Read the lesson overview
This 5-E lesson (based on Next Generation Science Standards) is designed to engage students in learning how to be responsible for changes in local water quality by indirectly studying cyanobacteria blooms (blue-green algae). Freshwater blooms are responsible for degradation of water quality in many lakes, streams and rivers nationwide. These blooms interfere with recreational activities, produce toxins that can be a health hazard for animals and humans, can cause low oxygen conditions which, in turn, can be a determining factor in local fish kills and can upset the balance of the waterway ecosystem. Cyanobacteria will not be directly used in this inquiry-based activity since they often produce toxic substances and should not be handled without appropriate protections. Students will learn about the structure and function of algal cells through models, compare duckweed (aquatic plant) to cyanobacteria, experiment with simple variables to alter growth of duckweed relating those variables to conditions that may also alter cyanobacteria growth and help scientists identify, from Chowan River micrographs, the number of nitrogen-fixing cells present in algal blooms over the past few years. It is assumed that facilitators will have basic classroom supplies and teaching equipment such as display tools, paper, pencils, etc., so those items have been omitted from suggested supplies for this unit.
IMPORTANT NOTE about using duckweed: It is recommended to use locally obtained duckweed that would be found in ponds and streams from your area (Lemna perpusilla for Northeastern North Carolina). There are many different species of duckweed, and unfortunately, the species that is sold from biological supply companies (Lemna minor) is considered an invasive species in North Carolina. If you use purchase Lemna minor, do not dispose of it by releasing back into the local environment – let it dry out and die, then put in the trash. If you are unsure of the species of duckweed you may have obtained even locally, best not to add to the invasive species concerns, but rather just dry out and dispose.
“Engage” transitions from “I tell them or show them” to helping students reflect on what they already know and ask questions about what they don’t yet understand, which propels them toward an initial feeling of dissatisfaction.
Engage Part 1 – Dish Duckweed
Set this up about a week or two in advance of the unit on cyanobacteria (before the students are asked questions about the differences and similarities between cyanobacteria and duckweed). Recommended time frame – early fall or late spring when the air temperatures are not too cold.
Materials: Petri dishes, tweezers, local duckweed if available, pond water (best) or fresh water (distilled is fine), hand lenses (optional), and growth chart for data collection.
Order duckweed from Carolina Biological if you cannot obtain locally (see note above).
Rationale: Most students have had very little opportunity to observe division of cells to create new growth. Duckweed can be effectively used to 1) encourage students to make long term observations about the growth of duckweed recording daily changes 2) help students discern the difference between duckweed and cyanobacteria (often duckweed appears as a surface scum on the water and can be mistaken for cyanobacteria blooms).
Procedure: Using tweezers, gently place one or two duckweed plants in a petri dish with local pond water. Record the count of duckweed fronds as the initial count. Place petri dish in a location that receives good sunlight. Students can make observations daily or every other day to record any new daughter fronds that are appearing (a hand lens may be helpful). Record observations.
Engage Part 2
When students have successfully observed reproduction and growth in duckweed plants (from above), divide students into small groups of 2-3 students. Pass out Post-it notes and markers.
Facilitator – Show students pictures of surface duckweed and blue-green algal blooms using video display.
Facilitator – Question to students: What ways could a person tell the difference between the blue-green algal blooms and the duckweed?
Allow students to use online materials/web resources to find out more about blue-green algae and duckweed (15-20 minutes). Each group will post one similarity between duckweed and blue-green algae and one characteristic of duckweed and of blue-green algae that is not similar.
Facilitator – Draw Venn diagram on surface, large enough for Post-it notes to be displayed (colors are optional).
This would be a good time to review or introduce any new terminology for students prior to their web search (this could be done as a handout so they understanding the meaning of some of the technical vocabulary they may come across as they research). Download Suggested Vocabulary list.
Ask student groups to place Post-it notes with their differences and similarities on the Venn diagram. Facilitator clarifies any misconceptions and emphasizes key points highlighting the structural differences and similarities, the ecological needs and impact of these organisms.
