A synthetic consortium built by co-culturing the marine cyanobacterium Synechococcus sp. PCC 7002 (Syn7002) — a model strain that lacks the gene cluster for vitamin B12 biosynthesis and therefore depends on other microbes for it — with a bloom-forming Microcystis community from Lake Taihu, then purifying the result. Metagenomics showed the early community consisted mainly of Syn7002, Mesorhizobium sp. TaiHu (MesTH) and Pseudomonas sp. TaiHu (PseTH); PseTH declined once the community stabilised, leaving the cyanobacterium and MesTH as the persistent pair. Electron microscopy found rod-shaped bacteria clustered around Syn7002 cells. Metatranscriptomics showed vitamin B12 biosynthesis and transport genes significantly upregulated in MesTH, and B12-positive control experiments supported complementarity between the two strains, so the community is a worked example of a vitamin auxotrophy being satisfied by a partner rather than by the medium.
Taxonomy
| Taxon | Ontology ID | Functional Roles | Abundance |
|---|---|---|---|
| Synechococcus sp. PCC 7002 (Syn7002) — ontology label: Picosynechococcus sp. PCC 7002 | NCBITaxon:32049 · TaxonMech · TraitMech |
PRIMARY_PRODUCER
|
N/A |
|
|||
| Mesorhizobium sp. TaiHu (MesTH) — ontology label: Mesorhizobium | NCBITaxon:68287 · TaxonMech · TraitMech |
CROSS_FEEDER
|
N/A |
|
|||
| Pseudomonas sp. TaiHu (PseTH) — ontology label: Pseudomonas | NCBITaxon:286 · TaxonMech · TraitMech | N/A | |
|
|||
| bloom-forming Microcystis community (source inoculum) — ontology label: Microcystis <cyanobacteria> | NCBITaxon:1125 · TaxonMech · TraitMech | N/A | |
|
|||
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.
Microcystis Disappearance During Salinity Passaging
Increasing salinity during Syn7002 co-cultivation with a Lake Taihu bloom inoculum was accompanied by disappearance of Microcystis and persistence of associated epibionts. This is a treatment-associated compositional result, not proof of niche partitioning, an exclusive salinity mechanism or the cause of later B12 complementarity. Microcystis is the source inoculum, not a confirmed member of the final pair.
Participating Taxa: Synechococcus sp. PCC 7002 (Syn7002), bloom-forming Microcystis community (source inoculum)
Evidence
-
PMID:41790499 - SUPPORT (IN_VITRO)"Syn7002 was co-cultured with algal sediment collected from a cyanobacterial bloom in Lake TaiHu "
-
PMID:41790499 - SUPPORT (IN_VITRO)"the salinity gradient was gradually increased, leading to the disappearance of Microcystis "
Syn7002-MesTH-PseTH Community Sorting
Across successive streak-purification rounds, Syn7002, MesTH and PseTH were the major early components; relative PseTH abundance declined and MesTH became dominant after the second round. This preserves compositional stabilization without asserting PseTH extinction, competitive exclusion, absolute viable growth or B12 availability as the isolated cause. Purification and changing conditions are part of the observation, not an ecological arrow to later assays.
Participating Taxa: Synechococcus sp. PCC 7002 (Syn7002), Mesorhizobium sp. TaiHu (MesTH), Pseudomonas sp. TaiHu (PseTH)
Evidence
-
PMID:41790499 - SUPPORT (IN_VITRO)"Syn7002, PseTH and MesTH constituted the major components of the community"
-
PMID:41790499 - SUPPORT (IN_VITRO)"MesTH became the dominant component after the second round"
Vitamin B12 Provision from Mesorhizobium TaiHu to Synechococcus PCC 7002
CROSS_FEEDINGHYPOTHESIZED - MesTH supplies B12 needed by auxotrophic Syn7002 in the washed, B12-free BG11 coculture. Positive growth rescue, B12-supplemented controls and genomic/transcript evidence support this proposed cross-feeding. Targeted LC-MS/MS did not detect free B12 in day-20 pelleted cultures; that is not an extracellular assay, proof of rapid utilization or demonstration of donor-to-recipient transfer. Delivery by release versus contact and the necessity of MesTH B12 biosynthesis were not selectively resolved.
Source Taxon: Mesorhizobium sp. TaiHu (MesTH)
Target Taxon: Synechococcus sp. PCC 7002 (Syn7002)
Metabolites: vitamin B12 (CHEBI:176843 · MediaIngredientMech · CMMMech)
Biological Processes:
- cobalamin biosynthetic process (GO:0009236 · PathwayMech)
HYPOTHESIZED - B12 supplied by MesTH could relieve the auxotroph limitation and explain positive Syn7002 growth rescue. Coculture and added-B12 controls support this mechanism without directly measuring vitamin delivery or selectively disrupting the donor pathway.
