# How corals build tissue and color **URL:** \ **Category:** Kenny Lin\ **Created:** [September 8, 2026, 9:10pm UTC](https://community.reefnbid.com/t/how-corals-build-tissue-and-color/252 "2026-09-08T21:10:19Z")\ **Posts on this page:** 1\ **Showing post:** 1
**Author:** ![KennyL](https://yyz2.discourse-cdn.com/flex028/user_avatar/community.reefnbid.com/kennyl/32/91_2.png) [@KennyL](https://community.reefnbid.com/u/KennyL)\ **Post date:** [September 8, 2026, 9:10pm UTC](https://community.reefnbid.com/t/how-corals-build-tissue-and-color/252/1 "2026-09-08T21:10:19Z")
We focus a lot of our attention on coral’s skeleton building process. But that is just half of the battle. The part we actually look at, the appearance and colors, is in the living tissue. This is considered a whole different task. It uses different materials, it can run at a different speed than the skeleton. A tank can have perfect alkalinity and colors still look off. Let me preface that I’m not a scientist. This is a hobby-grade interpretation of published work combined with my personal experience. Details vary by species. Some of this is still being worked out. The broad picture is solid enough to use. ### Two tasks, two sets of materials Skeleton is a mineral task. It consumes calcium, carbonate, and energy. A little magnesium and strontium ride along in the crystal. Tissue is a protein and membrane task. A living cell is built from: - Carbon, the backbone - Nitrogen, about 13% of a cell by dry weight, and the key element in proteins and in DNA - Phosphorus, for DNA, ATP, and cell membranes - Sulfur, for some amino acids - Potassium, magnesium, sodium, for enzymes and cell stability Nothing listed is utilized by the skeleton. You need the right tools for the job, or in this case, the right ingredients for the final product. ### Sugar is a fuel, not the ingredients The algae inside the coral, the zooxanthellae, cannot live well on their own in open ocean water. Nutrients out in the vast ocean are scarce. The coral feeds them with its own waste. In return, once the algae have fed themselves, they hand sugars back to the host. Up to 30% of a polyp’s tissue can be algae material. Those sugars are carbon, hydrogen, and oxygen. They are calories, not nitrogen, phosphorus. nor sulfur. A coral living on light alone can have plenty of fuel and still be short of building material. Every nitrogen, phosphorus, and sulfur atom in that animal come externally: dissolved nitrate and phosphate from the water, or food the coral catches. ### Color is a protein, so color costs nitrogen The colors in corals that we chase are fluorescent proteins. Proteins are chains of amino acids. Amino acids contain nitrogen. Some also contain sulfur. Here’s what we need to remember: no nitrogen, no pigment. Under [blue light](https://community.reefnbid.com/t/what-blue-light-actually-does-to-corals/163), coral receives the signal to turn the pigment genes on, and turns them down when the signal fades. A signal only produces a protein if the materials are there. These are the two conditions and both have to be true: - Blue light acting as the signal. - Nitrogen acting as the material. A coral in a nitrogen-poor tank under heavy blue gets the signal but has nothing to build with. A coral sitting in high nitrate under weak or the wrong light has the materials but no signal to start. The question “what light are you using use to get that color?” only looks at half the picture when someone posts a colored up coral. Also, a coral does not make pigment at one fixed rate. It turns the amount up or down. Eyal and colleagues studied corals at 50 to 60 meters in the Red Sea, where very little light reaches. Those corals carry extra copies of the pigment genes, and they control how many copies stay on. More copies on, more pigment. Some species turned them down when they were kept in the dark. The authors described heavy pigmentation as expensive. In dim conditions, a coral is often better off staying pale and spending those resources on something else. So remember, color costs nitrogen. Light tells the coral whether to make pigment, but only if nitrogen is available. ### What you see is two layers The color you see from the coral is the sum of two things. One is the golden brown of the zooxanthellae. The other is the coral’s own fluorescent proteins and chromoproteins. Both are built from the same nitrogen and phosphorus. Raise nutrients and both go up. Past a point the algae multiply faster than the host pigment, and the coral browns out. The pigment did not go away. It is buried under algae. Strip nutrients away and you can starve both. The algae is gone and so is the pigment. The coral appears pale and bleached. It’s a seesaw that you’re fighting with to find balance all the time. This is why I like a real fish load and good feeding more than a tank polished down to zero, or supplement by dosing. If you run your tank with nutrient in mind after having invested in the proper equipments to filter it, the tank will tell you if it’s enough. Excess nutrients are usually easy to address with water changes, a bigger skimmer, a refugium, or carbon dosing. In today’s hobby with its equipments, it is often easier to overdo it when it comes to filtering your tank. ### The impact of phosphate is shared between skeleton and tissue Phosphate can be incorporated into growing skeleton and get in the way of a clean crystal, slowing down calcification. Some of it ends up in the skeleton as defects. To understand how this happens, we go back to how bicarbonate and carbonate come together to make skeleton. Calcium has a positive charge and carbonate has a negative charge, being opposite they attract, lock together and build the crystal. Phosphate is a strong negative ion. On a growing skeleton there are open calcium sites waiting for the next carbonate. Phosphate can sit on those same sites. Carbonate needs those same spots to keep the crystal growing. While phosphate is sitting there, the next carbonate cannot dock cleanly. Some of it gets buried as the stone grows, which is how phosphate ends up in the skeleton as a defect. When high phosphate affects skeleton growth, it causes branches to become denser rather to longer, so it appears as if the growth has stopped. It has not, but just slowed and growth goes elsewhere such as the bottom. If you keep dosing calcium and alkalinity at the old rate while that is happening, the unused alkalinity stays in the water, causing your alk test to read higher. On the flip side, elevated alkalinity and pH cause phosphate to read lower on your test kit. With high pH, calcium carbonate starts to precipitate on surfaces such as heaters, pumps, and rock. Those growing surfaces also bind phosphate and pull it out of the water. While elevated phosphate is unsuitable in coral calcification, it is still a required nutrient for tissue growth. It is part of the building block for membranes, DNA, and ATP. Bottoming it out is not what we want here. You want enough for the living layer, and not so much that the skeleton gets crowded with unwanted defects from phosphate. One way I tell if phosphate may be getting high enough to interfere with skeleton growth is by looking at the growth branches, as those tips often round off, and the pale growth tips darken or disappear. Calcification is still happening, as the branch grows thicker under the tip. Skeleton is growing, but it is not shooting out new branches the way it used to. The branch is just getting denser instead of gaining growth tips. When we manage phosphate, the goal should be for the corals, not around the [nuisance algae](https://community.reefnbid.com/t/how-i-manage-algae/137). Once the cleanup crew and the refugium are doing their job, phosphate stops being an algae lever for me. It is a calcification lever. ### The NP ratio Buckingham and colleagues ran _Acropora polystoma_ through four combinations of nitrogen (N) and phosphorus (P). - High N, high P: Algae stayed high. Growth tips or extension as they call it in the paper, and calcification were far faster than the other groups. The skeleton came out most porous. Fast, but brown and fragile. - High N, low P: The corals bleached. Phosphorus starvation. Growth tanked and the skeleton thickened. - Low N, high P: In between. They held symbionts better than the high-N low-P group, but growth dropped and the skeleton thickened. - Low N, low P: Bleaching again, the slowest growth, and the densest skeleton. An off-balance ratio did much the same thing to the skeleton as starvation. High nitrate with phosphate stripped out is a common hobby predicament. So