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Frequently Asked Questions

Mineral Accretion, or the Biorock® process, is a new method that uses low voltage direct current electricity to grow solid limestone rock structures in the sea and accelerate the growth of corals providing homes for reef fish and protecting the shoreline. The electrical current causes minerals that are naturally dissolved in seawater to precipitate and adhere to a metal structure.

The result is a composite of limestone and brucite with mechanical strength similar to concrete. Derived from seawater, this material is similar to the composition of natural coral reefs and tropical sand beaches.

Invente d by architect Wolf Hilbertz for construction purposes, mineral accretion has been applied to coral reef restoration by Hilbertz and coral scientist, Tom Goreau. Mineral accretion structures can be built in any size or shape.

This patented process increases the growth rate of corals well above normal, giving them extra energy that allows them to survive in conditions that would otherwise kill them. At the same time these structures attract huge numbers of fish, and also provide breakwaters that get stronger with age.

Biorock reefs, with their lush coral swarming with fish, have become major ecotourism attractions. Hotels in the Maldives, Indonesia, and Panama have built their advertising around the fact that they can offer great snorkeling right in front of their beaches.

Reefs die for many reasons: rising water temperatures, sewage, eutrophication, disease and negligence. A reef ecosystem that took hundreds of years to grow can be destroyed in a single afternoon by dredging, dynamite or cyanide fishing.

Corals around the world have been severely affected by global warming. High temperatures cause corals to turn white (or “bleach“). If it remains too hot for too long, the corals die of heat shock.
Record high temperatures are killing corals across the globe, with only a few of the hardiest corals surviving. Mineral accretion has proved to be a remarkable new method that increases coral growth rates and their ability to resist environmental stresses.

When coral reefs die, fish populations disappear; beaches and shorelines are damaged. Unprotected by breakwaters, fragile land areas become vulnerable to erosion, saltwater intrusion and destruction from waves. For an already damaged reef, regeneration is very slow taking several decades, even under ideal conditions.

Global warming has caused significant reef mortality around the world. The prognosis is that oceans will continue to warm until world leaders recognize the long-term consequences of turning a blind eye to the problem.

A few governments have tried to address the problem by building sea walls out of concrete, steel, coral rubble or sand bags. But these materials soon rust, corrode, collapse and need to be rebuilt. In contrast only breakwaters and reefs made of mineral accretion can provide permanent, cost-effective protection capable of keeping pace with rising global sea levels


Mineral accretion growth rates are typically from one to several centimeters of new rock per year, depending on the surface area of the structure. The rate at which the coral grows depends on the amount of current, the size of the structure and the species of coral. Typically, growth rates are about 3 to 5 times faster than normal.


Artificial reefs are typically made from manmade materials like sunken ships, planes, cars, concrete, rubber tires and trash. On land, this material might be called junk. Although fish will hide behind or within any structure that provides shelter and although certain sponges and soft organisms will sometimes settle on these materials, they never turn into a true coral reef.


To build a Biorock reef, an electrically conductive frame, usually made from readily available construction grade rebar or wire mesh, is welded together, submerged and anchored to the sea bottom. Sizes and configurations are infinite and are varied to fit the setting. A low voltage direct current is then applied. (Power sources can include chargers, windmills, solar panels or tidal current generators.) This initiates an electrolytic reaction causing mineral crystals naturally found in seawater, mainly calcium carbonate and magnesium hydroxide, to grow on the structure.


The structure is built from ordinary construction materials typically available almost anywhere in the world. This can include steel rods, pipe, or rebar. Other materials necessary for the project include electrical cables and epoxy or silicone sealants to protect the electric connections. While the main structure serves as the cathode, another electrode, the anode, is a special titanium mesh that does not corrode.

To power the mineral accretion process, a low DC voltage is necessary. The source of this current generally depends upon the environment near the reef. If a ready supply of electricity is available, cables can be attached to the structure. In more remote areas, solar collectors are usually the energy source. These panels will generally be set up on the shore to feed current to the Biorock reef via submerged cables. Power can also be supplied by other non-polluting sources such as windmills or tidal current generators.

In practice, a low voltage direct current is fed to the reef via cables. The structure acts as a cathode. A special inert material is used as the anode to complete the electrical circuit. The low power is completely safe for swimmers and marine life.

In most coral reef environments, structures sit on limestone bedrock where they eventually cement themselves solidly to the hard bottom. Usually structures are held in place against wave forces by their own weight or by filling them with rocks. In hurricane regions, where there is a tradeoff between how long it takes to get the structure solidly cemented to the bottom and when the first hurricane hits, we drill holes for vertical rebar supports; a couple of feet is usually adequate. In sand, we anchor reefs using rebar pounded into the substrate.

