Desmids are intriguing. When you look at them through the microscope, you can see why. Various shapes and details show themselves almost as if they were constructed to fulfill our needs of the aesthetic eye. The external structures vary from spindles to spheres, and the internal structures can be quite stunning!
It would go too far to explain them all to you. So I will discuss, in short, two or maybe three desmids that are regularly found in clear, (semi)aquatic sites of the Netherlands. I will mainly explain the external and boundary structures, and partly the internal structures. I will start with the species variation Hyalotheca dissiliens var. dissiliens. It consists of grouped cells (filaments), within a layer of mucilage.
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| Hyalotheca dissiliens var. dissiliens |
In the photograph above, you can find the following details:
Cell filaments are enclosed by a thick mucilage sheath, in eukaryote cells called a glycocalyx. It is mainly composed of polysaccharides. The glycocalyx contributes to the reception of chemical signals in the colonial coherence and the outer environment. It also defines the development of biofilms and microbial mats. The glycocalyx helps to increase the chance of dispersal by sticking to bodies of insects or to be washed away to other sites. One of the other important functions of the glycocalyx is protection of the internal body with an antigen character.
The sticky, little spikes on the surface of the desmid could be cilia or maybe excretion from the cell. The desmid Hyalotheca dissiliens is sometimes parasatized by the straminipilous fungi called Olpidium hyalothecae - though this desmid is particularly well protected by the glycocalix to keep infections of bacteria and fungi at bay.
Vacuoles are membrane-bound sacs of fluid or solid particles to be excreted, digested, or stored. The membrane of a vacuole has selective permeability to various substances. If levels of fluid and particles rise in the vacuole, water will be drawn from the cytoplasm. The vacuole swells and this gives a certain pressure to the rigidity of the algae structure. Under the light microscope it is sometimes possible to see the pumping movements of the vacuoles.
Grana are found in algae and plant cells. Chloroplasts are the photosynthetic drive for converting sunlight into chemical energy. Within the chloroplast, the inner membrane system is called stroma. The stroma consists of thylakoids, a kind of flattened vesicles in the stroma. These thylakoids are stacked and each complete stack of thylakoids is defined as a granum (plural = grana). These structures carry the green pigment chlorophyll.
Hyalotheca dissiliens has a layered cell wall, which surrounds the fragile cytoplasm or cell membrane. Its main task is to provide rigidity to the cell. This is vital to protect it of differences in pressure between the internal and external environment. The cell wall is also a barrier to large molecules and carrier of antigenic properties.
The empty cell in the string reveals the cytoskeleton, which is made of protein. It is important for 'the architecture' within the cell, like metabolism structures. The cytoskeleton is vital to cell transportation: movements within the cell (intracellulair transport) or between cells (intercellulair transport). The intricate cytoskeleton needs to be a dynamic, continuously reorganizing framework, to cope with cell division. The cytoskeleton is also what's left behind when algae die.
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| Hyalotheca dissiliens var. minor Note! This is a different variation of the species Hyalotheca dissiliens. The centre of the chloroplast of each cell (pyrenoid) can be clearly seen as a big mid-dark-green sphere in the middle of each cell. |
In the photograph above you may notice the pyrenoids, structures lying in the chloroplast. They are usually spherical or ellipsoidal. They contain few or no thylakoids. Reserve polysaccharides, like starch granules, are formed near them. The stored reserve polysaccharides are used in the growing season to improve growth, or when the organism temporary lacks nutrients.
So far, so good. This was an introduction to the structures of Hyalotheca dissiliens. For more information about this particular desmid, I can recommend the following link: http://www.desmids.nl/maand/english/mayeng03.html
Please follow the introductions in the next episode.
Literature
Coesel P, Meesters K (2007) Desmids (Desmidiaceae) in the Netherlands http://www.desmids.nl/
Microbiowiki (2008) What is a Eukaryotic Cell? http://microbiowiki.wetpaint.com/page/What+is+a+Eukaryotic+Cell%3F
Van Dam-Mieras MCE et. al (cursusteam 1999) Cursusboeken Biologie van Cellen I, II, III en IV, Open Universiteit Nederland, Heerlen, Boek I 37-65.
All photographs were taken with Nikon Inverted Microscope Eclipse Ti-U (100w), Nikon DS-Fi1 camera, NIS Elements BR 3.22.00 at 600x magnification. Mastered in Photoshop Elements 8. Photographs: Marta Demarteau/GWL
