GEOL 3213
Introduction & Classification
Diatoms – Biology & Ecology
• Unicellular & eukaryotic; lack flagella; asexual and sexual
• May be single celled or in chains
• Photosynthetic, productive even in cold nutrient-rich water
(Antarctic, etc.)
• Base of the food chain = “Grass of the sea”
• Planktic and benthic forms
Diatoms – Biology & Ecology
• Can move on & within the sediment with mucus
streamers associated with a groove called the raphe
• Can live heterotrophically in the dark
• Freshwater (lacustrine, swamp, marsh, riverine, even
terrestrial) & marine; also found in hot springs,
hypersaline lakes, & melt-water pools on icebergs
• Diversity of benthic forms is high worldwide
Diatom Classification
• Diatoms are “golden brown” algae
• Some classify them in the Phylum Bacillariophyta
• Other workers put them, with the coccolithophores, in
the Phylum Chrysophyta
• Frustrule shapes vary greatly, so there is no simple
summary of morphology
• Two major subdivisions are recognized:
– Centrales (centric) = circular, oblong, hemicircular, triangular,
or quadrangular, with surface features arranged around a
central point
– Pennales (pennate) = elongate with major features at right
angles to the median line (long axis). (These are further
subdivided into 7 subgroups.)
• Range:
Diatom Skeletons
– Jurassic(?) marine, Mid-Cretaceous to Recent
– Oldest nonmarine forms are Eocene
• Skeleton called a frustrule
– Overlapping bivalves
• Box = hypotheca
• Lid = epitheca
• Overlapping sides = girdle
Porous (called punctae);striae & costae
Raphe = groove
Marine dominantly centric forms (Cretaceous to Recent)
Nonmarine dominantly pennate forms (Paleocene to Recent)
As small as 5 um and up to 500 or more um
Account for 70-90% siliceous particles suspended in oceanic water
Siliceous skeleton resistant to solution
Their solution and reprecipitation leads to chert formation
Sediments: oozes, diatomaceous earth, diatomite
• Alternation of sexual and asexual reproduction
• Girdle views of diatom valves through several reproductive
phases. Note the progressive decrease in soze of some forms.
Diatom Morphology
• Striae = line
of punctae
• Costae =
raised ridge
parallel to
Examples of Centrales
Examples of Pennales
Diatom Diversity Through Time
• Centric forms
– Cretaceous to Recent
– Dominate the plankton
• Pennate forms
– Paleocene to Recent
– Dominate the benthos
Diatoms Uses
• Applications of diatom studies
– Age-dating and correlation
• Especially for the Tertiary
• Many short-ranged species
Paleoenvironmental studies
Used a lot by Quaternary geologists
O18/O16 ratios used for paleotemperature studies
Used to detect polluted water because of
environmental sensitivity, e. g., to nitrate and
phosphate nutrient enrichments
Diatom Lab Exercise
Name: ______________
1. Examine the slides with marine diatoms
2. Examine the slides with freshwater diatoms
3. Determine which of the unknowns is from a
freshwater deposit and which is from a marine
deposit. State your reasons:
Unknown Marine? Nonmarine?
End of File
• Which are pennate and
which are centric
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