A natural acidic iron-cycling microbial community from the Rio Tinto River in southwestern Spain, distinguished by exceptional eukaryotic diversity (>65% of total biomass) in an extremely acidic environment (mean pH 2.2). While prokaryotic diversity is low with 80% of bacteria belonging to three iron-cycling genera (Leptospirillum, Acidithiobacillus, Acidiphilium), eukaryotic diversity is remarkably high including algae (diatoms, Chlorophyta, Euglenozoa, Rhodophyta), fungi (Hortaea, Acidomyces), and diverse protists (ciliates, amoebae, heliozoans, cercomonads, stramenopiles). The river has constant acidic pH (mean 2.2, range 1.0-2.5) and high metal concentrations (Fe 2.3 g/L, Cu 0.11 g/L, Zn 0.22 g/L) throughout its 100 km length, driven by natural oxidation of massive sulfide deposits in the Iberian Pyrite Belt. The prokaryotic community drives iron cycling through Fe²⁺ oxidation (Leptospirillum, Acidithiobacillus) and heterotrophic carbon cycling (Acidiphilium), creating stromatolite-like iron oxide precipitates. Eukaryotes contribute primary production (photosynthetic algae), heterotrophic consumption (protists, fungi), and trophic structure (ciliate predators, heliozoan top predators). The ecosystem represents a natural analog for early Earth conditions and extraterrestrial iron-rich acidic environments, supporting astrobiology research. Rio Tinto demonstrates that complex food webs with eukaryotic dominance can thrive in extreme acid-metal conditions, challenging assumptions about habitability limits.
Taxonomy
| Taxon | Ontology ID | Functional Roles | Abundance |
|---|---|---|---|
| Leptospirillum ferrooxidans | NCBITaxon:180 · TaxonMech · TraitMech |
PRIMARY_DEGRADER
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DOMINANT |
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| Acidithiobacillus ferrooxidans | NCBITaxon:920 · TaxonMech · TraitMech |
PRIMARY_DEGRADER
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ABUNDANT |
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| Acidiphilium | NCBITaxon:522 · TaxonMech · TraitMech |
CROSS_FEEDER
SECONDARY_FERMENTER
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ABUNDANT |
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| Bacillariophyceae | NCBITaxon:33849 · TaxonMech · TraitMech |
PRIMARY_PRODUCER
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COMMON |
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| Chlorophyta | NCBITaxon:3041 · TaxonMech · TraitMech |
PRIMARY_PRODUCER
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COMMON |
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| Basidiomycota | NCBITaxon:5204 · TaxonMech · TraitMech |
PRIMARY_DEGRADER
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RARE |
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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.
Iron Oxidation and Acid Generation
PARTIAL: the 2003 river survey and isolate physiology support L. ferrooxidans and A. ferrooxidans as principal iron oxidizers. Their conversion of ferrous to ferric iron supplies the iron cycle; ferric iron also contributes to acidic buffering. Iron oxidation itself consumes protons and must not be equated with sulfuric-acid production. The field-derived model does not demonstrate mutual benefit or a universal species abundance, iron concentration or pH.
Participating Taxa: Leptospirillum ferrooxidans, Acidithiobacillus ferrooxidans
Metabolites: Fe(II) (CHEBI:29033 · MediaIngredientMech · CMMMech), Fe(III) (CHEBI:29034 · MediaIngredientMech · CMMMech)
PARTIAL: oxidation supplies ferric iron that Acidiphilium can use as an electron acceptor during organic-carbon respiration, connecting the oxidative and reductive arms of the authors' iron-cycle model. This is not a measured transfer of autotroph-derived organic carbon or a quantified in situ exchange flux.
Evidence
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doi:10.1128/aem.69.8.4853-4865.2003 - PARTIAL (IN_VIVO)"all related to the iron cycle, accounted for most of the prokaryotic microorganisms detected"
Heterotrophic Carbon-Iron Coupling
PARTIAL: the 2003 model assigns Acidiphilium organic-carbon respiration coupled to ferric-iron reduction, producing ferrous iron, particularly under anoxic or microaerobic conditions. It does not assign ferrous-iron oxidation to this heterotrophic role. The survey combines taxon detection and physiological knowledge; it does not trace algal exudates, fungal products or autotroph biomass into Acidiphilium, quantify in situ carbon/iron flux, or establish a closed mutually beneficial nutrient cycle.
