Practical 22 Marine Biology CAPS block 45 · Term 2 · 3 hours

Cyanobacteria and Phytoplankton — Identify, Draw, Divide, Bloom

Identify twelve things from a plankton sample — and catch the four that are not phytoplankton at all. Draw the four groups the way a scientist does. Follow a diatom family as its glass boxes shrink, generation by generation. Then grow a bloom in a jar, and watch what happens when it crashes.

What CAPS asks for

Investigations column, block 45: "Photo identification exercise (or with a microscope if suitable samples can be obtained) · Include drawing exercise in the practical." Resources: "Phytoplankton ID charts · Light microscopes or USB microscope device." The key concepts ask three times for "Diagram and photos to show structures" — for diatoms, coccolithophores and dinoflagellates.

The identification exercise is Investigation 1 and the drawing exercise is Investigation 2. Investigation 3 models CAPS diatom concepts 7 and 8 — the shrinking chain and the auxospore. Investigation 4 grows a bloom to answer the booklet's questions on harmful blooms.

What you need

🌊 shore A plankton sample: tow the stocking net you built in Practical 17 off a jetty or through a rock pool, and keep the catch cool in a jar
🟠 hobby A student microscope (×100–×400) or a USB microscope — the same one used in Practicals 14, 17 and 18 — with slides and coverslips, a dropper and a little vinegar
🟢 kitchen A sharp pencil, plain paper, a ruler and an eraser for scientific drawings
🟢 kitchen Four identical clear jars (1 litre), water from a garden pond or a rock pool, liquid plant fertiliser, a teaspoon, a sunny windowsill, and the phone light meter and torch from Practical 20
🟢 kitchen Graph paper, a calculator and coloured pencils

No microscope, or no sample? That's fine.

Investigation 1 supplies twelve described specimens — what each looks like under the microscope, with its size — and the key to identify them. Investigation 2 lists the structures to draw. Investigation 3 needs only paper. Investigation 4 has two weeks of supplied readings from four jars. No mark on this sheet depends on owning a microscope.

Safety — water samples, fertiliser and night shores Pond and rock-pool water carry bacteria: never drink it, cover cuts, and wash your hands after handling samples. Wear gloves with liquid fertiliser and keep it away from small children and pets. Pour the jars out onto the garden or down a drain when you finish — never into a river, vlei or the sea, where a bloom you have grown could spread. If you go to see glowing surf at night, go with an adult, on a falling tide, and stay out of the water.

1 Identify a Plankton Sample CAPS: "Photo identification exercise (or with a microscope if suitable samples can be obtained)"

Aim

To identify the main groups of phytoplankton in a sample, using a key — and to separate them from the other small things a plankton net catches.

Method

Put one drop of your plankton sample on a slide, lower a coverslip, and scan the whole slide at ×100, then look closer at ×400.
For each different kind of organism, note its shape, its size (the field of view at ×400 is about 0.45 mm = 450 µm across), its colour, whether it moves and how, and what its wall looks like. Add a drop of vinegar beside the coverslip: chalky (calcium carbonate) parts fizz and dissolve.
Use the key below to identify each one, and write down the route through the key (for example 1b → 2b → 3a → 4a).
No microscope? Use the twelve described specimens.

A key to a plankton sample.
1a Cells too small to see as separate cells even at ×400 (under 2 µm); the water may look faintly tinted → cyanobacteria (picoplankton)
1b Cells or bodies clearly visible → 2
2a Many cells: legs, eyes or a gut — an animal → zooplankton (not phytoplankton)
2b A single cell (it may be joined in a chain) → 3
3a Inside a hard, clear glass (silica) case that does not fizz in vinegar → 4
3b No glass case → 5
4a The case is two closely fitting halves, like a pillbox or a boat; golden-brown chloroplasts inside → diatom
4b The case is an open lattice ball with spines; no chloroplasts; it catches food → radiolarian (a protist, not phytoplankton)
5a Covered with chalky plates or a chalky shell that fizzes in vinegar → 6
5b Not chalky → 7
6a A tiny sphere (under 20 µm) covered in overlapping round plates; chloroplasts inside → coccolithophore
6b A shell of many chambers, coiled; no chloroplasts; thin threads stream out → foraminiferan (a protist, not phytoplankton)
7a Two whips (flagella), one lying in a groove around the middle of the cell → dinoflagellate — with chloroplasts it is phytoplankton; without them it feeds like an animal
7b Anything else → another protist

