International audienceAragonitic microbialites, characterized by a reticulate fabric,were discovered beneath lacustrine microbial mats on the atoll ofKiritimati, Republic of Kiribati, Central Pacific. The microbialmats, with cyanobacteria as major primary producers, grow inevaporated seawater modified by calcium carbonate and gypsumprecipitation and calcium influx via surface and/or groundwaters.Despite the high aragonite supersaturation and a high photosyntheticactivity, onlyminor aragonite precipitates are observed in thetop parts of the microbial mats. Instead, major aragonite precipitationtakes place in lower mat parts at the transition to the anoxiczone. The prokaryotic community shows a high number of phylotypesclosely related to halotolerant taxa and/or taxa with preferenceto oligotrophic habitats. Soil- and plant- inhabiting bacteriaunderline a potential surface or subsurface influx from terrestrialareas, while chitinase-producing representatives coincide with theoccurrence of insect remains in the mats. Strikingly, many of theclones have their closest relatives inmicroorganisms either involvedin methane production or consumption ofmethane or methyl compounds.Methanogens, represented by the methylotrophic genusMethanohalophilus, appear to be one of the dominant organisms inanaerobic mat parts. All this points to a significant role of methaneand methyl components in the carbon cycle of the mats. Nonetheless,thin sections and physicochemical gradients through themats,as well as the 12C-depleted carbon isotope signatures of carbonatesindicate that spherulitic components of the microbialites initiatein the photosynthesis-dominated orange mat top layer, and furthergrow in the green and purple layer below. Therefore, thesespherulites are considered as product of an extraordinary highphotosynthesis effect simultaneous to a high inhibition by pristineexopolymers. Then, successive heterotrophic bacterial activityleads to a condensation of the exopolymer framework, and finallyto the formation of crevice-like zones of partly degraded exopolymers.Here initiation of horizontal aragonite layers and verticalaragonite sheets of the microbialite occurs, which are consideredas 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 leadsto thin fibrous aragonite veneers. While sulfate reduction, methylotrophy,methanogenesis and ammonification play an importantrole in element cycling of the mat, there is currently no evidencefor a crucial role of them in CaCO3 precipitation. Instead, photosynthesisand exopolymer degradation sufficiently explain theobserved pattern and fabric of microbialite formation
The identity of photobionts from 20 species of the Physciaceae from different habitats and geographical regions has been determined by ITS rDNA sequence comparisons in order to estimate the diversity of photobionts within that lichen group, to detect patterns of specificity of mycobionts towards their photobionts and as a part of an ongoing study to investigate possible parallel cladogenesis of both symbionts. Algal-specific PCR primers have been used to determine the ITS rDNA sequences from DNA extractions of dried lichens that were up to 5 years old. Direct comparisons and phylogenetic analyses allowed the assignment of Physciaceae photobionts to four distinct clades in the photobiont ITS rDNA phylogeny. The results indicate a diversity within the genus Trebouxia Puymaly and Physciaceae photobionts that is higher than expected on the basis morphology alone. Physciaceae photobionts belonged to 12 different ITS lineages of which nine could unambiguously be assigned to six morphospecies of Trebouxia. The identity of the remaining three sequences was not clarified; they may represent new species. Specificity at the generic level was low as a whole range of photobiont species were found within a genus of Physciaceae and different ranges were detected. The photobionts of Physcia (Schreb.) Michaux were closely related and represented one morphospecies of Trebouxia, whereas the algal partners of Buellia De Not and Rinodina (Ach.) Gray were in distant lineages of the ITS phylogeny and from several Trebouxia morphospecies. Photobiont variation within a genus of Physciaceae may be due to phylogeny, geographical distance or because photobionts from neighbouring lichens were taken (“algal sharing“). At the species level Physciaceae mycobionts seem to be rather selective and contained photobionts that were very closely related within one morphospecies of Trebouxia.
Lichens from the genus Umbilicaria were collected across a 5,000-km transect through Antarctica and investigated for DNA sequence polymorphism in a region of 480-660 bp of the nuclear internal transcribed spacer region of ribosomal DNA. Sequences from both fungal (16 ascomycetes) and photosynthetic partners (22 chlorophytes from the genus Trebouxia) were determined and compared with homologs from lichens inhabiting more temperate, continental climates. The phylogenetic analyses reveal that Antarctic lichens have colonized their current habitats both through multiple independent colonization events from temperate embarkation zones and through recent long-range dispersal in the Antarctic of successful preexisting colonizers. Furthermore, the results suggest that relichenization-de novo establishment of the fungus-photosynthesizer symbiosis from nonlichenized algal and fungal cells-has occurred during the process of Antarctic lichen dispersal. Independent dispersal of algal and fungal cultures therefore can lead to a successful establishment of the lichen symbiosis even under harsh Antarctic conditions.
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.
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 up 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.
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