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17.7: Bacillariophyta - The Diatoms

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  • Diatoms are an incredibly diverse group of unicellular organisms containing anywhere from 20,000 to 2 million species.

    • Morphology: Unicellular
    • Cell wall composition: Silica frustule
    • Chloroplasts: 4 membranes, pigments are chlorophyll a, chlorophyll c, and fucoxanthin
    • Storage carbohydrate: Chrysolaminarin
    • Life cycle: Diplontic
    • Ecology: Everywhere! Marine, freshwater, and terrestrial.

    We are still trying to figure out how to determine what a diatom "species" is and, so far, they have been classified based on the morphology of their frustules, a silica shell made from two distinct valves that enclose the plasma membrane. Using this classification, there are two major groups of diatoms: centric (have radial symmetry) and pennate (have bilateral symmetry). Draw and label an example of each type below.

    Diatoms are a major component of the phytoplankton (phyto- meaning plant, plankton meaning ‘to wander’). These are photosynthesizing, microscopic organisms in aquatic environments and include members from many of the other groups covered in general botany, including cyanobacteria, Rhodophyta, and the green algae.

    Would any members of Phaeophyta be considered phytoplankton? Why or why not?

    Considering that phytoplankton are primary producers, what important roles might they have in aquatic and/or terrestrial environments?

    In addition to morphology, diatoms can also be classified by where they occur. Free-floating diatoms are planktonic. Diatoms attached to other organisms (like giant kelp) are epiphytic. Benthic diatoms tend to dwell toward the bottom of a body of water. Try to find some planktonic diatoms by preparing slides from fresh samples of pond or seawater. Look for epiphytic diatoms by mounting a piece of algae and searching along its edges. Note: I have the best luck with epiphytic diatoms on intertidal red algae. In fact, you can usually find diatoms on prepared slides of Polysiphonia, our model organism from the Rhodophyta.

    Draw what you find below. Label frustules and chloroplasts and identify whether the diatom is centric or pennate.

    Diatoms primarily reproduce asexually by binary fission, similar to prokaryotes. During binary fission, the two valves of the frustule are separated and each new cell forms a new valve inside the old one. However, the new valve is always smaller. If diatoms only reproduce in this way, it results in a continual decrease in average size. When some minimal size is reached, this can trigger sexual reproduction. When diatoms sexually reproduce, they have a diplontic life cycle and produce a very large auxospore. Stay on the lookout for these strange, enlarged diatoms!


    Apparently, diatoms can produce auxospores through asexual or sexual reproduction.

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