(A) Photosynthetic pigments

The photosynthetic pigments are located in chloroplasts. The pigment common to all algae is chlorophyll a. Other types of chlorophyll are present such as chlorophyll b, c, and d. Chlorophyll a is the site of photosynthetic light reactions. The other chlorophylls transfer their light energy to chlorophyll a and are therefore called accessory pigments. Accessory pigments absorb light at a different wavelength than chlorophyll a, usually at slightly shorter wavelengths and transfer the energy via fluorescence. In addition to some of the chlorophyll pigments, other groups of pigments are considered accessory, and include carotenoid pigments (usually yellow and orange in color), xanthophylls (yellow-green and brown colors), and phycobilins (either bluish or red in color). The production of accessory pigments can often obscure the underlying green chlorophyll pigment. In addition, individual species may vary in the production of accessory pigments depending upon their location in the intertidal or their stage of growth.

(1) Chloroplast Shapes

Prepare whole mounts and examine the live material of Chlorella (or Ulothrix), Spirogyra, Oedogonium, and Cladophora under your compound microscope. Notice the variation in the shape of the chloroplast. The most common appearance in the green algae is called a parietal chloroplast which means the chloroplast lies against the cell wall. It may or may not surround the entire cell, sometimes it is located only on one side. Which alga(e) have parietal chloroplasts? What do you notice about the shape of the chloroplast in Spirogyra? Can you guess how this alga got its name? (This alga so impressed a young musician taking a biology course that he named his jazz group after it!) When the chloroplast becomes heavily divided it appears net-like or reticulate. Which alga(e) exhibit this type of chloroplast? Can you theorize on the evolutionary significance of various chloroplast shapes? Now examine the live material of a red alga, if available, under your compound microscope. What shape are the chloroplasts? When many small chloroplasts are found in a single cell, we call them discoid. Nevertheless, they are often found along the cell wall in the parietal position. Most higher plants have discoid chloroplasts.

(2) Colors

Compare the colors of chloroplasts observed above with that of a diatom and a thin section of a kelp (Fucus or Laminaria). Compare the colors with the chart on accessory pigments. What colors can be attributed to which pigment names? What are the advantages of the various accessory pigment combinations?

Demonstrate the presence of chlorophyll a within the brown and red algae by taking a piece of each one and dipping it in boiling water to extract the accessory pigment. What color does the water become? What color remains in the algae tissue? Now place the tissue in a beaker of cold acetone. What color is the solvent and algal tissue after extraction? What does this procedure demonstrate about the solubility and chemical nature of the two types of pigments?