Amorphous calcium carbonate (Acc) is a precursor of crystalline calcium carbonates that plays a key role in biomineralization and polymorph evolution. Here, we show that several bacterial strains isolated from a Hungarian cave produce Acc and their extracellular polymeric substance (epS) shields ACC from crystallization. The findings demonstrate that bacteria-produced ACC forms in water-rich environment at room temperature and is stable for at least half year, which is in contrast to laboratoryproduced ACC that needs to be stored in a desiccator and kept below 10 °C for avoiding crystallization. the Acc-shielding epS consists of lipids, proteins, carbohydrates and nucleic acids. in particular, we identified large amount of long-chain fatty acid components. We suggest that ACC could be enclosed in a micella-like formula within the EPS that inhibits water infiltration. As the bacterial cells lyse, the covering protective layer disintegrates, water penetrates and the unprotected Acc grains crystallize to calcite. our study indicates that bacteria are capable of producing Acc, and we estimate its quantity in comparison to calcite presumably varies up to 20% depending on the age of the colony. Since diverse bacterial communities colonize the surface of cave sediments in temperate zone, we presume that Acc is common in these caves and its occurrence is directly linked to bacterial activity and influences the geochemical signals recorded in speleothems. Amorphous calcium carbonate (ACC) is known as a precursor phase of crystalline CaCO 3 that plays a key role during calcium carbonate precipitation and biomineralization 1. It is the least stable CaCO 3 modification that rapidly transforms to crystalline calcium carbonate polymorphs. Laboratory-synthesized ACC crystallization can be delayed by keeping the physisorbed H 2 O below the critical level and storing the material in a desiccator and keeping it below 10 °C 2. Additives such as Mg 2+ , phosphate, and organic macromolecules can retard its crystallization 3-5. According to Purgstaller et al. 6 , the metastability of Mg-ACC is associated with the formation kinetics (pH and the Mg/Ca ratio) of the less soluble crystalline phase, i.e., the physico-chemical conditions of the environment. Biogenic activity can also modify the physico-chemical conditions, and thus can enhance the preservation of ACC. It has been reported from tissues of various eukaryotic organisms and several organic molecules have been associated with its occurrence. According to Aizenberg et al. 7 , the skeletal parts of calcareous sponge Clathrina and the spicules of ascidian Pyura pachydermatina contain ACC and its formation is associated with polysaccharides and proteins enriched in glutamic acid (and/or glutamine), serine and glycine. ACC was also described from the intraskeletal organic matrix of numerous scleractinian corals 8 , the spicules of the embryos of Strongylocentrotus purpuratus sea urchins 9 and the shell of Biomphalaria glabrata snail embryos 10. Amines, glycosylated proteins and phosp...

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We describe Chloroparva pannonica Somogyi, Felfö ldi & Vö rö s gen. et sp. nov., a new trebouxiophycean picoplanktonic alga isolated from a turbid, shallow soda pan in Hungary. The cells are spherical to oval, less than 2 mm in diameter, with simple ultrastructure typical to small green algae. Cells divide by autosporulation, forming two daughter cells per autosporangium. Cell wall structure consists of an outer trilaminar layer, an inner microfibrillar layer and an electrontransparent layer covering the plasma membrane. The trilaminar layer of the mother cell wall often persists around the autospores. Typical chlorophyte pigments have been found, including chlorophyll a and b and lutein as the dominant carotenoid. The main fatty acid was oleic acid. The phylogenetic position of the new chlorophyte confirms the proposal of a new genus within the Trebouxiophyceae. Based on its 18S rRNA gene sequence, this isolate is distantly related to Nannochloris eucaryotum UTEX 2502, Chlorella minutissima C-1.1.9 and C. minutissima SAG 1.80 (# 97.6% 18S rRNA gene pairwise similarities).

