This is the second part of the ecocolumn project. in this lesson, students will be constructing the ecocolumns according to the experimental design guidelines they established in the previous lesson.
I will explain here how I intend to build the ecocolumn, though there are certainly many ways to do so. Please see the bottle biology website for a more complete description of the construction process as well as many other optional methods to make your ecocolumn.
As discussed in the previous lesson, certain details of the construction of the ecocolumns will be determined by the class as a whole, so I include here only my suggested guidelines for construction along with certain safety considerations.
Connection to Standard: This project will give students the opportunity to both design and follow a complex multistep experimental procedure. Additionally, they will practice writing over an extended time frame to document their sustained research.
Prepping the bottle
The first step to constructing both chambers is cleaning out the remaining contents and taking the labels off of the 2 liter bottles. I use a hairdryer to warm up the glue seam so that the label can be pulled off easily. Be careful not to warp the bottle with excess heat (at least a week prior to this lesson, I recommend students bring in at least one bottle* per student: with groups of 4 this provides at least 2 “back up” bottles in case of mistakes). I bring in a hairdryer from home, and although it might move things along faster if you had a second one, I just keep that single hairdryer plugged in at the front of the room near my desk and groups come up when they're ready to use it.
*It's important that the bottles students use are of roughly the same size and shape. I mean no disrespect to the Coca-Cola Company, they make a great cola, but their 2 Liter bottles are too curvy to work. When I first announce this project to my students a few weeks before we begin, I bring in two generic soda bottles from the local supermarket, encouraging students to use them because they can easily buy 2 for less than the price of a single name brand bottle. Whatever the brand, as long as they can fit together then there should be no problem.
Cutting the bottle
This chamber is a bit simpler to construct than the terrestrial chamber and needs to be constructed first as it ends up being the base of the whole ecocolumn. To make the aquatic chamber, you use the bottom of a 2 Liter bottle. Even though this is made from the bottom of a bottle and the aquatic chamber will be made from the top, I don’t recommend trying to use the same bottle for both chambers as you may need the additional length of the “body” of both bottles to establish your ecocolumn.
When students have established where they want to cut the bottle (my students chose to cut a line at 20 cm from the base of the bottle), I instruct them to have one group member turn the bottle slowly while another student holds a dry erase marker to the bottle and marks a line around the circumference of the bottle. When a group has marked their line around the bottle, I come by with a box cutter and start the cut on their line and let them complete the cut with their scissors.
Once your students have cut the bottle to use as their aquatic chamber, they will need to add a hole to insert the pipette for analyzing the microorganisms (which will be discussed in the next lesson). Different methods abound for doing this, including using a bunsen burner to heat a nail or teasing needle, but I recommend using a simple hole punch. Depending on the size, you may have to make a few punches to make the hole large enough to fit the pipette in (you might want to encourage students to check to make sure a pipette can fit before proceeding). Yes, it doesn’t look as neat as a hole made with a hot poker, but it’s easier to deal with. An important consideration here is to make sure the hole is above the water level or else you will have a very leaky bottle!
Stocking the aquatic chamber
Once the hole has been made, students can add gravel (again, they determine the amount). I recommend being fairly liberal with the amount of gravel in the aquatic chamber as it provides a bit of stabilizing bottom weight (which, trust me, you’re going to want to have if you have several of these in your class and you have multiple groups of students eager to see what the kids in environmental science are up to).
After you’ve added the gravel, your students add water (and any sand or silt if they chose to include it as part of the design). It’s entirely up to your class if you want to use tap water, bottled water, or water taken from the same sample that provided any sand or algae.
I have a large gallon water bottle filled with pond water and algae that students can pour through a strainer into a bucket (or sink if you’re not saving that water). The strainer or wire mesh will separate the filamentous algae from the water and allow students to make an accurate measurement of the mass of the algae they add to their chamber.
Although I do mention it’s ok for students to use any water they wish, I recommend encouraging them to add at least some of the water from the same source as the algae as it’s likely to be home to a more diverse group of species than those lucky enough to cling to the algae when their habitat went rushing through a strainer (generally speaking: greater species diversity = more interesting project).
This chamber is a bit more complicated than the aquatic chamber, but it follows many of the same steps as the aquatic chamber.
Cutting the bottle
Students make the terrestrial chamber out of an inverted top of the 2 Liter bottle. Since the seeds must be planted in soil that packs into the “bottom” (actual top), the bottle must be cut to separate the bottom and the top. I recommend cutting somewhere near the actual bottom of the bottle (where the bottom of the label would be) as this gives your students the most flexibility to cut it down to their preferred size with scissors (my students measured a line 20 cm from the bottle cap). Students follow the same guidelines described in the aquatic chamber section to remove their bottle’s label and mark their cutting line. Again, for safety reasons, I start the cut with box cutters and have students complete the cutting with scissors.
This chamber needs a drain
The cap of the bottle needs to have a largish hole punched into it to allow for drainage into the aquatic chamber. You could use a drill, an awl, or a heated teasing needle to make this hole, but I simply set the bottle cap top down on a small 2x4 and hammer a standard nail through it. For obvious safety reasons, you will want to do this rather than having your students do so. You also may want to do this ahead of time depending on how many groups you have. On the other hand, the occasional loud hammering noise does provide some sensory reinforcement that we're really building something.
Students can use push pins or a small nail to put additional drainage holes into the curved plastic at the bottom of their terrestrial chamber, just make sure students are aware that unless adding such holes is the specific independent variable they’re investigating, their holes need to be constant with the other groups and the teacher’s control group ecocolumn.
Stocking the chamber
After their bottle is cut and any holes punched, they attach the bottle cap and add the predetermined amount of gravel to the bottom of the chamber. After the gravel, they add the potting soil (the amount which, again, is up to your class to determine, but I recommend at least 10 cm of potting soil).
Finally, they’ll need to add the seeds and any additional ingredients (fertilizer, leaves, worms, etc). I recommend following the guidelines on the seed packet regarding depth to plant the seed (I use a chopstick to push them down into the soil about 1-2 cm). I recommend putting maybe a few more seeds than you think you may need, as it will be pretty disappointing for the students if none of their seeds germinate, but if your extensive gardening experience exceeds mine (which is only slightly greater than nil) and you have a better method to suggest to your students then, again, you do you (and please let me know about it in the comments… thanks!).
Once both chambers are assembled, putting them together is a fairly simple task. Slowly fit the inverted bottle of the terrestrial chamber inside the walls of the body of the aquatic chamber. I say slowly because it’s important that the cap at the bottom of the terrestrial chamber remains above the water level of the aquatic chamber. If the water level is too high, students should remove some of the water (keep in mind that this should be consistent for all groups).
Once the terrestrial chamber is snug within the walls of the aquatic chamber, you could use packing tape to secure both chambers together, but I have found that just pushing the terrestrial chamber down into the aquatic chamber until it's snug works fine (this also gives them more access to the aquatic chamber should they want to take a sample out of the range of a pipette).
Once your students are done, have them add any irrigating water and/or additional steps of their design and then place their completed ecocolumns in their appropriate locations (all of my groups chose to line them along the length of a large window in my classroom, wherever your decide to put them, you might want to remind your students to make sure they maintain a controlled experiment by ensuring all ecocolumns receive equal light).
Or... you could just tell them how to do it
One final consideration is the possibility of just giving the class predetermined specifications. It really depends on time. All told, the entire process from conception to completion of construction took almost 5 hours of class time. This probably could have been accomplished in 2 hours if I had limited discussion to what variable you'd like to test and what you think the variable's effect might be. Personally though, I think the time spent was worth it. The students really took ownership of the project and I think that will end up paying for itself in terms of the quality of their work going forward... this is their project.
This photo shows the final specs my students decided upon. I left this on the board as a reference for all students to use and had each copy their own version in their journal.
Whether you want to provide these specifications for reference or as instructions to your students, here's what my design team came up with: