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Phytoplankton Unit (Topic 10A-1) – page 1
Name:
Phytoplankton Unit
Section:
About Phytoplankton and Photosynthesis
Phytoplankton are tiny (one-celled) algae, plant-like organisms that use sunlight as an energy
source to make their own food in a process called photosynthesis (which means “making with
light”). To carry out photosynthesis, they need large amounts of water and the gas carbon
dioxide, both abundant in the ocean, to make carbohydrates (the “food” molecules).
Sunlight + Water + Carbon Dioxide
Carbohydrates + Oxygen
("Sugars")
The carbon in the carbon dioxide is combined with water to create a carbohydrate (“hydro”
means water). The oxygen in the carbon dioxide is simply released.
To carry out photosynthesis, phytoplankton also need small amounts of nutrients, molecules that
they use to build their bodies or molecular “tools” like chlorophyll (the “green stuff” in plants)
that are needed to carry out photosynthesis, but are not used up in the process. A hammer is a
tool that can be used in the process of building a chair, but the hammer is not part of the chair at
the end of the process. It can be re-used again and again to build more chairs. A spatula is used to
make food like hamburgers, but does not become part of the hamburger and can be used to make
many hamburgers. Examples of nutrients used in photosynthesis include nitrates and phosphates.
Think of nutrients as “fertilizing” the phytoplankton. Nutrients are not “food” or “eaten” by
phytoplankton. (Phytoplankton make their own food.) As you will observe under the microscope,
phytoplankton have no tentacles, arms, or other structures with which to grab the extremely tiny
nutrients. Instead they rely upon chance: nutrient molecules drift into their bodies through the
holes in their shells or cell walls.
1. What is the purpose of photosynthesis? In other words, why do algae like phytoplankton
carry out photosynthesis?
2. What do algae like phytoplankton need to carry out photosynthesis?
3. Give 2 examples of nutrients that are needed for photosynthesis.
4. How do phytoplankton get their nutrients?
Phytoplankton Unit (Topic 10A-1) – page 2
Kinds of Phytoplankton
There are many different kinds of phytoplankton. Some have calcium carbonate shells (e.g.,
coccolithophores), some have silica shells (e.g., diatoms), and some have no hard shells at all and
can swim (e.g., dinoflagellates). Silica dissolves more rapidly in warm water than cold water, so
silica-shelled phytoplankton tend to prefer living in colder surface water. Calcium carbonate
dissolves more rapidly in cold, carbon-dioxide-rich water (more acidic), so calcium-carbonateshelled phytoplankton tend to prefer living in warmer surface water. (There are many examples
of calcium-carbonate-shelled phytoplankton and zooplankton who live in cold water, and silicashelled phytoplankton and zooplankton who live in warm water, so this is a preference, not an
absolute rule.) Silica is also considered a nutrient, because there are many places in the ocean
where there is not enough silica dissolved in the water for silica-shelled phytoplankton to grow,
so like other nutrients its absence limits the growth of phytoplankton.
Phytoplankton, the Food Chain, and Primary Productivity
Sunlight and nutrients are the hardest things for phytoplankton to obtain, so wherever there are
both sunlight and nutrients, phytoplankton will be abundant. Animals will be attracted there too,
because phytoplankton are at the bottom of the ocean food chain. For example, zooplankton will
go there to eat the phytoplankton, and small fish will go there to eat the zooplankton. Unlike
animals, phytoplankton make their own food, so we call them “primary producers:” they make
(“produce”) organic material from inorganic material (carbon dioxide, water). We call the
organic carbon created primary productivity. This is the first or “primary” step in the food chain.
Animals are secondary, tertiary, etc. producers depending upon their level in the food chain.
They make the organic material of their bodies from organic material that they eat.
5. What substances do phytoplankton make their shells out of?
6. Give an example of a nutrient that phytoplankton use for their shells.
7. Which kind of shells dissolves rapidly in cold water?
8. Which kind of shell dissolves rapidly in warm water?
9. Which kind of organism makes its own food and thus is at the bottom of the ocean food
chain? In other word, which kind of organism begins the food chain?