Supplied results — twelve specimens from a sample off a Gqeberha jetty

A A round, flat glass case 60 µm across, seen from above as a disc with rows of tiny pores running out from the centre like spokes. Golden-brown chloroplasts inside. Seen side-on it looks like a pillbox.

B A boat-shaped glass case 45 µm long, with a slit along its middle. Golden-brown. It glides slowly across the slide.

C A chain of twelve short glass cylinders joined end to end, each 15 µm across. Golden-brown chloroplasts in each.

D A tiny sphere 8 µm across, covered in overlapping oval plates like hubcaps. The plates fizz away in vinegar, leaving a small green-brown cell.

E A cell 60 µm long covered in armour plates, with three long horns — one forward, two back. A whip lies in a groove around its middle and a second trails behind. Golden-brown; it swims in a slow spiral.

F A colourless, balloon-like cell 0.8 mm across — just visible to the naked eye — with one short tentacle and food particles inside it. At night, when the jar was tapped, the sample flashed blue.

G No cells visible at ×400, but the water has a faint tint. Filtered through a very fine filter and viewed under a special (fluorescence) microscope, it shows countless particles about 1 µm across, glowing orange.

H A pear-shaped cell 30 µm long with a groove around the middle and two whips, and no armour plates. Golden-brown; it swims fast. It came from a reddish-brown patch of water.

I An open lattice ball of glass 150 µm across with long spines. The cell inside has no chloroplasts and catches smaller plankton on sticky threads.

J A coiled shell of many chambers, 200 µm across, that fizzes in vinegar. Thin threads stream out through its holes. No chloroplasts.

K A tiny animal 250 µm long with one dark eye and three pairs of legs, jerking through the water.

L Needle-shaped glass cells 120 µm long and only 5 µm wide, lying in bundles side by side. Golden-brown chloroplasts.

Results

Table 1.1 — identification
Specimen	Route through the key	Group	Phytoplankton? (yes / no)
A
B
C
D
E
F
G
H
I
J
K
L

Analysis

Complete Table 1.1 for all twelve specimens — the route and the group. (One mark each.) 12
Four of the twelve are not phytoplankton. Name them, and say in each case what the specimen is and the feature that rules it out. 4
Specimen F keys out as a dinoflagellate, yet it is not counted as phytoplankton. Explain why, using your booklet's statement that "many, but not all" dinoflagellates are autotrophic. 2
Specimen I has a glass case like a diatom, and J a chalky shell like a coccolithophore. Give one feature that tells each apart from the phytoplankton it resembles. 2
About how many G cells would fit side by side across specimen A? Use G's size to explain why cyanobacteria are "not available to conventional food chains", and what happens to their food instead. 3
Name the kingdom CAPS gives for diatoms, coccolithophores and dinoflagellates, and the group cyanobacteria belong to. 4
Specimen L is a diatom, yet some needle diatoms like it make a toxin that builds up in mussels. Explain why a mussel farmer must monitor diatoms as well as dinoflagellates. 2

2 The Drawing Exercise CAPS: "Include drawing exercise in the practical" · "Diagram and photos to show structures"

Aim

To make labelled scientific drawings of the four groups, showing the structures CAPS and the booklet name, and to link each structure to its function.

