Printer Friendly
Activity: Algae Identification with Dichotomous Key
NGSS Science and Engineering Practices
NGSS Crosscutting Concepts
NGSS Disciplinary Core Ideas


  • Different types of algae
  • Seawater
  • Containers for algae
  • Dichotomous key to local algae taxa (genus level). Table 2.6 is a key to common Hawaiian intertidal and subtidal algae
  • Additional information about algae species (e.g., books and online resources)
  • Petri dishes or microscope slides
  • Vinegar
  • Plastic pipettes or droppers
  • Scissors



  1. Choose an algae species to examine.
  2. Use the dichotomous key (Table 2.6 is a key to common Hawaiian intertidal and subtidal algae) to determine the genus of the algae species you have chosen to examine.
    1. Read the first two lines of the dichotomous key and choose the most appropriate statement about your alga. Record your response and proceed to the numbered step given at the end of the line you chose. Note that the first step in the dichotomous key asks whether or not the alga is calcified. Proceed to step 2b if you are unsure whether your alga is calcified, otherwise proceed to step 2c.
    2. Determine if the alga is calcified.
      1. Cut a small piece of the alga thallus, approximately 1 cm x 1 cm in size, and place in a petri dish or on a microscope slide.
      2. Using a dropper or a pipette, apply 3–5 drops of vinegar to the alga.
      3. Observe the sample for bubble formation. Bubble formation indicates that calcium carbonate is present.
      4. Decide if your alga is calcified.
    3. Proceed to the next set of two steps in the dichotomous key indicated from step 2a. Choose the most appropriate statement about your alga. Continue following the dichotomous key and recording your process as you work until you reach an alga genus. If you are unsure of a particular step, mark that step, and then use your best judgment to proceed.
  3. Once you have determined an alga genus, refer to additional resources and compare your identification with the available information.
    1. If the identification does not seem reasonable, go back to the recorded steps you made when using the dichotomous key. Consider any steps you were unsure about and try again.
    2. Repeat steps 2–3 of the procedure.
  4. If there is time, repeat Steps 1–3 for additional algae.


Activity Questions
  1. What are the advantages and disadvantages using a dichotomous key to identify species?
  2. You validated your algae identification from the dichotomous key by comparing your identification to other available resources. How accurate were your identifications? If you felt your identification was not accurate, explain why and what steps you took to correct your identification.
  3. How did your actions using the dichotomous key reflect those of a professional scientist?
  4. Why is it important to be able to identify organisms?
  5. Optional: Using the resources available, learn about the typical characteristics of the genera you identified. Identify aspects of the habitat, morphology, and/or life history.
Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.