What Lives in the Classroom Aquaponics Tank?

Use microscope and jar-test evidence to find the living things in a fish-and-lettuce system.

Part 1: The samples and the jar test

Read the passage and both tables. Use them to answer questions 1–5.

The Living Loop

1In the back corner of Mr. Obi Lindgren's sixth-grade classroom sits an aquaponics system, a way of raising fish and plants together. A 200 L tank holds twelve tilapia. A small pump lifts tank water into a grow bed filled with clay pebbles, where lettuce and basil grow with their roots in the wet pebbles. The water trickles back down into the fish tank, and the loop starts again. Fish waste would harm the fish if it built up, but Mr. Lindgren says tiny living things on the pebbles change the waste into nutrients the plants can use.

2Ximena Hart was not convinced. "I only see fish, lettuce and basil," she said. "The rest is water and rocks." So the class collected samples from around the system and looked at them under the microscope. They also used prepared slides of tilapia gill, muscle and blood from a science supply company, so no classroom fish was harmed.

3To test the slime on the pebbles, the class set up two jars. Jar 1 got slime rubbed off a pebble from the grow bed, plus tank water. Jar 2 got a brand-new clay pebble, plus tank water that an adult had boiled and cooled to kill anything living in it. Each jar was kept at room temperature with a loose lid. Every day for three days, the class put one drop from each jar on a slide and counted the tiny rods they saw in one field of view.

4One more sample came from the fish food. The class soaked a food pellet in water for a day. It swelled and turned mushy, but nothing in it changed in number or shape when they looked again the next day.

Table 1: The Class Microscope Log (400×; pebble slime at 1,000×)
Sample	What the class saw	Nucleus seen?
Green film on the tank glass	Separate round green cells; some split	Yes
Slime from a grow-bed pebble	Tiny separate rods, about 2 µm long	No
Lettuce root tip	Boxy cells in rows; long root hair cells	Yes
Tilapia slides	Gill, striped muscle and red blood cells	Yes
Soaked food pellet	Flakes and bits of old plant cell walls	No
Chip of a new clay pebble	Rough grains, no cell outlines	No

Table 2: Rods in One Field of View (1,000×)
Day	Jar 1 rods	Jar 2 rods
0	15	0
1	31	0
2	59	0
3	121	0

Use Table 2. Round to the nearest whole number.

On Day 3, Jar 1 held about times as many rods in view as on Day 0.

Use paragraph 3. Why did the class set up Jar 2?

A. To give the rods from the slime more space and water so that they could spread out
B. To check whether rods appear from a pebble and water alone, with no rods to start from
C. To show that boiled water helps the rods split faster than ordinary tank water does
D. To measure how much slime a brand-new clay pebble gives off when it soaks for a day

Which observation best shows that the green film is made of many one-celled organisms, rather than a sheet of cells from one large organism?

A. Each green cell lives apart from the others, and some split in two.
B. The film is green, like the leaves of the lettuce in the grow bed.
C. Each green cell has a nucleus, like the tilapia's red blood cells.
D. The film grows on the glass of the tank, close to the tilapia.

Circle each observation from Table 1 that is evidence that one organism is made of many different kinds of cells.

Gill, striped muscle and red blood cells on the tilapia slidesSeparate round green cells on the tank glass, some splittingBoxy cells in rows and long root hair cells in the lettuce rootTiny separate rods in the slime from a grow-bed pebbleFlakes and bits of old plant cell walls in the soaked pellet

The rods in Jar 1 roughly doubled each day. Suppose they keep doubling at the same pace. Use the Day 3 count.

On Day 4 the class would count about rods in one field of view.

Part 2: Living or not, one cell or many

Use the passage and both tables as evidence for questions 6–10.

Match each question the class asked to the observation that would answer it best.

Is a tilapia made of more than one kind of cell?
Is the slime on the pebbles alive?
Is each green speck on the glass a whole organism?
Is the fish food pellet alive now?

A. Compare the cells on the gill, muscle and blood slides.
B. Count the rods in a jar of pebble slime for several days.
C. See whether anything in the soaked pellet grows or divides.
D. See whether one green cell lives and splits on its own.

A chip of a new clay pebble showed no cells, but a used pebble from the grow bed was coated in slime full of rods. What is the best conclusion about the used pebble?

A. The pebble is nonliving, but one-celled organisms live and multiply on its surface.
B. The pebble makes new rods from its grains, since the rods appeared on its surface.
C. The pebble is a multicellular organism, since its slime holds many rods together.
D. The pebble came to life in the grow bed, since it is now coated in living rods.

Ximena says, "The food pellet shows bits of plant cell walls, so the pellet is a living thing." Which is the best reply?

A. Not quite: the pellet was alive while dry, and soaking it in water killed it.
B. You're right: a sample that shows parts of cells under a microscope is a living thing.
C. You're right: the pellet swelled in water, and swelling shows it is growing.
D. Not quite: those bits came from plants that were once alive, but nothing in it grows now.

Which list names only living things in the aquaponics system that the class found evidence for?

A. Lettuce, the food pellet, the clay pebbles and the tilapia
B. Tilapia, lettuce, the green film cells and the food pellet
C. Tilapia, lettuce, the green film cells and the pebble rods
D. Tilapia, lettuce, the clay pebbles and the pebble rods

10.

Ximena said, "I only see fish, lettuce and basil. The rest is water and rocks." Use cell evidence to argue whether she is right. Write a claim, give at least three pieces of evidence from the passage and both tables, and explain your reasoning about one-celled and many-celled living things.