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Coral reefs look like colorful underwater gardens, but they are built by animals that farm tiny algae inside their tissues. This partnership powers the growth of entire reefs, feeds fish, and supports a huge web of life. If you keep a reef tank, the same teamwork between coral and algae is happening in your glass box, day and night. Understanding how it works helps you give your corals what they need to thrive.
This guide explains the coral–algae partnership in clear, beginner-friendly language. You will learn what each partner does, how light and nutrients drive the relationship, what causes bleaching, and how to set up and care for a home reef so the partnership stays strong.
What is the coral–algae partnership?
Most reef-building corals host microscopic algae inside their cells. These algae are dinoflagellates from the family Symbiodiniaceae, often called zooxanthellae. The coral and the algae live in symbiosis: both partners benefit and need each other under normal conditions.
The algae use sunlight to make sugars and other energy-rich molecules. They share much of that energy with the coral. In return, the coral gives the algae a safe home, carbon dioxide, and nutrients like nitrogen and phosphorus. With that steady energy and building materials, the coral can grow a skeleton of calcium carbonate and create a reef.
Scientists sometimes call the coral, the algae, and the coral’s microbes together the coral holobiont. Healthy reefs rely on a healthy holobiont, not just the coral animal alone.
Meet the partners: coral polyps
A coral is a colony of many small animals called polyps. Each polyp has a mouth and tentacles to catch food. Inside and around the polyp’s tissues is a special space where the algae live. Beneath the tissue, the coral builds a hard skeleton of calcium carbonate. As the colony grows and branches, it creates complex shapes that protect fish and other reef life.
Corals feed in two ways. They hunt tiny plankton with their tentacles, especially at night, and they “farm” their algae during the day. This mixed diet helps them survive when light or nutrients are limited.
Meet the partners: symbiotic algae
Symbiodiniaceae are tiny algae that live inside coral cells. There are many types with different strengths. For example, some Cladocopium types grow fast in stable, moderate temperatures, while some Durusdinium types handle heat better but may grow slower. The type of algae can influence color, growth, and stress resilience.
These algae are masters of photosynthesis in blue light, which penetrates seawater best. They turn carbon dioxide and water into sugars and oxygen using sunlight, then share the energy with the coral host.
How photosynthesis powers a reef
During the day, the algae absorb light and perform photosynthesis. They produce sugars, lipids, and other compounds. Corals use those products to fuel life processes: building tissue, running cells, and depositing skeleton. Without this energy supply, most reef-building corals cannot grow reefs, especially in the clear, nutrient-poor waters where reefs usually occur.
At night, photosynthesis stops. Both coral and algae still need energy, so they switch to using stored compounds and oxygen for respiration. This day-night rhythm drives changes in oxygen and pH inside the coral tissue and around the reef.
The nutrient exchange
The algae provide the coral with sugars such as glucose and glycerol, plus some amino acids and lipids. This “photosynthate” can supply a large share of the coral’s daily energy needs. In exchange, the coral gives back carbon dioxide from its own respiration, ammonium from its metabolism, and phosphate. The algae use the CO2 as a carbon source and the nitrogen and phosphorus to build proteins and DNA.
Because nutrients are scarce on reefs, the tight recycling inside the coral is key. Almost nothing goes to waste. The coral’s waste becomes algae food, and the algae’s excess energy feeds the coral.
Building the limestone city
Reef corals build their skeletons from calcium carbonate (aragonite). To do this, they need calcium ions and carbonate species from seawater. Inside the coral, special cells raise the pH and control ions in a thin space where the skeleton forms. Photosynthesis helps by using CO2 and pushing the chemistry toward more carbonate availability, making it easier to lay down aragonite.
This is why light can speed calcification. Strong, appropriate light encourages the algae to photosynthesize more, which supports faster skeleton growth and reef building, as long as other needs (alkalinity, calcium, magnesium, and nutrients) are met.
