The world is a budget.Matter goes round, energy runs out.

The conserved quantities, their forms, and how they move. Three currencies: one that flows, two that cycle.

1.1 Conserved quantities

• Two conserved matter elements — carbon (C) and nitrogen (N). Each exists as inorganic (dissolved, in local fields) or organic (in biomass / dead matter).
• Total C and total N across the whole board are fixed. Nothing is minted or destroyed.
• Energy is a one-way flux, never a stored pool. In as light, degrades at every conversion, out as heat. Banked only as reduced (organic) carbon.
• Organic matter returns to inorganic by two paths: excretion — a living cell dumping the element it ate but can't use (§1.4) — and death. Death deposits the body's organic C/N into an organic detritus pool; only decomposition then returns it to inorganic. Death never mineralizes directly: a dead cell's carbon still holds its energy, so skipping the organic step would destroy that energy and starve decomposers.
• Division conserves. Daughter cells partition the parent's structure and reserve — their C, N and stored energy sum to the parent's. (When/how to divide is §3/§5; the conservation is §1's.)

1.2 Spatial modellocal fields + diffusion

Inorganic C and N sit in local per-cell fields, coupled by diffusion. The diffusion rate is a single dial — and global / well-mixed is just its high-diffusion limit, so we model one mechanism, not two.

Guardrail: no directional transport that can move matter permanently out of reach. Plain diffusion spreads regenerated nutrients back out — which is exactly what avoids the prior detritus-stranding failure.

1.3 Biomass: reserve, structure & stoichiometryreserve / structure

Biomass isn't one lump. Following dynamic-energy-budget theory, a cell is split into two compartments:

• Reserve — mobilizable stores of C and N (and energy, banked as reserve carbon). Costs no maintenance. This is where variable stoichiometry lives: reserve C:N is whatever's been taken up, so luxury uptake and lean-time draw-down both happen here.
• Structure — the built body, at a near-fixed structural C:N. It costs maintenance in proportion to its amount, and it can't be reclaimed — burning structure to survive is death.
• Growth = converting reserve → structure, gated by Liebig × Droop: limited by the scarcest of {energy, reserve C, reserve N} measured relative to a subsistence minimum, via a saturating (hyperbolic) response. Below the subsistence reserve level, growth stops and the cell starves.
• Reserve is bounded — capped as a density (reserve per unit structure), not an absolute amount. To bank more you must build more structure, which costs maintenance — so hoarding is never free, and there are no immortal dormant hoarders locking matter out of circulation. At the cap, surplus uptake / fixation overflows to the local field (conserved — real carbon exudation by nutrient-limited cells).

• Max size emerges — because uptake scales with surface area but maintenance with volume, growth slows to an asymptote where upkeep eats all income. This retires the old hard "max quota" constant; the ceiling is now physics.
• Homeostasis emerges — a cell's stoichiometric flexibility is its reserve fraction. Reserve-heavy cells (autotrophs banking carbon) are flexible; structure-heavy cells (consumers) are naturally more homeostatic. The producer/consumer split falls out — it isn't coded.

This is the keystone addition. One extra compartment buys three things at once: a size cap that's earned not imposed, a clean definition of starvation (burn reserve; die when structure can't be maintained), and the producer–consumer homeostasis gradient for free. It also adds a real size tradeoff — bigger cells pay more just to exist.

1.4 Energy & matter bookkeepingper-tick budget

Maintenance is paid first and scales with structural biomass — bigger cells owe more just to persist (metabolic theory). If the budget can't cover it, the cell draws down reserve; when reserve can't cover it either, it dies.

Why no separate energy pool: energy storage is carbon storage in real bioenergetics. Folding it into carbon removes a state variable, makes energy's one-way nature structural rather than policed, and lets light fund immediate work directly (no fix-then-burn tax). Energy↔carbon is a fixed exchange rate — a constant.

Excretion & remineralization closing the matter loop

A consumer that eats off-ratio food (say, carbon-rich prey) keeps the element it's short on and excretes the surplus straight back to the inorganic field. This is mandatory for conservation — the atoms must go somewhere — and it's also the main way consumers and decomposers hand the limiting nutrient back to producers. Consumer-driven nutrient recycling is a real feedback, not a side effect.

Inter-cell transfer the multicellularity substrate

When clonally-bonded cells share resources, the physics is fixed even though the decision to bond or share is an effector (§3):

• Matter is conserved in transit — C and N moved between cells are never lost. A matter leak would be the mirror of minting matter, so it's forbidden.
• The bond is structure — it costs matter to build and upkeep to hold (the §1.4 maintenance ∝ structure). This is the always-on tax on staying together.
• Active transport burns energy — moving resources to a neighbour costs a per-unit energy drawn from the §1.4 budget, dissipated as heat. The loss lives in the energy currency, never in matter.

This is all §1 owes multicellularity: conservative transfer with a real cost. The adhesion and sharing effectors, and the benefits that make colonies pay (a predation size-refuge, division of labour), are §3/§4 — and they must stay emergent, never coded as a bonus.

1.5 Scope & deliberate omissions

• Energy sources: light or eaten organic carbon only. Chemoautotrophy (energy from chemical gradients) is out of scope — addable later as an energy-input effector, not a stored pool.
• No oxygen / redox. Real aquatic systems track O₂ too (photosynthesis makes it, respiration and decomposition consume it), which creates anoxic zones and redox-gradient niches. We model only C, N and energy — so those niches simply don't exist here. A clean, conscious omission, not an oversight.
• Transport is diffusion only — no directional currents/advection (consistent with the §1.2 anti-stranding guardrail).
• N is a representative second element. The biology would read the same with P (often the freshwater limiter); we picked one to keep it at two elements.

1.6 The emergent payoff — and its limita result, not a coded rule

Because energy is stored carbon, one genuine axis falls out of the mechanism: autotrophy. Autotroph → mixotroph → heterotroph is a real, monotonic gradient — the fraction of your carbon you fix yourself vs. acquire pre-made. (It collapses to a single axis only because we scoped out chemoautotrophy in §1.5; in full microbiology, carbon source and energy source are orthogonal.)

But that axis does not linearise the guilds. Predator, grazer and decomposer are all full heterotrophs — they sit at the same point on the autotrophy axis. What separates them is a second, orthogonal axis: the source / state of the organic carbon they eat — living autotroph (graze), living cell (predate), or dead matter (decompose).

So guild identity = (axis A position) × (axis B band), and the decomposer band exists because death now deposits a distinct organic detritus pool (§1.1). Both axes still emerge — neither is a hard-coded guild label; we classify after the fact, we don't build guilds in.

1.7 Open threads handed forward