How does rising atmospheric CO2 affect marine organisms?

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Center Experiment #1: Setup Directions


Setup Directions

Real-Time Results

Final Results


Pothos roots Our first experiment is designed to reveal the effects of atmospheric CO2 enrichment and depletion on the growth of the common Devil's Ivy or Golden Pothos (Scindapsus aureus) plant, with particular emphasis being given to a number of different root responses, for which purpose the Pothos plant is extremely well suited, as illustrated in the accompanying figure, which depicts the root systems of two such plants grown at mean CO2 concentrations of 196 and 752 parts per million in a prior experiment conducted by an eighth-grade science class.

The initial study produces sufficient preliminary data to be meaningful to students over a period of about six weeks; but the number of insights to be gained increases with time, so we collect and report data weekly for close to three months, after which we harvest our plants and make a number of other measurements on them.  As this experiment takes up a fair amount of space and requires the expenditure of a moderate amount of money, it is perhaps best suited for a school class project.  The next experiment is smaller in scope and much less costly; hence, it will be something an individual can easily and affordably do at home.

In conducting this experiment, we use six 10-gallon aquariums that are transformed into terrariums.  You can use more or less; but you should have at least three.  By maintaining the internal airspace of one of these "poor man's biospheres" at a CO2 concentration close to today's global mean of approximately 370 ppm, and by maintaining a second tank at a concentration about double that value, you will be able to see if Pothos plants are likely to fare any better or worse than they do now in the expected high CO2 world of the future.  Likewise, by maintaining the atmospheric CO2 concentration of the third aquarium at approximately half of today's mean global value, you will be able to see how Pothos plants performed at the peak of the last great Ice Age some 18,000 years ago, when the air's CO2 content was only about 182 ppm.  A fourth tank is also to be desired, for it is always interesting to see just how low an airspace CO2 concentration it is possible to achieve by the means we will use; and it is equally interesting to see the responses of plants to very low CO2 concentrations.

The basic container or "biospheric shell" for each unit of this experiment is the common 10-gallon aquarium, which can be purchased at nearly any pet supply store for about $10.  Each of these aquariums requires two 20-inch strip-lights, housing 15-watt fluorescent light bulbs.  These fixtures, costing just under $24 each, come with their own lights.  If desired, however, the bulbs they contain may be replaced with bulbs that mimic the visible spectrum of sunlight; but such bulbs can be expensive, costing on the order of $18 each, or $36 per aquarium.  We use such bulbs in this experiment, but the $216 they add to the cost of a six-unit study may be prohibitive to many of you.  For most purposes, the bulbs that come with the light fixtures should suffice.

While at the pet supply store, you should also purchase approximately 67 pounds of natural aquarium gravel for each of your experimental units at a cost of approximately $20 per 100-pound bag, as well as six feet of flexible air tubing per tank.  Silicone tubing is best, as it is most flexible.  It can be bought in 20-foot packets at $4 each.  Before leaving the store, you may also want to purchase a small thermometer for each aquarium.  Although it is not required for this study, it could prove useful at a future date, if the experiment were to be repeated at a significantly different temperature, which would enable you to explore the subject of CO2-temperature interactions.  Suitable thermometers can be purchased for approximately $3 each.

Other materials required for each tank in this experiment are two 0.5- to 1-liter water bottles or glasses made of plastic or glass, enough clear polyethylene plastic sheeting to cover the top of each tank, enough black felt to cover the front glass of each aquarium from side to side and from its bottom to a height of 18 cm, as well as a large role of clear cellophane tape and some masking tape.  The bottles or glasses may be obtained for less than a dollar each at a drug or food store, where the tape may also be obtained for very little.  The plastic sheeting, of 3- or 4-mil thickness, will most likely be found at a hardware or home supply store, where 10- by 25-foot sheets, packaged in a compact roll only 16 inches long, sell for approximately $6; while the felt can be purchased at a fabric store for about $5 a square yard.

Last of all are the plants.  We plant seven individual leaves cut from a vine of a mature Pothos plant in each of the aquariums we use.  These leaves should come from vines that allow you to have about a centimeter of vine on each side of the leaf stem where it joins the vine.  Determining how many of these leaves you will need will enable you to determine how many plants you should buy, once you see what they look like.  Local nurseries are good sources of Pothos plants, as are some home supply stores.  Individual plants may cost in the range of $5 to $15, and may supply enough leaves for one to three aquariums.

Once everything is assembled, as described in detail in our Experiment #1 Setup Directions, you will need two other items.  One is a ruler, which is used to make weekly measurements of the root lengths of two of the plants in each tank; and the other is a simple colorimetric test kit, which is used to estimate the CO2 concentration of the air to which the experimental plants are exposed.  For this job we employ the CO2 test kit of the TetraWerke corporation of Melle, Germany, which most large pet supply stores can obtain for you for about $13 per kit.  Over the course of the current experiment, you will need, on average, two of these kits per tank.  It is wise, however, to have some extra ones on hand; and you can always use the left-overs in subsequent studies.

Adding up all essential expenses, the cost of our first six-unit experiment runs about $660, or approximately $110 per "biosphere."  To this six-unit figure we add another $216, or $36 per tank, to trade out the bulbs that come with the light fixtures for bulbs that more closely mimic the solar spectrum.  Hence, as this first experiment is somewhat expensive, you may instead want to begin with our much less expensive second study (Experiment #2: Introduction).

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