Phytoplankton Unit (Topic 10A-1) – page 3
Why Phytoplankton Are Important
As discussed previously, phytoplankton are at the bottom of the ocean food chain. So, without
them, ocean animals would have little food, and there would be fewer fish and other animals for
us to catch and eat. Oceanographers can observe where phytoplankton are common in the ocean
using satellites (they typically make the water look more “green” instead of “blue”), and thus
predict where lots of fish and other animals will be found.
As part of photosynthesis, phytoplankton add oxygen to ocean water. A lot of this oxygen leaks
from the ocean into the atmosphere. At least half of the oxygen that you and I breathe was
produced by phytoplankton. In addition, phytoplankton remove carbon dioxide from ocean water
to carry out photosynthesis and make their shells (e.g., calcium carbonate). Humans have added
large amounts of carbon dioxide to the atmosphere by burning fossil fuels like oil and natural
gas, increasing it by over 33%. Carbon dioxide is a “greenhouse gas:” it absorbs infrared “light”
(“heat”) radiated from the warm Earth towards outer space, “trapping it” and thus keeping the
world warmer than it would otherwise be. This “greenhouse effect” is actually a good thing,
because it keeps our planet from becoming a giant ball of ice. However, by adding so much extra
“greenhouse gas” to the atmosphere, humans are likely to have caused the unusually fast
warming over the past century. A large amount of the carbon dioxide that we add to the
atmosphere leaks into the ocean where it no longer adds to the greenhouse effect. By removing it
from ocean water, phytoplankton create more “space” in the ocean water, allowing the ocean to
absorb more carbon dioxide. Most phytoplankton bodies end up being eaten or decomposed and
their carbon is released back into the ocean water, but a few become part of the sediments on the
bottom of the ocean; this permanently removes the carbon from the ocean-atmosphere system
and thus “cleans up” some of our carbon dioxide pollution.
Over vast amounts of time, the phytoplankton bodies in the sediments can become fossil fuels
like oil and natural gas. Diatoms, in particular, are thought to contribute, because they contain a
drop of oil in their bodies, a way for them to store food and lower their density so that they can
float in the sunlit waters at the surface of the ocean. So, you may be “burning” some
phytoplankton when you drive your car. Don’t forget to say “thank you.”
10. Describe 4 different ways in which phytoplankton affect your life.
Phytoplankton Unit (Topic 10A-1) – page 4
Bacteria
Bacteria are small, single-celled organisms (even smaller than phytoplankton). Bacteria use a
huge number of chemical processes to extract the energy necessary to sustain their lives from
inorganic and organic matter, including dead, decaying material and fecal matter that animals
cannot use as food. The bacteria decompose (“breakdown”) dead, decaying material into( carbon
dioxide and nutrients like phosphates and nitrates and release them, making them available to
phytoplankton to use again. This is important: without bacteria “recycling” nutrients, there would
be fewer phytoplankton in the ocean, and therefore fewer animals as well. However, many
bacteria need oxygen to decompose (“breakdown”) organic matter, so if too much dead organic
material is present in ocean water, it can lead to the growth of lots of bacteria which use up the
oxygen in the water; animals who cannot leave suffocate (die because they cannot breathe).
Each species of bacterium specializes in using one chemical process or a few. They have evolved
these differences over time. For example, some bacteria can “fix” nitrogen gas, making it into a
nutrient usable by phytoplankton. This is how soils naturally get nitrogen. Recall that the
atmosphere is nearly 80% nitrogen. Some bacteria can breakdown oil, cleaning the ocean.
11. What is the primary role of ocean bacteria in the ocean food chain?
How do ocean bacteria get the energy that they need to stay alive?
12. When bacteria decompose dead bodies and wastes, what do they release into ocean
water?
13. What do bacteria absorb from the water so that they can decompose dead bodies and
wastes?
