Anaerobic oxidation of ferrous iron by purple bacteria, a new type of phototrophic metabolism

Abstract

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 mmolVliter, 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 j,m) which frequently formed gas vesicles. In addition to ferrous iron, strain L7 used H2+C02, acetate, pyruvate, and glucose as substrates for phototrophic growth. Strain SW2 had small rod-shaped, nonmotile cells (0.5 by 1 to 1.5 tim). Besides ferrous iron, strain SW2 utilized H2+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

Nitrate‐dependent iron(II) oxidation in paddy soil

“…In turn, light-dependent iron(II) oxidation (Widdel et al, 1993;Ehrenreich and Widdel, 1994;Heising and Schink, 1998) could only take place in anoxic zones with light. Measurements of light intensities in soil cores from the paddy soil (data not shown) with microelectrodes developed by Ku È hl et al (1997) showed that light was only available in the first 0.5 mm of the paddy soil, where oxygen was still present (Fig.…”

Microbial anaerobic Fe(II) oxidation – Ecology, mechanisms and environmental implications

“…Anoxygenic phototrophic Fe(II)-oxidizers have since been isolated from both the purple sulfur (Gammaproteobacteria) and non-sulfur bacteria (Alphaproteobacteria), and from the green sulfur bacteria (see Table 2). Isolated purple nonsulfur bacteria include Rhodobacter ferrooxidans strain SW2 (Ehrenreich and Widdel, 1994), Rhodopseudomonas palustris strain TIE-1 (Jiao et al, 2005), Rhodovulum iodosum and Rhodovulum robiginosum (Straub et al, 1999;Wu et al, 2014). The only purple sulfur bacteria isolate available is Thiodycton sp.…”

“…Phototrophic Fe(II)-oxidizing bacteria are able to oxidize a variety of Fe(II) species (see previous paragraph) resulting in the formation of poorly soluble Fe(III) (oxyhydr)oxides (Ehrenreich and Widdel, 1994;Straub et al, 1999;Kappler and Newman, 2004;Jiao et al, 2005;Gauger et al, 2015). Several studies observed the transformation of these amorphous to low crystalline initial precipitates to higher crystalline and thermodynamically more stable Fe mineral phases, such as goethite, lepidocrocite and magnetite over time (Straub et al, 1999;Kappler and Newman, 2004;Jiao et al, 2005;Miot et al, 2009c;Schaedler et al, 2009;Wu et al, 2014).…”

“…Yet, the extent to which cells and minerals are associated seems to be strain-specific. Furthermore, varying cell-mineral aggregate morphologies, ranging from irregular and bulbous to more symmetric (e.g., flowerand star-like shapes) have been reported and seem to depend on the mineralogy and age of culture (Ehrenreich and Widdel, 1994;Kappler and Newman, 2004;Jiao et al, 2005;Posth et al, 2010;Wu et al, 2014;Gauger et al, 2015;Laufer et al, 2017).…”

Banded Iron Travertines at the Ilia Hot Spring (Greece): An interplay of biotic and abiotic factors leading to a modern Banded Iron Formation analogue?

“…Several hypotheses have been suggested concerning the biological influence on BIF deposition (Bekker et al, 2010(Bekker et al, , 2014: the anoxygenic F I G U R E 1 0 Model for the formation of banded iron travertine of Ilia. Switching from one mode to another might be controlled by variations in the flow of venting water, potentially controlled by volcanic pulses and whatever controls the fluid circulation in the hydrothermal system phototrophic iron-oxidizing bacteria, which can couple Fe (II) oxidation to the reduction of CO 2 (Ehrenreich & Widdel, 1994;Widdel et al, 1993), the cyanobacterial model for BIF formation (Cloud, 1973) or the chemolithoautotrophic iron-oxidizing bacteria living in a neutral pH environment (Holm, 1989). In the case of the BIT of Ilia, the latter model likely explains the process of iron precipitation in the Fe-rich bands.…”