Biology subjects

Nicol, G. W.

Publications and source records attributed to Nicol, G. W..

4 recordsLinked to original sources

bioRxiv2021

Ammonia-oxidizing archaea possess a wide range of cellular ammonia affinities

Nitrification, the oxidation of ammonia to nitrate, is an essential process in the biogeochemical nitrogen cycle. The first step of nitrification, ammonia oxidation, is performed by three, often co- occurring guilds of chemolithoautotrophs: ammonia-oxidizing bacteria (AOB), archaea (AOA), and complete ammonia oxidizers (comammox). Substrate kinetics are considered to be a major niche-differentiating factor between these guilds, but few AOA strains have been kinetically characterized. Here, the ammonia oxidation kinetic properties of 12 AOA representing all major phylogenetic lineages were determined using microrespirometry. Members of the genus Nitrosocosmicus have the lowest substrate affinity of any characterized AOA, which are similar to previously determined affinities of AOB. This contrasts previous assumptions that all AOA possess much higher substrate affinities than their comammox or AOB counterparts. The substrate affinity of ammonia oxidizers correlated with their cell surface area to volume ratios. In addition, kinetic measurements across a range of pH values strongly supports the hypothesis that - like for AOB - ammonia and not ammonium is the substrate for the ammonia monooxygenase enzyme of AOA and comammox. Together, these data will facilitate predictions and interpretation of ammonia oxidizer community structures and provide a robust basis for establishing testable hypotheses on competition between AOB, AOA, and comammox.

microbiology↗

bioRxiv2020

Methane-derived carbon flow through host-virus trophic networks in soil

The concentration of atmospheric methane continues to increase with microbial communities controlling soil-atmosphere fluxes. While there is substantial knowledge of the diversity and function of organisms regulating methane production and consumption, the frequency and impact of interactions with viruses on their activity in soil is unknown. Metagenomic sequencing of soil microbial communities has enabled identification of linkages between viruses and hosts. However, determining host-virus linkages through sequencing does not determine whether a virus or a host are active. In this study, we identified active individual interactions in situ by following the transfer of assimilated carbon from active hosts to viruses. Using DNA stable-isotope probing combined with metagenomic analyses, we characterized methane-fueled microbial networks in acidic and neutral pH soils, specifically primary and secondary utilisers of carbon, together with the recent transfer of methane-derived carbon to viruses. Sixty-three percent of viral contigs from replicated soil incubations contained genes associated with known methanotrophic bacteria. Genomic sequences from 13C-enriched viruses were present in clustered regularly interspaced short palindromic repeats (CRISPR) arrays of multiple, closely-related Methylocystis populations, revealing differences in their history of viral interaction. Viruses infecting non-methanotrophic methylotrophs and heterotrophic predatory bacteria were also identified through the analysis of shared homologous genes, demonstrating that carbon is transferred to a diverse range of viruses associated with methane-fueled microbial food networks.

microbiology↗

bioRxiv2020

Use and abuse of potential rates in soil microbiology

Potential rate assays are used in soil microbial ecology to determine the rates of a functional process in environmental samples under a defined set of conditions. While they can be used appropriately to provide mechanistic insights, potential rates are also often used to estimate the abundance of specific taxonomic groups and their in situ activity. These estimates incorrectly assume that all contributing organisms in a community are active at a maximum rate under one set of optimal incubation conditions and that potential rates reflect activity in the soil. While investigators now recognise that populations within communities are physiologically diverse, they often ignore the consequent suboptimal activity, or even inactivity, of the majority of community members performing that function. In this short perspective article, we discuss when potential assays can be informative and highlight the underlying conceptual problems under circumstances where potential assays are misused, using potential nitrification rate (PNR) as an example. PNR was originally developed to estimate the size of active ammonia oxidising communities in environmental samples. It is routinely determined in short-term shaken slurry incubations by measuring assumed maximum rates of nitrate or nitrite production under optimal, non-substrate-limiting conditions. As with other functional processes, it is now recognised that a broad diversity of organisms contribute to aerobic ammonia oxidation in terrestrial and other habitats, and this diversity represents a substantial range of physiologies, including variation in substrate affinity, ammonia tolerance, cell specific activity and substrate preference. Despite this, PNR, and other potential rate assays, are often inappropriately used in an attempt to determine an ecologically relevant measurement of activity in soil. As with any potential assay, PNR has inherent biases towards particular functional groups and its use in investigating the ecology of ammonia oxidisers in natural systems should be carefully considered.

microbiology↗

bioRxiv2020

Comparison of the in vitro activity of novel and established nitrification inhibitorsapplied in agriculture: challenging the effectiveness of the currently availablecompounds

Nitrification inhibitors (NIs) applied to soil reduce nitrogen fertilizer losses from agricultural ecosystems. Currently available NIs appear to selectively inhibit ammonia-oxidizing bacteria (AOB), while their impact on other groups of nitrifiers is limited. Ethoxyquin (EQ), a preservative shown to inhibit ammonia-oxidizers (AO) in soil, is rapidly transformed to 2,6-dihydro-2,2,4-trimethyl-6-quinone imine (QI) and 2,4-dimethyl-6-ethoxy-quinoline (EQNL). We compared the inhibitory potential of EQ and its derivatives in vitro with other established NIs that have been applied in an agricultural setting (dicyandiamide (DCD), nitrapyrin (NP), 3,4-dimethylpyrazole phosphate (DMPP)) by evaluating their impact on the activity and growth of five soil-derived strains (two AOB (Nitrosomonas europaea, Nitrosospira multiformis), two ammonia-oxidizing archaea (AOA) ("Candidatus Nitrosocosmicus franklandus", "Candidatus Nitrosotalea sinensis"), and one nitrite-oxidizing bacterium (NOB) (Nitrobacter sp.)). NIs degradation was also determined. AOA were more sensitive than AOB or NOB to EQ and its derivatives. Despite its transient character, QI was primarily responsible for AO inhibition by EQ, and the most potent NI against AOA. For AOB, QI was more potent than DCD but less than nitrapyrin and DMPP. AOA and NOB showed higher tolerance to the persistent compounds DCD and DMPP. Our findings benchmark the activity range of known and novel NIs with practical implications for their use, and the development of novel NIs with broad or complementary activity against all AO.

microbiology↗