Lighting — zooxanthellae, the Stokes shift and photoacclimation

Lighting deep dive — zooxanthellae, the Stokes shift and photoacclimation

Light is simultaneously an energy source, a threat, and a signal for a coral. The zooxanthellae symbiont converts photons into sugar, but the same process can produce reactive oxygen radicals that kill both the symbionts and the host cell. The coral has developed a complex system to balance these two needs — and this system explains everything about why bleaching and STN/RTN occur, why colours change, and why acclimation takes time.

This article covers the biological use of light — the zooxanthellae photosynthetic apparatus, saturation and inhibition points, the difference between PAR and PUR, the Stokes shift as a deep-water coral survival mechanism, the timeline and mechanism of photoacclimation, and the significance of DLI in nature and in the aquarium. The practical side — fixture selection, placement, photoperiod and acclimation protocol — is covered in the practice article.

1. The zooxanthellae photosynthetic apparatus

Zooxanthellae (dinoflagellates of the family Symbiodiniaceae) are the primary energy source for corals. They live within coral tissue as symbionts and produce glucose, glycerol and amino acids through photosynthesis, a significant portion of which — estimates range from 60–90% — is transferred directly to the host coral.

The photosynthetic pigments of zooxanthellae:

Chlorophyll a absorbs primarily at 430–450 nm (blue) and 620–680 nm (red) wavelengths, emitting in the 678–735 nm range. This is the main engine of photosynthesis.

Chlorophyll c₂ absorbs at 450–470 nm (blue) and in the 625–640 nm range. It supports chlorophyll a by expanding the absorption range.

Peridinin is a carotenoid pigment that absorbs at 478–550 nm (blue-green) and transfers energy to chlorophyll. Peridinin is an almost unique pigment to zooxanthellae and accounts for their yellowish-brown colour.

Xanthophylls — diatoxanthin, diadinoxanthin and β-carotene — absorb in the 449–500 nm range. Their role is twofold: they expand the absorption range for photosynthesis, but also act as photoprotection by dissipating excess energy.

This pigment profile explains why blue-violet wavelengths are critical in coral photosynthesis — but equally explains why pure 450 nm alone is insufficient. Chlorophyll c₂ and peridinin require a broader spectrum. Not all photons in the 400–700 nm range are equally valuable — the portion of PAR that is actually used in photosynthesis is PUR.

2. PAR vs. PUR — usable vs. measured radiation

PAR (Photosynthetically Active Radiation) measures all photons between 400–700 nm equally. It does not account for which wavelengths the coral’s photopigments actually absorb.

PUR (Photosynthetically Usable Radiation) is the portion of PAR that matches the active absorption peaks of the photopigments. The same PAR value can mean very different PUR values depending on the spectrum.

Example: Two fixtures, both at 200 µmol m⁻² s⁻¹ PAR. Fixture A is blue-dominant (main emission at 450 nm), fixture B is broad-spectrum (even distribution across 400–700 nm). The PUR ratio (PUR/PAR × 100%) might be 45% for fixture A and 65% for fixture B — even though PAR is identical. Fixture B produces 44% more usable radiation at the same PAR meter reading.

The ITC PARwice’s cPUR technology calculates the PUR:PAR ratio directly from the measured spectrum. This is one concrete reason why spectral analysis is useful beyond a simple PAR measurement.

3. Compensation point, saturation point and photoinhibition — the P/I curve

The coral’s photosynthetic response to light intensity is not linear. It follows an S-curve with three critical points.

Compensation point (Ic): The PAR level at which the energy produced by photosynthesis just covers the consumption of respiration — net energy balance is zero. Below this point the coral draws on its own reserves. For most aquarium corals this is 10–30 µmol m⁻² s⁻¹.

Saturation point (Ik): The PAR level at which net photosynthesis reaches its maximum. Additional light above this point does not increase the efficiency of photosynthesis — all extra photons must be dissipated by other means. For most reef coral zooxanthellae clades this is 150–300 µmol m⁻² s⁻¹, though significant variation exists between species and environments (Riddle 2004, How Much Light analysis).

