Phototrophs: Algae & Cyanobacteria
Most freshwater phototrophs belong to the groups listed in Table 1. Of primary interest to microbiologists are the cyanobacteria. Cyanobacteria have prokaryotic cells and belong to the Kingdom Monera, along with other bacteria.
Algae is the common name for photosynthetic eukaryotic organisms that lack true roots, stems, and leaves. Algae are placed in the Kingdom Protista. Algae may be found in the ocean and in fresh water and on moist tree bark and soil. Algae may be unicellular, colonial, filamentous, or multicellular. They exhibit a wide range of shapes: from giant brown algae or kelp and delicate marine red algae to spherical green algal colonies. Algae are classified according to pigments, storage products, chemical composition of their cell walls, and flagella.
While the growth of phototrophs is essential in providing oxygen and food for other organisms, some filamentous algae, such as Spirogyra, are a nuisance to humans because they clog filters in water systems. Phototrophs can be used to determine the quality of water. Polluted waters containing excessive nutrients from sewage or other sources have more cyanobacteria and fewer diatoms than clean waters. Additionally, the number of algal cells indicates water quality. More than 1,000 algal cells per milliliter indicates that excessive nutrients are present.
|Table 1||Some characteristics of major groups of phototrophs found in fresh water|
|Cell wall||Peptidoglycan||Lacking||Readily visible with regular markings||Visible|
|Flagella||Absent||Present||Absent||Present in some|
|Cell arrangement||Unicellular or filamentous||Unicellular||Unicellular or colonial||Unicellular, colonial, or filamentous|
|Produce O2||Yes; some use bacterial photosynthesis||Yes||Yes||Yes|
Pond water samples. Pond water samples can be collected from a nearby lake or steam or made using cultures purchased from a biological supply house.
A. Incubated in the light for 4 weeks.
B. Incubated in the dark for 4 weeks.
C. Nitrates and phosphates added; incubated in the light for 4 weeks. (Add a few drops of plant fertilizer to 100 ml pond water.)
D. Copper sulfate added; incubated in the light for 4 weeks. (Add a few drops of commercial algicide, copper sulfate, to 100 ml pond water.)
1. Observe each sample and record the color and density.
2. Put a drop of pond water on a slide and examine under a microscope. Carefully draw each different alga that you see. Record the relative amounts of each type of alga on a scale from 4+ (most abundant) to + (one representative seen).
3. Repeat the observations and data collection for the remaining pond water samples.
4. As a class, tabulate the number and various groups of algae observed in the following situations:
|Source||T. R. Johnson and C. L. Case. Laboratory Experiments in Microbiology, Brief edition, 4th edition. Menlo Park, CA: Benjamin/Cummings, 1995.|