Of mammals and bacteria in a rainforest: Temporal dynamics of soil bacteria in response to simulated N pulse from mammalian urine

Pulse-type perturbation through excreta by animals creates a mosaic of short-term high nutrient-load patches in the soil. How this affects microbial community composition and how long these impacts last are important for microbial community dynamics and nutrient cycling. Our study focused on the short-term responses to N by bacterial communities and ‘functional groups’ associated with the N cycle in a lowland evergreen tropical rainforest. We applied a single urea pulse, equivalent to urine-N deposition by medium-sized mammals to simulate N enrichment and changes in soil N availability, and analysed soil bacterial communities using molecular methods, before and after urea application. Urea addition increased mineral N availability and changed bacterial community composition, from phylum to operational taxonomic unit levels, however, taxon richness and diversity were unaffected. Taxa involved in the physiologically “narrow” processes of nitrification (e.g. Nitrosospira) and denitrification (e.g. Phyllobacteriaceae, Xanthomonadaceae and Comamonadaceae) increased their relative abundance, while N2-fixers (e.g. Rhodospirillales, and Rhizobiales) decreased after treatment. While a temporal legacy on both community composition and functional group profile was observable 58 and 159 days after treatment, at the latter date bacterial communities were already tending towards pre-treatment composition. We suggest that pulse-type perturbation by mammal urine that occurs on a daily basis has strong short-term effects on patch dynamics of soil microbiota and N availability. Such a spatio-temporally dynamic soil environment enhances overall microbial richness and diversity, and contributes to the apparent temporal resilience of community composition. A plain language summary is available for this article. © 2017 The Authors. Functional Ecology © 2017 British Ecological Societ

A Bioinformatics Classifier and Database for Heme-Copper Oxygen Reductases

Background: Heme-copper oxygen reductases (HCOs) are the last enzymatic complexes of most aerobic respiratory chains, reducing dioxygen to water and translocating up to four protons across the inner mitochondrial membrane (eukaryotes) or cytoplasmatic membrane (prokaryotes). The number of completely sequenced genomes is expanding exponentially, and concomitantly, the number and taxonomic distribution of HCO sequences. These enzymes were initially classified into three different types being this classification recently challenged. Methodology:We reanalyzed the classification scheme and developed a new bioinformatics classifier for the HCO and Nitric oxide reductases (NOR), which we benchmark against a manually derived gold standard sequence set. It is able to classify any given sequence of subunit I from HCO and NOR with a global recall and precision both of 99.8%. We use this tool to classify this protein family in 552 completely sequenced genomes. Conclusions: We concluded that the new and broader data set supports three functional and evolutionary groups of HCOs. Homology between NORs and HCOs is shown and NORs closest relationship with C Type HCOs demonstrated. We established and made available a classification web tool and an integrated Heme-Copper Oxygen reductase and NOR protein database (www.evocell.org/hco)

Nitrite reduction in Rhizobium "hedysari" strain HCNT1.

Rhizobium \u201chedysari\u201d strain HCNT 1 rapidly reduced nitrite to N2O, only slowly reduced nitrate to nitrite and did not exhibit nitrous oxide reductase activity. Nitrite reduction in this rhizobium strain may be a detoxification mechanism for conversion of nitrite, which inhibits O2 uptake, to non-toxic N2O. Concentrations of nitrite as small as 3 \ub5M diminished O2 uptake in whole cells. The bacterium did not couple energy conservation with nitrate or nitrite reduction. Cells neither grew anaerobically at the expense of these nitrogen oxides nor translocated protons during reduction of nitrite. Induction of nitrite reductase activity was not a response to the presence of nitrate or nitrite, but occurred instead when the O2 concentration in culture atmospheres fell to o, which is synthesized only in cells grown under O2-limited conditions, may account for the toxicity of nitrite in strain HCNT 1

Analysis of the Role of the nnrR gene Product in the Response of Rhodobacter sphaeroides 2.4.1 to Exogenous Nitric Oxide

Rhodobacter sphaeroides 2.4.1, which is incapable of denitrification, has been found to carry nnrR, the nor operon, and nnrS, which are utilized for denitrification in R. sphaeroides 2.4.3. The gene encoding nitrite reductase was not found in 2.4.1. Expression of β-galactosidase activity from a norB-lacZ fusion was activated when cells of 2.4.1 were incubated with NO-producing bacteria. This result indicates that the products of nnrR and the genes flanking it are utilized when 2.4.1 is growing in an environment where denitrification occurs

Nitrite reductase in bacteroids of Rhizobium "hedysari" strain HCNT1.

Ex planta, bacteroids of the sulla-symbiont Rhizobium “hedysari” strain HCNT 1 terminated reduction of nitrite at nitrous oxide irrespective of the presence or absence of acetylene. Nitrate was not reduced during the experimental period, but slight nitrate reductase activity occurred if incubation with nitrate was prolonged (up to 15 h). As was observed in free-living cells, exposure of the bacteroids to the metal chelator, diethyldithiocarbamate, prevented reduction of nitrite, indicating the presence of a copper-containing nitrite reductase. Pulses of 10–75 µM nitrite transiently impeded O2 uptake in bacteroids, which resumed consumption of O2 when the nitrite had been reduced. Exposure to >1.0 mM nitrite for 24h greatly inhibited nitrogenase activity (assayed as acetylene reduction activity) of bacteroids in planta. Exposure to the same concentrations of nitrite after 1h of incubation in the presence of acetylene almost completely stopped ongoing ethylene production in bacteroids of strain HCNT 1 extracted from nodules. Free cells of the non-nitrite-reducing R. “hedysari” strain CC 1335 were lacking in nitrogenase (acetylene-reduction) activity, whereas identically cultured (low-oxygen) strain HCNT 1 cells reduced both nitrite and acetylene