Phytoplankton Unit (Topic 10A-1) – page 5
More about Bacteria and Other Kinds of Tiny Ocean Life
In recent years, bacteria have been found to play 2 other important roles in ocean ecosystems.
First, some bacteria can carry out photosynthesis (make their own food using the energy of the
Sun). Second, bacteria are more abundant than previously thought, because many phytoplankton
bodies are “leaky.” Phytoplankton gain water, carbon dioxide, and nutrients by simply letting
them drift into their bodies, and remove wastes by letting them drift out of their bodies on their
own (not unlike a small child in a public swimming pool). Unfortunately, phytoplankton also
lose about ¼ of the potentially useful “dissolved organic carbon” (DOC) that they make too. If
they used the DOC to grow and for energy, then there would be more food for animals and
therefore more animals, but unfortunately they “lose” a significant amount of what they make.
Many bacteria can extract energy from this material, so the leaky phytoplankton lead to more
bacteria in the ocean. The big discovery was that there are many very small animals who
specialize in eating bacteria (bacteriovores), and that these animals are eaten by zooplankton
(who are eaten by small fish, who
Ocean
are….). Thus, bacteria make sure
Food
Chain
that the food “lost” by
& Bacteria
big fish
phytoplankton is not lost from the
food chain, and therefore they
small fish
dead, decaying
increase the number of fish and
bacteria
material,
other animals in the ocean (more
wastes
zooplankton
bacteriovores
food for us). This pathway in the
food chain is commonly called
phytoplankton
the “microbial loop” (the purple
nutrients
lines in the food web below).
Bacteria, fungi, and viruses are abundant in ocean water. However, because they are so small,
they are hard to capture and study. Aside from what I discussed above, relatively little is
understood about their role in ocean ecosystems. It is an active area of research, and I expect
some big discoveries in the not-too-distant future that will radically alter what I teach about
them. (Fungi are related to plants, but do not carry out photosynthesis. Most obtain their energy
from decomposing dead matter (like bacteria). Viruses are strands of RNA (DNA-like material)
that invade cells, take over their DNA, and use the cell’s own body to produce more virus
particles. These virus particles are then released, and try to infect more cells.)
14. True or false? “Some species of bacteria get their food by carrying out photosynthesis.”
15. True or false? “Some animals eat bacteria.”
Phytoplankton Unit (Topic 10A-1) – page 6
Sunlight and Floating
Sunlight is, of course, more abundant at the top of the ocean. As sunlight goes downward
through ocean water, it is absorbed by the water, warming the surface of the ocean but leaving
the deep ocean cold and dark. Anything that helps phytoplankton float like unusually cold and
salty water (high density water) at the surface of the ocean and upwelling (water moving upward
from the deep to replace surface water pushed away by winds) leads to the growth of more
phytoplankton. In fact, many phytoplankton are a little more dense than water (because of their
hard, heavy calcium carbonate or silica shells), so they sink. They rely upon waves or other
mixing mechanisms to bring them back up towards the sunlight at the surface. Their bodies are
shaped so that they fall slowly, allowing them to wait for that lucky wave. There is at least one
advantage to falling: their bodies are exposed to more nutrients.
16. Do phytoplankton live near the surface of the ocean or down deep? Why?
17. True or false? “Warm water helps phytoplankton get more sunlight.”
18. True or false? “Upwelling helps keep phytoplankton near the surface of the ocean where
they can get sunlight for photosynthesis.”
19. True or false? “Waves help keep phytoplankton near the surface of the ocean where they
can get sunlight for photosynthesis.”
Phytoplankton Unit (Topic 10A-1) – page 7
Phytoplankton Live Where the Nutrients Are
California
Nutrients enter the ocean when they are washed off the land
Land
by rain runoff. (The nutrients are in the soil on land. They
help land plants to grow.) Thus, phytoplankton like to live
near the coast, and nutrients are hard to find out in the middle
of ocean. As dead organisms and wastes sink towards the
"Gap"
bottom of the ocean, they are slowly decomposed by bacteria
or dissolved by ocean water, releasing nutrients back into the
Upwelling
water in the deep ocean. However, many nutrients are
released down deep, where phytoplankton cannot live
because of a lack of sunlight. Anything that brings these nutrients up to surface will lead to the
growth of lots of phytoplankton. Upwelling is the most effective means of bringing up nutrients,
but upwelling only happens in special places where the winds blow in the right directions.
