in the lichens associated with the intimate association of the fungal and photosynthetic cells that make Lip the lichen thallus? 2) Are the Multiple group I introns in the lichen-fungi of independent origins, or have existing introns spread into novel sites in the rDNA? 3) If introns have moved to novel sites, then does the exon context of these sites provide insights into the mechanism of intron spread? and 4) What is the pattern of intron loss in the small subunit rDNA gene of lichen-fungi? Our analyses show that group I introns in the lichen-fungi and in the lichen-algae (and lichenized cyanobacteria) do not share a close evolutionary relationship, suggesting that these introns do not move between the symbionts. Many group I introns appear to have originated in the common ancestor of the Lecanorales, whereas others have spread within this lineage (particularly in the Physciaceae) putatively through reverse-splicing into novel rRNA sites. We suggest that the evolutionary history of most lichen-fungal group I introns is characterized by rare gains followed by extensive losses in descendants, resulting in a sporadic intron distribution. Detailed phylogenetic analyses of the introns and host cells are required, therefore, to distinguish this scenario from the alternative hypothesis of widespread and independent intron gains in the different lichen-fungal lineages."],["dc.identifier.doi","10.1007/s00239-001-2305-x"],["dc.identifier.isi","000176243200007"],["dc.identifier.pmid","12165844"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/42237"],["dc.language.iso","en"],["dc.notes.status","final"],["dc.notes.submitter","Najko"],["dc.relation.issn","0022-2844"],["dc.title","Vertical evolution and intragenic spread of lichen-fungal group I introns"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","29"],["dc.bibliographiccitation.issue","1"],["dc.bibliographiccitation.journal","Geomicrobiology Journal"],["dc.bibliographiccitation.lastpage","65"],["dc.bibliographiccitation.volume","29"],["dc.contributor.author","Arp, Gernot"],["dc.contributor.author","Helms, Gert"],["dc.contributor.author","Karlinska, Klementyna"],["dc.contributor.author","Schumann, Gabriela"],["dc.contributor.author","Reimer, Andreas"],["dc.contributor.author","Reitner, Joachim"],["dc.contributor.author","Trichet, Jean"],["dc.date.accessioned","2018-11-07T09:15:30Z"],["dc.date.available","2018-11-07T09:15:30Z"],["dc.date.issued","2012"],["dc.description.abstract","Aragonitic microbialites, characterized by a reticulate fabric, were discovered beneath lacustrine microbial mats on the atoll of Kiritimati, Republic of Kiribati, Central Pacific. The microbial mats, with cyanobacteria as major primary producers, grow in evaporated seawater modified by calcium carbonate and gypsum precipitation and calcium influx via surface and/or groundwaters. Despite the high aragonite supersaturation and a high photosynthetic activity, only minor aragonite precipitates are observed in the top parts of the microbial mats. Instead, major aragonite precipitation takes place in lower mat parts at the transition to the anoxic zone. The prokaryotic community shows a high number of phylotypes closely related to halotolerant taxa and/or taxa with preference to oligotrophic habitats. Soil-and plant-inhabiting bacteria underline a potential surface or subsurface influx from terrestrial areas, while chitinase-producing representatives coincide with the occurrence of insect remains in the mats. Strikingly, many of the clones have their closest relatives in microorganisms either involved in methane production or consumption of methane or methyl compounds. Methanogens, represented by the methylotrophic genus Methanohalophilus, appear to be one of the dominant organisms in anaerobic mat parts. All this points to a significant role of methane and methyl components in the carbon cycle of the mats. Nonetheless, thin sections and physicochemical gradients through the mats, as well as the C-12-depleted carbon isotope signatures of carbonates indicate that spherulitic components of the microbialites initiate in the photosynthesis-dominated orange mat top layer, and further grow in the green and purple layer below. Therefore, these spherulites are considered as product of an extraordinary high photosynthesis effect simultaneous to a high inhibition by pristine exopolymers. Then, successive heterotrophic bacterial activity leads to a condensation of the exopolymer framework, and finally to the formation of crevice-like zones of partly degraded exopolymers. Here initiation of horizontal aragonite layers and vertical aragonite sheets of the microbialite occurs, which are considered as a product of high photosynthesis at decreasing degree of inhibition. Finally, at low supersaturation and almost lack of inhibition, syntaxial growth of aragonite crystals at lamellae surfaces leads to thin fibrous aragonite veneers. While sulfate reduction, methylotrophy, methanogenesis and ammonification play an important role in element cycling of the mat, there is currently no evidence for a crucial role of them in CaCO3 precipitation. Instead, photosynthesis and exopolymer degradation sufficiently explain the observed pattern and