propose environmental problems associated with the oxygen tolerance of nitrogen fixers and denitifiers
· subculture bacterial samples as necessary
· prepare media
· stabilize temperature of incubator
· media (see Madigan, Jbact, 1984)
· disposable 1 ml pipettes
· biohazard bags
· Clorox or a steam sterilizer
· autoclave gloves
· anti-bacterial soap
· ice bath
One day for introducing topic of nitrogen cycling and aseptic technique
One day for inoculating agar tubes with cultures
One day for observing results and writing laboratory reports
Engagement and Exploration (Student Inquiry Activity
Begin by asking students where the nitrogen in our atmosphere originates. How does it get there? Does it always comprise approximately 79% of natural air? What would happen if the nitrogen concentration in air decreased? Is the nitrogen cycle related to any other element cyles?
Lecture on the nitrogen cycle, emphasizing nitrogen fixation and its counterpart, denitrification. Does oxygen play a role in the cycling of nitrogen? Students will be surprised to learn that it does! Introduce the concept that bacteria carry out the processes of nitrogen fixation and denitrification. Are all bacteria capable of fixing nitrogen or denitrifying? If not, are there different tolerances to the presence of oxygen as it affects nitrogen-fixation?
Elaboration (Polar Applications)
Explain that the bacteria that live in the suboxic region of Lake Bonney in the Dry Valleys of Antarctica, are capable of denitrification. Provide a detailed description of the lake [two lobes (East and West) separated by a shallow sill; each lobe is 40 m and is permanently covered by 4 m of ice; bacterial populations peak at between 12 and 20 m). While denitrification is difficult to measure without sophisticated instrumentation, we can observe the opposite process of nitrogen-fixation.
Exchange (Students Draw Conclusions)
In their lab reports, students will answer the following questions:
1. Does oxygen play a role in the process of nitrogen-fixation?
2. Which organisms were the most oxygen tolerant? Which were the least oxygen tolerant?
3. Rate the oxygen tolerance of all of the organisms that you tested on a scale of 1 to ??? (this depends on the number of organisms that you included in the experiment) with 1 being the most tolerant.
4. What general conclusions can be drawn about the denitrifiers in Lake Bonney and their tolerance for oxygen? Can any conclusions be drawn?
Evaluation (Assessing Student Performance)