we dose nitrate to compensate, while letting a skimmer or GFO pull phosphate down, then decide nutrients are fine because nitrate looks good. The two have to move together. What about the Redfield ratio that’s often looked at as a target? It’s a misleading concept that has been perpetuated in the hobby. Any ratio at most can tell you something is off balance, but it shouldn’t be treated as a target. I’ve always maintained that each system is as unique as a fingerprint. Having a target ratio ignores all the other factors that make a system successful. Redfield ratio = atoms in plankton ≠ ppm in tank water ≠ a number you should chase. In truth, Redfield ratio is the average count in atoms of the mix of carbon, nitrogen, and phosphorus in ocean plankton. It tells us what cells are made of. It has nothing to do with how much N and P a coral wants in our water. Buckingham’s paper which is often cited for the application of the ratio really shows us that the coral suffers when one ingredient is missing. We should not treat 16:1 as a tank setting. Convert it to the numbers on a hobby kit which measures by weight, 16:1 atoms is about 10:1 nitrate to phosphate by weight. Most established tanks that we run, say 5 to 10 nitrate with 0.05 to 0.10 phosphate, are far richer in nitrogen than that. Corals also take a lot of their nitrogen and phosphorus as food, which no test kit sees. Natural reef water can test even lower. Papers usually report those numbers in micromoles per liter, which is a molecule count. Converted to a hobby kit, that is on the order of 0.01–0.02 ppm nitrate and 0.005–0.01 ppm phosphate. On a hobby-grade test kit that is only about 2:1, against the 10:1 that Redfield converts to. The actual N and P in the water simply do not adhere to the Redfield ratio. So I would not chase 16:1 or 10:1, or any ratio for that matter. Watch for trends, go for a range, and not a number. Your tank will tell you what is too low or too high, based on the observations laid out above. ### Two things worth taking away. The first is balance. Skeleton and tissue run on different materials and both have to be managed. Nitrate and phosphate also have to move together, since one running high while the other is stripped out is its own problem. Aim for a range, watch the trend, and do not chase a number. Having a healthy population of bioload and adequately feeding your fish can deliver balanced package of nitrogen, phosphorus, and sulfur to the tissue building task that needs them. The [blue-light-and-feeding](https://community.reefnbid.com/t/what-blue-light-actually-does-to-corals/163/2) work found _Stylophora_ put on the most extension when stronger blue was paired with more food. This is signal and materials working together in balance. The second is that phosphate is playing both sides. It’s required by tissue building for membranes, DNA, and ATP. Too much and it becomes a detriment to skeletal growth, because it gets in the way of the crystal formation. This is why at POTO, we only regularly test two numbers, alkalinity, ranging from 6.5-7.5, and phosphate, ranging from 0.05-0.15. ### References **[Eyal, G., Wiedenmann, J., Grinblat, M., D’Angelo, C., Kramarsky-Winter, E., Treibitz, T., Ben-Zvi, O., Shaked, Y., Smith, T.B., Harii, S., Denis, V., Noyes, T., Tamir, R. and Loya, Y. (2015). Spectral diversity and regulation of coral fluorescence in a mesophotic reef habitat in the Red Sea.](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0128697)** _PLoS ONE_ 10(6): e0128697. A fluorescence survey of a deep reef off Eilat, paired with darkness experiments. Source for corals still brightly fluorescent at 50 to 60 meters, for the pigment genes existing in multiple copies with the number switched on setting how much pigment the coral makes, for some species losing or altering their pigment in darkness while others keep it regardless, and for the description of heavy pigmentation as expensive. **[Buckingham, M.C., D’Angelo, C., Chalk, T.B. et al. (2022). Impact of nitrogen (N) and phosphorus (P) enrichment and skewed N:P stoichiometry on the skeletal formation and microstructure of symbiotic reef corals.](https://pmc.ncbi.nlm.nih.gov/articles/PMC10276130/)** _Coral Reefs_ 41: 1147-1159. The four-way nitrogen and phosphorus experiment on _Acropora polystoma_. Source for every one of the four combinations listed above: the far faster extension and most porous skeleton under high N and high P, the bleaching and phosphorus starvation under high N and low P, the thickened skeleton when growth was inhibited, and the densest skeleton of all under full starvation. Also the conclusion that an off-balance ratio does much the same thing to the skeleton as outright limitation. **[Wiedenmann, J., D’Angelo, C., Smith, E.G., Hunt, A.N., Legiret, F.