Growth of the obligate anaerobe Desulfovibrio vulgaris Hildenborough under continuous low oxygen concentration sparging: impact of the membrane-bound oxygen reductases.

Fanny Ramel,Laetitia Pieulle,Odile Valette,Gael Brasseur,Agnès Hirschler-Réa,Alain Dolla,Marie Laure Fardeau,Franck Chauvat

doi:10.1371/journal.pone.0123455

Abstract

Although obligate anaerobe, the sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough (DvH) exhibits high aerotolerance that involves several enzymatic systems, including two membrane-bound oxygen reductases, a bd-quinol oxidase and a cc(b/o)o3 cytochrome oxidase. Effect of constant low oxygen concentration on growth and morphology of the wild-type, single (Δbd, Δcox) and double deletion (Δcoxbd) mutant strains of the genes encoding these oxygen reductases was studied. When both wild-type and deletion mutant strains were cultured in lactate/sulfate medium under constant 0.02% O2 sparging, they were able to grow but the final biomasses and the growth yield were lower than that obtained under anaerobic conditions. At the end of the growth, lactate was not completely consumed and when conditions were then switched to anaerobic, growth resumed. Time-lapse microscopy revealed that a large majority of the cells were then able to divide (over 97%) but the time to recover a complete division event was longer for single deletion mutant Δbd than for the three other strains. Determination of the molar growth yields on lactate suggested that a part of the energy gained from lactate oxidation was derived toward cells protection/repairing against oxidative conditions rather than biosynthesis, and that this part was higher in the single deletion mutant Δbd and, to a lesser extent, Δcox strains. Our data show that when DvH encounters oxidative conditions, it is able to stop growing and to rapidly resume growing when conditions are switched to anaerobic, suggesting that it enters active dormancy sate under oxidative conditions. We propose that the pyruvate-ferredoxin oxidoreductase (PFOR) plays a central role in this phenomenon by reversibly switching from an oxidative-sensitive fully active state to an oxidative-insensitive inactive state. The oxygen reductases, and especially the bd-quinol oxidase, would have a crucial function by maintaining reducing conditions that permit PFOR to stay in its active state.

Highlights

Sulfate-reducing bacteria (SRB) are anaerobic microorganisms ubiquitously distributed, even in atypical environments for this physiological group, e.g. in aerobic layer of a stratified fjord [1], in aerobic wastewater biofilms [2,3], in oxic layers of microbial mats [4,5,6,7,8] or in oxic marine sediment layers close to the sediment surface [9]
Sigalevich and Cohen [34] reported that when an initially established chemostat coculture of Desulfovibrio oxyclinae and the facultative heterotrophic aerobe Marinobacter sp. strain MB grown under anaerobic conditions in lactate/sulfate medium was exposed to an oxygen flux, the sulfate reducing bacterium performed an incomplete oxidation of lactate to acetate
Wild-type Desulfovibrio vulgaris Hildenborough (DvH) and both single (Δbd, Δcox) and double (Δcoxbd) mutants were cultured in lactate/sulfate medium in Hungate tubes with constant sparging with 0.02% O2 in order to evaluate the role of the membrane bound oxygen reductases in the growth under these oxidative conditions

Summary

Introduction

Sulfate-reducing bacteria (SRB) are anaerobic microorganisms ubiquitously distributed, even in atypical environments for this physiological group, e.g. in aerobic layer of a stratified fjord [1], in aerobic wastewater biofilms [2,3], in oxic layers of microbial mats [4,5,6,7,8] or in oxic marine sediment layers close to the sediment surface [9]. Microscopy of roots and rhizomes revealed the presence of SRB in the sea grass rhizosphere sediments [10], on the surfaces [11], inside epidermal and exodermal cells [12], even deep into the cortex cells of aquatic plants roots [13] All these ecological niches can temporary be exposed to oxygen concentration up to saturation [4,14,15] and force SRB to cope with elevated oxygen tension. Other artificial oxygen gradient experiments revealed growth of SRB close to the oxic-anoxic interface [27,33] These data point out a positive aerotaxis that would enable bacteria to find environmental conditions favourable for their metabolic lifestyle. Even if none of the SRB isolated so far can either grow aerobically or reduce sulfate under high oxygen concentrations, several strains of Desulfovibrio species have been demonstrated to have the capability to couple oxygen reduction with proton translocation and energy conservation [35]

Methods

Results

Conclusion