Making stone out of seawater

by Janice Nigro

Stony corals fool us. We see only a big mountain of what appears to be lifeless rock before us, when in fact the mountain of rock is growing daily (in healthy reefs) due to the hard work of the macro animals and their algal symbionts, zooxanthellae, living inside.

Scientists have long known which molecules constitute the stony skeleton that we recognize as coral, a particular crystal form of calcium carbonate called aragonite, but there hasn’t been agreement on how it gets there. One view has been that the process is dominated by environmental/seawater conditions and basic physical/chemical principles. The other suggests that the process is largely driven by biology.

The distinction is important, especially today. One theory has rather ominous implications for the lifespan of our coral reefs based on poor ocean ecology. The other places a certain amount of destiny in the survival of our reefs on the coral animals themselves.

Studies have led scientists to recently conclude that formation of the coral skeleton is largely driven by biological events. Meaning, the coral polyp is in charge. High resolution imaging and microscopy revealed a stepwise process that begins with the generation of an organic foundation favorable for the formation of aragonite crystals. A specific set of proteins secreted by the coral polyp make up this organic layer and attract nanoparticles (magnesium) which are the seeds for aragonite crystal growth.

The coral animal creates the space, essentially directing localization of mineralization. It’s truly a bit of Harry Potter magic-making stone out of seawater-engineered by a pair of simple organismal collaborators.

A more relevant question then is what conditions stimulate and/or inhibit coral polyps in producing these proteins and influencing skeletal growth.


The results present the interesting conundrum that coral growth might be more robust than previously thought because of a level of independence from the seawater environment. Does coral just do its thing despite changes in seawater such as PH?

We don’t get off the hook to further assault our oceans. Warm temperatures and increased acidification affect stony corals, in part through the loss of their algal symbionts. The zooxanthellae are required for coral growth, and loss of the zooxanthellae is what we know as coral bleaching. The algae produce glucose, glycerol, amino acids, and oxygen, which are all essential nutrients for the coral polyps, as well as remove waste. The symbiont performs these functions through photosynthesis which incorporates the coral waste product of CO2, essentially recycling it just as trees and plants do for us on land.

I am always amazed at the delicate arrangements that have evolved between organisms to support life in our oceans. It makes sense that coral has developed capabilities to live under potentially more extreme changes in environment-it has survived after all for hundreds of millions of years.

At the same time, I think of something a guide once said to me at the site of some ancient temples in Sicily. “You would think that natural disasters would have had the greatest impact in the deterioration of these temples over time. But in fact it is man, our wars and our ability to pick the most vulnerable point in the structure, which can destroy an entire temple in an instant.”

So if you are wondering if you can rest your hand on that coral head, the answer is don’t do it. There is something in there. And it’s busy making magic.

©Janice Marie Nigro/

Looking for a scientific editor or writer? Contact Janice Nigro at Janice Nigro Scientific Editing and Writing. I have published in Cell, Science, and Nature, and articles I have edited have appeared in Cancer Research, PLoSONE, the Journal of Surgical Oncology, and Oncotarget.


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