Genomic Analysis Unravels Reduced Inorganic Sulfur Compound Oxidation of Heterotrophic Acidophilic Acidicaldus sp. Strain DX-1.

Yuanyuan Liu,Xueduan Liu,Hongying Yang,Yunhua Xiao,Xue Guo,Xian Zhang

doi:10.1155/2016/8137012

Abstract

Although reduced inorganic sulfur compound (RISC) oxidation in many chemolithoautotrophic sulfur oxidizers has been investigated in recent years, there is little information about RISC oxidation in heterotrophic acidophiles. In this study, Acidicaldus sp. strain DX-1, a heterotrophic sulfur-oxidizing acidophile, was isolated. Its genome was sequenced and then used for comparative genomics. Furthermore, real-time quantitative PCR was performed to identify the expression of genes involved in the RISC oxidation. Gene encoding thiosulfate: quinone oxidoreductase was present in Acidicaldus sp. strain DX-1, while no candidate genes with significant similarity to tetrathionate hydrolase were found. Additionally, there were genes encoding heterodisulfide reductase complex, which was proposed to play a crucial role in oxidizing cytoplasmic sulfur. Like many heterotrophic sulfur oxidizers, Acidicaldus sp. strain DX-1 had no genes encoding enzymes essential for the direct oxidation of sulfite. An indirect oxidation of sulfite via adenosine-5′-phosphosulfate was proposed in Acidicaldus strain DX-1. However, compared to other closely related bacteria Acidiphilium cryptum and Acidiphilium multivorum, which harbored the genes encoding Sox system, almost all of these genes were not detected in Acidicaldus sp. strain DX-1. This study might provide some references for the future study of RISC oxidation in heterotrophic sulfur-oxidizing acidophiles.

Highlights

Due to the evolutionary importance and applied perspectives of extreme acidophiles, researchers have paid much attention to these microorganisms [1,2,3]
Compared to other closely related bacteria Acidiphilium cryptum and Acidiphilium multivorum, which harbored the genes encoding Sox system, almost all of these genes were not detected in Acidicaldus sp. strain DX-1
Previous studies showed that Acidicaldus organivorus harbored some unique physiological characteristics, which differed from other acidophilic heterotrophs

Summary

Introduction

Due to the evolutionary importance and applied perspectives of extreme acidophiles (optimal growth at or below pH 3.0), researchers have paid much attention to these microorganisms [1,2,3]. Extreme acidophiles are widely used in the bioprocessing of minerals and bioremediation of acidic and metal-enriched water [4,5,6]. Many chemolithoautotrophic acidophiles, such as Leptospirillum spp. and Sulfolobus metallicus, can accelerate the dissolution of sulfide minerals by oxidizing ferrous iron or reduced inorganic sulfur compound (RISC) [7]. In addition to obligate autotrophs, many heterotrophic and mixotrophic acidophiles play key roles in iron and sulfur cycling, such as Ferrimicrobium acidiphilum, Sulfobacillus spp., Thermoplasma sp., and Acidiphilium spp. The closest phylogenetic relatives of Ac. organivorans Y008 are acidophilic heterotrophs, including Acidiphilium [14], Acidisphaera [15], and Acetobacter [16].

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