Carbonate: The Building Block of Marine Life

Seashells use calcium carbonate to build their shells. Photo Credit: Melissa Ward

Next time you are walking along a beach, pay attention to how many unique types of shells you see: scallops, gastropods (snails), oysters, pteropods, and many other marine organisms rely on shells for protection and housing. Just as humans build our homes from brick and wood, these marine organisms build their homes out of a chemical compound called calcium carbonate.

The hard parts of many marine organisms, like crabs, oysters and corals, are made of calcium carbonate. Image: North Carolina Aquarium
The hard parts of many marine organisms, like crabs, oysters and corals, are made of calcium carbonate. Image: North Carolina Aquarium

This material comes from the ocean itself: both calcium and carbonate exist as dissolved ions in the ocean, and both are needed in order for shell-building marine organisms to thrive (click on the image to the right for a short video about how OA affects shell formation).

However, ‘anthropogenic’ (human-generated) carbon dioxide is acidifying the ocean and decreasing the ocean’s concentration of available carbonate ions.  This makes it more difficult for marine organisms to build and maintain their homes; therefore, these marine organisms are becoming increasingly vulnerable (this short NOAA video explains the chemistry).

Seawater Carbonate Chemistry. Image: NOAA PMEL
Seawater carbonate chemistry. Image: NOAA PMEL

My labmates from the University of South Florida College of Marine Sciences and I are measuring the concentration of carbonate ion in seawater on this cruise using UV (ultraviolet) spectrophotometry. You probably already know that UV radiation can cause sunburns, but it also can be used to make measurements on a piece of specialized equipment called a spectrophotometer.

A spectrophotometer uses the interaction of a chemical with light – either visible light that you can see, or, in this case, UV light that you cannot see – to measure the concentration of that chemical in a sample.  In our case, the seawater that we sample contains a certain concentration of carbonate ions.  These carbonate ions will bind with lead ions to form a chemical that absorbs UV light, so we inject a small amount of a solution containing lead into each sample.  The carbonate ions and the injected lead ions bind to one another, and the concentration of this lead carbonate compound – and therefore the concentration of the carbonate ions – can be determined by measuring how much UV light is absorbed.

Basic design of a spectrophotometer. Image: http://chemwiki.ucdavis.edu
Basic design of a spectrophotometer. Image: http://chemwiki.ucdavis.edu

To determine the UV absorbance of the sample, the spectrophotometer aims a beam of UV light through the sample, and a sensor on the other side of the sample detects how much UV light transmits through the sample.  By subtracting the amount of transmitted light from the total amount of light from the beam, the spectrophotometer can tell us how much of the light was absorbed. This absorbance is proportional to the concentration of the carbonate ion in solution, so by knowing the length of the light path through the sample and a special property of the lead carbonate called the molar absorptivity, the concentration can be calculated.

Knowing the concentration of the carbonate ions in a seawater sample can be very useful to scientists as they work to predict ocean health.  If there are not enough carbonate ions in the seawater to maintain the homes of shell-building marine organisms, the shells are at risk of weakening, thinning, and developing holes. Additionally, because shell-forming organisms are generally lower on the food chain, their vulnerability will affect their predators as well.  Therefore, knowing the concentration of carbonate ions in seawater is necessary to predict changes occurring in ocean chemistry and to understand the implications of these changes on marine organisms.

Author: Katie Douglas

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