Method

Draw from your own microscope, or from the descriptions in Investigation 1 and the drawings on the topic page. Use a sharp pencil, clean single lines and no shading.
Make each drawing at least 6 cm across. Label with ruled lines that do not cross and have no arrowheads. Give each drawing a title and a magnification.
The structures to label. Centric diatom (draw a valve view and a girdle view): frustule, the two valves (one fitting inside the other), girdle band, pores, chloroplasts, oil droplets. Dinoflagellate: armour plates, transverse groove with its flagellum, longitudinal groove with its trailing flagellum, horns, chloroplasts. Coccolithophore: coccoliths (chalk plates), the cell inside, chloroplast. Cyanobacterium (your booklet's diagram): slime sheath, cell wall, cell membrane, thylakoid membranes, nucleoid (no nucleus), ribosomes.

Analysis

Draw a centric diatom in valve view and in girdle view, and label six structures. 6
Draw a dinoflagellate and label five structures. 5
Draw a coccolithophore and label three structures. 3
Draw and label a cyanobacterium, as your booklet asks, labelling five structures. 5
Specimen A is 60 µm across. Measure your diatom drawing and calculate its magnification. Show your working, with units. 2
Explain how each structure helps the organism: (a) the pores in a frustule; (b) the oil droplets in a diatom; (c) the two flagella of a dinoflagellate; (d) the coccoliths of a coccolithophore. 4
The topic's slide deck draws cyanobacteria deep in the abyssal zone, "no light". Explain what is wrong with this, and what really keeps cyanobacteria out of ordinary food chains. 2

3 The Shrinking Diatom CAPS diatom concepts 7 and 8 · binary fission, the shrinking chain, and the auxospore

Aim

To model what happens to the size of diatoms as a population divides inside rigid glass boxes — and to work out why they must, sooner or later, reproduce sexually.

How a diatom divides

A diatom's frustule is a pillbox: a larger lid that fits over a slightly smaller base. Glass cannot stretch. When the cell divides, each daughter keeps one half of the old box as its lid and builds a new base to fit inside it. A daughter that inherits the old lid is the same size as its parent; the daughter that inherits the old base uses it as a lid — so it is smaller.

The model's rule. A cell's size is the width of its lid. Each new base is 1 µm narrower than the lid it fits inside. So a cell of size s divides into one cell of size s and one of size s − 1. Start with one cell 50 µm wide.

Method

On a large sheet of paper, draw the starting cell (50) at the top. Below it draw its two daughters (50 and 49), then their four daughters, and so on, down to generation 4 (sixteen cells). Write each cell's size inside it.
If you like, model it with two paper strips per cell — a lid and a base — each cut 1 cm narrower than the one it fits inside.
Complete Table 3.1, then extend it to generation 10 using the pattern you find.

Supplied results — the first three generations

Generation 0: 50 · Generation 1: 50, 49 · Generation 2: 50, 49, 49, 48 · Generation 3: 50, 49, 49, 48, 49, 48, 48, 47.

In real diatoms the shrinking stops when cells reach roughly a third to a half of their largest size. Then sexual reproduction produces an auxospore, which swells to the full size before making a new frustule — and the cycle starts again. In good conditions a diatom can divide about once a day.

Results

Table 3.1 — sizes by generation
Generation	0	1
Number of cells	1	2
Largest (µm)	50	50
Smallest (µm)	50	49
Mean (µm)	50	49.5

Analysis

Complete the family tree to generation 4 and list all sixteen sizes. 3
Complete Table 3.1. Describe how the largest, the smallest and the mean size change with each generation, and write a rule for each in terms of the generation number n. 4
CAPS says diatoms divide "with new individuals emerging smaller and smaller". Use your tree to show that this is only partly true, and rewrite the statement correctly. 2
Suppose an auxospore forms only once a cell is smaller than 20 µm. After how many divisions does the first such cell appear, and how many days is that at one division a day? After how many generations does the mean cell reach 20 µm? 3
Explain why sexual reproduction and the auxospore are essential to a diatom population. 3
Plot the largest, mean and smallest sizes against generation from 0 to 10 on one graph. 4

One word to watch. Your booklet and CAPS describe the auxospore as a structure that "can stay dormant until conditions are favourable". In most biology texts the auxospore's job is to restore the full size; the stage that sits dormant through bad conditions is usually called a resting spore. For this course, give the booklet's definition when asked for "auxospore" — but know both ideas.

