An acidic warm spring microbial community from the Ngawha Geothermal Field (Northland Region, Aotearoa New Zealand) characterized by genome-resolved metagenomics and mercury speciation analysis. Two adjacent springs differ in mercury speciation, with dissolved total and methylated mercury concentrations among the highest reported from natural sources (250 to 16,000 ng/L total; 0.5 to 13.9 ng/L methylmercury). Sediment total mercury ranges from 1,274 to 7,000 ug/g. Despite ultrahigh mercury levels, the geothermal microbiome is unexpectedly diverse and dominated by acidophilic and mesophilic sulfur- and iron-cycling bacteria, mercury- and arsenic- resistant bacteria, and thermophilic and acidophilic archaea. A conceptual model integrates community structure and metagenomic potential with abiotic and biotic controls on Hg methylation, demethylation, and reduction to volatile Hg(0).
Taxonomy
| Taxon | Ontology ID | Functional Roles | Abundance |
|---|---|---|---|
| acidophilic sulfur- and iron-cycling bacteria — ontology label: Bacteria | NCBITaxon:2 · TaxonMech · TraitMech |
PRIMARY_DEGRADER
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DOMINANT |
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| mercury- and arsenic-resistant bacteria — ontology label: Bacteria | NCBITaxon:2 · TaxonMech · TraitMech |
PRIMARY_DEGRADER
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N/A |
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| thermophilic and acidophilic archaea — ontology label: Archaea | NCBITaxon:2157 · TaxonMech · TraitMech |
PRIMARY_DEGRADER
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N/A |
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Ecological Interactions
The graph shows only explicitly linked interaction participants. Community members without an asserted participant link are omitted; membership alone does not imply an interaction.
Acidic Warm-Spring Mercury Speciation Regime
Geochemical measurements characterize acidic warm Ngawha springs with high dissolved total mercury and methylmercury and high sediment mercury. These measured environmental pools constrain hypotheses about bioavailability, but total dissolved mercury is not synonymous with freely available Hg(II), and co-occurrence does not show that the regime caused the observed diversity.
Metabolites: mercury(2+) (CHEBI:16793 · MediaIngredientMech · CMMMech), methylmercury (CHEBI:49747 · MediaIngredientMech · CMMMech)
Evidence
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PMID:32414793 - SUPPORT (IN_VIVO)"acidic warm springs in the Ngawha Geothermal Field (<55°C, pH <4.5), Northland Region, Aotearoa New Zealand "
Metagenomic Mercury Transformation Potential
PARTIAL - Metagenomic merA/merB homologues support potential Hg(II) reduction and organomercurial demethylation. Cub Bath yielded hgcA/hgcB evidence, but hgcA-bearing sequences could not be assigned to a resolved genome or definitive taxon; Tiger Bath nondetection is sequencing-depth bounded. These are positive sequence findings, not measured transformation rates or identified active members for every reaction. HMM merA counts required paralog filtering.
Metabolites: mercury(2+) (CHEBI:16793 · MediaIngredientMech · CMMMech), methylmercury (CHEBI:49747 · MediaIngredientMech · CMMMech)
Evidence
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PMID:32414793 - PARTIAL (COMPUTATIONAL)"merB genes were identified"
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PMID:32414793 - SUPPORT (COMPUTATIONAL)"The Cub Bath hgcA sequences could not be binned"
Diverse Acidophile Guilds Under Ultrahigh Mercury
The sampled Ngawha microbiomes were unexpectedly diverse despite high mercury, with sulfur- and iron-cycling bacteria, Hg/As-resistance potential and acidophilic archaea; the two baths differed in composition. This metagenomic observation does not demonstrate temporal stability, realized niche partitioning, or a mercury-driven diversity increase.
Metabolites: mercury(2+) (CHEBI:16793 · MediaIngredientMech · CMMMech), sulfur atom (CHEBI:26833 · MediaIngredientMech · CMMMech)
Evidence
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PMID:32414793 - SUPPORT (COMPUTATIONAL)"In the context of such ultrahigh mercury levels, the geothermal microbiome was unexpectedly diverse"
Measured Gaseous Mercury With Unresolved Microbial Contribution
Gaseous Hg(0) was measured above Tiger and Cub Baths, alongside aqueous and sediment mercury measurements. The source also reports merA-based reduction potential. The measured atmospheric mercury is a positive environmental result, not an isolated microbial emission rate: deep geological inputs and abiotic redox processes also contribute, and their contributions were not separated from microbial reduction.
Metabolites: mercury(2+) (CHEBI:16793 · MediaIngredientMech · CMMMech), mercury(0) (CHEBI:16170 · MediaIngredientMech · CMMMech)
Evidence
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PMID:32414793 - SUPPORT (IN_VIVO)"Gaseous Hg0 emissions were also recorded from the baths"
Sulfur-Iron-Coupled Mercury Cycle Model
HYPOTHESIZED - The conceptual model links sulfur/iron redox cycling and cinnabar precipitation/dissolution to mercury bioavailability, with potential methylation, demethylation and Hg(II) reduction. These coupled biotic and abiotic reactions are an evidence-informed model, not demonstrated interspecies syntrophy or a measured taxon-to-taxon mercury flux.
Metabolites: mercury(2+) (CHEBI:16793 · MediaIngredientMech · CMMMech), sulfur atom (CHEBI:26833 · MediaIngredientMech · CMMMech)
Evidence
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PMID:32414793 - PARTIAL (COMPUTATIONAL)"illustrates how geothermal mercury cycling may couple to microbial community dynamics and sulfur and iron biogeochemistry "
Knowledge gaps & discussions (1)
KNOWLEDGE_GAP OPEN Which measurements establish the proposed causal mechanism in this exact system?
Retain measured mercury pools, atmospheric Hg(0), diverse microbiomes and positive mer/hgc sequence evidence. The hgcA sequences could not be binned, MerA HMM hits included paralogs, and no hgcAB was recovered from resolved sulfate-reducing bins. Absence at one sequencing depth is not biological absence. Environmental concentrations and mer/hgc potential do not isolate microbial rates or prove temporal stability. The sulfur/iron/cinnabar coupling is proposed; selective activity assays and source partitioning are needed. Preserve the earlier removal of four interpretation-workflow arrows and remove the remaining environmental-context arrow.
Attaches to: ecological_interactions#Acidic Warm-Spring Mercury Speciation Regime, ecological_interactions#Metagenomic Mercury Transformation Potential, ecological_interactions#Diverse Acidophile Guilds Under Ultrahigh Mercury, ecological_interactions#Measured Gaseous Mercury With Unresolved Microbial Contribution, ecological_interactions#Sulfur-Iron-Coupled Mercury Cycle Model
External Resources
| Name | Repository | Resource ID |
|---|---|---|
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Primary publication for the Ngawha Hg-cycling community
PubMed record for the Gionfriddo et al. 2020 Appl Environ Microbiol paper. |
OTHER | PMID:32414793 |
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DOI landing page
DOI link to the Applied and Environmental Microbiology paper. |
OTHER | doi:10.1128/AEM.00176-20 |
Environmental Factors
| Factor | Value | Unit |
|---|---|---|
| Mercury concentration | ultrahigh dissolved and solid Hg | N/A |
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| Spring chemistry | pH <4.5, temperature <55 C | N/A |
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