“Explore” moves away from thoughts such as “I give them,” “I demonstrate,” or “They look at a model” and toward students themselves unpacking the problem, developing a model, and gathering data.
Explore Part 1 – Display your Data
Continue to observe and collect data on the duckweed in a dish from above. Create displays of data collected (graphs, pictures, data tables, etc.).
Facilitator – Ask student groups to create a display of the duckweed growth data that they have been collecting. You may use large Post-it notes and markers (rulers may be helpful), white board or other projection. Provide examples of types of graphs for students if they have little experience with graphing – bar graphs and line graphs (with independent and dependent variables). Displays may include digital pictures that students have taken (will vary due to school or home resources). Post the student displays on classroom wall or electronically.
Explore Part 2 – Cells on a String
Materials: long pipe cleaners, small cup, bag of pony beads with fluorescence, bag of whitish colored regular pony beads, bag of oval-shaped pony beads and UV flashlight. Each group of students (2-3) gets a small cup filled with 20-25 white pony beads, 10-15 fluorescent pony beads, 5-10 oval pony beads and several pipe cleaners. Pony beads can be reused, but the pipe cleaners will need to be replaced each time this activity is repeated. This can also be individual work rather than group work – you will just need more materials.
Procedure: Students will make a model of a blue-green algal strand (Dolichospermum sp.) by putting as many beads as they want on their pipe cleaner. They can bend it anyway they would like when they are finished. After students have completed their cyanobacteria models, show digital micrographs of the many different forms of Dolichospermum sp. and compare student models to actual shapes of this type of cyanobacteria.
Place student models in a dark corner of the classroom, and use black light flashlight to highlight (fluoresce) the heterocysts on student models. Ask students to return to seats to sketch and label their model with the heterocysts, akinetes and vegetative cells to match their constructed model. Provide paper and colored pencils as needed. This is a model – be sure to clarify that heterocysts on real strands of Dolichospermum sp. do not fluoresce.
“Explain” no longer means turning and talking, having a carousel discussion, or asking questions like “What did” and “What was.” Now, it means digging deeply into where the question has been answered or the problem solved, and using evidence to support claims.
Explain Part 1 –Getting it straight
Teacher initiates and facilitates discussion by referencing the Post-it note student responses comparing the similarities and differences between blue-green algae and duckweed. Some suggested starter questions may be 1) How might duckweed and blue-green algae be similar in terms of structure, what are their basic requirements for living, how do they reproduce and what is their role in the ecosystem? 2) How might they be different (focused on the same questions as above)? The purpose of this discussion will be to clarify student understanding, resolve student misconceptions, and to direct students toward developing an investigation into environmental factors that contribute to successful growth of blue-green algae and/or duckweed.
Possible Facilitator Questions to Guide discussion:
- What are some of the more obvious physical differences between the cyanobacteria and the duckweed? A: duckweed are visible to eye, appear to be small plants with roots and leaves, cyanobacteria are microscopic.
- What are some of the essential requirements for growth for the cyanobacteria and the duckweed? Are they the same or different? A: Both need light and nutrients, both need a water environment, both have chlorophyll for photosynthesis.
- Is what other ways are the cyanobacteria similar to the duckweed? A: Both reproduce asexually, both can multiply cells rapidly.
- In what other ways are the cyanobacteria different from the duckweed? A: Cyanobacteria can produce toxins and duckweed do not produce toxins, some cyanobacteria are capable of producing their own nitrogen as needed when their environment is low in this nutrient.
Explain Part 2 – Team Investigations
Dig deeper with the discussion to ask students to generate ideas for controlling the algal blooms? What variables in the environment could be manipulated or controlled to reduce the growth of cyanobacteria? From student research and from duckweed observations, what evidence to you have that these variables may affect the growth of the cyanobacteria?
