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PHYCOLOGICAL SOCIETY OF AMERICA

Pseudo-nitzschia

Paleoecology & the Fossil Record

Diatom Diversity

Planktonic species of Pseudo-nitzschia are fairly well-studied whereas there is far less information on benthic species. Benthic species are harder to sample, collect, and count. At the present time, it is thought that Pseudo-nitzschia is a largely planktonic species. They rarely act as hosts for other algae and diatoms due to the mucilage sheath that sloughs off. It is possible, though, for these diatoms to act as hosts for endosymbiotic organisms like bacteria.

Paleoecology

The siliceous exoskeleton of the Pseudo-nitzschia 's frustule makes it decay resistant. It thus dissolves very slowly, though alkaline conditions are known to speed decay. When diatom shells become a part of sediment, the shells stay there and effectively create a history of the populations in both the benthic and pelagic regions. The marine diatoms are not as preserved as fresh water diatoms because marine frustules will decay before they become a part of the sediment (they must fall a greater depth, usually, than fresh water habitats).

These preserved frustules can be used in order to date natural events (like acidification of lakes) as well as pollution events. The ratio of oxygen isotopes can be measured in the silica. Overall, a greater understanding about the appearances, disappearances, and abundance of Pseudo-nitzschiaquantities can be gathered from the sediment record. Presently, there are projects that exist that drill into sediment and date the cores with radiometric and geomagnetic methods.

The Fossil Record

There is a well-documented fossil record that begins at the middle of the Cretaceous period. Deposits are in the form of fine white-grey colored material that is called "diatomite" by those in the industry. This "diatomite" can be used in food/drink processing, filtering agents, silver polish paint, cosmetics, insulation, and toothpaste (abrasive capacity). A cubic centimeter contains approximately 4.6 million diatom shells. A brief timeline of diatom existence as evidenced by the fossil record follows:

• Cretaceous: all marine diatoms, mostly centric
• Tertiary: high diversity of diatoms
• Miocene: flowering of centric marine diatoms, morphology forms similar to those today
• Late Miocene: extinction
• Late Eocene: earliest freshwater forms, morphology forms similar to those today

Taxonomical History

Where are distinctions made? Traditionally, taxonomists have been dubbed "splitters" and "lumpers" with respect to the manner in which they group species. Taxonomy in diatoms is usually based on the structure and shape of the valves rather than the differences in protoplast and reproduction. Especially with the advent of the electron microscope, the fine differences in valve structure can be visualized. SeeTaxonomy for more information!

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copyright Jennifer Shin 1999.