Mineral precipitation in microbial mats may have been the key to their preservation as fossil stromatolites, potentially documenting evidence of the earliest life on Earth. Two factors that contribute to carbonate mineral precipitation are the saturation index (SI) and the presence of nucleation sites. Both of these can be influenced by micro-organisms, which can either alter SI through their metabolisms, or produce and consume organic substances such as extracellular polymeric substances (EPS) that can affect nucleation. It is the balance of individual metabolisms within the mat community that determines the pH and the dissolved inorganic carbon concentration, thereby potentially increasing the alkalinity and consequently the SI. Sulfate-reducing bacteria (SRB) are an important component of this 'alkalinity engine.' The activity of SRB often peaks in layers where CaCO(3) precipitates, and mineral precipitation has been demonstrated in SRB cultures; however, the effect of their metabolism on the alkalinity engine and actual contribution to mineral precipitation is the subject of controversy. Here, we show through culture experiments, theoretical calculations, and geochemical modeling studies that the pH, alkalinity, and organomineralization potential will vary depending on the type of electron donor. Specifically, hydrogen and formate can increase the pH, but electron donors like lactate and ethanol, and to a lesser extent glycolate, decrease the pH. The implication of this for the lithification of mats is that the combination of processes supplying electron donors and the utilization of these compounds by SRB may be critical to promoting mineral precipitation.

We report on the construction, tests, calibrations and commissioning of an Optical Readout Time Projection Chamber (O-TPC) detector operating with a CO 2 (80%) + N 2 (20%) gas mixture at 100 and 150 Torr. It was designed to measure the cross sections of several key nuclear reactions involved in stellar evolution. In particular, a study of the rate of formation of oxygen and carbon during the process of helium burning will be performed by exposing the chamber gas to intense nearly mono-energetic gamma-ray beams at the High Intensity Gamma Source (HIγS) JINST 5 P12004facility. The O-TPC has a sensitive target-drift volume of 30x30x21 cm 3 . Ionization electrons drift towards a double parallel-grid avalanche multiplier, yielding charge multiplication and light emission. Avalanche-induced photons from N 2 emission are collected, intensified and recorded with a Charge Coupled Device (CCD) camera, providing two-dimensional track images. The event's time projection (third coordinate) and the deposited energy are recorded by photomultipliers and by the TPC charge-signal, respectively. A dedicated VME-based data acquisition system and associated data analysis tools were developed to record and analyze these data.The O-TPC has been tested and calibrated with 3.183 MeV alpha-particles emitted by a 148 Gd source placed within its volume with a measured energy resolution of 3.0%. Tracks of alpha and 12 C particles from the dissociation of 16 O and of three alpha-particles from the dissociation of 12 C have been measured during initial in-beam test experiments performed at the HIγS facility at Duke University. The full detection system and its performance are described and the results of the preliminary in-beam test experiments are reported.

No abstract

No abstract