Xiong Mai Yia, Shelobolina Evgenya S, Roden Eric E
Department of Geoscience, University of Wisconsin, and NASA Astrobiology Institute, University of Wisconsin, Madison, Wisconsin.
Astrobiology. 2015 May;15(5):331-40. doi: 10.1089/ast.2014.1233. Epub 2015 Apr 27.
Basaltic glass (BG) is an amorphous ferrous iron [Fe(II)]-containing material present in basaltic rocks, which are abundant on rocky planets such as Earth and Mars. Previous research has suggested that Fe(II) in BG can serve as an energy source for chemolithotrophic microbial metabolism, which has important ramifications for potential past and present microbial life on Mars. However, to date there has been no direct demonstration of microbially catalyzed oxidation of Fe(II) in BG. In this study, three different culture systems were used to investigate the potential for microbial oxidation of Fe(II) in BG, including (1) the chemolithoautotrophic Fe(II)-oxidizing, nitrate-reducing "Straub culture"; (2) the mixotrophic Fe(II)-oxidizing, nitrate-reducing organism Desulfitobacterium frappieri strain G2; and (3) indigenous microorganisms from a streambed Fe seep in Wisconsin. The BG employed consisted of clay and silt-sized particles of freshly quenched lava from the TEB flow in Kilauea, Hawaii. Soluble Fe(II) or chemically reduced NAu-2 smectite (RS) were employed as positive controls to verify Fe(II) oxidation activity in the culture systems. All three systems demonstrated oxidation of soluble Fe(II) and/or structural Fe(II) in RS, whereas no oxidation of Fe(II) in BG material was observed. The inability of the Straub culture to oxidize Fe(II) in BG was particularly surprising, as this culture can oxidize other insoluble Fe(II)-bearing minerals such as biotite, magnetite, and siderite. Although the reason for the resistance of the BG toward enzymatic oxidation remains unknown, it seems possible that the absence of distinct crystal faces or edge sites in the amorphous glass renders the material resistant to such attack. These findings have implications with regard to the idea that Fe(II)-Si-rich phases in basalt rocks could provide a basis for chemolithotrophic microbial life on Mars, specifically in neutral-pH environments where acid-promoted mineral dissolution and utilization of dissolved Fe(II) as an energy source is not likely to take place.
玄武玻璃（BG）是一种存在于玄武岩中的含铁（II）的无定形物质，而玄武岩在地球和火星等岩石行星上大量存在。先前的研究表明，BG中的铁（II）可以作为化学无机营养型微生物代谢的能量来源，这对火星过去和现在潜在的微生物生命具有重要意义。然而，迄今为止，尚未有直接证据证明BG中铁（II）的微生物催化氧化作用。在本研究中，使用了三种不同的培养系统来研究BG中铁（II）的微生物氧化潜力，包括：（1）化学无机自养型铁（II）氧化、硝酸盐还原的“施特劳布培养物”；（2）兼养型铁（II）氧化、硝酸盐还原生物弗氏脱硫杆菌G2菌株；（3）来自威斯康星州一条河床铁渗漏处的本土微生物。所使用的BG由来自夏威夷基拉韦厄TEB流的新鲜淬火熔岩的粘土和粉砂大小的颗粒组成。可溶性铁（II）或化学还原的NAu-2蒙脱石（RS）用作阳性对照，以验证培养系统中的铁（II）氧化活性。所有三个系统都证明了可溶性铁（II）和/或RS中结构铁（II）的氧化，而未观察到BG物质中铁（II）的氧化。施特劳布培养物无法氧化BG中的铁（II）这一点尤其令人惊讶，因为这种培养物可以氧化其他不溶性含铁（II）矿物，如黑云母、磁铁矿和菱铁矿。尽管BG对酶促氧化具有抗性的原因尚不清楚，但无定形玻璃中缺乏明显的晶面或边缘位点似乎使得该物质能够抵抗这种攻击。这些发现对于玄武岩中富含铁（II）-硅的相可以为火星上的化学无机营养型微生物生命提供基础这一观点具有启示意义，特别是在不太可能发生酸促进矿物溶解和利用溶解的铁（II）作为能量来源的中性pH环境中。
