Cyanobacteria have affected major geochemical cycles (carbon, nitrogen, and oxygen) on Earth for billions of years. In particular, they have played a major role in the formation of calcium carbonates (i.e., calcification), which has been considered to be an extracellular process. We identified a cyanobacterium in modern microbialites in Lake Alchichica (Mexico) that forms intracellular amorphous calcium-magnesium-strontium-barium carbonate inclusions about 270 nanometers in average diameter, revealing an unexplored pathway for calcification. Phylogenetic analyses place this cyanobacterium within the deeply divergent order Gloeobacterales. The chemical composition and structure of the intracellular precipitates suggest some level of cellular control on the biomineralization process. This discovery expands the diversity of organisms capable of forming amorphous calcium carbonates.

Two BRCA2-like sequences are present in the Arabidopsis genome. Both genes are expressed in flower buds and encode nearly identical proteins, which contain four BRC motifs. In a yeast two-hybrid assay, the Arabidopsis Brca2 proteins interact with Rad51 and Dmc1. RNAi constructs aimed at silencing the BRCA2 genes at meiosis triggered a reproducible sterility phenotype, which was associated with dramatic meiosis alterations. We obtained the same phenotype upon introduction of RNAi constructs aimed at silencing the RAD51 gene at meiosis in dmc1 mutant plants. The meiotic figures we observed strongly suggest that homologous recombination is highly disturbed in these meiotic cells, leaving aberrant recombination events to repair the meiotic double-strand breaks. The ‘brca2' meiotic phenotype was eliminated in spo11 mutant plants. Our experiments point to an essential role of Brca2 at meiosis in Arabidopsis. We also propose a role for Rad51 in the dmc1 context

The geomicrobiology of crater lake microbialites remains largely unknown despite their evolutionary interest due to their resemblance to some Archaean analogs in the dominance of in situ carbonate precipitation over accretion. Here, we studied the diversity of archaea, bacteria and protists in microbialites of the alkaline Lake Alchichica from both field samples collected along a depth gradient (0–14 m depth) and long-term-maintained laboratory aquaria. Using small subunit (SSU) rRNA gene libraries and fingerprinting methods, we detected a wide diversity of bacteria and protists contrasting with a minor fraction of archaea. Oxygenic photosynthesizers were dominated by cyanobacteria, green algae and diatoms. Cyanobacterial diversity varied with depth, Oscillatoriales dominating shallow and intermediate microbialites and Pleurocapsales the deepest samples. The early-branching Gloeobacterales represented significant proportions in aquaria microbialites. Anoxygenic photosynthesizers were also diverse, comprising members of Alphaproteobacteria and Chloroflexi. Although photosynthetic microorganisms dominated in biomass, heterotrophic lineages were more diverse. We detected members of up to 21 bacterial phyla or candidate divisions, including lineages possibly involved in microbialite formation, such as sulfate-reducing Deltaproteobacteria but also Firmicutes and very diverse taxa likely able to degrade complex polymeric substances, such as Planctomycetales, Bacteroidetes and Verrucomicrobia. Heterotrophic eukaryotes were dominated by Fungi (including members of the basal Rozellida or Cryptomycota), Choanoflagellida, Nucleariida, Amoebozoa, Alveolata and Stramenopiles. The diversity and relative abundance of many eukaryotic lineages suggest an unforeseen role for protists in microbialite ecology. Many lineages from lake microbialites were successfully maintained in aquaria. Interestingly, the diversity detected in aquarium microbialites was higher than in field samples, possibly due to more stable and favorable laboratory conditions. The maintenance of highly diverse natural microbialites in laboratory aquaria holds promise to study the role of different metabolisms in the formation of these structures under controlled conditions.

The shallow submarine hydrothermal field of the Prony Bay (New Caledonia) discharges hydrogen- and methane-rich fluids with low salinity, temperature (< 40°C) and high pH (11) produced by the serpentinization reactions of the ultramafic basement into the lagoon seawater. They are responsible for the formation of carbonate chimneys at the lagoon seafloor. Capillary electrophoresis single-strand conformation polymorphism fingerprinting, quantitative polymerase chain reaction and sequence analysis of 16S rRNA genes revealed changes in microbial community structure, abundance and diversity depending on the location, water depth, and structure of the carbonate chimneys. The low archaeal diversity was dominated by few uncultured Methanosarcinales similar to those found in other serpentinization-driven submarine and subterrestrial ecosystems (e.g. Lost City, The Cedars). The most abundant and diverse bacterial communities were mainly composed of Chloroflexi, Deinococcus-Thermus, Firmicutes and Proteobacteria. Functional gene analysis revealed similar abundance and diversity of both Methanosarcinales methanoarchaea, and Desulfovibrionales and Desulfobacterales sulfate-reducers in the studied sites. Molecular studies suggest that redox reactions involving hydrogen, methane and sulfur compounds (e.g. sulfate) are the energy driving forces of the microbial communities inhabiting the Prony hydrothermal system.

