Coral Polyp Photosynthesis-Irradiance Curve

Environment

Irradiance (PAR) 800 µmol·m⁻²·s⁻¹

Water temperature 28.0 °C

Symbiont density 1.00×

Photosynthesis output

P–I curve (Platt model) — dot marks current state

O₂ production, P0.00 µmol·cm⁻²·h⁻¹

Carbon to host (CZAR)0 %

Calcification (light : dark)1.00×

How it works

Each coral polyp's gastrodermis hosts thousands of Symbiodinium (zooxanthellae) per cm². Their photosynthetic rate P follows the classic photosynthesis–irradiance (P–I) curve, using the Platt et al. (1980) formulation with photoinhibition:

P(I) = Ps·[1 − exp(−αI/Ps)]·exp(−βI/Ps)

α = initial slope (light-use efficiency)
β = photoinhibition parameter (rises with heat stress)
Ps = light-saturated capacity, Ps ≈ Pmax(T)
Pmax(T) = Pmax₀·exp(−(T−Topt)²/2w²)

At low irradiance, P rises almost linearly with light (slope α). Near the saturation irradiance Ek ≈ Pmax/α, extra photons stop helping. Push irradiance further and the exponential β term bends the curve back down — photoinhibition, the same photochemical damage to Photosystem II that, under sustained heat, leads toward bleaching.

Most of the O₂ and photosynthate (mainly glycerol, glucose and amino acids) is translocated from algae to host — the fraction is summarized as CZAR, the "contribution of zooxanthellae to animal respiration," commonly 20–90% of the coral's daily energy budget. The surplus carbon and the local pH rise from CO₂ drawdown both enhance calcification: reef corals typically deposit calcium carbonate 2–4× faster in the light than in the dark — the light-enhanced calcification (LEC) effect modeled here as a simple function of P.

Irradiance — PAR reaching the coral (0 at night, up to ~2000 µmol·m⁻²·s⁻¹ at a shallow midday reef).
Water temperature — shifts Pmax along a thermal-performance curve and steepens photoinhibition above the ~28 °C optimum, mirroring how heat stress lowers the photosynthetic ceiling.
Symbiont density — scales total flux per unit host tissue, as coral colonies vary in algal cell density.
Auto day cycle — sweeps irradiance through a sinusoidal daylight curve so you can watch P, CZAR and calcification track the sun.

Sunlight (photons) O₂ released Photosynthate to host Symbiodinium (algae)

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Coral Polyp Photosynthesis–Irradiance Curve