Chlorophyta Lab

GREEN ALGAE · DIVISION CHLOROPHYTA
Introduction
Of the approximately 16,000 species of green algae, 90% are restricted to the freshwater
environment: damp soil, rivers, lakes, ponds, puddles, tree bark, and even the hair of polar bears. The
marine representatives are limited to relatively few orders and are common in the intertidal and upper
subtidal regions.
Like vascular plants, green algae have chlorophylls a and b in addition to a variety of carotenes and
xanthophylls that act as accessory pigments. Nutrition is autotrophic, with the reserve carbohydrates
stored in plastids in the form of starch. Green algae exhibit a wide variety of thallus forms, ranging
from single cells to filaments to parenchymatous thalli. In tropical and subtropical waters, many
forms may be calcified.
Reproduction occurs asexually by fragmentation or by the production of spores that develop directly
into new individuals, or sexually by the union of two gametes. While the sexual gametes and asexual
spores may look very similar, they can be differentiated by the number of flagella; sexual gametes
have two flagella and asexual spores have four flagella. In many species the entire thallus becomes
reproductive, a term called “holocarpic.”
Systematics
A few distinguishing characteristics separating key orders for marine Chlorophyta are below.
Order
Thallus
Chloroplasts
Ulvales
parenchymatous
single, parietal,
several pyrenoids
Cladophorales
coenocytic cells
united end-end
Bryopsidales
much branched,
coenocytic,
nonseptate
parietal, reticu late,
few to many
pyrenoids
numerous, discoid,
w/o pyrenoids
Reproduction & Life History
(asexual; sexual)
Examples
zoospores; gametes iso/
anisogomous. mostly iso morphic
stages
frag mentation, zoospores;
biflagellate gametes
Ulva
none; biflagellate gametes
Codium,
Bryopsis
Cladophora,
Chaetomorpha
Morphology
The green algae are well represented in the marine plankton and damp terrestrial environments, with
many species occurring as unicellular organisms. These include motile vegetative cells that produce
vegetative spores and/or gametes (e.g. Chlamydomonas), non- motile vegetative cells that produce
flagellated reproductive structures (spores/gametes) (e.g. Chlorococcum), and lastly non- motile
vegetative cells without motile reproductive structures (e.g. Chlorella).
Colonial green algae (e.g. Volvox) are not commonly encountered in the marine environment, though
they are common residents of soil and standing freshwater. Colonies (synobia) are made up of
distinct single cells held together by mucilage.
A giant evolutionary step was made in the green algae when the cells that resulted from a mitotic
event remained contiguous. Connected cells enabled both multicellularity and the complex functions
this required. Filamentous green algae are well represented in the sea either as unbranched, simple
uniseriate (single thread) filaments of cells that show little or no thallus differentiation (e.g.
Chaetomorpha, Ulothrix), or as branched filaments (e.g. Cladophora).
If a single cell divides in only one plane, a filament is formed. However, if that single cell divides and
remains adherent in two or three planes, a parenchymatous multicellular thallus is formed (e.g. Ulva).
A variation of the filamentous thallus that is tubular and lacks cross walls (except when delimiting
reproductive structures) is called siphonous. The siphonous filaments may occur singly (uniaxial, e.g.
Bryopsis) or may be aggregated into a pseudoparenchymatous multiaxial system (e.g. Codium). Most
of the siphonous marine green algae are tropical, but a few genera are prominent in the flora off the
west coast of North America (e.g. Codium).
Objectives
In today’s lab, you will examine green algae that represent a wide range of morphologies: from
colonial, to filamentous, to parenchymatous, to siphonous. Examine representatives from each group
and make careful drawings of the key features that separate them.
Notebook Requirements (15 drawings)
1)
2)
3)
4)
5)
6)
7)
8)
Chlamydomonas-drawing of different life stages (at least 2)
Volvox- draw different steps in life cycle (at least 2)
Chaetomorpha linum- 2 drawings (thallus & single cell)
Cladophoracolumbiana- 2 drawings (thallus & single cell)
Ulva taeniata- 2 drawings (thallus and cross section)
Codium fragile and Codium setchellii -3 drawings (thallus of both & squash of one)
Unknown1- 1 drawing & steps to key out
Unknown 2- 1 drawing & steps to key out
A. Unicellular Forms
Key Organism: Chlamydomonas (Class Chlorophyceae, Order Volvocales)
Unicellular forms can be motile or non-motile. Chlamydomonas is a single-celled motile alga
commonly found in damp soil, ditches, tide pools and occasionally in salt marshes. The organism is
typically egg-shaped and has a large cup-shaped chloroplast containing a proteinacious body—the
pyrenoid, which functions in starch polymerization. The nucleus is often difficult to see because of the
prominent chloroplast.
