Department of Earth Sciences, The University of Western Ontario, London, Ontario N6A 5B7, Canada.
Geochem Trans. 2007 Dec 5;8:13. doi: 10.1186/1467-4866-8-13.
This study provides experimental evidence for biologically induced precipitation of magnesium carbonates, specifically dypingite (Mg5(CO3)4(OH)2.5H2O), by cyanobacteria from an alkaline wetland near Atlin, British Columbia. This wetland is part of a larger hydromagnesite (Mg5(CO3)4(OH)2.4H2O) playa. Abiotic and biotic processes for magnesium carbonate precipitation in this environment are compared.
Field observations show that evaporation of wetland water produces carbonate films of nesquehonite (MgCO3.3H2O) on the water surface and crusts on exposed surfaces. In contrast, benthic microbial mats possessing filamentous cyanobacteria (Lyngbya sp.) contain platy dypingite (Mg5(CO3)4(OH)2.5H2O) and aragonite. Bulk carbonates in the benthic mats (delta13C avg. = 6.7%, delta 18O avg. = 17.2%) were isotopically distinguishable from abiotically formed nesquehonite (delta13C avg. = 9.3%, delta 18O avg. = 24.9%). Field and laboratory experiments, which emulated natural conditions, were conducted to provide insight into the processes for magnesium carbonate precipitation in this environment. Field microcosm experiments included an abiotic control and two microbial systems, one containing ambient wetland water and one amended with nutrients to simulate eutrophic conditions. The abiotic control developed an extensive crust of nesquehonite on its bottom surface during which [Mg2+] decreased by 16.7% relative to the starting concentration. In the microbial systems, precipitation occurred within the mats and was not simply due to the capturing of mineral grains settling out of the water column. Magnesium concentrations decreased by 22.2% and 38.7% in the microbial systems, respectively. Laboratory experiments using natural waters from the Atlin site produced rosettes and flakey globular aggregates of dypingite precipitated in association with filamentous cyanobacteria dominated biofilms cultured from the site, whereas the abiotic control again precipitated nesquehonite.
Microbial mats in the Atlin wetland create ideal conditions for biologically induced precipitation of dypingite and have presumably played a significant role in the development of this natural Mg-carbonate playa. This biogeochemical process represents an important link between the biosphere and the inorganic carbon pool.
本研究为蓝藻从不列颠哥伦比亚省阿特林附近的碱性湿地中生物诱导沉淀镁碳酸盐（特别是水菱镁矿，Mg5(CO3)4(OH)2.5H2O）提供了实验证据。该湿地是更大的水镁石（Mg5(CO3)4(OH)2.4H2O）playa 的一部分。比较了该环境中镁碳酸盐沉淀的生物和非生物过程。
野外观察表明，湿地水的蒸发会在水面上产生水菱镁矿（MgCO3.3H2O）的碳酸盐薄膜和暴露表面上的壳层。相比之下，具有丝状蓝藻（Lyngbya sp.）的底栖微生物席包含片状水菱镁矿（Mg5(CO3)4(OH)2.5H2O）和文石。底栖席中的块状碳酸盐（delta13C avg. = 6.7%，delta 18O avg. = 17.2%）与非生物形成的水菱镁矿（delta13C avg. = 9.3%，delta 18O avg. = 24.9%）在同位素上可区分。为了深入了解该环境中镁碳酸盐沉淀的过程，进行了野外和实验室实验，这些实验模拟了自然条件。野外微宇宙实验包括一个非生物对照和两个微生物系统，一个系统含有环境湿地水，另一个系统用营养物质进行了补充以模拟富营养条件。非生物对照在其底部表面形成了广泛的水菱镁矿壳，在此过程中，[Mg2+]相对于起始浓度下降了 16.7%。在微生物系统中，沉淀发生在席内，而不是简单地由于从水柱中沉降的矿物颗粒的捕获。镁浓度分别在微生物系统中下降了 22.2%和 38.7%。使用来自阿特林现场的天然水进行的实验室实验产生了水菱镁矿的玫瑰花状和片状球状团聚体，与从该地点培养的丝状蓝藻为主的生物膜有关，而非生物对照再次沉淀出水菱镁矿。
阿特林湿地中的微生物席为水菱镁矿的生物诱导沉淀创造了理想的条件，并可能在这种天然镁碳酸盐 playa 的形成中发挥了重要作用。这个生物地球化学过程代表了生物圈和无机碳库之间的重要联系。
