ANAEROBIC OXIDATION OF FERROUS IRON BY PURPLE BACTERIA, A NEW-TYPE OF PHOTOTROPHIC METABOLISM
ANAEROBIC OXIDATION OF FERROUS IRON BY PURPLE BACTERIA, A NEW-TYPE OF PHOTOTROPHIC METABOLISM
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DOI：
10.1128/aem.60.12.4517-4526.1994
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发表时间：
1994-12-01
影响因子：
4.4
通讯作者：
WIDDEL, F
中科院分区：
文献类型：
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作者：
EHRENREICH, A;WIDDEL, F
储存在阳光下的富含铁的缺氧沉积物样本显示出棕色、锈斑的发展。在含有亚铁(10 mmol/L，大部分以FeCO3形式沉淀)的特定矿物介质中的继代培养产生了大量利用亚铁作为光合作用唯一电子供体的厌氧光养细菌。以菌株L7和SW2为代表，分离到两株不同类型的紫色细菌，它们在光下缺氧条件下将无色亚铁氧化为棕色铁。菌株L7有杆状、不活动的细胞(1.3×2~3微米)，经常形成气泡。除亚铁外，菌株L7还利用H-2+CO2、乙酸盐、丙酮酸和葡萄糖作为光营养生长的底物。菌株SW2有小的杆状、不活动的细胞(0.5×1~1.5微米)。除了亚铁外，菌株SW2还利用H-2+CO2、一元羧酸、葡萄糖和果糖；两株菌都不利用游离硫化物，但都生长在黑色硫化亚铁(FeS)上，黑色硫化亚铁转化为铁和硫酸盐。光异养生长的菌株L7和SW2分别呈紫红色到棕红色和黄褐色，吸收光谱显示出细菌叶绿素a的特征峰。迄今为止，根据16S rRNA序列检测到的菌株L7和SW2的最近的光营养亲缘关系分别是色素属(变形杆菌的伽马亚类)和红杆菌属(α亚类)。在矿物介质中，新菌株的细胞干重为每摩尔Fe(II)氧化7.6g，这与利用亚铁作为电子供体进行CO2固定的光自养利用是一致的。在致密的细胞悬浮液中，亚铁氧化对光和二氧化碳的依赖性也被证明。在同时含有亚铁和有机底物(如醋酸盐、葡萄糖)的培养基中，菌株L7同时利用亚铁和有机化合物；相反，菌株SW2只有在消耗有机电子供体后才开始氧化亚铁。从能量学的角度来看，无氧光养细胞氧化亚铁是可以理解的。与酸性溶液中存在的Fe3+/Fe2+对(E(0)=+0.77V)相反，在含重碳酸盐的环境中，相应的pH为7时的氧化还原对Fe(OH)(3)+HCO3-/FeCO3的E(0)为+0.2V。因此，在pH为7时，亚铁可以向缺氧光养生物的光系统提供电子，而紫色细菌的中点电位约为+0.45V。亚铁氧化缺氧光生生物的存在可能为太古宙假定的缺氧生物圈中早期条带铁的沉积提供了解释。
Anoxic iron-rich sediment samples that had been stored in the light showed development of brown, rusty patches. Subcultures in defined mineral media with ferrous iron (10 mmol/liter, mostly precipitated as FeCO3) yielded enrichments of anoxygenic phototrophic bacteria which used ferrous iron as the sole electron donor for photosynthesis. Two different types of purple bacteria, represented by strains L7 and SW2, were isolated which oxidized colorless ferrous iron under anoxic conditions in the light to brown ferric iron. Strain L7 had rod-shaped, nonmotile cells (1.3 by 2 to 3 mu m) which frequently formed gas vesicles. In addition to ferrous iron, strain L7 used H-2+CO2, acetate, pyruvate, and glucose as substrates for phototrophic growth. Strain SW2 had small rod-shaped, nonmotile cells (0.5 by 1 to 1.5 mu m). Besides ferrous iron, strain SW2 utilized H-2+CO2, monocarboxylic acids, glucose, and fructose; Neither strain utilized free sulfide; however, both strains grew on black ferrous sulfide (FeS) which was converted to ferric iron and sulfate. Strains L7 and SW2 grown photoheterotrophically without ferrous iron were purple to brownish red and yellowish brown, respectively; absorption spectra revealed peaks characteristic of bacteriochlorophyll a. The closest phototrophic relatives of strains L7 and SW2 so far examined on the basis of 16S rRNA sequences were species of the genera Chromatium (gamma subclass of proteobacteria) and Rhodobacter (alpha subclass), respectively. In mineral medium, the new isolates formed 7.6 g of cell dry mass per mol of Fe(II) oxidized, which is in good agreement with a photoautotrophic utilization of ferrous iron as electron donor for CO2 fixation. Dependence of ferrous iron oxidation on light and CO2 was also demonstrated in dense cell suspensions. In media containing both ferrous iron and an organic substrate (e.g., acetate, glucose), strain L7 utilized ferrous iron and the organic compound simultaneously; in contrast, strain SW2 started to oxidize ferrous iron only after consumption of the organic electron donor. Ferrous iron oxidation by anoxygenic phototrophs is understandable in terms of energetics. In contrast to the Fe3+/Fe2+ pair (E(0) = +0.77 V) existing in acidic solutions, the relevant redox pair at pH 7 in bicarbonate-containing environments, Fe(OH)(3)+HCO3-/FeCO3, has an E(0) of +0.2 V. Ferrous iron at pH 7 can therefore donate electrons to the photosystem of anoxygenic phototrophs, which in purple bacteria has a midpoint potential around +0.45 V. The existence of ferrous iron-oxidizing anoxygenic phototrophs may offer an explanation for the deposition of early banded-iron formations in an assumed anoxic biosphere in Archean times.