1. Examine mating strains of Chlamydomonas. Identify the steps in the life cycle. Draw 2
different life stages in detail. Be sure to draw and label chloroplast.
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B. Colonial Forms
Key Organism: Volvox (Class Chlorophyceae, Order Chlamydomonadales)
While colonial green algae are not common in the marine environment, they are widespread in soil
and standing fresh water. Some are motile flagellated species (e.g. Gonium, Pandorina, Eudorina,
Volvox), while others are non- motile colonies that produce flagellated reproductive cells (e.g.
Chlorococcum).
1. Identify the different colony types of Volvox. Draw at least 2 different colony types and describe
where they fit into the life cycle of Volvox.
C. Filamentous Forms
Key Organism (simple filament): Chaetomorpha sp.
(Class Ulvophyceae, Order Cladophorales)
Chaetomorpha is a very common unbranched filamentous alga in the intertidal areas of central
California. It is usually attached to pier pilings or to rocks and looks like green hair.
Chaetomorpha linum
1. Examine Chaetomorpha under the dissecting (thallus) and compound (single cell) microscopes.
Draw the thallus and a single cell. What shape is the chloroplast?
2. See if you can observe holocarpic reproduction. To do this, place a piece of the thallus under a
light source and observe under a scope. As the thallus heats up, it will begin releasing all the
pigmented material inside. Can you think of a benefit of reproducing this way?
Key Organism (branched filament) : Cladophora columbiana
(Class Ulvophyceae, Order Cladophorales)
Cladophora is a branched filamentous alga, its common name is the “pin cushion algae” because it
grows in small clumps on rocks and resembles pin cushions used for sewing. There are freshwater
and salt water species, including a free-living, unattached ball forming species. The ball forming
Cladophora are sometimes called marimo balls.
Cladophora columbiana
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1. Draw the thallus (include the extensive branching) and a single cell of Cladophora.
2. Observe the chloroplast in living cells of Cladophora. How does this chloroplast shape differ
form the chloroplast in Chaetomorpha?
3. Iodine stain reacts with starch to form dark patches. Try staining this alga with iodine stain.
What do you see? Was the iodine stain positive or negative? What does the result tell you
about the carbon storage product utilized by this alga?
C. Parenchymatous Forms
Key Organisms: Ulva taeniata
(Class Ulvophyceae, Order Ulvales)
Ulva spp. are common and widespread throughout the world.
1.
Observe and draw the thallus morphology of Ulva taeniata with the naked eye and under the
dissecting scope. Where does growth appear to occur? Is it diffuse, or localized to a meristem?
2.
Make a thin cross section of Ulva taeniata and draw the internal details. How many cells thick
is the blade? What is the shape of the chloroplast? Where is the chloroplast located within the
thallus? How many chloroplasts are there per cell?
3.
The gametes of Ulva are positively phototactic, whereas the zygotes and zoospores are
negatively phototactic. Can you hypothesize a reason for this? For ideas, look over the diagram of
Ulva’s life history at the end of this lab.
D. Siphonous Forms
Key Organisms: Codium fragile and Codium setchellii
(Class Ulvophyceae, Order Bryopsidales)
Codium fragile
Codium setchellii
1. Codium is a siphonous alga which is multinucleate and unicellular; the siphons are
intertwined to form a spongy, pseudoparenchymatous thallus. Observe and draw the overall
external morphology of Codium (both species).
2.
Squash a small piece of Codium sp. on a microscope slide and look at it under the compound
scope. Draw what you see. Do you see cross walls? Are there chloroplasts in the siphons?
Where are they located? What is their shape? Do you see any reproductive structures?
3. Look over the diagram of the life history and close-up showing utricles. Did you see any
utricles in your squash above? What is the function of the utricles?
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E. Identifying Algae to Species
Draw and key out the unknown specimen from the division Chlorophyta. It is important to write
down the steps you take as you work through the key. This way, if you are incorrect, you can retrace
your steps much more quickly and find where you went wrong.
If you desire more practice working with the Chlorophyta dichotomous key you may key out
individual samples presented to you as “knowns” from this lab.
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Life Cycle of Cladophora columbiana
Isomorphic a lt ernation of gene rations
meiosis
occu rs in
apical cells
1N zoosp or e
2N
f ilamen tou s
sporophyte
1N
filamentous
gametoph ytes
1N gametes
2N zyg ote
syngamy
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Life C ycle of
o Codium fragile
Anima l-like (diplonticfragile
)
syngamy
1N gametes
meiosi s
occur s i n
utricles
2N adu lt
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2N
zygote
Life Cycle of Ulva spp.
Isomorphic alternation of generations
syngamy
1N gametes
2N
zygote
1N gametophytes
2N
sporophyte
meiosis
occurs in
cells of
sporophyte
1N zoospores
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