4 Grow a Bloom CAPS dinoflagellate concept 3 (harmful algal blooms) · booklet "Test your knowledge" questions 1–4

Aim

To find out how extra nutrients change the growth of the microscopic algae in a sample of water — and what happens when a bloom dies.

Method

Fill four identical jars with 1 litre each of pond or rock-pool water, well stirred so each gets the same algae. Label them 0, low, medium and high.
Add liquid plant fertiliser: none; ¼ teaspoon; ½ teaspoon; 1 teaspoon. Stir, and stand all four side by side on a sunny windowsill.
Every two days, stir each jar, stand it on the phone's light sensor in a dark room, shine the torch down from a fixed height, and record the reading. Express it as a percentage of the reading on day 0. The greener the water, the less light gets through.
Carry on looking — and sniffing — for a week after the readings stop.

Supplied results — two weeks of four jars, and what came after

Data set 4.1 — light passing through each jar (% of day 0)
Day	0	2	4	6	8	10	12	14
0 (no fertiliser)	100	99	97	95	93	92	91	90
low	100	98	94	86	74	62	55	52
medium	100	97	89	72	52	38	31	29
high	100	97	88	70	49	35	30	31

After day 14. By day 18 the "high" jar had a green scum on top and a layer of dead, greyish algae on the bottom. By day 20 its water had turned brown-grey and smelt of rotten eggs. The "low" jar was still green and smelt only of pond.

Analysis

Plot the light passing through each jar against time, as four lines on one graph. 5
Describe and explain the differences between the four jars. 3
The "medium" and "high" jars ended almost the same, although "high" had twice the fertiliser. Suggest why. 2
Why did the light through the jar with no fertiliser fall at all? 1
Explain, step by step, what happened in the "high" jar between days 14 and 20, and why it smelt of rotten eggs. Link it to the rock-lobster walkouts your booklet describes. 4
The booklet's question 4: red tides seem to happen more often now than 50 or more years ago. Suggest three possible causes. 3
The booklet's question 3: write a paragraph explaining why phytoplankton can be both beneficial and harmful, with at least two advantages and two disadvantages. 4
The booklet calls the blue glow of dinoflagellates in breaking waves "phosphorescence", and defines phosphorescence as "a glow in the dark caused by exposure to radiation". Explain why that definition does not fit the breaking-wave glow, and give the better term. 3
The booklet's questions 1 and 2. Give the term for: (a) the protective, silica-rich walls secreted by diatoms; (b) a special structure formed by diatoms to remain dormant until favourable conditions return; (c) the ability of certain organisms to emit light from their bodies. Then define: (d) photosynthetic; (e) niche partitioning; (f) protists. 6

Memo — worked answers and mark allocation (does not print)

Total: 106 marks — Investigation 1: 29 · Investigation 2: 27 · Investigation 3: 19 · Investigation 4: 31.

