A two-member mutualistic consortium consisting of the green alga Chlamydomonas reinhardtii and the methylotrophic bacterium Methylobacterium aquaticum, isolated from a freshwater environment. This natural association demonstrates reciprocal metabolic exchange under nitrogen stress conditions, mediated by indole-3-acetic acid (IAA) signaling. The alga provides photosynthetically fixed carbon and oxygen, while Methylobacterium modulates IAA levels through both production and degradation, influencing algal physiology and stress responses. This system serves as a model for understanding algae-bacteria interactions in freshwater ecosystems and potential applications in sustainable algal biotechnology under nutrient limitation.
Taxonomy
| Taxon | Ontology ID | Functional Roles | Abundance |
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| Chlamydomonas reinhardtii | NCBITaxon:3055 · TaxonMech · TraitMech |
PRIMARY_PRODUCER
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N/A |
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| Methylobacterium aquaticum | NCBITaxon:270351 · TaxonMech · TraitMech |
SYNTROPHIC_PARTNER
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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.
Algal Glycerol Provision in Proline-Dependent Mutualism
MUTUALISMIn the 2018 CYB/proline experiment, C. reinhardtii CC-1690 and M. aquaticum GR16T grew together when neither monoculture grew. Algal glycerol provision and bacterial ammonium production support reciprocal carbon/nitrogen exchange. This is not evidence of an oxygen-mediated benefit or of glycerol-dependent IAA turnover in the distinct 2024 assay.
Source Taxon: Chlamydomonas reinhardtii
Target Taxon: Methylobacterium aquaticum
Metabolites: glycerol (CHEBI:17754 · MediaIngredientMech · CMMMech)
Biological Processes:
- photosynthesis (GO:0015979 · PathwayMech)
- response to nitrogen starvation (GO:0006995 · PathwayMech)
HYPOTHESIZED - algal photosynthates could support the bacterial carbon/nitrogen balance needed for cooperative IAA removal, as proposed in the 2024 study. Its mediator is unresolved; the 2018 glycerol result in CYB/proline does not prove this connection in acetate-containing T-N medium.
Evidence
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PMID:29385570 - SUPPORT (IN_VITRO)"However, we discovered that algal growth on these substrates is supported in association with Methylobacterium spp., and that a mutualistic carbon-nitrogen metabolic exchange between Chlamydomonas and Methylobacterium spp"
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PMID:29385570 - SUPPORT (IN_VITRO)"CO2 photosynthetically fixed by Chlamydomonas yields glycerol"
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PMID:38269098 - PARTIAL (IN_VITRO)"algal-derived photosynthates may restore carbon/nitrogen balance and enhance bacterial growth"
Cooperative IAA Degradation
MUTUALISMThe 2024 study reports cooperative IAA removal by C. reinhardtii CC-5325 and M. aquaticum: neither monoculture depleted auxin, and both populations attained higher densities together on supplied IAA. Algal LAO1-dependent L-tryptophan conversion produces IAA; bacterial IAA production is not established here. The degradation pathway and algal contribution remain unresolved, and its Salkowski assay measures indoles rather than uniquely identifying IAA.
Source Taxon: Methylobacterium aquaticum
Target Taxon: Chlamydomonas reinhardtii
Metabolites: indole-3-acetic acid (CHEBI:16411 · MediaIngredientMech · CMMMech)
Biological Processes:
- indole-3-acetic acid catabolic process (GO:0042436 · PathwayMech)
PARTIAL - cooperative IAA removal can relieve the inhibition of algal cell multiplication and chlorophyll degradation. IAA addition and coculture comparisons support this connection, while the exact catabolic pathway, necessity of particular metabolites and assay specificity remain unresolved.
Evidence
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PMID:38269098 - SUPPORT (IN_VITRO)"both microbes are necessary for auxin to be eliminated from the media in a cooperative fashion"
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PMID:38269098 - SUPPORT (IN_VITRO)"coculturing with exogenously added IAA resulted in higher cell density for both microbes compared with the respective monocultures"
IAA-Associated Algal Cell Division and Chlorophyll Responses
In the 2024 nitrogen-limited experiments, high IAA inhibited algal cell multiplication and chlorophyll degradation. In coculture with M. aquaticum, the tested IAA-associated growth and chlorophyll effects were absent. These measured responses do not establish a broad stress-tolerance or survival mechanism, nor do they imply bacterial IAA production.
Participating Taxa: Chlamydomonas reinhardtii, Methylobacterium aquaticum
Metabolites: indole-3-acetic acid (CHEBI:16411 · MediaIngredientMech · CMMMech)
Biological Processes:
- response to nitrogen starvation (GO:0006995 · PathwayMech)
- response to auxin (GO:0009733 · PathwayMech)
- interspecies interaction between organisms (GO:0044419 · PathwayMech)
Evidence
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PMID:38269098 - SUPPORT (IN_VITRO)"there was no inhibition of growth by IAA and no effect on chlorophyll content"
Knowledge gaps & discussions (1)
KNOWLEDGE_GAP OPEN Which algal factor enables IAA removal, and how much of the growth benefit requires that removal?
PMID:38269098 proposes photosynthate support but leaves the degradation pathway and alternative algal metabolites/proteins unresolved. Production HPLC/MS assays differ from Salkowski degradation assays. No detected ammonium in bacterial monoculture does not exclude coculture mineralization. Do not combine acetate-containing T-N/CC-5325 IAA tests with the 2018 CYB/proline/CC-1690 exchange assay. Non-graph origin, medium, hydroxyproline and taxonomy-role claims require separate correction.
Attaches to: ecological_interactions#Algal Glycerol Provision in Proline-Dependent Mutualism, ecological_interactions#Cooperative IAA Degradation, ecological_interactions#IAA-Associated Algal Cell Division and Chlorophyll Responses
Environmental Factors
| Factor | Value | Unit |
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| Nitrogen Limitation | Nitrogen-depleted conditions | N/A |
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| Light | Illuminated culture conditions | N/A |
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| Aquatic Environment | Freshwater medium | N/A |
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Growth Media
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