The acid test

Oceans do a good job of removing CO2 from the atmosphere but many marine organisms are paying the price. The International Panel on Climate Change has estimated that every day the oceans soak up over 25mt of CO2 produced by burning fossil fuels. This is having an effect on ocean chemistry, making the waters more acidic and reducing the levels of calcium carbonate minerals needed by many marine organisms to build shells and skeletons. Using models and modern sea data, the average surface pH of seawater is estimated to have dropped from 8.2 by 0.1 units since the 1900s and is predicted to fall to about 7.8 by 2100. ‘For the next few decades, we have a pretty good idea of what is going to happen. As models go, the chemistry of the oceans can be predicted pretty well,’ says Scott Doney, from the Woods Hole Oceanographic Institution in Massachusetts, US.

Carbon dioxide dissolves in seawater to form carbonic acid, H2CO3, which rapidly dissociates to give H+, bicarbonate (HCO3–) and carbonate (CO32–) ions. The carbonate system acts as a buffer for ocean acid–base chemistry. Thus adding CO2 to seawater pushes up the hydrogen ion concentration and drives down pH (pH = –log10[H+]. As CO2 dissolves, carbonate ions (CO32– ) pull out some of the H+ ions. Unfortunately, the resulting drop in levels of carbonate ions is not good news for the corals and other calcifying organisms that need them to build hard structures.

Ocean scientists use the CaCO3 ‘saturation state’ (Ω) to describe how much carbonate material is available to organisms. Ω= [Ca2+ ][CO32– ]/Ksp where Ksp is the thermodynamic solubility product, which varies with temperature, pressure and mineral form, such as aragonite, the main form used by corals; calcite; or magnesium calcite. Ocean surface waters are currently ‘supersaturated’ for aragonite and calcite – the value of Ω exceeds one – which is good for marine organisms such as corals. But when Ω drops below one, as it commonly does in deep water, shells and skeletons begin to dissolve. Surface ocean CaCO3 saturation states are falling in all of the world’s oceans.

The drop in carbonate levels affects organisms in different ways. Tropical corals are particularly susceptible and, as reef structures weaken and disappear, the many fish species which depend on them are also affected. For other species, the damaging effects are more subtle. For example, recent studies on clownfish by an international team led by Stephen Simpson at the University of Bristol, UK, found evidence that CO2-enriched waters can damage fish hearing by hindering the growth of carbonate structures in the inner ear. The clownfish uses a suite of sensory capabilities to navigate through life. Hearing is crucial, especially around coral reefs, where ‘clicks and chirps’ guide the fish to a good habitat while helping it to steer clear of predators. Simpson’s team found that fish raised in ambient-CO2 waters steered clear of tank areas where a sound recording of a reef rich in predators was played whereas those raised in high- CO2 waters swam on regardless.

‘For some organisms it is the actual reduction of pH that is stressful to them,’ says Chris Langdon from the University of Miami’s Rosenstiel School of Marine and Atmospheric Science in Florida, US. ‘For simple organisms, when you change the pH of the external environment it forces the chemistry inside the body to change as well. They have mechanisms to compensate for that but it takes additional energy. That energy is then not available to put into reproduction so it has knock-on effects.’

Emma Davies, soci.org, 11 July 2011. Full article.


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