Photoinhibition point: The light intensity at which excess energy begins to damage the photosystem. Net photosynthesis begins to decline even as light increases. The mechanism is reactive oxygen species (ROS) production — a biological “surge protector” that trips.

This P/I curve is the fundamental reason why “more light = more growth” is a flawed assumption. Riddle (2004) found in his analysis that 200–300 µmol m⁻² s⁻¹ is sufficient to saturate photosynthesis in most Hawaiian reef corals — and that higher intensity can slow growth or trigger STN/RTN or bleaching.

4. The xanthophyll cycle — natural excess energy management

When PAR exceeds the saturation point, zooxanthellae activate the xanthophyll cycle. Diadinoxanthin is converted to diatoxanthin in a reactive process that dissipates excess energy as heat rather than leading to ROS formation.

This is dynamic photoinhibition (Non-Photochemical Quenching, NPQ) — a correctable safety valve. Photosynthetic quantum efficiency decreases (the Fv/Fm value measurable by PAM fluorometry decreases), but the process recovers in darkness. If excess energy is sustained and the xanthophyll cycle is insufficient, the result is chronic photoinhibition and ultimately zooxanthellae expulsion — bleaching.

5. The Stokes shift — a deep-water coral’s light collector

In one of the most fascinating phenomena in light biology, a short wavelength is absorbed and a longer wavelength is emitted. This is called the Stokes shift.

Coral fluorescent proteins (GFP-type compounds) absorb short wavelengths — typically 400–500 nm blue or violet — and emit longer wavelengths: green (516 nm), orange or red. The emitted energy is always at a longer wavelength than the absorbed (energy loss in the Stokes shift).

Biological significance in deep water: In deep water the light spectrum is heavily weighted toward short wavelengths — blue and violet penetrate deep, while red and green are absorbed in shallow water. Deep-dwelling coral species, such as Leptoseris spp. (known from depths of up to 165 m), have developed a specialised system of GFP-type pigments that converts the blue light penetrating to depth into longer wavelengths — precisely those that chlorophyll a uses most efficiently in photosynthesis. The Stokes shift acts as a biological spectrum converter — a light harvester (Aslett 2024, Bollati et al. 2017).

Biological significance in shallow water: The same fluorescent proteins are used for the opposite purpose in shallow water. In high-intensity environments, GFP types dissipate excess energy through fluorescence — they act as photoprotection. The same molecule, two different roles depending on light intensity (Gittins et al. 2015).

This explains why colourful SPS coral species typically require higher PAR to express their colours — fluorescence is activated at full intensity near the saturation point, not below it.

6. Photoacclimation — mechanism and timeline

A coral is not a passive recipient of light. It actively regulates its zooxanthellae density and pigment concentration according to light intensity.

Two main mechanisms:

Zooxanthellae density regulation: In low light, the coral increases the number of zooxanthellae cells per unit of tissue — more light collectors. In high light, cell numbers decrease — fewer cells, lower photoinhibition susceptibility per cell.

Pigment concentration regulation: An individual zooxanthellae cell regulates the amount of chlorophyll and accessory pigments within itself. In low light, more chlorophyll per cell (darkening); in high light, less (lightening).

Timeline: Photoacclimation is not instantaneous. DiPerna et al. (2018) demonstrated that Pachyseris speciosa adapts to changing DLI in 3–5 days, but Acropora millepora adapts slowly — more than 20 days. This is a species-specific trait, not an exception.

Practical consequence: a new Acropora coral requires several weeks of acclimation, not a few days. Polyp extension on the following day alone does not indicate that acclimation has occurred.

7. DLI in nature vs. in the aquarium

On natural reefs in shallow zones (1–5 m), DLI is 30–42 mol m⁻² day⁻¹ (Tonk et al. 2014, based on Hill et al. 2012 data). This corresponds to approximately 800–1,200 µmol m⁻² s⁻¹ peak intensity on a sunny day in the tropics.

These figures look dramatic compared to aquarium values — and they are. But they should not lead to the conclusion that an aquarium should aim for the same levels.

Why aquarium readings are lower:

On a natural reef, light is not constant — clouds, waves, shading and diel variation create a dynamic DLI in which the peak lasts for hours, not the entire day. Brief instantaneous spikes (shimmer lines, water surface lens effect) can be up to three times the background intensity — but last milliseconds. In an aquarium, intensity is continuous and stable.

Additionally, aquariums typically house species originating from a range of depths and biotopes. A shallow-water Acropora humilis from 1 m depth and a deeper-dwelling Acropora carduus do not require the same DLI — and the hobbyist generally does not know the exact collection depth.

Practical DLI targets for the aquarium (repeated from the practice article for reference):

8. Colour temperature and spectrum — what corals actually use

Colour temperature (Kelvin) is the human eye’s perception of the “tone” of light — it describes what light looks like, not what wavelengths it contains. 20,000 K looks blue, 6,500 K white, 3,000 K warm yellow. Two different fixtures can have the same Kelvin rating but contain very different wavelength distributions.

What corals actually absorb:

Chlorophyll a needs 430–450 nm and 620–680 nm. Chlorophyll c₂ needs 450–470 nm. Peridinin needs 478–550 nm. Xanthophylls need 449–500 nm. This means photosynthetic pigments cover the entire visible spectrum — including green, yellow and red — even though absorption is stronger in the blue.

The “full spectrum” myth:

Many manufacturers market their products under the “full spectrum” label meaning something other than a biologically balanced spectrum. Luca (Quanta/Atlas LED) stated directly on the Real Reef Talk panel (2025): “Most of the competitors’ LED output is mainly in the visible spectrum — and many don’t even reach 720 nm. They call it full spectrum. It’s a great injustice to the animals.”

Practical test: measure the spectrum with an ITC PARwice. If you see a narrow peak around 450 nm with no significant green or red component, the fixture is blue-dominant regardless of marketing claims.

The role of red wavelengths:

Red light (620–700 nm) is absorbed rapidly in water — at just a few metres depth it is practically absent. In natural reef environments, deeper-dwelling coral species therefore do not “expect” red light. Some hobbyists avoid red entirely, others use it judiciously to support growth. Aslett (2024) notes that red colonies reflect red inefficiently but reflect blue and violet well — and blue-purple SPS species need the most light, because higher pigmentation requires fluorescent proteins that activate near photosaturation.

9. Light damage in the aquarium — STN, RTN and bleaching

In the aquarium, light-induced tissue damage manifests in two ways that are important to distinguish. They look superficially similar — tissue disappears, skeleton is exposed — but the mechanism, speed and treatment differ.

STN (Slow Tissue Necrosis) and RTN (Rapid Tissue Necrosis):

STN and RTN are tissue necrosis processes in which the animal tissue of coral polyps dies and detaches from the skeleton. Light overload is one triggering factor among other stressors — the mechanism runs through reactive oxygen species (ROS): superoxide (O₂⁻) and hydrogen peroxide (H₂O₂) damage Photosystem II (PSII) and the host cell’s mitochondria. When cellular respiration is disrupted, tissue enters an apoptotic programme — STN as a slower process (days to weeks), RTN more rapidly (hours to a day). Light-induced STN/RTN is rarer as a sole cause in the aquarium — most often it is a combination of excessively high PAR and other stressors (pH spike, mechanical damage, bacterial infection). The skeleton is exposed locally and necrosis advances linearly.

Bleaching — a different phenomenon:

Bleaching refers to the expulsion of zooxanthellae from the coral or the breakdown of zooxanthellae pigments — the host tissue remains alive. The result is a white or pale yellow coral that has lost its symbionts but is not dead. Bleaching is a reversible condition if the stressor is removed quickly enough — the coral can re-colonise with zooxanthellae within weeks. In the aquarium, a typical trigger is temperature rise combined with high PAR, not either factor alone.

The combined effect of temperature and light in bleaching:

Thermal stress significantly lowers the photoinhibition threshold — the same PAR level that is normally safe is sufficient to trigger bleaching in a coral that is simultaneously under thermal stress. This explains the timing of mass bleaching events on natural reefs — they do not result from high temperature or high light alone, but from the combination of both.

Skeletal reflection feedback loop (bleaching feedback loop):

When zooxanthellae density falls, the aragonite skeleton is partially exposed. White skeleton reflects light extremely efficiently — considerably more so than living tissue. This increases the light load on remaining zooxanthellae further, accelerating the process (skeletal light-scattering feedback loop, Enríquez et al. 2005). Bleaching is thus a self-reinforcing process once it has begun.

Summary — light biology through a practical lens

The coral is evolutionarily optimised to live in an environment of variable light — not under sustained high intensity. Its zooxanthellae partner has developed a complex system for light harvesting, photoprotection and spectral conversion. In the aquarium this system works when three conditions are met:

First, the spectrum is broad enough to cover the absorption ranges of all photopigments — 450 nm alone is insufficient.

Second, intensity is at or below the saturation point of the coral species — not continuously above it, which leads to STN/RTN risk or bleaching.

Third, DLI (daily dose) corresponds to the ecological origin of the coral species — a shallow-water species needs more than a deep-water species.

A PAR meter and spectral analysis are the only way to ensure that these three conditions are met.

References

Peer-reviewed studies

• DiPerna, S., Hoogenboom, M., Noonan, S. & Fabricius, K. (2018). Effects of variability in daily light integrals on the photophysiology of the corals Pachyseris speciosa and Acropora millepora. PLOS ONE, 13(9): e0203882.
• Gittins, J. R., D’Angelo, C., Oswald, F., Edwards, R. J. & Wiedenmann, J. (2015). Fluorescent protein-mediated colour polymorphism in reef corals: multicopy genes extend the adaptation/acclimatization potential to variable light environments. Molecular Ecology, 24, 453–465.
• Bollati, E., D’Angelo, C., Alderdice, R., Pratchett, M., Ziegler, M. & Wiedenmann, J. (2020). Optical feedback loop involving light-harvesting and photoprotective functions of fluorescent proteins resolves photoacclimation conflict in corals. Current Biology, 30, 1–10.
• Kuhl, M., Cohen, Y., Dalsgaard, T., Jørgensen, B. B. & Revsbech, N. P. (1995). Microenvironment and photosynthesis of zooxanthellae in scleractinian corals studied with microsensors for O₂, pH and light. Marine Ecology Progress Series, 117, 159–172.
• Enríquez, S., Méndez, E. R. & Iglesias-Prieto, R. (2005). Multiple scattering on coral skeletons enhances light absorption by symbiotic algae. Limnology and Oceanography, 50(4), 1025–1032.
• Tonk, L., Sampayo, E. M., Weeks, S., Magno-Canto, M. & Hoegh-Guldberg, O. (2013). Host-specific interactions with environmental factors shape the distribution of Symbiodinium across the Great Barrier Reef. PLOS ONE, 8(8): e68533.

Hobby literature and brand documentation

• Aslett, C. G. (2024). Teach a Person to Fish… SPS Academy Part XII. reefranch.co.uk.
• Riddle, D. (2004). How Much Light? Analyses of Selected Shallow-Water Invertebrates’ Light Requirements. Advanced Aquarist / CORAL Magazine.
• Real Reef Talk (2025). Lighting panel: Halides, LEDs, UV & Full Spectrum. Tulio (Reef Brite) × Luca (Quanta). 24-hour stream transcript.
• Joshi, S. (2005). Feature Article: Underwater Light Field and its Comparison to Metal Halide Lighting. Advanced Aquarist, August 2005.

Books and textbooks

• Delbeek, J. C. & Sprung, J. (2005). The Reef Aquarium: Science, Art, and Technology. Two Little Fishes, Miami Gardens.
• Kirk, J. T. O. (1994). Light and Photosynthesis in Aquatic Ecosystems. Cambridge University Press.
• Hill, R., Larkum, A., Prášil, O., Kramer, D., Szabó, M., Kumar, V. & Ralph, P. (2012). Light-induced dissociation of antenna complexes in the symbionts of scleractinian corals correlates with sensitivity to coral bleaching. Coral Reefs, 31(4), 963–975.