Yellow-green and Golden-brown Algae and the Diatoms (Division Chrysophyta)
Most of the species in these three groups of algae placed together in the division Chrysophyta are some shade of yellow or brown caused in part by a predominance of carotenoids.
This rather heterogeneous assemblage of algae contains chlorophyll A and chlorophyll C, beta carotene and xanthophylls. Starch is absent, the food substances being stored primarily as other carbohydrates or as oils. Silica rather than cellulose is the major component of the cell walls.
It should be noted however that despite these similarities, the Diatoms are quite different in many ways from the yellow-green or the golden-brown algae. Eventually it may be necessary to put them in separate division of their own.
Among the Chrysophytes, Diatoms are probably of greater importance in the energy cycle of natural waters. These algae are frequently considered among the foremost of synthetic producers and are found to a great extent in fast regions of upland streams. Diatoms are typically unicellular although in some forms the individuals may be aggregated or variously shaped colonies such as the radiating colonies of Asterionella or the chain-like colony of Tabellaria or Melosira.
A unique characteristic of Diatoms is the enclosure of the cell in two silicaceous shells or frustules which fit together in overlapping or “pill box” fashion. The frustules are frequently decorated with highly ornate and precise sculpture consisting of fine striations, pits and shallow depressions.
Diatoms are generally microscopic, but some reach a size of nearly 200 microns. They occur abundantly as floating forms in plankton and on submerged objects such as stones, larger plants and animals. In streams, the slick nature of stones may be due to the dense Diatom populations and their secretions.
The plankton of streams is derived almost wholly from the scouring of the substrate by stream action and this plankton usually consists in a large part of Diatoms. They often serve as the principle food source for many small stream animals.
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