Using the student generated list of independent variables, students (teams) will develop an experimental idea to accelerate the growth of duckweed. Several experiments (in your classroom) can be conducted at the same time depending on classroom space (assign students to work in groups on these investigations – number of students in each group is determined by how many separate investigations are ongoing), and additional materials may be used depending on student experimental design (simplify by using a bright windowsill rather that a plant grow light, etc.). The facilitator needs to spend time with student groups to formulate a clear problem statement or question, state a hypothesis identifying dependent and independent variables in their investigation including comparisons (or controls). Students may also need some guidance on describing and implementing their methodology, and ways to document observational data. Allow several weeks for daily data collection on student investigations.
Possible Student Investigations:
- Students compare duckweed growth in light, partial shade and total shade conditions.
- Students compare duckweed growth under artificial light (grow-light) to natural sunlight
- Students compare duckweed growth with fertilizer added to water to growth with no fertilizer.
- Students vary the amount of fertilizer pellets to compare growth.
- Students compare duckweed growth in petri dishes placed in areas with different temperatures.
Introduce the Great Heterocyst Count to students and show them how to participate. Optionally, the teacher can include additional content knowledge depending on grade level of students focused on the following:
Cyanobacteria Biology/Ecological Impact (Some content resources are provided) in Reference section.
- Cell structure – prokaryotic, other cellular organs and their importance. Specialized cells of the colony that have unique jobs (N2 fixation heterocysts, akinetes and other)
- Appearance in the waterways – clumps, strings and a variety of different shapes of the cells as they grow together. Coloration and how they look in the water column without magnification. View pictures of morphological types for Dolichospermum.
- Ecology – short description of ecological role of cyanobacteria as well as unique capability of N2 fixation in some genera depending on the limiting nutrient in waterways. Describe bloom conditions that are ideal for cyanobacteria growth and the corresponding depletion of O2 in the water as the cells die.
- Impact of Algal blooms – discussion of hazards from toxins produced, overview of areas that have significant ecological disruption due to cyanobacteria growth (Florida, Lake Erie, etc.), water supply contamination, recreational and economic concerns of localized algal blooms.
“Elaborate” is less about reading, watching or introducing new ideas, and more about forging the incredibly valuable concept-to-self, concept-to-concept and concept-to-world connections that help tie anchor and investigative phenomena together.
Elaborate Part 1
Student data from duckweed investigations is analyzed and each group responsible for one of the duckweed investigations should prepare a presentation to share with the class including the basic format of scientific design: Introduction and Rationale, Investigative Problem or Question, Hypothesis, Materials and Methodology, Results (with data tables and graphs), Discussion of outcomes, and Conclusion.
Elaborate Part 2
Each student group should include in their presentation a plan for reducing the potential of cyanobacteria blooms – what would they recommend as strategies for minimizing algal growth? Each plan should have two specific suggestions that could be implemented locally to control bloom outbreaks.
Groups may choose to prepare their presentation using Powerpoint©, Prezi©, or other similar graphics tool. Allow each group 10 minutes to share their work orally with the entire class. Print graphic presentations as posters or other formats if possible for display.
“Evaluate” cannot simply mean vocabulary assessments or graded journals anymore; now it means reflecting critically on the investigative process, the hypothesis, and the anchor phenomena.
Evaluate Part 1 – Self Assessment
After all the presentations have been given, each group should complete the following self-assessment by talking through each of the following questions;
- Was our investigative question or problem clearly stated and implemented?
- What was the clearest outcome from our investigation?
- What was somewhat confusing from our investigation?
- What might be changed to make this a better investigation?
Evaluate Part 2 – Group Feedback
Teacher provides each group an identifying number that only that team (group) will know. Teacher anonymously assigns other groups (at least 2 groups) to provide feedback to the specific investigation by providing the above questions on a feedback form to be completed, turned in to the teacher who will read for completeness/accuracy/integrity, and then will make the constructive feedback available to each team.
Copyright Colleen Karl 2020