Daily rhythms and light
The partnership follows the sun. As light rises in the morning, photosynthesis increases. Around midday, in very bright light, algae can experience photoinhibition, where too much light can damage their photosystems. Corals have ways to manage this, like filtering light with pigments, producing antioxidants, or adjusting algae densities. In the evening, photosynthesis slows and respiration dominates.
In aquariums, a gentle ramp up and down in light intensity can mimic this natural cycle and reduce stress. Sudden light changes can shock the symbionts and trigger bleaching in sensitive corals.
Depth, water clarity, and spectrum
Blue light penetrates deepest in the ocean, so shallow corals can handle wider spectra, while deeper corals depend more on blue wavelengths. The algae’s light-harvesting pigments are tuned to blue-violet light around 420–470 nm. In tanks, blue-heavy spectra often support healthy photosynthesis and coral coloration.
Water clarity also matters. Clear water lets more UV and visible light through. Turbid water filters light and can protect corals from midday stress but reduces total energy. Matching light intensity and spectrum to the coral’s origin helps the partnership work smoothly.
Feeding still matters: coral heterotrophy
Even with good light, many corals benefit from catching food. Plankton and fine particles provide nitrogen, phosphorus, and trace elements that the algae alone cannot fully supply. Feeding has been shown to increase growth, improve color, and boost resilience to stress.
Corals use mucus to trap particles and their tentacles to grab prey. At night, many extend farther to feed when predators are less active and currents deliver more plankton. In tanks, targeted feeding after lights dim often works well.
Stress and bleaching
Bleaching happens when the partnership breaks down. Heat waves, strong light after cloudy periods, pollution, sediment, or sudden chemistry swings can lead to the buildup of reactive oxygen species in the algae. To protect themselves, corals expel the stressed algae or the algae die off. Without their symbionts, corals turn pale or white because their tissues are clear over white skeletons.
If stress eases soon, corals can take up new algae and recover. If stress lasts too long, they can starve, get diseased, or die. On natural reefs, prolonged temperatures just 1–2°C above normal for weeks can trigger mass bleaching.
Can corals recover?
Yes, many corals can recover if the environment stabilizes. Some take up more heat-tolerant algae types like Durusdinium. These can help during warm periods but sometimes slow growth when conditions are normal. Recovery speed depends on coral species, stress severity, and food availability. Feeding and steady water quality increase the chances of full recovery.
The wider holobiont: bacteria, viruses, and mucus
Corals host diverse bacteria and other microbes on their surfaces and inside their tissues. These microbes help cycle nutrients, produce vitamins, and fight pathogens. The coral’s mucus is both armor and pantry: it protects tissue, traps food, and supports helpful microbes that crowd out harmful ones.
When the partnership is stressed, the microbiome can shift toward harmful bacteria, increasing disease risk. Stable, clean conditions and balanced nutrients help keep the microbial team on the coral’s side.
Reproduction and symbiont transmission
Corals reproduce sexually by releasing eggs and sperm into the water or by brooding larvae inside the colony. Some species pass algae directly to their offspring (vertical transmission). Others produce algae-free larvae that pick up symbionts from the environment later (horizontal transmission). This difference affects how quickly young corals can settle and how flexible they are in choosing symbionts for changing conditions.
Why this matters for aquarists
In a reef tank, you are the weather, the water, and the nutrient supply. If you understand what the coral and algae need from each other, you can shape light, flow, nutrients, and chemistry to support the partnership. Strong symbiosis means stronger colors, faster growth, and better resistance to stress.
Lighting basics for reef tanks
A blue-heavy spectrum supports photosynthesis. Common ranges that work well include strong output around 420–470 nm. Aim for light intensity appropriate to your coral types, measured as PAR (photosynthetically active radiation). As a simple guide: soft corals often do well at 50–150 PAR, many LPS at 75–200 PAR, and many SPS at 200–350 PAR. Always acclimate corals by starting at lower intensity and ramping up over 2–4 weeks.
