BACTERIALLY INDUCED PRECIPITATES OF MINERALS

摘要

It has been abundantly demonstrated that bacteria have the ability to induce the precipitation of mineral matter. Previous laboratory studies have indicated that bacteria can induce mineral precipitation due to photosynthesis, aerobic and anaerobic oxidation of organic nitrogen compounds, aerobic and anaerobic oxidation of carbon compounds, and sulfate reduction. With regard to karst conditions, the primary minerals of concern are carbonates, and to a lesser degree, iron- (Fe) and manganese-rich (Mn) minerals. The bacteria influence the micro-environment immediately surrounding them and also within the abundant extrapolysacchrides (EPS) that they produce. The EPS is extruded as a copious enveloping mass, in part, to buffer the bacterial cells from any changes in their immediate environment. The EPS is commonly calcified. In addition, bacteria have a negatively charged outer cell wall, thus they attract Ca, as well as other cations. Bicarbonate ions in the surrounding waters can readily bind with these cations to start mineralization. It is necessary to recall that the initiation of mineral growth is the difficult part of the process, and once the initial seed crystal is formed, it takes much less energy for continued growth. The bacteria facilitate this initial growth, which in some ways is anti-Darwinian, i.e., they entomb themselves within the mineral matter. However, bacteria in general, make very poor fossils, once entombed they commonly burst their cell and collapse. Within a mineral precipitate, this results in micron-scale pore spaces, the former homes of the bacteria.