Evidence
-
PMID:41790499 - PARTIAL (IN_VITRO)"The results further suggest that MesTH promotes the growth of Syn7002 in the community by providing the small amount of vitamin B12 needed for its growth"
Close Spatial Association of Partner Bacteria with Syn7002 Cells
SEM showed rod-shaped bacteria clustered around Syn7002 in fixed, dehydrated coculture material. MesTH morphology is compatible with the rods, but morphology alone does not independently resolve every cell identity. Close association is a positive observation, not experimentally demonstrated colonization facilitation, mandatory contact or vitamin transfer.
Participating Taxa: Synechococcus sp. PCC 7002 (Syn7002)
HYPOTHESIZED - proximity could facilitate local B12 availability or contact-associated delivery. The source leaves extracellular release versus direct contact unresolved; microscopy does not establish that proximity is required or sufficient for transfer.
Evidence
-
PMID:41790499 - SUPPORT (IN_VITRO)"Electron microscopy revealed numerous rod-shaped bacteria clustered around Syn7002 cells, indicating close spatial associations between species"
MesTH Vitamin B12 Biosynthesis and Transport Response
MesTH showed B12 biosynthesis/transport-associated transcript changes during community formation, while Syn7002 had lower reported transporter expression. Figure 4 presents log2 fold changes relative to the initial stage; these are not absolute cross-species vitamin-output measurements or a matched coculture-versus-monoculture induction test. Transcription supports the proposed B12 role, not secretion, uptake flux or pathway necessity. Nitrogen-related expression is likewise not direct nitrogen fixation or assimilation.
Participating Taxa: Synechococcus sp. PCC 7002 (Syn7002), Mesorhizobium sp. TaiHu (MesTH)
Metabolites: vitamin B12 (CHEBI:176843 · MediaIngredientMech · CMMMech)
Biological Processes:
- cobalamin biosynthetic process (GO:0009236 · PathwayMech)
HYPOTHESIZED - MesTH biosynthetic and transport capacity could support B12 availability to Syn7002. Transcript profiles are consistent with this proposal but do not establish that their changes drive secretion or donor-to-recipient transfer.
Evidence
-
PMID:41790499 - PARTIAL (IN_VITRO)"vitamin B12 biosynthesis and transport-related genes were more highly expressed in MesTH compared to Syn7002 "
MesTH Growth Rescue of B12-Limited Syn7002
Adding washed MesTH to washed Syn7002 in B12-free BG11 promoted the cyanobacterial growth endpoint relative to the unsupplemented alga-only condition; supplied B12 was the positive control. Growth was followed by absorbance and culture appearance over 20 days. Preserve the positive rescue without claiming directly measured photosynthetic flux, reciprocal benefit, universal long-term survival or that B12 is the sole mediator. Washing does not by itself quantify residual or intracellular vitamin stores.
Participating Taxa: Synechococcus sp. PCC 7002 (Syn7002), Mesorhizobium sp. TaiHu (MesTH)
Metabolites: vitamin B12 (CHEBI:176843 · MediaIngredientMech · CMMMech)
Evidence
-
PMID:41790499 - SUPPORT (IN_VITRO)"the presence of MesTH promoted the growth of Syn7002 compared to the group without vitamin B12 supplementation "
Knowledge gaps & discussions (1)
KNOWLEDGE_GAP OPEN Does MesTH-derived B12 mediate rescue, and does spatial association contribute?
All six observations remain. Remove salinity-to-sorting and sorting-to-microscopy/transcript arrows because they describe assembly or analysis order. Retain three source-consistent B12 links as hypotheses. Pellet non-detection does not prove rapid uptake; separate extracellular assays, donor perturbation/rescue and contact separation would test delivery. PseTH decline is not extinction or isolated competition; nitrogen transcripts are not functional fixation. Preserve B12-free and added-B12 controls, canonical genus groundings and #583 guidance. #1597 repairs graph claims; non-graph vitamin certainty and PseTH/source wording remain #1600. Complete primary main text/captions read, not separate Table S1, images or raw data.
Attaches to: ecological_interactions#Microcystis Disappearance During Salinity Passaging, ecological_interactions#Syn7002-MesTH-PseTH Community Sorting, ecological_interactions#Vitamin B12 Provision from Mesorhizobium TaiHu to Synechococcus PCC 7002, ecological_interactions#Close Spatial Association of Partner Bacteria with Syn7002 Cells, ecological_interactions#MesTH Vitamin B12 Biosynthesis and Transport Response, ecological_interactions#MesTH Growth Rescue of B12-Limited Syn7002
Environmental Factors
| Factor | Value | Unit |
|---|---|---|
| Vitamin B12-positive control supplementation | N/A | |
|
||
Growth Media
Cross-Mech Suite Links
Explore related entities across the CultureBotAI X-Mech knowledge bases: