Population Growth in Euglena: A Student-Designed Investigation Combining Ecology, Cell Biology, & Quantitative Analysis
Oswald, Christine, Kwiatkowski, Stephen, The American Biology Teacher
The National Science Education Standards (NSES) stress "science as inquiry" as crucial to science education at all levels (National Research Council, 1996). Often, a tension exists between allowing students the freedom to pursue questions entirely of their own design and the practical considerations associated with materials, space, and the instructor's ability to keep track of many diverse experiments, particularly in a class with multiple sections. Here, we describe the use of Euglena for student-designed investigations of environmental factors that affect population growth. This project allows each group of students to choose topics of interest to them, but within the confines of a relatively simple cell culture system that requires little space for each student group, and at a fairly low cost. We have used this as a term-long project in an introductory college course for biology majors (six sections of 24 students), but it could be adapted for use in high school classes as well.
The NSES specifies several fundamental abilities and concepts that underlie the "science as inquiry" standard. This project addresses several of them, including identification of questions that guide scientific investigation, design and performance of scientific investigations, communication and defense of a scientific argument, and use of mathematics to improve investigations. The NSES content standards state that "mathematics is essential in scientific inquiry."
The repeated use of calculations throughout data collection and the application of population-ecology mathematical models to the final data set address this standard in a direct way. Students collect a fairly large amount of data for statistical analysis at whatever level the instructor thinks appropriate for the students' grade level. The NSES content standards also refer to the relationship between the techniques and technology used and the quality of the results obtained. In this project, students have ample opportunity to assess their data-collection techniques, particularly sampling procedures, and discuss how they may influence the outcome.
An additional educational benefit of this project stems from its long-term nature. Students collect data from multiple replicates over several weeks. This provides them with a more realistic taste of scientific research than 3-hour lab exercises do. This also generates a great deal of raw data that must be organized, tabulated, and analyzed using a spreadsheet.
Students develop questions ranging from straightforward investigation of one particular nutrient on Euglena growth to more complex questions about the interactions of multiple factors, such as light and carbon source. The effect of pollutants known to be harmful in aquatic ecosystems is a popular choice. This is also a very appropriate choice, given the importance of Euglena as an organism for bioassay of environmental toxins (Danilov & Ekelund, 2001; Streb et al., 2002; Ahmed & Hader, 2010). This practical application gives real-world meaning to their experiments and enhances the interest of many students in the project.
In addition to the experience of designing, conducting, and reporting on their own scientific investigation, students gain specific knowledge and skills in the following areas:
* Techniques for handling cultures
* Use of micropipettes
* Preparation of dilutions and performance of calculations related to dilutions
* Counting cells in a sample and calculation of an estimate of the population density
* Knowledge of some different types of liquid culture media and how to prepare them
* Information on the biology of Euglena
* Application of population growth models to their data set
* Calculation of percent error
In addition, we stress the importance of searching the literature for previous studies related to the students' experimental question. …