Set a photoperiod of around 8–10 hours of strong light with gentle ramping at the start and end. Avoid sudden major changes in spectrum or intensity, especially after water changes or cleaning that increases water clarity.
Water movement and gas exchange
Flow brings oxygen and nutrients to the coral and carries away waste and excess oxygen. It also keeps pH and carbonate chemistry steady around the coral surface. Random, turbulent flow is better than a constant blast. Many tanks aim for total flow rates of 10–40 times the tank volume per hour, adjusted for coral type and placement.
Good surface agitation helps oxygen levels and stabilizes the day-night swings in pH that come with photosynthesis and respiration.
Nutrients, not zeros
Ultra-low nutrients starve the symbionts and can fade color or trigger dinoflagellate pests. Too high nutrients fuel nuisance algae and can stress corals. For most mixed reefs, a practical target is nitrate around 2–10 ppm and phosphate around 0.02–0.08 ppm. Keep these levels steady rather than chasing exact numbers daily.
Feed fish and corals modestly and consistently. Export nutrients with a protein skimmer, refugium with macroalgae, or regular water changes. If levels are near zero, increase feeding, reduce aggressive filtration, or dose small amounts of nitrate or phosphate as needed, and monitor.
Chemistry for calcification
Corals need alkalinity, calcium, and magnesium to build skeleton. Balanced starting targets for many tanks are alkalinity 7–9 dKH, calcium 400–450 ppm, and magnesium 1280–1400 ppm. Keep pH stable in the 8.1–8.4 range. Stability matters more than perfect numbers.
As your corals grow, they use more. Plan for dosing via two-part solutions, kalkwasser, or a calcium reactor. Test regularly, adjust slowly, and avoid big swings in alkalinity, which can stress the algae and the coral.
Temperature and salinity stability
Most tropical corals are happiest around 24–26°C (75–79°F). Short spikes above 28–29°C (82–84°F) can trigger stress, especially if combined with intense light. Use a reliable heater and, in warm climates, consider a fan or chiller. A temperature controller adds safety.
Keep salinity steady around 35 ppt (specific gravity 1.025–1.026 at 25°C). Use an auto top-off (ATO) to replace evaporated freshwater and prevent salinity swings that can harm the partnership.
Feeding corals to support the symbiosis
Supplemental feeding helps, especially for LPS and many soft corals. Offer small zooplankton like rotifers or enriched brine for fine feeders, and slightly larger foods (0.5–1 mm) for big-polyp corals. Amino acid supplements can support tissue growth in low-nutrient systems, but dose lightly and observe.
Feed after lights dim or when you see tentacles extended. Turn off flow briefly to let corals capture food, then restore flow to keep oxygen up.
Preventing and responding to bleaching in tanks
Prevention starts with stability: steady temperature, consistent light, and balanced nutrients. Avoid rapid increases in PAR, especially after glass cleaning or swapping lights. Quarantine new corals to reduce disease and pest risks that can stress the holobiont.
If a coral pales quickly, reduce light intensity by 20–30% or shade it, maintain nutrients in the target range, run fresh activated carbon to remove irritants, and perform a moderate water change. Do not make multiple big changes at once. Feed lightly to help energy balance, and keep hands out of the tank to minimize additional stress.
Choosing and placing corals by symbionts
We seldom know the exact algae type in a coral at purchase, but general rules help. Many SPS corals from bright, shallow habitats prefer high light and strong, random flow. Many LPS enjoy moderate light and gentle to moderate flow. Soft corals often prefer lower to moderate light and can handle more nutrients.
Place new corals low and in shade, then move them upward gradually over weeks. Watch polyp extension, color, and growth as feedback on the partnership’s comfort.
Common myths and quick answers
Myth: Corals are plants. Reality: Corals are animals that host photosynthetic algae. They still need food and oxygen and can starve even under bright light.