Investigation 1 — identify a plankton sample

Q1 (12): One mark each for the correct group with a sensible route. A — 1b→2b→3a→4a, diatom (centric). B — 1b→2b→3a→4a, diatom (pennate). C — 1b→2b→3a→4a, diatom (a chain). D — 1b→2b→3b→5a→6a, coccolithophore. E — 1b→2b→3b→5b→7a, dinoflagellate (armoured, with chloroplasts). F — 1b→2b→3b→5b→7a, dinoflagellate without chloroplasts (Noctiluca, which feeds on other plankton). G — 1a, cyanobacteria. H — 1b→2b→3b→5b→7a, dinoflagellate (unarmoured; a red-tide former). I — 1b→2b→3a→4b, radiolarian. J — 1b→2b→3b→5a→6b, foraminiferan. K — 1b→2a, zooplankton (a copepod larva). L — 1b→2b→3a→4a, diatom (needle-shaped).
Q2 (4): F — a dinoflagellate with no chloroplasts, feeding on other plankton; I — a radiolarian: glass lattice but no chloroplasts, catches food; J — a foraminiferan: chalky chambers, no chloroplasts, feeding threads; K — an animal (copepod larva): many cells, eye and legs (1 each).
Q3 (2): Phytoplankton are the plankton that photosynthesise (1). F has no chloroplasts and has food particles inside — it eats other plankton, so it is a heterotroph, even though its body plan is a dinoflagellate's (1).
Q4 (2): I — an open lattice with spines, not a two-part pillbox; and no chloroplasts (1). J — a large coiled, many-chambered shell (200 µm), not a tiny sphere of separate plates; no chloroplasts; feeding threads (1).
Q5 (3): 60 µm ÷ 1 µm = about 60 cells (1). They are far too small for filter-feeders and zooplankton such as copepods to catch (1); instead they are eaten by tiny protists and their material is recycled by bacteria in the microbial loop, which returns nutrients to the water (1).
Q6 (4): Diatoms — Chromista; coccolithophores — Chromista; dinoflagellates — Protista; cyanobacteria — bacteria (prokaryotes, Kingdom Monera / Bacteria) (1 each).
Q7 (2): Mussels filter phytoplankton and concentrate any toxin they contain, so the toxin can reach people who eat them (1). Harmful blooms are not only dinoflagellates — some diatoms make toxins too — so a farm must watch every toxic group (1).

Investigation 2 — the drawing exercise

Q8 (6): Two views drawn with clean lines (1); any five labels correctly placed: frustule, the two valves (one fitting inside the other), girdle band, pores, chloroplasts, oil droplets (1 each, max 5).
Q9 (5): Any five correctly placed: armour plates (theca), transverse groove, transverse flagellum, longitudinal groove, trailing flagellum, horns, chloroplasts (1 each).
Q10 (3): Coccoliths drawn as overlapping round plates (1); the cell inside labelled (1); chloroplast labelled (1).
Q11 (5): Any five correctly placed: slime sheath (mucilage), cell wall, cell membrane, thylakoid membranes, nucleoid (no nucleus), ribosomes (1 each).
Q12 (2): Magnification = drawing size ÷ real size (1), e.g. 60 mm = 60 000 µm; 60 000 ÷ 60 = ×1 000 (1). Accept any correct calculation from the learner's own measurement, with units converted.
Q13 (4): (a) Pores let dissolved gases, nutrients and wastes pass in and out through the glass (1). (b) Oil is less dense than water, so it keeps the heavy glass cell floating where there is light (1). (c) The flagella let it swim up and down the water column — towards light by day and nutrients deeper down (1). (d) The chalk plates shade the cell from very bright light near the surface, letting it live where others cannot — niche partitioning (they may also protect it) (1).
Q14 (2): Cyanobacteria photosynthesise, so they live in the sunlit (euphotic) zone, not in the dark abyss (1). It is their tiny size — 0.2–2 µm — not their depth that keeps them out of ordinary food chains (1).

