Vent fluid rising from the seafloor carries dissolved H₂S but essentially no oxygen; the surrounding deep water carries dissolved O₂ but no H₂S. Neither reactant alone can sustain sulfide-oxidizing chemosynthesis — the reaction only runs where turbulent entrainment mixes the two into the same parcel of water, so the habitable zone is a shell around the plume, not the plume core itself.
Each field is modelled as a turbulent buoyant plume with a Gaussian cross-section that widens with height:
σ(y) = σ₀ + k·y (plume spreading with height)
[H₂S](r,y) = [H₂S]v · e^(−y/L) · e^(−r²/2σ²) (decays outward & with height)
[O₂](r,y) = [O₂]sw · (1 − 0.95·e^(−r²/2σ²)) (depleted only inside the plume core)
Chemosynthesis rate follows dual-substrate Monod kinetics — the standard form for a bacterium that needs two co-limiting reactants (used for the real overall reaction 2H₂S + O₂ → 2S + 2H₂O, energy released and fixed as biomass via the Calvin cycle):
R(r,y) = Rmax · [H₂S]/(Km,S + [H₂S]) · [O₂]/(Km,O2 + [O₂])
This product of two saturating (Michaelis–Menten-shaped) terms is zero when either reactant is absent and peaks only where both are simultaneously present in moderate amounts — exactly the annular "goldilocks" shell the orange colonies occupy below. The bacterial mat biomass then relaxes toward that instantaneous rate with a first-order lag, dB/dt = growthRate·(R̄norm − B), so the visible colony grows or recedes smoothly as you change the chemistry rather than jumping instantly.
- Vent sulfide flux — how reducing the source fluid is; more H₂S pushes the inner (anoxic) boundary of the shell outward.
- Ambient oxygen — deep-water O₂ concentration; more O₂ pushes the outer (H₂S-starved) boundary of the shell outward, widening the habitable band.
- Turbulent mixing rate — how fast the plume entrains ambient water; faster mixing dilutes H₂S sooner but also delivers O₂ into the core sooner.
- Bacterial response rate — how quickly the mat's biomass tracks changes in the reaction rate (a real colony reproduces on a timescale of hours–days, not instantly).