Environmental Engineering Reference
In-Depth Information
Chrysophycophyta division is also diversified in cell wall chemistry and flagellation.
The division is divided into three major classes: golden-brown algae (Chrysophyceae),
yellow-green algae (Xanthophycae), and diatoms (Bacillariophyceae). Some
Chrysophyta lack cell walls; others have intricately patterned coverings external to
the plasma membrane, such as walls, plates, and scales. The diatoms are the only
group that has a hard cell wall, called a
frustule
, which is composed of pectin, cellu-
lose, or silicon and consists of two valves: the epitheca and the hypotheca. Two anteri-
orly attached flagella are common among Chrysophyta; others have no flagella. Most
Chrysophyta are unicellular or colonial. Asexual cell division is the usual method
of reproduction in diatoms, but other forms of Chrysophyta can reproduce sexually.
Diatoms have direct significance for humans. Because they make up most of the phy-
toplankton of the cooler ocean parts, they are the ultimate source of food for fish.
Water and wastewater operators understand the importance of their ability to function
as indicators of industrial water pollution. As water quality indicators, their specific
tolerances to environmental parameters such as pH, nutrients, nitrogen, concentration
of salts, and temperature have been determined.
Note:
Diatoms secrete a silicon dioxide shell (frustule) that forms the fossil deposits
known as diatomaceous earth, which is used in filters and as abrasives in polishing
compounds.
Phaeophyta (Brown Algae)
With the exception of a few freshwater species, all algal species of this division exist
in marine environments as seaweed. They are a highly specialized group, consisting
of multicellular organisms that are sessile (attached and not free-moving). These
algae contain essentially the same pigments seen in the golden-brown algae, but they
appear brown because of the predominance of and the masking effect of a greater
amount of fucoxanthin. Brown algal cells store food as the carbohydrate laminarin
and some lipids. Brown algae reproduce asexually. Brown algae are used in foods,
animal feeds, and fertilizers and as a source for alginate, a chemical emulsifier added
to ice cream, salad dressing, and candy.
Pyrrophyta (Dinoflagellates)
The principal members of this division are the dinoflagellates. The dinoflagellates
comprise a diverse group of biflagellated and nonflagellated unicellular eukaryotic
organisms. The dinoflagellates occupy a variety of aquatic environments, with the
majority living in marine habitats. Most of these organisms have a heavy cell wall
composed of cellulose-containing plates. They store food as starch, fats, and oils.
DID YOU KNOW?
Cell division in dinoflagellates differs from most protistans, with chromo-
somes attaching to the nuclear envelope and being pulled apart as the nuclear
envelope stretches. During cell division in most other eukaryotes, the nuclear
envelope dissolves.