Investigation 3 — the shrinking diatom

Q15 (3): Generation 4 (16 cells): 50, 49, 49, 48, 49, 48, 48, 47, 49, 48, 48, 47, 48, 47, 47, 46 — that is one 50, four 49s, six 48s, four 47s and one 46 (3 for all correct; 2 for one slip; 1 for a partly correct tree).
Q16 (4): Table: cells 4, 8, 16, 1 024; largest always 50; smallest 48, 47, 46, 40; mean 49, 48.5, 48, 45 (2). The largest stays at 50; the smallest falls by 1 µm each generation: 50 − n; the mean falls by half a micrometre: 50 − n ÷ 2 (2).
Q17 (2): At each division only one of the two daughters is smaller; the other is the same size as its parent (1). Correct version: "at each division, one daughter is smaller than the parent, so the average size of the population falls and some cells get smaller and smaller" (1).
Q18 (3): The smallest cell is 50 − n µm, so it first falls below 20 µm when 50 − n = 19: after 31 divisions (accept 30) (1) — about a month at one a day (1). The mean reaches 20 µm when 50 − n ÷ 2 = 20: after 60 generations (1).
Q19 (3): Without it, the cells would keep shrinking until they were too small to survive (1). Sexual reproduction produces an auxospore that grows to full size and builds a new, full-size frustule, restoring the population's size (1). It also mixes genes, producing variation (and, as the booklet says, can carry the population through unfavourable conditions) (1).
Q20 (4): Axes labelled (1); three lines — largest flat at 50, mean falling gently, smallest falling twice as steeply (2); a key (1).

Investigation 4 — grow a bloom

Q21 (5): Four lines on one graph (1); a key (1); axes labelled with units (1); sensible scales (1); accurate plotting (1).
Q22 (3): The more fertiliser, the faster and further the light fell — the water turned greener (1). Fertiliser supplies the nutrients (nitrates and phosphates) that limit algal growth, so the algae multiplied faster (1). Growth was slow at first, then fast (days 4–10), then levelled off — the pattern of a bloom (1).
Q23 (2): Beyond a certain amount, nutrients were no longer the limiting factor (1); something else — light (the algae shade each other), carbon dioxide or space — limited the growth, so extra fertiliser made no difference (1).
Q24 (1): The pond or rock-pool water already contained some nutrients and algae, so a little growth happened anyway.
Q25 (4): The algae used up the nutrients (and shaded each other) and began to die and sink (1). Bacteria decomposed the dead algae, and their respiration used up the oxygen in the water — it became anoxic (1). Bacteria that work without oxygen took over and produced hydrogen sulphide, the rotten-egg smell (1). The same thing on a huge scale in the sea — a bloom dies off and bacteria strip the oxygen from the water — is why rock lobsters walk out onto the beach to escape (1).
Q26 (3): Any three: more nutrients reaching the sea from fertiliser runoff and sewage (eutrophication); warmer, calmer, more stratified surface water as the climate warms; ships' ballast water carrying dinoflagellate cysts to new places; more aquaculture and coastal development; better monitoring, so more blooms are noticed and recorded than before.
Q27 (4): Two advantages — any two: they are the primary producers that feed almost all marine food chains; they produce between 50 % and 80 % of the world's oxygen; they take up carbon dioxide; their remains form ooze and, in time, rock such as chalk (2). Two disadvantages — any two: blooms that die off cause anoxia and mass deaths; some form harmful algal blooms whose toxins harm animals and the people who eat filter-feeders; dense blooms shade out seaweeds and clog gills (2).
Q28 (3): The glow happens when the water is agitated — by a wave, paddle or propeller — not after exposure to radiation (1). The light is made by a chemical reaction inside living cells (1). The correct term is bioluminescence; "phosphorescence" is an old sailors' name for it (1).
Q29 (6): (a) frustule; (b) auxospore; (c) bioluminescence (bioluminescent) (3). (d) Photosynthetic — able to make its own food from carbon dioxide and water using light energy (1). (e) Niche partitioning — dividing up resources or living space (for example different depths) so that species avoid competing with each other, as coccolithophores do near the bright surface (1). (f) Protists — single-celled (or simple) eukaryotic organisms, some autotrophic and some heterotrophic, placed in the Kingdom Protista (1).

Marking note. The four non-phytoplankton in Investigation 1 are the point of the exercise: a radiolarian has glass like a diatom, a foraminiferan has chalk like a coccolithophore, and Noctiluca is a dinoflagellate that eats. A learner who identifies by one feature will fall into each trap; one who follows the whole key will not. In Q17 the learner corrects CAPS's own wording — only half the daughters shrink — and in Q28 the booklet's definition of phosphorescence.