Abstract. The terrestrial hyperalkaline springs of Prony Bay (southern lagoon, New Caledonia) have been known since the nineteenth century, but a recent high-resolution bathymetric survey of the seafloor has revealed the existence of numerous submarine structures similar to the well-known Aiguille de Prony, which are also the location of high-pH fluid discharge into the lagoon. During the HYDROPRONY cruise (28 October to 13 November 2011), samples of waters, gases and concretions were collected by scuba divers at underwater vents. Four of these sampling sites are located in Prony Bay at depths up to 50 m. One (Bain des Japonais spring) is also in Prony Bay but uncovered at low tide and another (Rivière des Kaoris spring) is on land slightly above the seawater level at high tide. We report the chemical composition (Na, K, Ca, Mg, Cl, SO 4 , dissolved inorganic carbon, SiO 2 (aq)) of 45 water samples collected at six sites of high-pH water discharge, as well as the composition of gases.Temperatures reach 37 • C at the Bain des Japonais and 32 • C at the spring of the Kaoris. Gas bubbling was observed only at these two springs. The emitted gases contain between 12 and 30 % of hydrogen in volume of dry gas, 6 to 14 % of methane, and 56 to 72 % of nitrogen, with trace amounts of carbon dioxide, ethane and propane.pH values and salinities of all the 45 collected water samples range from the seawater values (8.2 and 35 g L −1 ) to hyperalkaline freshwaters of the Ca-OH type (pH 11 and salinities as low as 0.3 g L −1 ) showing that the collected samples are always a mixture of a hyperalkaline fluid of meteoric origin and ambient seawater. Cl-normalized concentrations of dissolved major elements first show that the Bain des Japonais is distinct from the other sites. Water collected at this site are three component mixtures involving the highpH fluid, the lagoon seawater and the river water from the nearby Rivière du Carénage. The chemical compositions of Published by Copernicus Publications on behalf of the European Geosciences Union. C. Monnin et al.: Low temperature hyperalkaline hydrothermal system of Prony Baythe hyperalkaline endmembers (at pH 11) are not significantly different from one site to the other although the sites are several kilometres away from each other and are located on different ultramafic substrata. The very low salinity of the hyperalkaline endmembers shows that seawater does not percolate through the ultramafic formation.Mixing of the hyperalkaline hydrothermal endmember with local seawater produces large ranges and very sharp gradients of pH, salinity and dissolved element concentrations. There is a major change in the composition of the water samples at a pH around 10, which delimitates the marine environment from the hyperalkaline environment. The redox potential evolves toward negative values at high pH indicative of the reducing conditions due to bubbling of the H 2 -rich gas. The calculation of the mineral saturation states carried out for the Na-K-Ca-Mg-Cl-SO 4 -DIC-SiO 2 -H2O system shows...

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Active carbonate chimneys from the shallow marine serpentinizing Prony Hydrothermal Field were sampled 3 times over a 6 years period at site ST09. Archaeal and bacterial communities composition was investigated using PCR-based methods (clone libraries, Denaturating Gel Gradient Electrophoresis, quantitative PCR) targeting 16S rRNA genes, methyl coenzyme M reductase A and dissimilatory sulfite reductase subunit B genes. Methanosarcinales (Euryarchaeota) and Thaumarchaea were the main archaeal members. The Methanosarcinales, also observed by epifluorescent microscopy and FISH, consisted of two phylotypes that were previously solely detected in two other serpentinitzing ecosystems (The Cedars and Lost City Hydrothermal Field). Surprisingly, members of the hyperthermophilic order Thermococcales were also found which may indicate the presence of a hot subsurface biosphere. The bacterial community mainly consisted of Firmicutes, Chloroflexi, Alpha-, Gamma-, Beta-, and Delta-proteobacteria and of the candidate division NPL-UPA2. Members of these taxa were consistently found each year and may therefore represent a stable core of the indigenous bacterial community of the PHF chimneys. Firmicutes isolates representing new bacterial taxa were obtained by cultivation under anaerobic conditions. Our study revealed diverse microbial communities in PHF ST09 related to methane and sulfur compounds that share common populations with other terrestrial or submarine serpentinizing ecosystems.

The role of microorganisms in microbialite formation remains unresolved: do they induce mineral precipitation (microbes first) or do they colonize and/or entrap abiotic mineral precipitates (minerals first)? Does this role vary from one species to another? And what is the impact of mineral precipitation on microbial ecology? To explore potential biogenic carbonate precipitation, we studied cyanobacteria–carbonate assemblages in modern hydromagnesite-dominated microbialites from the alkaline Lake Alchichica (Mexico), by coupling three-dimensional imaging of molecular fluorescence emitted by microorganisms, using confocal laser scanning microscopy, and Raman scattering/spectrometry from the associated minerals at a microscale level. Both hydromagnesite and aragonite precipitate within a complex biofilm composed of photosynthetic and other microorganisms. Morphology and pigment-content analysis of dominant photosynthetic microorganisms revealed up to six different cyanobacterial morphotypes belonging to Oscillatoriales, Chroococcales, Nostocales and Pleurocapsales, as well as several diatoms and other eukaryotic microalgae. Interestingly, one of these morphotypes, Pleurocapsa-like, appeared specifically associated with aragonite minerals, the oldest parts of actively growing Pleurocapsa-like colonies being always aragonite-encrusted. We hypothesize that actively growing cells of Pleurocapsales modify local environmental conditions favoring aragonite precipitation at the expense of hydromagnesite, which precipitates at seemingly random locations within the biofilm. Therefore, at least part of the mineral precipitation in Alchichica microbialites is most likely biogenic and the type of biominerals formed depends on the nature of the phylogenetic lineage involved. This observation may provide clues to identify lineage-specific biosignatures in fossil stromatolites from modern to Precambrian times.