Participating Taxa: Acidiphilium
Metabolites: organic molecular entity (CHEBI:50860 · MediaIngredientMech · CMMMech), Fe(II) (CHEBI:29033 · MediaIngredientMech · CMMMech), Fe(III) (CHEBI:29034 · MediaIngredientMech · CMMMech)
Biological Processes:
- organic substance catabolic process (GO:0009056 · PathwayMech)
Evidence
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doi:10.1128/aem.69.8.4853-4865.2003 - PARTIAL (IN_VIVO)"all related to the iron cycle, accounted for most of the prokaryotic microorganisms detected"
Eukaryotic Primary Production
Phototrophic eukaryotes, including diatoms and green algae represented by the canonical Bacillariophyceae and Chlorophyta entries, support primary production in acidic Tinto biofilms. The 2013 review summarizes photosynthesis measurements and the 2011 primary abstract confirms biofilm photosynthesis-versus-irradiance assays. This is not a diatom-to-green-alga cross-feeding experiment, nor a demonstration that their fixed carbon reaches Acidiphilium. Broad eukaryotic biomass estimates must not be assigned to these two taxa alone.
Participating Taxa: Bacillariophyceae, Chlorophyta
Biological Processes:
- photosynthesis (GO:0015979 · PathwayMech)
Evidence
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PMID:25369810 - PARTIAL (REVIEW)"Most the photosynthetic eukaryotes in the Tinto ecosystem are forming biofilms"
Complementary Iron and Sulfur Oxidation
PARTIAL: the 2003 field-derived model includes ferrous-iron oxidation by L. ferrooxidans and A. ferrooxidans, with sulfur oxidation also assigned to A. ferrooxidans. These complementary physiological capacities do not establish a mutualistic partner benefit, selective removal of a passivating layer, or a uniform 25 percent abundance across river sites. No new sulfur-to-iron causal direction is added from physiological plausibility alone.
Participating Taxa: Leptospirillum ferrooxidans, Acidithiobacillus ferrooxidans
Metabolites: Fe(II) (CHEBI:29033 · MediaIngredientMech · CMMMech), Fe(III) (CHEBI:29034 · MediaIngredientMech · CMMMech), sulfuric acid (CHEBI:26836 · MediaIngredientMech · CMMMech)
Evidence
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doi:10.1128/aem.69.8.4853-4865.2003 - PARTIAL (IN_VIVO)"all related to the iron cycle, accounted for most of the prokaryotic microorganisms detected"
Knowledge gaps & discussions (1)
KNOWLEDGE_GAP OPEN Which phototrophic or fungal products actually support Acidiphilium carbon and iron metabolism?
The primary fungal isolation abstract (PMID15644909) includes basidiomycete and ascomycete yeasts; Basidiomycota presence is not rejected. It does not establish the removed exchange/scaffolding mechanism. Hortaea werneckii belongs to Ascomycota (NCBITaxon91943), so it cannot justify that Basidiomycota assignment. Taxon-specific carbon tracing and coordinated fungal identity curation are needed before reinstating a causal exchange. Non-graph identities, abundances and redox descriptions remain issue #1898.
Attaches to: ecological_interactions#Heterotrophic Carbon-Iron Coupling, ecological_interactions#Eukaryotic Primary Production
1 evidence item(s)
Within the decomposers, fungi are very abundant and exhibit great diversity, including yeast and filamentous forms
Review supports a fungal decomposer guild generally, not Basidiomycota-to-Acidiphilium exchange or Hortaea membership in Basidiomycota.
Environmental Factors
| Factor | Value | Unit |
|---|---|---|
| pH | 2.2 | pH units |
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| Iron Concentration | 2.3 | g/L |
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| Eukaryotic Biomass Dominance | 65 | % of total biomass |
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| Prokaryotic Diversity | 80% | % in 3 genera |
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