Examples include the Equator, along west coasts at mid- and low latitudes, and in the middle of
the Southern (Antarctic) Ocean. In most parts of the ocean, nutrients are brought up by waves
and other mixing mechanisms that stir up the nutrients from below. Waves can only bring up
nutrients if the surface water is not too warm. Cold, nutrient-rich water brought up from below
cannot float in warm surface water – its density is too high – so it will sink back down rather
than mix with the surface water.
Warm
Few Nutrients
Cold
Lots of Nutrients
Waves have difficultly bringing up nutrients if the surface
water is too warm. When a wave tries to mix the surface
water, the cold water tries to dive back down, the warm water
tries to pop back up.
20. Why are nutrients more abundant near the coast?
21. Where are phytoplankton more abundant, near coasts or in the middle of the ocean?
Why?
22. What brings up more nutrients in most parts of the ocean, upwelling or waves?
23. True or false? “Upwelling brings up nutrients from below.”
24. True or false? “Waves bring up more nutrients from below when the water at the surface
of the ocean is warm.”
Phytoplankton Unit (Topic 10A-1) – page 8
Why Deep Water is Rich in Nutrients
Deeper water has more nutrients than surface water, because nutrients at the surface of the ocean
are being removed from the water by phytoplankton, but nutrients are being added to the deeper
water when dead, decaying matter (e.g., phytoplankton bodies) sinking from the surface are
decomposed by bacteria. Unlike at the surface, nutrients are not being “used up” by
phytoplankton, so they slowly build up in deeper water over time.
The water beneath the surface mixed layer (a layer called the “thermocline”) is particularly rich
in nutrients. At the top of the thermocline, the water is much colder (and often saltier) than the
water of the mixed layer, so it is has a higher density than the mixed layer. Therefore, it gives
more support, so when sinking bodies reach the thermocline, they begin to fall slower. The
bodies “clump up” near the top of the thermocline, so more nutrients are released when they are
decomposed by bacteria.
mixed
layer
LOTS of
oxygen
few
nutrients
thermocline
little
oxygen
LOTS of
nutrients
deep
layer
plenty of
oxygen
plenty of
nutrients
Sinking dead, decaying matter (like the bodies of phytoplankton),
carry nutrients away from the surface and into the deep ocean.
The oxygen concentration is the reverse of the nutrient concentration: it is higher at the surface
where the oxygen is being made by phytoplankton and lower in the thermocline because of
bacteria use oxygen to decompose the dead bodies and wastes than are sinking down.
Deep in the ocean, there are no waves or strong currents. The primary mixing mechanism which
stirs the water is the swimming of animals. This mixing is very weak, but over time their
swimming helps nutrients come up to the algae at the surface (VERY slowly)!
25. What is the surface layer of the ocean called?
26. What is the layer of the ocean below the surface layer called?
Phytoplankton Unit (Topic 10A-1) – page 9
27. Which layer of the ocean contains the most nutrients, the mixed layer or the thermocline?
28. What kind of organism adds nutrients to ocean water?
How or why does the organism add nutrients to the water? As a part of what process?
29. What kind of organism removes nutrients from ocean water?
How or why does the organism remove nutrients to the water? As a part of what process?
30. Which layer of the ocean contains the most oxygen, the mixed layer or the thermocline?
31. What kind of organism adds oxygen to ocean water?
How or why does the organism add nutrients to the water? As a part of what process?
32. What kind of organism removes oxygen from ocean water?
How or why does the organism remove nutrients to the water? As a part of what process?
Phytoplankton Unit (Topic 10A-1) – page 10
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