A. Chrysophytes with golden-brown chloroplasts, containing chlorophylls
a and c, and predominantly carotenes and xanthophylls, including fucoxanthin.
B. Lack silicified cell walls. Most forms are naked protoplasts, but
some have a lorica.
C. Food reserves include chrysolaminarin, a modified laminarin
(leucosin) and oils.
D. Flagellated forms have two dissimilar flagellae.
E. Reproduction: motile forms divide by fission, while non-motile forms
produce motile zoospores. There is also a special type of spore unique
to this group, known as a statospore. It is spherical with a plug,
which is popped out as the spore germinates.
Isogamous sexual reproduction is rare.
F. Habitat is primarily cold freshwater sites.
H. Economic and ecological significance - slight value in the food chain,
some like Dynobryon and Synura may cause foul water tastes.
A. Unicellular or colonial forms with silicified cell walls.
B. Habitat - common in all aquatic situations, but especially in cooler
waters. They make up much of the plankton, but there are some attached
forms.
C. Food reserves are chrysolaminarin (modified laminarin) and
oils.
D. Structure - two overlapping halves - epivalve and epicingulum
forming the epitheca, and a hypovalve and hypocingulum
forming
the hypotheca. The cigulums form the girdle.
Frustule
is the term for the entire "shell". Centric and pennate
types of
diatoms. Pennate forms show a rapha or slit.
E. Reproduction
1. Asexual by cell division, in which each daughter retains half of
the original wall, which becomes the epitheca of the new cell. Thus half
of the daughter cells decrease in size. This can only go on for a limited
time. Sexual reproduction allows for the return to full size.
2. Sexual reproduction involves a special cell known as an auxospore.
The precise mechanism varies, but this is a typical procedure: the 2n vegetative
cells release the protoplast from the cell wall, and undergo meiosis, followed
by syngamy, which restores the 2n condition. This 2n protoplast enlarges
to the full size for the species. a cell wall forms around this protoplast,
forming an auxospore with cell walls unlike those of the typical cell wall.
Mitosis now produces two or more new cells which will form new cell walls
typical for the species, and of full size. There are many variations on
this general theme.
F. Economic Importance.
1. Plankton, especially in cool oceans, where it is the major primary
producer.
2. Diatomaceous earth. Huge deposits, up to 3000 feet thick are mined.
Lompoc, California is a noted mine, with an area of many square miles and
depths of 700 feet. Uses include:
a. filtration, especially in sugar refining, aquariums, etc;
b. fine polish for silver, toothpaste, etc;
c. paint additive to increase reflectivity;
d. insulation, especially in furnaces with temperatures in excess of
1000 degrees F.
G. Ecology
1. Spring diatom increase - many invertebrates hatch at this time,
and one species is known which secretes a substance which actually induces
hatching in some Balanus.
2. Diversity is greatest in pelagic areas, where there is low individual
density (also in infertile lakes), while diversity is low in coastal areas
and fertile lakes, where there is high individual density.
3. Because many diatoms have very specific growth requirements, monitoring
of diatom species is a good indicator of water quality.
4. Diatoms form mats on docks, boats, etc., as the second stage in
the fouling process, which culminates in barnacles and oysters.
5. Significant numbers in soil.
6. Have resumed growth after 48 years of dry storage!
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