How does rising atmospheric CO2 affect marine organisms?

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McKemy Junior High School:
Pothos plants in six 10-gallon aquariums



Introduction

Experiment Description

Results


Experiment Description

Our first experiment was conducted between 1 September 1997 and 1 December 1997.  It was essentially identical to Center Experiment #1, so I will not repeat here the setup instructions that are described there, other than to say that the study was launched by a group of students who volunteered to come in and set it up after school, which process took two 1.5-hour sessions.  I have subsequently had other classes set up experiments during regular classroom periods, and this approach has worked well too.  In all cases, however, the Monday, Wednesday and Friday biospheric airspace CO2 concentration measurements have always been made by the students at the start of regular class periods.

Figure 1The results of this first study were truly amazing.  The students were able to see significant differences in the plants of the different CO2 treatments just by looking at them, such as the lengths of the new vines that sprouted from the parent leaves of each experimental unit and the numbers and sizes of the new leaves produced by these vines.  Figure 2 The most visually striking observations, however, were reserved for the time of harvest, when the experiment was terminated and the plants were removed from the aquariums that housed them, exposing their vastly different root systems (see Figures 1-6).  Figure 3 In each of these figures, the experiment-long average airspace CO2 concentration is shown just below a typical plant retrieved from that CO2 treatment.  Figure 4 In the case of Figure 1, however, no plant is shown, because the average airspace CO2 concentration in that unit was too low to produce any new growth.

Figure 5Harvesting procedures were much more time-consuming than experimental setup procedures, requiring several after-school sessions to obtain all the data that could be derived from the plants.  Figure 6 The students began by removing the polyethylene film that covered each aquarium (see Figure 7), carefully loosening the sand from around each plant's root system while slowly removing the plant from the tank (see Figure 8), and then further removing any remaining sand from the roots in a pan of clean tap water (see Figure 9).

Figure 7Figure 8For each of the eight plants of each aquarium, the students then measured the lengths of each primary, secondary and tertiary root, which also gave them the numbers of each of these types of roots.  Figure 10 depicts this procedure, where two students are measuring root lengths and their partners are recording the data they call out to them.  Figure 9 For very large plants with many roots, this procedure takes a fairly long time.  Hence, to keep the plants from drying out while the root length data are being acquired, the plant is typically kept in a shallow pan of water, while its root system is removed piece by piece.  Figure 10 As this step is completed for each segment of the root system, the individual batches of roots are placed in a cup in a balance and weighed (see Figure 11).  Figure 11 These root batches are then placed in paper bags -- one bag per tank for all primary roots, one bag for all secondary roots, and one bag for all tertiary roots -- after which the bags of roots are placed in an oven and dried prior to weighing them a second time to determine plant dry weight production over the course of the experiment.

Figure 12The students next removed each new leaf that was produced during the study, traced its outline on a sheet of paper (see Figure 12), and obtained its individual fresh weight (see Figure 13), after which all new leaves from all of the plants in a specific aquarium were put in a paper bag for drying and ultimate re-weighing to obtain the dry weight of new leaf material produced in the tank.  Figure 13 They then measured the length of each new vine, obtained its fresh weight, and put all of the vines thus obtained from all plants in each aquarium into a paper bag for drying and ultimate re-weighing.

When all plant material was finally in the oven and drying, the students returned to the leaf outlines they had traced on paper and cut them all out and weighed them.  Then they cut out several square pieces of the same type of paper of known area - 2 cm x 2 cm = 4 cm2, 3 cm x 3 cm = 9 cm2, etc. - and weighed them.  By this means they "calibrated" the balance, so that by weighing the paper cutouts of the leaves they could determine each leaf's area.


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