# Biorock

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Biorock [1] is a method for creating artificial reefs that is designed to promote the growth and resilience of the resident corals.

The mechanism used by Biorock is based on the electrolysis of water [2]. When two electrodes are placed in seawater and an electric current runs between them, water reacts at one electrode (the anode) to produce chlorine gas, and at the other electrode (the cathode) to produce hydrogen gas:

• Anode: ${\displaystyle {\ce {2 Cl- -> Cl2 + 2 e-}}}$
• Cathode: ${\displaystyle {\ce {2 H2O + 2 e- -> H2 + 2 OH-}}}$

Depending on the concentration of ${\displaystyle {\ce {Cl-}}}$ ions the anode can also produce oxygen gas, though the reaction taking place at the anode doesn't really matter for the purpose here. The product of interest is the ${\displaystyle {\ce {OH-}}}$ hydroxide ions produced at the cathode: these react with ${\displaystyle {\ce {H+}}}$ ions to form ${\displaystyle {\ce {H2O}}}$, alkalizing the water surrounding the cathode. The alkaline water has a higher concentration of carbonate ions (${\displaystyle {\ce {CO3^2-}}}$), which has two effects:

• Solid calcium carbonate and other minerals accrete onto the cathode, building up over time.
• More carbonate ions make it easier for corals to precipitate their own calcium carbonate and grow more easily.

The Biorock method is to submerge a large framework made of steel or another conductor which will serve as the cathode, along with a much smaller anode made from a oxidation-resistant material such as titanium [3]. The cathode and anode are connected to a power supply, and the reactions above proceed. While Biorock's technology has been around since the 1970s, its effectiveness is difficult to evaluate from existing literature, with few studies performed.

One series of studies [4] [5] compared growth and survival rates of small coral fragments with and without this technique. Fragments treated with this technique had higher survival rates than untreated fragments, as corals can more easily attach to accreted calcium carbonate than to bare metal, and to improve growth rates for the fragments. Growth rates improved the most closest to the cathode (improvements in fragment base girth were much greater than improvements in fragment length), and after six months accretion stopped and the supply of electricity was shutoff. Afterwards there was no difference in growth between (formerly) treated and untreated fragments.

In contrast, the developers of the technique found that accretion continued for at least three years after installation, and that transplanted corals continued growing robustly for a while [3]. A lack of experimental controls makes it hard to evaluate what effect the technique had on coral growth rates. Still, Biorock installations have been made in over twenty countries [6] and are reportedly very successful at improving both growth rates and resilience of corals to bleaching events [7] [8].

It would be very interesting to try to reconcile these very different results, either by doing more experiments to validate the technique or by better pinning down the theoretical basis of the technique. The strength of the technique relies on its ability to alkalize the water in and around coral tissues, and yet there is little information available about this aspect of the design. A couple questions:

• What is the electrical resistance of accreted calcium carbonate and of living coral? Electrical resistance ties in directly with the efficiency of the system: the cathodic reaction can only occur in the presence of both water and electrons delivered from the cathode, and so can only occur after the electrons have been transported from the cathode itself to the surface of any accreted material or corals. In [4] [5] resistance in the accreted material stopped the reaction after only six months, whereas in [3] accretion continued for at least three years and produced layers of calcium carbonate up to 20cm thick.
• How does pH change with distance from the cathode? In [4] it is claimed that the carbonate ions fall within a few millimeters of the cathode, but it isn't clear what this is based on. With knowledge of the electrical resistances and models of where the cathodic reaction takes place, this could be used to estimate how much attached corals will benefit from the alkalized water.

## References

1. http://biorock.org/
2. https://en.wikipedia.org/wiki/Electrolysis_of_water
3. Goreau, T.J. and W. Hilbertz, Reef restoration using seawater electrolysis in Jamaica, 8th International Coral Reef Symposium, Panama (1996).
4. Sabater, M.G. and H.T. Yap, Growth and survival of coral transplants with and without electrochemical deposition of CaCO3, Journal of Experimental Marine Biology and Ecology Volume 272, Issue 2, 24 June 2002, Pages 131-146
5. Sabater, M.G. and H.T. Yap, Long-term effects of induced mineral accretion on growth, survival and corallite properties of Porites cylindrica Dana, Journal of Experimental Marine Biology and Ecology Volume 311, Issue 2, 16 November 2004, Pages 355-374
6. http://www.biorock.org/faq-page
7. http://www.biorock-indonesia.com/project/pemuteran-bali/
8. http://www.bbc.com/future/story/20120905-save-our-dying-coral-reefs