Algae & Cyanobacteria

Not all photosynthetic organisms are plants. Plants have a specific set of well defined features, which you'll learn about later in this lab. Algae are simpler photosynthetic organisms that lack some of the anatomical and other features that define plants. The term "algae" is not well-defined; the algae are not a monophyletic group. We won't study algae in detail in Bio 6A; mainly, you should look at algae from the point of view of contrasting them with plants. In fact, the algae are a fascinating group, filled with strange cellular and biochemical surprises; however, they don't have much of the kind of anatomical structure we can see in this lab.

Objectives

• Explain why algae aren't considered plants, in terms of essential plant features not present in algae.
• Explain how the structures of algae are adapted to life in water, but not in air.
• Explain how the life cycle of Chlamydomonas (Chlorophyta) differs from the life cycles of land plants.

Since we're contrasting algae with plants, the full significance of what you see in these specimens might not be obvious until you go on to the next page and look at plants.

Specimens

• Cyanobacteria: prepared slides and live specimens of Oscillatoria and Anabaena.
• Green algae (Chlorophyta): Live specimens of eukaryotic green algae such as Spirogyra and Hydrodictyon; prepared slides of Spirogyra and Chlamydomonas.

Cyanobacteria

Cyanobacteria are photosynthetic bacteria, which means that they are prokaryotes. Most biologists wouldn't call cyanobacteria algae, because algae usually refers to eukaryotes. However, I'm including it in this page because cyanobacteria are photosynthetic organisms that aren't plants.

This photo  shows live Oscillatoria, a cyanobacterium. It grows as long chains of similar-looking cells with no differentiation.  Cyanobacteria are sometimes called blue-green algae, both names refer to the blue-green color of these photosynthetic prokaryotes.The color comes from the cells' unique set of photosynthetic pigments.

Since they are prokaryotes, they do not have nuclei or chloroplasts in their cells. Each cell is filled with green photosynthetic pigments, but the pigments are not confined to chloroplasts like they are in eukaryotes. Don't spend a lot of time looking for cellular details; you won't see them under our microscopes. The main reason for you to look at this specimen is to see what it doesn't have; compare these to the green algae in the next section.

Viewing this specimen, you should recognize:

• Why this isn't a plant.
• Why this isn't a eukaryote.

Green Algae (Chlorophyta)

Evidence from cell structure, biochemistry, and genetics all shows that green algae are closely related to land plants. One obvious similarity is that green algae are green, and land plants are also green. This is not a coincidence; the similar color is a result of the fact that both groups use similar photosynthetic pigments.

However, green algae are very different from land plants in terms of their anatomy. Green algae are either unicellular, or grow as strings or sheets of similar-looking cells. Unlike land plants, green algae don't have complex organs such as leaves and stems.

Chlamydomonas

The picture at right shows a prepared slide of Chlamydomonas, a unicellular green alga. In this picture you see clusters of large oval cells. These cells are stuck together on the slide, but they normally live independently. Inside each cell, you can see some small structures; these are membrane-bound compartments such as the nucleus, vacuole, and chloroplasts. Refer to Campbell for more detail on these structures. You don't need to be able to identify them now; the functions of these intracellular structures will be discussed in Bio 6B. For now, recognizing that there are membrane-bound structures within the cell is important in one key respect: it tells you that these cells are eukaryotic. Prokaryotic cells do not have a system of membranes dividing the cell into compartments (an endomembrane system). Another clue that these are eukaryotes is their large size; at around 50 microns, these cells are much bigger than most prokaryotes.

In this prepared slide, the color comes from a stain that was applied to the preserved cells; the color is more greenish when the cells are alive.

The Chlamydomonas life cycle

It's important to understand how the life cycles of algae are different from those of land plants. You won't be able to see all the stages in our specimens in lab, but you should study the Chlamydomonas life cycle diagram in Campbell (figure 28.23 in the 10th edition).

Spirogyra

This picture shows part of a gigantic single cell of Spirogyra. This organism is unusual because the chloroplasts form a long spiral inside the cylindrical cells.

Spirogyra is called a filamentous alga, because it grows as long, filament-like chains of cells. This body plan provides a ratio of surface area to volume, which is important because each cell must independently absorb nutrients and sunlight directly from the environment. Although the cells are linked together, they live more or less independently.

Key points about green algae:

• Photosynthetic eukaryotes. You should be able to identify the chloroplasts, proving that these are eukaryotes. In unstained specimens, you won't be able to see the nucleus, but you can still recognize these cells as eukaryotic.
• Closely related to green plants.
• Many are unicellular, but some have simple multicellular body plans.
• Do not have differentiated tissues other than reproductive cells. Each cell must obtain resources (nutrients, water, gases, and sunlight) directly from the environment, since there are no specialized structures for absorption or transport.
• Because they lack differentiated tissues and other adaptations to life on land, they must live and reproduce in the water.