Science 7–10 · Years 7–8

Pond water under the microscope: protists, algae and the kingdoms of life

Science understanding: Biological sciences

PracticalLow risk

The idea

Living things include single-celled organisms that move, feed and photosynthesise, and they are classified into kingdoms by cell structure rather than by size.

Safety card

Low riskAn adult supervises

Hazards

Pond water can carry pathogens
Broken coverslips

Controls

No mouth pipetting; cover cuts with waterproof dressings
Wash hands after the lesson and wipe benches with disinfectant
Water and slides go to the technician for disposal, not down the sink untreated
Nothing from the sample is cultured

Note

No hazardous chemicals; record a RiskAssess risk assessment for the activity as school procedure requires.

What you need

Pond or aquarium water with a little bottom sediment and a strand of water weed, 100 mL in a jar, collected within a day of the lesson
Plastic pipettes, plain slides, cavity slides, coverslips, and a cotton wool fibre or a drop of methyl cellulose solution to slow fast swimmers
Compound microscope with 4x, 10x and 40x objectives and a 10x eyepiece
Pictorial guide to freshwater microorganisms, and Atlas of Living Australia for checking algae and invertebrate names

How to do it

Draw a drop from the sediment layer with the pipette and place it on a slide with a coverslip; make a second slide from water squeezed off the weed.
Scan at 40x total magnification to find moving objects, then centre one and change to 100x and 400x.
Add a cotton fibre under the coverslip or a drop of methyl cellulose if organisms cross the field too fast to observe.
For each organism record: does it move, and how (cilia, flagellum, crawling); is it green; is it a single cell or a filament or colony; is it segmented with limbs.
Sort the record into the groups the NSW Cells and classification content names for an investigation of this kind: bacteria (mostly too small to identify at 400x), fungi, plant cells (fragments of the water weed) and animal cells (rotifers, water fleas, worms), and add a group for the single-celled protists a pond sample also holds (diatoms, filaments such as Spirogyra, and protozoa such as Paramecium). A pond drop at 400x offers no character that separates a fungal thread from a colourless bacterial sheath, an oomycete thread or a fibre of plant debris, so record a colourless thread as an unidentified filament rather than as a fungus. Kingdom is one of the ranks in the classification hierarchy the syllabus lists; the syllabus does not set out a five-kingdom scheme, so record the cell structures that placed each organism in its group rather than the name of a kingdom.
Estimate the size of one organism from the field of view: at 400x with an 18 mm field number the field is 0.45 mm wide.
Make a hanging drop on a cavity slide to watch a slow organism over ten minutes without it drying out.

What you should see

A sediment drop usually shows moving single cells such as the ciliate Paramecium, flagellates, green filaments such as Spirogyra, diatoms with glassy cases, and often a rotifer or water flea. Paramecium species range from 0.06 to 0.3 mm long, so a large one spans up to about two-thirds of a 0.45 mm field at 400x (0.3 divided by 0.45). The learner knows it worked when each organism is placed in a group by its cell structure and movement, not by its size, and when at least one organism is measured against the field of view.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

All microorganisms are bacteria or germs.
A single cell cannot move, feed or respond on its own.
Anything green is a plant.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC4-CLS-01 SC4-WS-01
Australian Curriculum v9AC9S7U01AC9S8U01AC9S7I03

Sources

The pages the author read to write this activity.

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