New insights into the energy metabolism and taxonomy of Deferribacteres revealed by the characterization of a new isolate from a hypersaline microbial mat

Assessing the Influence of HGT on the Evolution of Stress Responses in Microbial Communities from Shark Bay, Western Australia

Pustular microbial mats in Shark Bay, Western Australia, are modern analogs of microbial systems that colonized peritidal environments before the evolution of complex life. To understand how these microbial communities evolved to grow and metabolize in the presence of various environmental stresses, the horizontal gene transfer (HGT) detection tool, MetaCHIP, was used to identify the horizontal transfer of genes related to stress response in 83 metagenome-assembled genomes from a Shark Bay pustular mat. Subsequently, maximum-likelihood phylogenies were constructed using these genes and their most closely related homologs from other environments in order to determine the likelihood of these HGT events occurring within the pustular mat. Phylogenies of several stress-related genes-including those involved in response to osmotic stress, oxidative stress and arsenic toxicity-indicate a potentially long history of HGT events and are consistent with these transfers occurring outside of modern pustular mats. The phylogeny of a particular osmoprotectant transport gene reveals relatively recent adaptations and suggests interactions between Planctomycetota and Myxococcota within these pustular mats. Overall, HGT phylogenies support a potentially broad distribution in the relative timing of the HGT events of stress-related genes and demonstrate ongoing microbial adaptations and evolution in these pustular mat communities.

Trichlorobacter ammonificans, a dedicated acetate-dependent ammonifier with a novel module for dissimilatory nitrate reduction to ammonia

Dissimilatory nitrate reduction to ammonia (DNRA) is a common biochemical process in the nitrogen cycle in natural and man-made habitats, but its significance in wastewater treatment plants is not well understood. Several ammonifying Trichlorobacter strains (former Geobacter) were previously enriched from activated sludge in nitrate-limited chemostats with acetate as electron (e) donor, demonstrating their presence in these systems. Here, we isolated and characterized the new species Trichlorobacter ammonificans strain G1 using a combination of low redox potential and copper-depleted conditions. This allowed purification of this DNRA organism from competing denitrifiers. T. ammonificans is an extremely specialized ammonifier, actively growing only with acetate as e-donor and carbon source and nitrate as e-acceptor, but H2 can be used as an additional e-donor. The genome of G1 does not encode the classical ammonifying modules NrfAH/NrfABCD. Instead, we identified a locus encoding a periplasmic nitrate reductase immediately followed by an octaheme cytochrome c that is conserved in many Geobacteraceae species. We purified this octaheme cytochrome c protein (TaNiR), which is a highly active dissimilatory ammonifying nitrite reductase loosely associated with the cytoplasmic membrane. It presumably interacts with two ferredoxin subunits (NapGH) that donate electrons from the menaquinol pool to the periplasmic nitrate reductase (NapAB) and TaNiR. Thus, the Nap-TaNiR complex represents a novel type of highly functional DNRA module. Our results indicate that DNRA catalyzed by octaheme nitrite reductases is a metabolic feature of many Geobacteraceae, representing important community members in various anaerobic systems, such as rice paddy soil and wastewater treatment facilities.

Succinyl-CoA:acetate CoA-transferase functioning in the oxidative tricarboxylic acid cycle in Desulfurella acetivorans

Desulfurella acetivorans is a strictly anaerobic sulfur-reducing deltaproteobacterium that possesses a very dynamic metabolism with the ability to revert the citrate synthase version of the tricarboxylic acid (TCA) cycle for autotrophic growth (reversed oxidative TCA cycle) or to use it for acetate oxidation (oxidative TCA cycle). Here we show that for heterotrophic growth on acetate D. acetivorans uses a modified oxidative TCA cycle that was first discovered in acetate-oxidizing sulfate reducers in which a succinyl-CoA:acetate CoA-transferase catalyzes the conversion of succinyl-CoA to succinate, coupled with the activation of acetate to acetyl-CoA. We identified the corresponding enzyme in this bacterium as the AHF96498 gene product and characterized it biochemically. Our phylogenetic analysis of CoA-transferases revealed that the CoA-transferase variant of the oxidative TCA cycle has convergently evolved several times in different bacteria. Its functioning is especially important for anaerobes, as it helps to increase the energetic efficiency of the pathway by using one enzyme for two enzymatic reactions and by allowing to spend just one ATP equivalent for acetate activation.