In this study, scanning electron microscopy (SEM) and 16S rRNA gene-based phylogenetic approach were applied to reveal the morphological structure and genetic diversity of thermophilic prokaryotic communities of a thermal karst well located in Budapest (Hungary). Bacterial and archaeal diversity of the well water (73.7 °C) and the biofilm developed on the inner surface of an outflow pipeline of the well were studied by molecular cloning method. According to the SEM images calcium carbonate minerals serve as a surface for colonization of bacterial aggregates. The vast majority of the bacterial and archaeal clones showed the highest sequence similarities to chemolithoautotrophic species. The bacterial clone libraries were dominated by sulfur oxidizer Thiobacillus (Betaproteobacteria) in the water and Sulfurihydrogenibium (Aquificae) in the biofilm. A relatively high proportion of molecular clones represented genera Thermus and Bellilinea in the biofilm library. The most abundant phylotypes both in water and biofilm archaeal clone libraries were closely related to thermophilic ammonia oxidizer Nitrosocaldus and Nitrososphaera but phylotypes belonging to methanogens were also detected. The results show that in addition to the bacterial sulfur and hydrogen oxidation, mainly archaeal ammonia oxidation may play a decisive role in the studied thermal karst system.

The Buda Thermal Karst System is an active hypogenic karst area that offers possibility for the analysis of biogenic cave formation. The aim of the present study was to gain information about morphological structure and genetic diversity of bacterial communities inhabiting the Diana-Hygieia Thermal Spring (DHTS). Using scanning electron microscopy, metal accumulating and unusual reticulated filaments were detected in large numbers in the DHTS biofilm samples. The phyla Actinobacteria, Firmicutes and Proteobacteria were represented by both bacterial strains and molecular clones but phyla Acidobacteria, Chlorobi, Chlorofexi, Gemmatimonadetes, Nitrospirae and Thermotogae only by molecular clones which showed the highest similarity to uncultured clone sequences originating from different environmental sources. The biofilm bacterial community proved to be somewhat more diverse than that of the water sample and the distribution of the dominant bacterial clones was different between biofilm and water samples. The majority of biofilm clones was affiliated with Deltaproteobacteria and Nitrospirae while the largest group of water clones was related to Betaproteobacteria. Considering the metabolic properties of known species related to the strains and molecular clones from DHTS, it can be assumed that these bacterial communities may participate in the local sulphur and iron cycles, and contribute to biogenic cave formation.

The Molnár János Cave is part of the northern discharge area of the Buda Thermal Karst System, and is the largest active thermal water cave in the capital of Hungary. To compare the prokaryotic communities, reddish-brown cave wall biofilm, black biogeochemical layers, and thermal water samples from the phreatic mixing zone of the cave were subjected to three investigative approaches, scanning electron microscopy, cultivation, and molecular cloning. According to the SEM images, multilayer network structures were observed in the biofilm formed by iron-accumulating filamentous bacteria and mineral crystals. Cultivated strains belonging to Aeromonadaceae and Enterobacteriaceae were characteristic from both water and subaqueous biofilm samples. The most abundant molecular clones were representatives of the phylum Chloroflexi in the reddish-brown biofilm, the class Gammaproteobacteria in the black biogeochemical layer, and Thiobacillus (Betaproteobacteria) in the thermal water samples. The reddish-brown biofilm and black biogeochemical layer's bacterial communities proved to be somewhat more diverse than that of the thermal water. The archaeal 16S rRNA gene clone libraries were dominated by thermophilic ammonia-oxidizer Nitrosopumilus and Nitrososphaera phylotypes in all three habitats. Considering the metabolic characteristics of known species related to the detected clones, it can be assumed that these communities may participate in the local sulfur and nitrogen cycles and may contribute to microbial mediated sulfuric acid speleogenesis.

18S rDNA sequences of six Naviculaceae species [Amphora montana, Gomphonema parvulum, Eolimna minima (syn. Navicula minima), Eolimna subminuscula (syn. Navicula subminuscula), Navicula veneta and Phaeodactylum tricornutum] were determined in order to assess the monophyly of this important group of diatoms, to date not included in 18S rDNA databases, and also that of the recently described genus Eolimna. Phylogenetic trees were constructed using other known diatom 18S rDNA sequences, and best tree topologies obtained were tested against alternative trees for their reliability. The analyses do not reject the monophyly of Naviculaceae and strongly support the separation of the genus Eolimna from Navicula sensu lato. The two species of Eolimna, however, do not appear to be each other 's closest relatives among the species investigated : rather, E. subminuscula shows affinities to A. montana, and E. minima to P. tricornutum. A. montana, a species which it has been proposed should be transferred into a separate taxon from the other five species, was found to have grouped well within them in all analyses.

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