MU Science Blog

Carbonic Acid: Not Just for Coca-Cola Anymore. April 30, 2011

We sit at an critical point in time with the looming threat of global warming. The world is being changed, but the exact extent of that change is now coming to fruition. Ocean Acidification is one facet of global change that is not being addressed at the same level as, say global warming. Nevertheless, oceanic acidification is going to become a global concern in the next twenty years because its effects are very damaging.

How bad is Ocean Acidification?

Oceanic acidification is not a new phenomenon. According to a February 2009 article in Scientific American:

Oceans naturally absorb the greenhouse gas; in fact, they take in roughly one third of the carbon dioxide released into the atmosphere by human activities. When CO₂ dissolves in water, it forms carbonic acid, the same substance found in carbonated beverages. New research now suggests that seawater might be growing acidic more quickly than climate change models have predicted.

"Present day" (1990s) sea surface pH

This article explains that the ocean is responsible for the bulk of the work in recycling atmospheric gases. The problem, though, occurs in the rate of carbonic acid formation. “Research at the University of South Florida has shown that in the 15-year period 1995-2010 alone, acidity has increased 6 percent in the upper 100 meters of the Pacific Ocean from Hawaii to Alaska,” according to an article on Ocean Acidification from Plumbot.com

The carbon cycle is a popular topic today. We talk about emissions and about the amount of CO2 given off by an SUV versus a Prius, but what we do not talk about is how detrimental CO2 can be to oceanic processes. The ocean recycles CO2 by converting it into carbonic acid via the reaction:

CO₂ + H₂O  H₂CO₃

Is There a Consensus?

Representation of the carbon cycle.

Carbonic acid is not necessarily a bad thing, but concentration influences its danger. I can drink a can of soda and I won’t see any detrimental effects. The sugar may cause problems, but not to a level of lethality. In the ocean, though, the stakes are higher. According to Jason Hall-Spencer, a researcher at the University of Plymouth, “Many of the marine species having calcium carbonate based external skeletons, including corals and mollusks, are affected because, as water becomes ever more acidic, calcium carbonate concentrations in the water decrease, leaving them with little resources to build their skeletons on.” Also, “Marine ecologist J. Timothy Wootton of the University of Chicago…and his team discovered that the balance of ecosystems shifted: populations of large-shelled animals such as mussels and stalked barnacles dropped, whereas smaller-shelled species and noncalcareous algae (species that lack calcium-based skeletons) became more abundant.” This trend is also true of herring populations. According to an articlein the Seattle Times, “For example, computer models suggest that, if acidification reduces one type of plankton eaten by herring, herring populations may go down. But if acidification hits a different plankton species, the number of the fish could in fact increase. In another hypothetical scenario, potential declines in invertebrates such as urchins and sea cucumbers might be less than first expected because their predators — sea stars — decline, too.” Dr. Busch is saying here that the effects of Ocean Acidification are so complex, that it will be difficult to really predict what will be affected.

How Does this Affect Me?

It is clear that, though we would not necessarily be directly affected by ocean acidification, the organisms that feed fish we use commercially could decline, resulting in a detrimental effect on the fishing industry in general. That alone may spark much interest into determining the root cause of oceanic acidification and move individuals into steps geared at remedying this problem. According to Cheryl Logan, in an article from BioScience: “Changes in ocean chemistry will probably affect marine life in three different ways: (1) decreased carbonate ion concentration could affect the calcification process for calcifying organisms (e.g., corals); (2) lowered pH could affect acid-base regulation, as well as a variety of other physiological processes; and (3) increased dissolved CO₂ could alter the ability of primary producers to photosynthesize.”

But I Live in Indiana!

The research that was done here, though it has many implications for the future, does not necessarily focus on the problem of fresh water resources. The ocean, by far, is the largest CO2 sink due to its size, but not much research has really been put into freshwater testing of acidification, other than the testing of acidification by direct dumping. The research that Maria Solis and I performed this year at Marian University attempted to test this theory, that freshwater resources would experience the same process of acidification.

Though we did not definitively prove any new groundbreaking theories about acidification, we think that we are on the right track. For us, the ideas about ocean acidification do not hit very close to home in land-locked Indiana, but we know that lakes are commonplace. We wanted to do something that not many have done before, look at natural acidification based on dissolved CO2 compared with chemical dumping.

For our experiment, we wanted to observe the effects of high and low CO2 concentrations on plant growth rate and snail shell formation. When looking at plant growth rate, we hypothesized that the increasing levels of CO2 would increase the growth rate in plants at lower CO2 levels. The rate would increase to a point, until acidification would lead to a decrease in plant metabolic functions. Testing photosynthetic rate, or in our case growth rate, is a good measure of CO2 metabolism. Photosynthesis depends on sunlight and CO2, so increasing the level of substrates would definitely increase the level of metabolism in the plants that we chose to use. We chose to use three types of plants to get a range of growth rates. We used a common aquarium plant, Egeria densa. For a secondary plant species, we chose Elodea densa.  Finally, for use as a invasive species control, we chose to use Vallisneria, a freshwater species of eelgrass. Eelgrass is an invasive species, that according to Gabriel Garche in his article entitled “Water Acidification Process Reveled by Marine Life,” “seagrass exploiting the excess of carbon dioxide seems to be thriving.” Also, to test the effects of carbonic acid on benthic organisms, we also included mystery snails (a species of Pomacea bridgesii).

A direct image of tanks used during our experiment

To establish an effective experiment, we obtained six, ten gallon tanks, into which we placed plants into the first three. We placed around 4-5 snails into each of the six tanks. We wanted to simulate the effects of dissolved CO2, so we placed stone bubblers into four of the tanks, into which we bubbled varying amounts of CO2. For two of the four tanks, we used stone bubblers that had room air bubbled into them. So, in total, we had three tanks with plants, all six with snails, four with CO2, and two with room air bubblers. See Photos below:

We were unable to measure dissolved CO2, so we used Vernier dissolved O2 sensors to measure the change in dissolved oxygen as a function of time. Also, we used pH probes to measure the change in acidity as a function of time. To measure photosynthetic rate, or rather metabolic rate, we measured all plants prior to experiment starting time, to develop a before-and-after measurement that would confirm growth rate. Also, we weighed all snails as a function of tank, measuring all by mass and volume to determine shell growth  rate. These measurements gave us a benchmark from which we would determine the level of growth as a function of tank. The experiment was carried out for several days.

Low CO2 (Snails and Plants)

Unfortunately, due to time constraints. We were unable to conclude much from the experiment itself.

Due to the fact that the water we used was fresh water, the pH sensors, based on their configuration for measuring ions, did not register much of a pH change. We will need to find a better method for measuring pH in non-alkaline solutions. An interesting effect we observed was in the snail populations. We observed that all snails in the high CO2 environments died, most likely due to the lack of oxygen. This result was not in keeping with our hypothesis of reduced shell growth, but does speak to the effects of a high CO2 environment on snails. The snails in the tank with low CO2 and no plants died as well. We saw some die in the tank with low CO2 that included plants, but not all died. This seems to indicate that the plants in the tank were able to utilize enough of the CO2 as to provide the snails with oxygen. The tanks with air bubbled in showed all living snails. 

The dissolved O2 sensors were sporadic at best. They needed water movement to best determine the dissolved O2. We ran out of CO2 early in the experiment, so without movement, our sensors were unable to register consistent measurements of dissolved O2. We will, in the future use bigger CO2 tanks to get a more prolonged test, so that our O2 sensors may become more effective in giving us detailed results. We also observed plant growth in all tanks. So, we were not successfully able to quantitatively determine what we set out to determine, i.e. pH and dissolved O2, the death of our snails and the growth of our plants gave us a qualitative result that demonstrated that the plants grew in this environment, but that the snails were unable to thrive.

The experiment, if it could be carried out for a longer period of time, would likely demonstrate a trend. This trend would show that the tanks that had high CO2 bubbled into it with plants would show a slower trend of dissolved O2 trending toward a higher CO2 rate. The plants would show growth at a rate higher than the control tank that had room air bubbled into it. The snails would probably not show much change in size, but would most likely thrive better in the tanks that contained the plants that had room air bubbled into. The rate of CO2 bubbling would need to be scaled back, so that our snails would have a chance to thrive in the high CO2 tanks. That way we would be able to measure relative growth rates based on mass and volumetric displacement. The high CO2 tank that contained snails that had no plants would most likely show death of snails, if no growth rate at all.

With these results, we would prove that acidification of freshwater can occur, but most likely not to the level observed in the ocean. This is due to a lack of calcium carbonate in the water itself, a molecule that interacts with CO2 to form carbonic acid.

With an understanding of the crisis that awaits us if CO2 is continually added to the water supply, we must begin to take steps to mediate acidification. One way to do this is to stop adding more CO2, allowing the algae and other CO2 metabolizing organisms to work to reduce the oceanic concentration. Hopefully, with the boom in growth rate that would be observed, the rate of acidification can be slowed to a degree that would diminish detrimental effects. Only time will tell if acidification of both the ocean and freshwater resources will be as detrimental as projected, or if mankind can do something about it. This crisis will affect all of us, if not directly. We need to think and act now.