Inferring pathways leading to organic-sulfur mineralization in the Bacillales

Abstract

Microbial organic sulfur mineralization to sulfate in terrestrial systems is poorly understood. The process is often missing in published sulfur cycle models. Studies on microbial sulfur cycling have been mostly centered on transformations of inorganic sulfur, mainly on sulfate-reducing and inorganic sulfur-oxidizing bacteria. Nevertheless, organic sulfur constitutes most sulfur in soils. Recent reports demonstrate that the mobilization of organic-bound-sulfur as sulfate in terrestrial environments occurs preferentially under high temperatures and thermophilic Firmicutes bacteria play a major role in the process, carrying out dissimilative organic-sulfur oxidation. So far, the determinant metabolic reactions of such activity have not been evaluated. Here, in silico analysis was performed on the genomes of sulfate-producing thermophilic genera and mesophilic low-sulfate producers, revealing that highest sulfate production is related to the simultaneous presence of metabolic pathways leading to sulfite synthesis, similar to the ones found in mammalian cells. Those pathways include reverse transsulfuration reactions (tightly associated with methionine cycling), and the presence of aspartate aminotransferases (ATs) with the potential of 3-sulfinoalanine AT and cysteine AT activity, which ultimately leads to sulfite production. Sulfite is oxidized to sulfate by sulfite oxidase, this enzyme is determinant in sulfate synthesis, and it is absent in many mesophiles.

• Aspartate transaminase
• dissimilatory organic-sulfur oxidation
• enzymes of cysteine and methionine pathways
• sulfite oxidase
• sulfite reductase

Declaration of interest

The corresponding author, Margarida Santana, was supported by the program “Ciência 2008” from “Fundação para a Ciência e a Tecnologia”. J. M. G. acknowledges funding from the Spanish Ministry of Economy and Productivity (Consolider CSD2009-00006) which included cofinancing through FEDER funds. The authors report no declarations of interest.

Supplementary material available online

Supplementary Figures S1–S8