Tuesday, May 27, 2014

Ocean Acidification Explained

Curious where carbon dioxide ends up after we send it to the heavens?

One major place is the ocean.

You can read my shpeal to have an in depth explanation or watch the inspiring video which covers the important aspects.  Plus, you can read (or skim, or pick out sections that interest you) the Royal Society report on the issue.


The Chemistry

Where the ocean and atmosphere meet, carbon dioxide can enter the surface waters.  Carbon dioxide interacts with water forming carbonic acid.

CO2+H2O <--> H2CO3

Carbonic acid can dissociate into hydrogen ions and bicarbonate ions.

H2CO3 <-->  H+ + HCO3-

With that increase in hydrogen ions, the water will become more acidic or have a lower pH (-log of hydrogen ions)

However, the ocean has a natural buffering system.  Carbonate ions either in the water or pulled from the sediment, bind the excess hydrogen forming bicarbonate ions.

H+ +CO3(2-)  <--> HCO3-

The ocean doesn't have an unlimited supply of carbonate ions (especially region specific) to sustain an ever increasing carbon dioxide concentration so the buffering capacity of the ocean is diminishing.

Biological Implications

Now to clear up why acidification is a problem for calcifying organisms (corals, urchins, sea butterflies, certain plankton, crustaceans, most larval stages-a popular strategy in the ocean).

General Calcification (as understood today):

2HCO3- + Ca2+ <--> CaCO3 + CO2 + H2O

So most calcifyers are using bicarbonate, why is less carbonate ions a problem??

Calcium carbonate dissolves into calcium and carbonate.

CaCO3 <--> Ca2+ + CO3(2-)

When chemical reactions occur in an equilibrium instead of a one direction reaction, removing a substance from one side will drive the reaction in that direction to balance out the removal.  Thus, calcium carbonate shells will be dissolved in the corrosive (high CO2, low pH) waters.  It will be more energetically costly to maintain a calcium carbonate shell.  Organisms will have to direct that energy away from other metabolic pathways (growth, reproduction, movement) putting them at high risk for not surviving.

CaCO3 --> Ca2+ + CO3(2-)

Some of these organisms are phytoplankton (like Coccolithophores) which provide us with around half of the oxygen we need for survival.

We are all connected directly or indirectly to the organic and inorganic substances of the Earth.  If we want to protect our own lives we need to be considerate of the Earth.  Burning dead material for energy is not the answer.

Interesting Predictions

Another aspect of calcification is that organisms create two different structures of calcium carbonate: aragonite and calcite.

Aragonite, because of its structure, is more readily dissolved.

Coral skeletons and pteropod shells (sea butterflies which are a large component of the Southern Ocean ecosystem) use aragonite.

It is predicted that with current levels of CO2 emissions, the Southern Ocean's aragonite saturation horizon (the depth below which aragonite dissolution is chemically favored) will be at the surface by 2100.  Basically, all aragonite forms of calcium carbonate will dissolve.

What we can do:

- promote sustainable energy sources (powered by wind, light, tides, and geothermal heating)
- buy sustainable fish
- use/buy less plastic  (reuseable water bottles and bags)
- bike, board, or walk
- use transportation powered by renewable sources
- properly dispose of trash (compost or recycle too)
- be less wasteful!

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