Originally appears in the Spring 2011 issue

The ocean is home to a wide range of organisms, many which are of great value to people. Fisheries such as tuna and anchovy support national economies throughout the world. In addition, some marine organisms are vital to traditional cultures, such as whales to the Maori, while others have become icons in pop-culture—think Shamu, Jaws, or Nemo. These familiar ocean critters, however, would not exist without tiny marine organisms that are invisible without magnification. These microscopic organisms not only form the base of the marine food web but they also recycle essential nutrients, such as nitrogen, phosphorous, and carbon.

Microscopic life in the ocean is highly diverse, consisting of the single-celled prokaryotes, (bacteria and archaea), single-celled eukaryotes (like phytoplankton and protists) and multi-celled zooplankton. Each of these groups of organisms plays an essential role in the marine environment. Phytoplankton are primary producers, providing energy for the entire food web. Zooplankton and protists are consumers, and in turn, they serve as food for larger zooplankton or small fish. Bacteria and archaea are decomposers and recyclers, allowing important nutrients contained in waste to be recycled and used again by phytoplankton (known as the “microbial loop”). Plankton and bacteria together create an entirely microscopic food web composed of several trophic levels.

Since students may have difficulty conceptualizing what they cannot see, the following series of activities use an inquiry-based approach to help bring to life this “invisible”, yet essential world of marine organisms to any grade 6-12 classroom. Students are given the task of determining which types of microscopic organisms live in a local body of water – whether marine, brackish, or fresh water. They then design tools and techniques to collect data and draw conclusions about the different groups of aquatic microorganisms and how these groups interact.

Activities

Formulate a question

Show students a water sample from your local beach, bay, or estuary.  (These activities could alternatively be done with fresh water from a pond or lake, as the same general principles apply.) Ask if they can see anything living in the sample. Then ask if they think there is anything living in the sample that they cannot see. Have students brainstorm how they can figure out whether the sample contains living organisms and how they might determine what these organisms are.

Design a collection device

Students are given the task of designing a net or another device to collect a concentrated sample of plankton as a first step to answering the guiding questions described above.

Objectives:  Students will be able to design and use a tool to collect data.  They will gain/improve skills in scientific inquiry and gathering evidence.

Materials: Embroidery hoops, nylon stockings, string, scissors, tape, strong rubber bands, key rings, small plastic jars with lids and a lip around the edge.

Procedure:

1. Tell students their job is to figure out how to collect a sample containing enough microscopic organisms to examine effectively.
2. Students can use any of the materials supplied to “catch” organisms.
3. Let the students choose supplies and create their collection device; resist the urge to give students the “right answer”—the goal is for students to use reasoning to create a tool with a purpose.
4. When everyone has their first “prototype” constructed, allow students to test their devices in a local body of water.
5. Allow students to make changes and repeat collection until they feel they have attained a satisfactory sample, or as long as time allows.
6. Compare student nets to a real plankton net (photo, or the real thing), and if available, conduct a plankton tow with a real net.
7. Save samples for later analysis.

Hints: Students may be frustrated by the notion that there is no right or wrong way to complete the task. Having students write down their design rationale can help guide their thought process. Also, keep in mind that nylon stocking mesh will only collect the largest fraction of microscopic organisms. Ideally, you should also collect several samples with a fine-mesh plankton net for a more thorough analysis in the next activity.

 

Who’s who in the microbial world?

Students use microscopes to determine which types of microscopic organisms are present in their samples. The goal is for students to use reasoning based on observations to draw conclusions about how organisms in the samples “make their living.”

Objectives: Students will have a basic understanding of “planktonic” organisms, and that form is related to function.  They will be able to classify marine plankton and draw conclusions by thinking critically about observations.

Materials: Plankton samples (from their home-made nets as well as from a fine-mesh plankton net), classroom compound and/or dissecting microscopes, slides, slide covers, pipettes, KimWipes, sheets of blank paper, colored pencils, plankton ID guide for your local area.

Procedure

1. Ask students what the next step should be to determine whether microscopic organisms are present in their samples, and if so what types of organisms the samples contain.
2. Supply students with microscopes and outline proper use and safety procedures.
3. Have students examine samples under microscopes and sketch in color each organism they observe on a different piece of paper, labeling any features that may indicate how the organism moves, obtains energy, etc. Have students use their own samples first, then allow them to examine samples obtained with a real plankton net so that they can observe a greater diversity of organisms.
4. Place students in groups. Have each group think of 2 or 3 categories by which their organisms can be categorized, then have students group their organism sketches according to these categories. Typically, students will organize according to plant-like organisms, animal-like organisms, and in between or undetermined.
5. Allow groups to share their findings. Discuss how students decided on each category (e.g. based on form/function). Discuss the concepts of producers and consumers.
6. Have students use plankton identification guides to identify the organisms in their sketches to support or challenge their conclusions.

Hint: If you are unable to collect samples, you can use our series of plankton photographs for teachers and students that are posted at www.greenteacher.com in the table of contents page for this issue, or photographs of plankton found online elsewhere, or prepared slides available through science supply vendors.  In any case, have students follow the same procedure above.

Extension: What about the little guys?

Although bacteria are so small we cannot even see them with most microscopes, we can still determine whether they are present in a sample by culturing (growing) bacteria into colonies that are visible. Refer to web resources for instructions on culturing bacteria (if permitted at your school) to determine how many bacteria are in sample (e.g. Culturing Bacteria: www1.coseecoastaltrends.net/modules/marine_bacteria/access_classroom_resources/culturing_bacteria/)

 

The microbial food web

Students use sketches of organisms from Activity 2 to construct a microscopic food web.

Objective: Students will be able to use logic and reasoning to determine functional relationships.

Materials: Sketches from activity 2, tape, string

Procedure:

1. Divide students into groups.
2. Have students organize their sketches from the previous activity based on who they think each organism eats or is eaten by. Then have students tape their sketches onto a wall/board and connect sketches with string based on functional relationships to illustrate the microbial food chain.
3. Have groups present their food chain/web, explaining their design rationale. Discuss concepts of producers, consumers, and interdependence.
4. Ask students what would happen if any organism were removed.
5. Ask students if any organism is missing because they were not able to observe them under the microscope (bacteria, for example). Have students create sketches of bacteria to add to the food web.
6. Discuss the importance of microbes to larger ocean organisms; compare to land food webs.

Hint: Younger students may tend to group organisms by trophic level, connecting each trophic level with one piece of string. More advanced students can be challenged to consider multiple connections between organisms and to create a more complex food web.

Extension: It’s all connected

This activity illustrates the concepts of ecological balance, interdependence, and trophic energy transfer in marine food webs. Students play the roles of organisms representative of the different trophic levels under various conditions and manipulate organism ratios to achieve a balanced marine ecosystem. Available at: http://www1.coseecoastaltrends.net/ocean_science/ocean_primary_production/marine_food_web/it_s_all_connected_trophic_tag/

 

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Cassie Gurbisz is the Program Manager and Laura Murray, the Director of COSEE (Centers for Ocean Science Education Excellence) Coastal Trends in Cambridge, Maryland.   Debbie Hinkle is a Faculty Research Assistant, and Byron Crump is an Associate Professor at the University of Maryland Center for Environmental Science (UMCES) in Cambridge.

A team of educators and scientists from the COSEE Coastal Trends and UMCES developed the above activities. They were tested by over 450 students through the UMCES Horn Point Lab Science, Technology, Engineering, and Math (STEM) Education Program.