Our divers never damage an existing, healthy reef to populate a Biorock structure. In all cases, we transplant broken fragments of live coral that have been damaged by waves, storms, anchors or by other means. These pieces would almost certainly die as the fragments roll over in heavy waves and become buried in sand.

Coral fragments are wedged into crevices and holes within the structure or attached using plastic cable ties or steel binding wire.

Within days to weeks, as the mineral accretion grows around the attached coral fragments, corals begin to grow at accelerated rates. Their rapid growth is directly attributable to the electrical current in the underlying steel framework.

Coral larvae, which are millimeter-sized freely-swimming baby corals, will only settle and grow on clean limestone rock. This is why conventional artificial reefs made of tires or concrete rarely exhibit hard coral growth.
But, when these coral larvae find a limestone surface, they attach themselves and start to grow skeletons. Mineral accretion is exactly what they are searching for. As a result, there are very high rates of natural coral settlement on Biorock structures.

Like an oasis in the desert, all forms of coral life are quickly attracted to Biorock reefs. Many forms of reef life have been observed to be attracted to the structures, and none repelled.

However it is the organisms with limestone skeletons, such as corals, clams, oysters, barnacles, tube worms and sand-producing algae that are especially benefited, allowing them to outgrow weedy algae that often smothers and kills corals in polluted waters.


The result is that mineral accretion structures quickly become real coral reefs dominated by corals with a wide variety of normal reef creatures.

Corals grow at accelerated rates with mineral accretion because the electricity flowing through the structure creates chemical conditions (high pH) at the surface of the growing limestone crystals and at the surface of the coral’s limestone skeleton, greatly speeding up their growth.

Corals normally have to spend a large part of their energy to create these conditions in order to grow their skeleton, but mineral accretion provides the right conditions for free, leaving the coral with much more energy for tissue growth, reproduction, and resisting environmental stresses.


Corals attached to a mineral accretion structure are typically more brightly colored and extend their tentacles to feed more often. Because they have more energy for growth and reproduction they are much healthier and are able to survive environmental stresses that would otherwise kill them (excessive temperatures, sedimentation, and pollution).


During 1998, when more than 95% of the corals in the natural reefs in the Indian Ocean died, only 20-40% of the corals on the five mineral accretion structures at Ihuru in the Maldives died. The difference of less than 5% survival on the natural reef versus 60-80% survival on mineral accretion reefs was a dramatic demonstration of just how well this process works in a stressed environment.


Just as a gardener pulls up weeds that would overgrow the flowers, undesirable weedy organisms and certain sponges and algae that could overgrow corals are periodically removed. Organisms that kill corals, such as the crown of thorns starfish and certain coral-eating snails are eliminated.

Biorock reefs also need to be periodically checked to ensure that cables and connections are intact. If these wires are broken, growth of mineral accretion will stop and growth rates of corals will decrease to normal values and lose their special ability to resist adverse conditions. If problems are found with a cable, it is repaired or replaced as needed.


The longer a project runs, the more corals will grow and be protected from future hot episodes. A project can have its power turned off at any time, but then the special advantages of growth, strength, self-repair, accelerated coral growth and survival will be lost. However, once the structure is sufficiently strong, the power can be reduced to maintenance levels.

Discarded tires quickly become breeding mosquitoes. Under the false guise of helping the marine habitat, tires are often wired together and dumped in the ocean. This method of disposal has several consequences: 

Tires release chemicals that are toxic to marine life for a long time until the tires have been completely overgrown with marine organisms. 

Organisms that eventually do settle on rubber tires are largely "weedy" organism like stinging hydroids, sponges, and fire coral. Tires never seem to generate a typical coral reef community. 

Tires have a large surface area and very little weight so they are easily moved by storm waves, especially in a hurricane zone. 

Rubber tire reefs perform so poorly that they often have to be removed at great expense. Broward County in Florida is in the middle of a very costly effort to remove rubber tire artificial reefs that they had misguidedly put down many years ago, with few beneficial results. 

On the other hand, rubber tire reefs do provide lots of holes for fishes and can provide a habitat for fish if they are placed in habitat where there is no other shelter for fish to hide (i.e. far from natural coral reefs).

Could a shipwreck become a Biorock reef?

In theory, yes. But because of the large amount of steel, a significantly higher amount of electricity would be needed. A shipwreck powered by mineral accretion would not rust or corrode, making the structure permanent. In addition a much more natural coral reef ecosystem would develop. However, most shipwrecks are so far from shore that very long cables and high power would be needed.


