Carbon dioxide is a compound with extremely important biological connotations. It is a reactant in photosynthesis, the chemical process by which plants produce energy. Carbon dioxide is also, however, a byproduct of many industrial reactions, and the earth is struggling to try to find a place to put all this excess anthropogenic carbon dioxide. In particular, some of the carbon dioxide is being sopped up by the world’s oceans. Nonmetal oxides such as carbon dioxide are acid anhydrides in water, and as a result of this absorption, the oceans are becoming more acidic and the average pH of the earth’s oceans has dropped by a quantifiable margin. This phenomenon has aptly been entitled ocean acidification, and it has some rather grave implications. However, the additional carbon dioxide may, to some degree, improve the photosynthetic output of some marine plants, given the compound’s aforementioned role in the reaction.
Diatoms are unicellular algae that are significant producers in many marine food chains. The effects of ocean acidification on diatoms have been a hot topic of research within the past seven years given their importance. In one study, scientists bubbled varying levels of carbon dioxide into colonies of the coastal phytoplankton skeletonema costatum. The colony that received 350 ppm of carbon dioxide grew 1.6 times as much as the control group. A colony given 1000 ppm grew 2.1 times as much. The 1000 ppm culture produced more chlorophyll than the 350 ppm group, and the effectiveness of photosynthesis was enhanced by the additional carbon dioxide as well.
Another experimented conducted entailed the creation of an equilibrium of atmospheric carbon dioxide with bubbled aqueous carbon dioxide. When the carbon dioxide was made to be twice that of normal conditions, consumption increased by 27%. When the carbon dioxide was tripled, the diatoms’ consumption was 39% higher. Estimates say that such carbon dioxide consumption as that described here may in have kept atmospheric levels to 90% of what they would be otherwise since start of the industrial revolution. In yet another study, it was found that certain species of diatoms grow 20% faster when exposed to increased carbon dioxide.
This potentially positive consequence of the increase in atmospheric carbon dioxide is not nearly enough to outweigh the negative results of anthropogenic carbon dioxide. Some algae do not, in fact, benefit from increased levels of carbon dioxide. Zooxanthellae, for example, exist symbiotically with coral reefs. If the zooxanthellae colonies grow too large, then they will be doing so at the expense of their coral homes. Some species of phytoplankton may react poorly to the increased acidity. Then we must factor in things such as coral bleaching, coastal erosion, decalcification, and the loss of biodiversity. Indeed, for every possible upside that comes from ocean acidification, it seems that there are two potentially devastating ramifications.
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With the increasing amount of CO2 being absorbed into the oceans, one can only imagine the impact it is and will have, and the breadth and depth of the effects of ocean acidification. Effects on surface water chemistry, calcification, various biological processes, and coral reef growth provide an idea of the breadth of impacts, but observing something specific, like phytoplankton, can be used to show the depth of negative effects.
The basis behind ocean acidification is that the ocean is becoming more acidic as a result of increased CO2 levels, which are being absorbed from the emissions in the atmosphere. Acids have higher concentrations of H+ ions, and this is exactly what is happening as CO2 is absorbed into the ocean: there are more H+ ions. Essentially, the CO2 combines with water to form carbonic acid (H2CO3). This carbonic acid then breaks down, forming bicarbonate (HCO3) and a remaining H+ ion, the ion found in acids.
Of course a more acidic environment could prove detrimental to ecosystems and organisms alike simply because of a change in the environment. But as we see, a decreased pH will even have effects on nutrient acquisition, and it starts with the nutrients themselves. The nutrients taken in by organisms are essentially chemical compounds. And because we have changed the chemistry of the ocean, it makes sense that the compounds themselves, which are taken in by organisms, are now also changed. Iron, among many trace metals, is an essential element for different biological processes. Unfortunately, a decreased pH causes dissolved iron to less bioavailable, and as low iron levels limit primary production in Phytoplankton, they now experience a limiting growth factor.
Other nutrients, such as phosphate, exhibit similar behavior in regards to their chemical form as result of a change in pH. These elements can bind to organic compounds, and the rate and type of binding occur relative to the pH. So Phytoplankton will have to use enzymes to biochemically remove the phosphate from the organic compound. It so happens that the enzymatic activity is also affected by the pH, and the enzymes which cleave phosphate or other nutrients from the organic compounds are located externally; they are directly affected with the pH change.
Additional to the nutrients being altered, increased CO2 levels increase Phytoplankton synthesis and primary production, and has been shown in various studies. Further studies have shown that the composition of what Phytoplankton produce in higher CO2 levels have greater carbon to nitrogen rations (C:N). A higher organic carbon ratio can mean the Phytoplankton have higher nutritional values, and since our Phytoplankton are at the bottom of the sea trophic levels, consumers can potentially expect an increased growth and reproduction rate. Looking at a study on bottom-up affects on trophic levels, we see that if trophic level 1 is increased, above trophic levels experience increases relative to the ratio of change in trophic level 1. If Phytoplankton encounter increased nutritional values, above trophic levels will reach a new population density equilibrium, as reproductions can also be affected at higher trophic levels.
However, other studies argue that even after comparing eight different species of phytoplankton, a decrease in pH, even to the predicted pH for the end of the century (7.8), showed neither an increase or decrease in growth or production rates. This is not to dismiss our theoretical situation where consumers reach a new population density equilibrium, but to note that sufficient studies are not yet available and to show that predications of what ocean acidification can do are fairly difficult, as the potential impacts cover a large scope.
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