Goethite Formed in the Periplasmic Space of Pseudomonas sp. JM-7 during Fe Cycling Enhances Its Denitrification in Water

Abstract

Denitrification-driven Fe­(II) oxidation is an important microbial metabolism that connects iron and nitrogen cycling in the environment. The formation of Fe­(III) minerals in the periplasmic space has a significant effect on microbial metabolism and electron transfer, but direct evidence of iron ions entering the periplasm and resulting in periplasmic mineral precipitation and electron conduction properties has yet to be conclusively determined. Here, we investigated the pathways and amounts of iron, with different valence states and morphologies, entering the periplasmic space of the denitrifier Pseudomonas sp. JM-7 (P. JM-7), and the possible effects on the electron transfer and the denitrifying ability. When consistently provided with Fe­(II) ions (from siderite (FeCO3)), the dissolved Fe­(II) ions entered the periplasmic space and were oxidized to Fe­(III), leading to the formation of a 25 nm thick crystalline goethite crust, which functioned as a semiconductor, accelerating the transfer of electrons from the intracellular to the extracellular matrix. This consequently doubled the denitrification rate and increased the electron transport capacity by 4–30 times (0.015–0.04 μA). However, as the Fe­(II) concentration further increased to above 4 mM, the Fe­(II) ions tended to preferentially nucleate, oxidize, and crystallize on the outer surface of P. JM-7, leading to the formation of a densely crystallized goethite layer, which significantly slowed down the metabolism of P. JM-7. In contrast to the Fe­(II) conditions, regardless of the initial concentration of Fe­(III), it was challenging for Fe­(III) ions to form goethite in the periplasmic space. This work has shed light on the likely effects of iron on environmental microorganisms, improved our understanding of globally significant iron and nitrogen geochemical cycles in water, and expanded our ability to study and control these important processes.

Thermodynamic Inhibition of Microbial Sulfur Disproportionation in a Multisubunit Designed Sulfur-Siderite Packed Bioreactor

“…Even after undergoing the sequence of S 0 DP, aeration, and acidification periods, the total percentage of the top 13 SOB genera did not decrease; instead, it increased from 52.1 to 74.5% (Figure 7B). In comparison, heterotrophic denitrifiers, such as Pseudomonas and Simplicispira, 34,35 accounted for more than 15% during the S 0 AD period (Figure 7C), attributed to the presence of residual organics in the influent. They were further enriched during the S 0 DP period, possibly due to the release of soluble microbial products from decaying cells.…”