About one-quarter of the carbon dioxide (CO2) released into the atmosphere as a result of human activity has been absorbed by the oceans, where it dissolves in seawater producing carbonic acid. This leads to ocean acidification, one of the major threats to marine ecosystemsand particularly to calcifying organisms such as coralsand coccolithophores.
If we keep emitting CO2 at today rates, global surface ocean pH will decrease by 0.3-0.5 units by 2100 that means the average surface ocean pH will have fallen from 8.2 to around 7.8 – average levels the Earth has probably never experienced for more than 20 million years.
Coccolithophores are phytoplankton that live in large numbers throughout the upper layers of the oceans and are an important component of the Earth system. Their short-term effect on the environment is somewhat complex. They contribute between ~1% and 10% of marine primary production, are responsible for a large portion of oceanic calcium carbonate flux, while they also affect the global climate by increasing the oceans’ albedo. Their possible susceptibility to ocean acidification is, therefore, of major concern.
The creatures use calcium carbonate, the material that makes up chalk and seashells, to build tiny plates on their exterior, releasing CO2. But because these organisms photosynthesize they also consume CO2. It is the balance between calcification and the consumption of CO2 by photosynthesis that determines whether coccolithophores act as a sink or as a source of CO2 to the atmosphere.
New research shows that that not all types will be affected equally, or even in the same way. Researchers (some of them participants of Sea- Surface Ocean Acidification Consortium) attempted to find out how the coccolithophore species Emiliania huxleyi responds to seasonal changes. Emiliana huxleyi is by far the most abundant of the coccolithophores on a global basis, and is extremely widespread, inhabiting almost all oceans, from the tropics to sub-polar waters.
The study which was based on monthly sampling from deep oceanic waters in the Bay of Biscay and over the course of a year showed that, contrary to the earlier predictions, the overcalcified E. huxleyi cells were more numerous than the normal type when conditions are most acidic. The findings of the study illustrate the difficulty of projecting how specific calcifying processes will respond to an acidifying ocean, and raise further questions about the fate of coccolithophores in a high- CO2 world.
Helen E. K. Smith, Toby Tyrrell,Anastasia Charalampopoulou, Cynthia Dumousseaud, Oliver J. Legge, Sarah Birchenough, Laura R. Pettit, Rebecca Garley, Sue E. Hartman, Mark C. Hartman, Navjit Sagoo, Chris J. Daniels, Eric P. Achterberg, and David J. Hydes
Predominance of heavily calcified coccolithophores at low CaCO3 saturation during winter in the Bay of Biscay PNAS 2012 109 (23) 8845-8849; published ahead of print May 21, 2012, doi:10.1073/pnas.1117508109
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