Myth: Zero nutrients give the best color. Reality: Very low nutrients often fade colors, stress the symbionts, and invite pests. Balanced, low-to-moderate nutrients with good export are better.
Myth: More light is always better. Reality: Too much light can cause photoinhibition and bleaching. Match intensity to coral type and acclimate slowly.
Myth: High flow damages corals. Reality: Constant, laminar blasts can harm tissue, but random, turbulent flow helps gas exchange and nutrient delivery and supports healthy symbiosis.
A simple setup plan for beginners
Weeks 0–2: Set up tank, rock, and sand. Start the cycle. Dial in stable temperature and salinity. Choose lighting that can deliver your target PAR, and plan for controllable flow pumps.
Weeks 3–6: When ammonia and nitrite are zero, add a small clean-up crew and a hardy soft coral. Set photoperiod to about 9 hours with blue-heavy spectrum. Keep nitrate 2–10 ppm and phosphate 0.02–0.08 ppm. Test alkalinity, calcium, and magnesium weekly.
Weeks 6–10: Add a few beginner corals (softies or forgiving LPS). Target modest PAR (80–120) and moderate random flow. Feed fish and corals lightly but consistently. Begin small, regular water changes and adjust nutrients if drifting to zero.
Weeks 10+: Start adding more demanding corals if you want. Raise PAR slowly in steps of 10–15% with acclimation. Begin dosing alkalinity and calcium if tests show drops between water changes. Keep records and change one thing at a time.
Putting it all together: how the partnership works
The algae inside coral cells capture light and turn it into energy-rich compounds. They share those with the coral, which uses them to build tissue and skeleton. The coral returns CO2 and nutrients to the algae, and the loop keeps turning. Light, flow, and water chemistry shape how smoothly the loop runs. Feeding adds missing nutrients. Stability protects the partnership from stress.
In the ocean, this tight recycling lets corals flourish where nutrients are low but light is strong. In a tank, you recreate that balance with measured lighting, gentle but steady nutrient availability, and careful control of temperature and chemistry.
Signs the partnership is healthy
Good polyp extension, stable or improving color, visible growth margins on stony corals, and steady nutrient and alkalinity consumption are all positive signs. Daytime oxygen levels are robust, nighttime levels remain safe with adequate flow, and pH swings stay modest.
Watch for early warning signs: sudden paling, tissue recession, retracted polyps, or brown slime. These can point to lighting shock, nutrient imbalance, pests, or chemical irritants that threaten the partnership.
Advanced notes for the curious
Corals usually calcify from bicarbonate (HCO3-) rather than CO3 directly, converting it near the skeleton as pH rises in the calcifying fluid. Enzymes like carbonic anhydrase help shuttle carbon species. Photosynthesis removes CO2, nudging the carbonate system toward conditions that favor aragonite deposition. This is why daytime calcification often outpaces nighttime, though well-fed corals keep growing after dark too.
Different Symbiodiniaceae lineages bring trade-offs. Heat-tolerant algae can reduce bleaching risk but sometimes limit growth or alter color under normal conditions. Over time, colonies can shuffle which algae dominate depending on the environment, a form of acclimatization that can aid survival through changing seasons or heat events.
Conclusion
The partnership between algae and coral is a masterclass in cooperation. Algae turn sunlight into fuel; corals build cities from stone. Together they create reefs that feed and shelter a third of ocean life. In your reef tank, the same daily dance plays out. When you give the holobiont the right light, flow, nutrients, and stable chemistry, the symbiosis hums. Colors deepen, growth accelerates, and your corals weather challenges more easily.
Keep your approach simple and steady. Match light to your corals and acclimate slowly. Provide flow that moves water everywhere without blasting tissue. Aim for balanced nutrients instead of zeros, and keep alkalinity, calcium, and magnesium within stable ranges. Feed your reef modestly and consistently. With these habits, you support the algae–coral partnership at the heart of every thriving reef, from the tropics to your living room.