Microbially promoted calcite precipitation in the pelagic redoxcline: Elucidating the formation of the turbid layer.

Large bell-shaped calcite formations called "Hells Bells" were discovered underwater in the stratified cenote El Zapote on the Yucatán Peninsula, Mexico. Together with these extraordinary speleothems, divers found a white, cloudy turbid layer into which some Hells Bells partially extend. Here, we address the central question if the formation of the turbid layer could be based on microbial activity, more specifically, on microbially induced calcite precipitation. Metagenomic and metatranscriptomic profiling of the microbial community in the turbid layer, which overlaps with the pelagic redoxcline in the cenote, revealed chemolithoautotrophic Hydrogenophilales and unclassified ß-Proteobacteria as the metabolic key players. Bioinformatic and hydrogeochemical data suggest chemolithoautotrophic oxidation of sulfide to zero-valent sulfur catalyzed by denitrifying organisms due to oxygen deficiency. Incomplete sulfide oxidation via nitrate reduction and chemolithoautotrophy are both proton-consuming processes, which increase the pH in the redoxcline favoring authigenic calcite precipitation and may contribute to Hells Bells growth. The observed mechanism of microbially induced calcite precipitation is potentially applicable to many other stagnant sulfate-rich water bodies.

High Spatiotemporal Dynamics of Methane Production and Emission in Oxic Surface Water.

The discovery of methane (CH4) accumulation in oxic marine and limnic waters has redefined the role of aquatic environments in the regional CH4 cycle. Although CH4 accumulation in oxic surface waters became apparent in recent years, the sources are still subject to controversial discussions. We present high-resolution in situ measurements of CH4 concentration and its stable isotope composition in a stratified mesotrophic lake. We show that CH4 accumulation in surface waters originates from a highly dynamic interplay between (oxic) CH4 production and emission to the atmosphere. Laboratory incubations of different phytoplankton types and application of stable isotope techniques provide a first unambiguous evidence that major phytoplankton classes in Lake Stechlin per se produce CH4 under oxic conditions. Combined field and lab results show that the photoautotroph community is an important driver for CH4 production and its highly dynamic accumulation in oxic surface waters.