fabric of microbialite formation."],["dc.identifier.doi","10.1080/01490451.2010.521436"],["dc.identifier.isi","000301982400004"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/27707"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.relation.issn","0149-0451"],["dc.title","Photosynthesis versus Exopolymer Degradation in the Formation of Microbialites on the Atoll of Kiritimati, Republic of Kiribati, Central Pacific"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","195"],["dc.bibliographiccitation.journal","The Lichenologist"],["dc.bibliographiccitation.lastpage","204"],["dc.bibliographiccitation.volume","39"],["dc.contributor.author","Hauck, Markus"],["dc.contributor.author","Helms, Gert"],["dc.contributor.author","Friedl, Thomas"],["dc.date.accessioned","2018-11-07T11:07:01Z"],["dc.date.available","2018-11-07T11:07:01Z"],["dc.date.issued","2007"],["dc.description.abstract","In two lichen species, Hypogymnia physodes and Lecanora conizaeoides, often used as model organisms for pollution-sensitive and pollution-tolerant epiphytic lichens, respectively, the hypothesis was tested that the toxitolerance of the Trebouxia photobiont limits the tolerance of the entire lichen symbiosis. Being lecanoralean-trebouxioid associations, H. physodes and L. conizaeoides represent the most common type of lichens. Photobionts of both lichen species deriving from microhabitats with varying supply of S and heavy metals were identified using nuclear ITS nrDNA sequencing. The photobiont of L. conizaeoides was identified as T. simplex, whereas the photobiont of H. physodes belongs to an undescribed Trebouxia species, related to T. jamesii subsp. angustilobata and provisionally named as T. hypogymniae Hauck & Friedl ined. Since T. hypogymniae ined. is also known from Lecidea silacea, which is characteristic of rock and slag with high heavy metal content, a high sensitivity of this alga to pollutants is unlikely to be a key factor for the relatively low toxitolerance of H. physodes. Furthermore, the photobiont cannot be crucial for the extremely high toxitolerance of L. conizaeoides, as T simplex is also known from pollution-sensitive lichens of the fruticose genus Pseudevernia. These findings suggest that the photobiont is not generally a key factor determining pollution sensitivity in the most common type of lichen symbiosis. The high specificity for T. simplex in L. conizaeoides in existing populations of L. conizaeoides suggest that already established thalli could be a source of photobiont cells for re-lichenization."],["dc.identifier.doi","10.1017/S0024282907006639"],["dc.identifier.isi","000244955300010"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/52454"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Academic Press Ltd Elsevier Science Ltd"],["dc.relation.issn","0024-2829"],["dc.title","Photobiont selectivity in the epiphytic lichens Hypogymnia physodes and Lecanora conizaeoides"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","434"],["dc.bibliographiccitation.issue","4"],["dc.bibliographiccitation.journal","Journal of Molecular Evolution"],["dc.bibliographiccitation.lastpage","446"],["dc.bibliographiccitation.volume","60"],["dc.contributor.author","Simon, Dawn"],["dc.contributor.author","Moline, Jessica"],["dc.contributor.author","Helms, Gert"],["dc.contributor.author","Friedl, Thomas"],["dc.contributor.author","Bhattacharya, Debashish"],["dc.date.accessioned","2018-11-07T11:12:22Z"],["dc.date.available","2018-11-07T11:12:22Z"],["dc.date.issued","2005"],["dc.description.abstract","The wide but sporadic distribution of group I introns in protists, plants, and fungi, as well as in eubacteria, likely resulted from extensive lateral transfer followed by differential loss. The extent of horizontal transfer of group I introns can potentially be determined by examining closely related species or genera. We used a phylogenetic approach with a large data set (including 62 novel large subunit [LSU] rRNA group I introns) to study intron movement within the monophyletic lichen family Physciaceae. Our results show five cases of horizontal transfer into homologous sites between species but do not support transposition into ectopic sites. This is in contrast to previous work with Physciaceae small subunit (SSU) rDNA group I introns where strong support Was found for multiple ectopic transpositions. This difference in the apparent number of ectopic intron movements between SSU and LSU rDNA genes may in part be explained by a larger number of positions in the SSU rRNA, which can Support the insertion and/or retention of group I introns. In contrast. we suggest that the LSU rRNA may have fewer acceptable positions and therefore intron spread is limited in this gene."],["dc.identifier.doi","10.1007/s00239-004-0152-2"],["dc.identifier.isi","000228836500003"],["dc.identifier.pmid","15883879"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/53649"],["dc.language.iso","en"],["dc.notes.status","final"],["dc.notes.submitter","Najko"],["dc.relation.issn","0022-2844"],["dc.title","Divergent histories of rDNA group I introns in the lichen family Physciaceae"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dspace.entity.type","Publication"]]