-E., Postle, A.D. and Achterberg, E.P. (2013). Nutrient enrichment can increase the susceptibility of reef corals to bleaching.](https://www.nature.com/articles/nclimate1661)** _Nature Climate Change_ 3: 160-164. Source for why high nitrogen with low phosphate is the damaging pairing rather than just an unbalanced one. The imbalance starves the symbiotic algae of phosphorus, which leaves the coral more likely to bleach under heat and light. **[Simkiss, K. (1964). Phosphates as crystal poisons of calcification.](https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-185X.1964.tb01166.x)** _Biological Reviews_ 39: 487-504. The original description of phosphate settling onto a growing crystal surface and interfering with the lattice, which is where the term crystal poisoning comes from. **[de Kanel, J. and Morse, J.W. (1978). The chemistry of orthophosphate uptake from seawater on to calcite and aragonite.](https://www.sciencedirect.com/science/article/abs/pii/0016703778900388)** _Geochimica et Cosmochimica Acta_ 42: 1335-1340. Laboratory work on how phosphate binds to calcite and aragonite in seawater. Source for the mechanism described above: calcium sitting at carbonate sites on the crystal surface is what phosphate binds to, which is why the open calcium sites waiting for the next carbonate are the same sites phosphate can occupy. **[Phosphate bound to calcareous sediments hampers skeletal development of juvenile coral (2021).](https://royalsocietypublishing.org/rsos/article/8/3/201214/96207/Phosphate-bound-to-calcareous-sediments-hampers)** _Royal Society Open Science_ 8: 201214. Source for phosphate above roughly 5 µM inhibiting aragonite formation both in a beaker and in the skeleton of juvenile _Acropora digitifera_, and for phosphate reaching the calcifying space, adsorbing onto the skeletal surface, and leaving a more porous and structurally weaker skeleton behind. **[Adverse effects of total phosphate load from the environment on the skeletal formation of coral juveniles (2024).](https://www.sciencedirect.com/science/article/abs/pii/S0025326X24013729)** _Marine Pollution Bulletin_. Source for phosphate as low as 0.5 µM measurably impairing skeletal formation in coral juveniles, and for the point that it is the accumulated phosphate load that matters rather than the reading on any single day. **[Dunn, J.G., Sammarco, P.W. and LaFleur, G. (2012). Effects of phosphate on growth and skeletal density in the scleractinian coral _Acropora muricata_: a controlled experimental approach.](https://www.sciencedirect.com/science/article/abs/pii/S0022098111004588)** _Journal of Experimental Marine Biology and Ecology_ 411: 34-44. Controlled phosphate enrichment on an established acro rather than a juvenile. Source for growth and skeletal density both dropping under added phosphate. **[Lin, Y.-C., Zhang, S.-C., Wu, C.-Y., Hsieh, Y.E., Meng, Y.-Z., Wong, J.-W.M., Yang, S.-H. and Fan, T.-Y. Effects of blue light and feeding on the physiological performance of reef corals, _Stylophora pistillata_ and _Pocillopora damicornis_.](https://zoolstud.sinica.edu.tw/issue2.php?Vol=65&no=06)** _Zoological Studies_ 65: 6. Two blue light intensities crossed with two feeding levels. Source for _Stylophora pistillata_ growing most under the combination of stronger blue light and heavier feeding, and for both species showing reduced color scores under the higher light. **Redfield, A.C. (1958). The biological control of chemical factors in the environment.** _American Scientist_ 46: 205-221. Where the 16:1 figure comes from. Source for it being an average atomic ratio describing the composition of ocean plankton, which is what makes it the wrong tool for setting nitrate and phosphate targets in an aquarium. --- _[View the full topic](https://community.reefnbid.com/t/how-corals-build-tissue-and-color/252)._