Normally we use shore based DC power sources (chargers, solar panels, windmills etc.) Because of voltage drops in the cable to the structure we prefer to work within 100 yards of the power source. But there is no problem going further if one is willing to boost the voltage at the source to compensate for voltage drops. We have a coral reef structure in the Maldives that is more than 400 meters from shore.

There is no limit to the depth. Normally we build structures in shallow water (5 to 25 feet bottom depth) because corals grow best in brightly lit shallow water, but we also try to have them deep enough that boats can't run into them.

There are many requests for mineral accretion projects from marine conservation groups around the world, but unfortunately there is little funding available from governments, international philanthropic agencies or private foundations.

As a result, most mineral accretion reefs are pilot projects that demonstrate the process. The results are so unexpectedly spectacular that one must see them directly to appreciate their value.

There has been a recent surge of promotion selling "coral calcium pills". Selling ground up coral skeleton as a natural source of calcium is an endlessly recurring scam that hucksters pick up on every few years like clockwork.  My colleague Wolf Hilbertz has a bottle of coral tablets someone was peddling in the 1960s.
There are many natural sources of dietary calcium that don't require killing corals. It is important to keep a balance between your calcium and magnesium intake. One recent coral calcium ad I got said that people in Okinawa are healthy because they get loads of calcium in their drinking water, but of course this is true anyplace in the world where the water is hard because it comes from aquifers in limestone rocks. The negative side is that they can have very high rates of kidney and bladder stones, as well as gastric cancer because limestone groundwaters are loaded with nitrate. Oyster shell calcium should be fine, and the oyster wasn't killed for the shell (somebody else ate the meat and chucked the shell), but organic forms of calcium are probably more readily absorbed.

Coral skeletons are very pure calcium carbonate, unless the coral is from a very muddy place and has included sediment grains, whose composition will vary from place to place. It contains no meaningful nutrients except a lot of calcium with traces of magnesium and strontium, but it is fundamentally no different nutritionally than any other form of limestone, including oyster shells, or ground up limestone rocks or marbles, which are certain to be nutritionally richer in trace metals.  Actually the best form to take calcium is in organically chelated form, say calcium gluconate, which is much more readily absorbed by the stomach. The claim that corals build strong bones in humans than other forms of calcium is pure hype! 

Corals are vanishing everywhere and any mining of live corals for this trade should be stamped out. Mining of dead corals should not be a problem unless they are part of a dead reef that is still helping to protect the shore from erosion. There are such vast supplies of limestone on land, that these shouldn't have to be mined either.

A recent web site presents information on coral mining in Okinawa, see, "coral wars", claiming that there is large scale mining of both reef coral sediments, and limestone formations on land, which are referred to as "dirty" limestone. It further claims that they are mining coral sand without harming corals and that the reefs are in excellent condition.  While it is true that limited amounts of sand mining can be carried out without harming reefs, in most cases the mining operations increase suspended sediment turbidity, and plumes of this turbid water drift onto corals, where they severely stress or kill them. My late friend Zenji Yoshimine, lost his life photographing the long-term decline of Okinawa coral reefs. Most were killed by mud washing from land after unwise large scale deforestation and land bulldozing operations. Some died from sewage pollution. And in 1998, almost all the surviving corals died from high temperatures caused by global warming. Sadly, only a very small part of the once stunningly beautiful Okinawa coral reefs now remain in good condition, and the few still remaining are threatened by dredging and land clearance to make a new US air force base.

The report claims that the coral being mined has an optimal ratio of two parts of calcium to one part of magnesium. I published the first measurements of seasonal variations of magnesium and strontium in coral skeletons, and assert that no coral skeleton has this ratio. Only around 1% of the coral skeleton is magnesium.  Higher "optimal" ratios are common in old limestone formations on land. Although corals themselves have very little magnesium, other marine organisms, especially sea urchins and coralline red algae make different mineral forms of limestone that may have up to 10 or 20 % magnesium.  If it is being called "coral" limestone this must be an inaccurate claim for publicity purposes. Any limestone with a 2:1 ratio of calcium to Magnesium is not coral skeleton; it is limestone from other organisms, or ancient rock formations that contain the mineral dolomite, a limestone with a 1:1 Magnesium to Calcium ratio mixed with calcium carbonate.  

Effects are already visible in every ecosystem: coral bleaching, changes in flowering seasons, changes in bird and insect and mammal and fish migrations, melting of glaciers, thinning of polar ice caps, disappearance of cloud forests, warming of groundwater, changes in tree ting thickness, etc.

