Lesson Plan: The Chalk

Subject areas: Biology, Earth science, chemistry, history of science Recommended level: Ages 12 to 18 (lower and upper secondary), adaptable for introductory undergraduate Duration: One to two 50 minute lessons

Learning objectives

By the end of this lesson, students will be able to:

1. Explain that chalk is calcium carbonate made from the skeletal remains of microscopic marine organisms (coccolithophores and foraminifera).

1. Describe how living things act as geological and chemical agents, building rock and influencing ocean chemistry and the carbon cycle.

1. Recount how Thomas Huxley used a piece of chalk to reconstruct deep geological history and to argue that science is organized common sense.

1. Summarise the 2026 finding that a gut bacterium helps marine fish produce calcium carbonate (ichthyocarbonate), and why that matters for ocean health.

1. Discuss biomineralization and symbiosis as recurring themes, and how closer observation keeps revealing smaller actors.

Background for the teacher

Chalk is a soft white limestone, almost pure calcium carbonate. In 1868 the biologist Thomas Henry Huxley, known as Darwin's bulldog, gave a famous public lecture, On a Piece of Chalk, to the working men of Norwich. He showed that chalk fizzes in acid (it is calcium carbonate) and, under a microscope, is built from the shells of countless microscopic sea creatures, chiefly single-celled algae called coccolithophores and protozoa called foraminifera. From this he reconstructed an entire vanished world: the chalk downs of England were once the floor of a warm sea where these tiny shells slowly rained down. He also noted that the same ooze is still forming on the deep-sea floor today. The deep idea is that life is a planetary builder: organisms pull carbon from seawater and air and lock it into stone.

For over a century, this carbonate-building was credited mainly to plankton. But bony fish also make calcium carbonate: to survive in salt water they drink seawater, and their guts excrete the excess calcium as solid pellets called ichthyocarbonates, a recognised contributor to the ocean's carbonate balance, studied notably by Martin Grosell's group at the University of Miami. In May 2026 a PLOS Biology study led by Anthony Bonacolta found that a gut bacterium, the vibrio Photobacterium damselae, lives in and around these pellets in the Gulf toadfish and carries genetic machinery linked to making carbonate, suggesting the fish do it in partnership with hidden microbes. The story rhymes with Huxley: the builders of the planet keep turning out to be smaller than we thought, and rarely working alone.

Warm up (10 minutes)

Give each group a piece of blackboard chalk and a few drops of vinegar. Watch it fizz. Ask: what gas is being released, and what does the fizzing tell us about what chalk is made of? Reveal that the white solid is calcium carbonate, the same as seashells.

Core activities

Activity 1: Chalk is alive (15 minutes). Show microscope images of coccoliths and foraminifera alongside a photo of the White Cliffs of Dover. Students estimate, roughly, how many microscopic shells might make up a single piece of chalk, and discuss what it means that a cliff is a graveyard of plankton.

Activity 2: Reading the world from a stone (15 minutes). Students list everything Huxley could infer from chalk: it formed in the sea, the sea was once where England is, vast time passed, life builds rock. Discuss the phrase science is organized common sense.

Activity 3: The fish stonemason (15 minutes). Diagram how a bony fish drinks seawater, removes excess calcium, and excretes calcium carbonate pellets. Add the 2026 twist: a gut bacterium helps. Students discuss what symbiosis means and why a partner organism might be hard to notice.

Discussion questions

1. How can living things create rock? Name two organisms that do it.

1. Huxley said the commonplace is a locked door. What did he mean, and how does the chalk example show it?

1. Why is it significant that fish, not just plankton, help make the ocean's carbonate? How might this matter as carbon dioxide rises and the ocean acidifies?

1. The carbonate was long credited to the fish alone. Why do hidden partners (like gut microbes) often go unnoticed in science?

Extension

• Research ocean acidification and how calcium carbonate helps buffer seawater chemistry.

• Investigate other examples of biomineralization (coral reefs, shellfish, the human skeleton) and of symbiosis (coral and algae, gut microbiomes).

Glossary

• Calcium carbonate (CaCO3): the compound that makes up chalk, limestone, and seashells.

• Coccolithophore: a single-celled marine alga that builds tiny calcite plates (coccoliths).

• Foraminifera: single-celled marine organisms with calcium carbonate shells.

• Biomineralization: the process by which living things produce minerals such as carbonate.

• Ichthyocarbonate: a pellet of calcium carbonate produced and excreted by bony fish.

• Symbiosis: a close, often mutually beneficial relationship between different organisms.

• Carbon sink: a process or reservoir that stores carbon and removes it from circulation.