SCIENCE · INVESTIGATE AND EXPLAIN

Close carbon across a donor and a receiving partner

Cross-feeding occurs when material released by one population becomes available to another. A metabolic product can be another organism’s substrate, but a shared metabolite does not guarantee mutual benefit. The donor, receiver and whole-community boundaries must be distinguished, as must atoms, molecules and energy.

Learning outcome: Draw donor, receiver and combined boundaries. Reconstruct all carbon terms for two allocations, then identify which energy or mechanism claims cannot be obtained from an atom ledger alone.

Starting recap

Distinguish community composition, observation bias and process contribution; conserve explicit material currencies and bound causal or useful-application claims.

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Try

With I=24 mol C and donor fraction 0.5, donor biomass is 12, donor carbon dioxide 6 and intermediate 6. The receiver produces 2.4 biomass C and 2.4 carbon dioxide C, leaving 1.2. Final carbon 12+2.4+8.4+1.2=24; the internal transfer 6 is not another final output.

Open the experiment

Predict

Why does counting an internal transfer as an additional output break a whole-community balance?

Your prediction and reason

Experiment

Draw donor, receiver and combined boundaries. Reconstruct all carbon terms for two allocations, then identify which energy or mechanism claims cannot be obtained from an atom ledger alone.

Close a cross-feeding carbon budget across two partners

Worked teaching model · synthetic values · Original atom-basis budget: donor retains fraction b as biomass; half remainder goes to CO2 and half to extracellular metabolite carbon. Receiver assimilates40% of that metabolite carbon, respires40%, leaves20%. No molecular/energy yield inferred.

Donor carbon input: 24 mol C Donor biomass allocation: 0.5 fraction

Reset keeps your prediction, captures and practice history.

The intermediate flux is internal to the two-partner boundary and must not be counted twice as a final output.

Original atom-basis budget: donor retains fraction b as biomass; half remainder goes to CO2 and half to extracellular metabolite carbon. Receiver assimilates40% of that metabolite carbon, respires40%, leaves20%. No molecular/energy yield inferred.

Original model quantities, not actual field or treatment results
Quantity	Value	Unit
Input carbon	24	mol C
Donor biomass carbon	12	mol C
Intermediate carbon transferred	6	mol C
Receiver biomass carbon	2.400000	mol C
Total carbon dioxide carbon	8.400000	mol C
Unconsumed intermediate carbon	1.200000	mol C
Final carbon total	24.00000	mol C

No biological handling, cultivation, process operation, field sampling or safe-effluent determination is requested.

The intermediate flux is internal to the two-partner boundary and must not be counted twice as a final output.

Observation to attach to a capture

Captures preserve recipes and replay their calculated results. They are synthetic model evidence, not physical measurements.

Understand

Define the community evidence

Connect mechanism and boundary

The original record supplies carbon amount I to a donor. Fraction f enters donor biomass. Half of the remaining carbon becomes carbon dioxide and half becomes an extracellular intermediate. The receiver converts 40% of that intermediate carbon to biomass,40% to carbon dioxide and leaves 20% unconsumed. All amounts are moles of carbon atoms, not an unspecified number of metabolite molecules.

For the two-partner boundary, intermediate transfer is internal. Final outputs are donor biomass, receiver biomass, total carbon dioxide carbon and residual intermediate carbon. Adding the transferred intermediate again would double-count carbon already allocated to the receiver’s outputs. The ledger does not determine energy capture, redox-carrier balance or a universal biomass yield.

Real cross-feeding may depend on spatial access, transport, thermodynamic feasibility and other community members. A carbon-closed record is necessary but not sufficient evidence of a viable ecological exchange. The comparison provides no fermentation setup, organism pairing or biological optimization method.

Reconstruct the original record

Predict and investigate

Predict first: Why does counting an internal transfer as an additional output break a whole-community balance?

Draw donor, receiver and combined boundaries. Reconstruct all carbon terms for two allocations, then identify which energy or mechanism claims cannot be obtained from an atom ledger alone.

Defend a changed case

Apply the mechanism to a changed boundary

Use I=12 mol C and donor fraction 0.25. Find all final outputs.

Retain the first response, units, uncertainty and any prior feedback exposure in the notebook before comparing this public explanation.

Compare a worked response

Donor biomass 3; remainder 9 gives donor carbon dioxide 4.5 and intermediate 4.5. Receiver biomass 1.8, receiver carbon dioxide 1.8 and residual 0.9 yield final 3+1.8+6.3+0.9=12 mol C.

Retain bounded evidence

Preserve the original record, first reasoning, units, sources and feedback exposure in the Microbiology evidence workspace. All numerical records are original fictional evidence, not measured field or treatment performance. No culture, process setup, biological handling or safety certification is requested.

Primary mechanism context, checked 31 October 2026. Original teaching and numerical examples; no source operating procedures reproduced.

Apply

Public practice checks a specific claim. It does not establish independent mastery.

Use I=12 mol C and donor fraction 0.25. Find all final outputs.

Your explanation and limitations

Continue

Completion requires a saved prediction, at least one capture and a reflection. Presence checks do not grade your reasoning.

Skipping retains experiments and awards no mastery.

Previous: Separate a community from its observation process Next: Track nitrogen species without losing nitrogen atoms

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