CO2 is building up from oil, coal, and gas burning faster than can be taken up by the oceans, forests and soils, whose capacity to absorb it, is steadily being reduced by human actions. The effects will take hundreds to thousands of years to be fully felt even if we used no more fossil fuels starting now.

This is enough to kill almost all coral reefs now existing, as they are right at their upper limit. Ocean circulation changes will have dire impacts on marine fisheries, which are only starting to be understood.


Joe needs to stop swallowing the false propaganda that government and industry bombard him with and start thinking and researching the issue for himself. What is an immediate effect we are experiencing today which is due to global warming?


Effects are already visible in every ecosystem: coral bleaching, changes in flowering seasons, changes in bird and insect and mammal and fish migrations, melting of glaciers, thinning of polar ice caps, disappearance of cloud forests, warming of groundwater, changes in tree ting thickness, etc.

It is too late to stop global warming from having severe future impacts so we need to focus on restoring damaged habitats now.


There is hardly a reef left anywhere that has not already been severely affected, and most have hardly any corals left. Fisheries are collapsing, beaches and shorelines washing away. Coral reefs are the most sensitive of all ecosystems to rising temperature and pollution, so the effects are first and worst, but all others will be affected as temperatures rise and as global ocean circulation patterns change.


Biorock technology allows us to grow corals faster than normal that are more resistant to environmental stress like sediments, pollution, and high temperature. For example, we had 16 to 50 times higher survival from high temperature in the Maldives. These reefs are swarming with much higher densities of fish than surrounding reefs. We can keep coral reefs and fisheries alive where they would die and restore them where they cannot recover naturally. No other method can do so. Yet, despite the results that can be seen in nearly a hundred projects in over a dozen countries, there has never been a penny of support form governments or large funding agencies for these efforts, and valuable time is being wasted while money is being thrown away on methods that can't possibly work in the long run as global warming and pollution increase.

The electricity drives the electrolysis of water that caused the chemical reactions growing dissolved minerals out of sea water. It also prevents all rusting of the metal. And it provides energy for the organisms exposed to the electrical field, causing faster growth and greater resistance to environmental stress. Without the electrical current these advantages are no longer present. We don't have much of a problem with algae blooms. These are temporary when water nutrients are too high, but go away by themselves, at least on Biorock projects (but not in natural reefs with excess nutrients from sewage and fertilizer, where the reefs are killed by algae).


Solar panels work fine if their terminals are sealed properly against corrosion, which many are not. But the power is not very much and is very expensive. We also use windmills, tidal energy turbines, wave energy generators, and transformers as power sources. Wind will work fine in many places, as long as they are strong and frequent enough, which is often not the case in the tropics.


With regard to acidity the effects are neutral in that equal amounts of acidity and of alkalinity are made at the opposite terminals. These tend to be neutralized very quickly within a very short distance and are not easily measurable in the water. The alkalinity is neutralized by limestone mineral growth, and the acidity by dissolution of limestone sediments. But because the alkaline area is much larger, we can grow reefs even if the water was acid. With regard to the salinity, the effects are very small. That is because while we can precipitate out some of the calcium, magnesium, and bicarbonate in sea water, the much larger amounts of sodium, chloride, and sulphate are not affected, and these make up the vast bulk of the salinity.

No! It is a source.
This is a complex issue which seems seductively but misleadingly simple, and which so many people have gone astray on.

It seems intuitively obvious that since limestone deposition is removing dissolved inorganic carbon from the ocean, that this should be compensated by one molecule of atmospheric CO2 dissolving in the ocean, but in fact the opposite happens.

The reason is that there is much more dissolved inorganic carbon in the ocean, in the form of bicarbonate ion, than there is CO2 in the atmosphere, and the ocean is a pH buffered system due to dissolution of limestone sediments and also acid base reactions involving weathering of oceanic basalts to clay minerals. So the predominant reaction is:

Ca++  +  2HCO3-   =  CaCO3  + H2O  + CO2

That is to say, in order to preserve pH and charge balance, for each molecule of bicarbonate precipitated as limestone in the ocean, one molecule is released as CO2 to the atmosphere. On a geological time scale, this is the major source of atmospheric CO2 along  with volcanic gases. The CO2 dissolves in rain water

H2O   +  CO2   =  H+   + HCO3-

The Hydrogen ion then is neutralized by dissolving limestone on land

2H+  + CaCO3  = Ca++   +   2HCO3-

Or it weathers igneous and metamorphic rock minerals, releasing metal cations and bicarbonate, which flow through rivers into the sea, building up Calcium and bicarbonate until they precipitate out as limestone, completing the cycle.

To put this into perspective, the amount of CO2 released to the atmosphere from all the limestone in coral reefs worldwide every year (which is about half the net storage of limestone in the oceans, at least back when our reefs were alive and growing) is only about one part in one hundred of the amount of the amount we put in the atmosphere every year from burning oil, coal, and natural gas, which shows how severely we have perturbed the natural carbon cycle!

With land vegetation it is a bit different. Plants take up CO2 in photosynthesis and store it in organic carbon. When they are eaten, burned, or rot, the CO2 is returned to the atmosphere. So they are a net sink only if the biomass is increasing, but in fact we are steadily reducing it, adding to the atmosphere. There is around 4 times more carbon in soils than in vegetation, and even more in sediments, so these are the real carbon sinks. Soil carbon is decreasing due to bad land management, but this could easily be reversed if we had the will to do so, and is the most effective way of sequestering carbon. Sediments are a large, but not very efficient sink, as most is oxidized by bacteria back to CO2. Sediments are good carbon sinks only if they are anoxic, and the lack of oxygen needed only covers a tiny part of the ocean (but rapidly increasing with global warming, as it makes oxygen less soluble). But for sediments to become a major sink we would need to turn large parts of the bottom water anoxic, killing the fish and marine invertebrates.

Each project is totally different, and all are done on a shoestring with not nearly enough funds. Usually we only get in-kind donations of materials, room and board, and if we are lucky a ticket to get there.
The costs for any project are very dependent on its size and many site-specific factors that must be assessed beforehand and incorporated into the design before we can come up with a budget. Up until now all our support has come from local groups, and except for one project in the Turks and Caicos Islands, they have never been funded by any government or large funding agency. Typical small pilot projects are done for a few hundred to thousands of dollars for all materials and equipment, but that does not include our travel, room and board. If we were paid for our time in a way consistent with our skills and knowledge, it would be much more. We plan to stop doing little projects except with community-based groups, and focus on large funded projects in the future, but that has not yet happened. 99% of the people who request our help do not follow through once they realize that there is a real cost for materials and equipment and that they should pay for our expenses and time (they think we are rich and expect us to pay). The best way volunteers can help is to develop projects by going back to the sites they dive and talk to the hotels and dive shops to convince them that they need to start restoring their degraded reef. Ask them to contact us. We'll work with all serious partners who want more corals, more fishes, and more beaches. How can I volunteer to help GCRA reef restoration projects?.

We are constantly developing coral reef restoration, fisheries restoration, shore protection, ecotourism, mariculture, and oyster restoration projects all over the world.
When projects come through they usually are very small and happen on very short notice. Local groups find the money for a ticket and materials, and provide us a bed and food. We almost never get paid. The volunteers on these projects are almost all locals. The vast majority of these projects are initiated by local people who find us because they remember how their reef used to be, realize it is almost gone, want it back, recognize that any funding from outside will be too late, and that they only thing they can do to make a difference is to grow more corals themselves right away.  These situations do not lend themselves to outside volunteers because these projects rarely can be organized long in advance, and anyway our key task is to train locals to care for or maintain the project, because this is the key to long term success, and outside volunteers (including us) will not stay for that. We have trained hundreds of people, mostly Indonesian students, but until governments and funding agencies fund large scale reef restoration, there will be no jobs for our students, marine biologists, and divers to use their skills and knowledge to grow reefs back. Only such funding will allow large projects to be organized ahead of time in a way in which outside volunteers would be useful in the construction and installation phase, although our focus must be on developing long-term local skills. We are also trying to develop commercial projects for the technology (for example shore protection, fisheries, artificial islands, etc.) in which trained people could be paid rather than using volunteers.  We know that divers around the world love coral reefs and want to help restore them. And many of our students want to work with local communities to restore their corals and fisheries. Around once a year we do hold training workshops in reef restoration. Watch our website for more details.

GCRA is a non-profit, all-volunteer, 501 (c)(3) corporation; donations to Global Coral Reef Alliance are fully tax-deductible.  No salaries or board perks are ever taken out of any income raised—100% of funds raised goes directly to studying, building and restoring coral reefs around the world.

If you would like to contribute to GCRA’s program for saving reefs, please send your tax deductible contributions to:

Global Coral Reef Alliance
37 Pleasant Street
Cambridge MA  02139

Mineral accretion projects been setup in more than 20 countries including: 


Papua New Guinea
Saya de Malha
The Marshall Islands