Diatoms, the "brown algae"

Diatoms. The familiar brown incrustations on glass and plant leaves and rocks that cause trouble in freshwater tanks, which we sometimes still miscall "brown algae," are the diatoms. Swimming pool owners call diatoms "mustard algae" or "yellow algae." Like them, you may be more interested in diatom control than their role in the ecology of the freshwater aquarium.
 
Diatoms (Bacillariophyta) are photosynthesizing single-celled organisms. Their golden brown color comes from a carotenoid, fucoxanthin, unique to the chrysophytes, which masks their chlorophylls' green: think of the yellowish olive brown color of kelp, a giant marine chrysophyte. Diatoms are a major component of the freshwater plankton. They occur in all still or moving water, even clinging, with the mucilage they secrete, to wet rock surfaces. They turn up among damp mosses, and even in hot springs! They encrust gravel and aquatic sediments, they coat other algae and higher plants and the shells of crustaceans or turtles. Many planktonic diatoms drift free in the current. They form a major food source for microorganisms and larvae, including many fish fry. Diatoms are everywhere in freshwater.
 
On the exterior of their cellular membrane diatoms form an exoskeleton in two parts, looking under the microscope like a fantastic macramé box with a snug-fitting cover (di-atom: "two units"). Each valve of the diatom test is composed of organic material impregnated with silica. Technically it's hydrated silica in an opaline state, the same stuff that makes gem opals. Partly becauise there's much more dissolved silica in freshwater than in the ocean, freshwater diatoms on average contain ten times the silica of their marine counterparts. In life the silica shell is covered with an organic skin; in death fossilized diatom tests form the diatomaceous earth used in water-polishing mechanical filtration.
 
Chrysophtyta and diatoms. Diatoms are often now placed in a phylum of Chrysophyta, which includes yellow-green and golden-brown algae, even the gigantic multicellular marine ones such as kelp, and which also includes the dinoflagellates, another group of unicellular photosynthesizers loosely called "algae." Among the Chrysophytes, some biologists would even include water molds, such as Saprolegnia. Not everyone agrees: other biologists think that the Chrysophytes in fact are a mixed bag of photosynthesizing "plant-like" protists, not all genealogically related.
 
Briefly, the Chrysophytes produce an accessory chlorophyll, chlorophyll c, that's unique to them; it differs from the accessory chlorophyll used by green algae — and their descendants, the plants — which is denoted chlorophyll b. The differences among the chlorophylls that separate the Chrysophytes from the green algae were early clues that these major photosynthesizing groups weren't as closely related as people had thought. Some biologists would go a step farther. Lynn Margulis is among the biologists who want to see diatoms distinguished from other Chrysophytes into a separate phylum of their own: "For years they were classified with the golden-yellow algae... In life history, cell structure and division, the diatoms differ greatly from the other golden-yellow algae. The diatoms make up such an easily-distinguished and large natural group that, in the light of modern information, we provide them a phylum separate from the other organisms that have golden-brown plastids." (Five Kingdoms, p. 58)
 
Aside from the ubiquitous diatoms, chrysophytes are almost exclusively marine — the few freshwater chrysophyte algae are rare curiosities found in very clear, cold waters, where they form an important basic food source for zooplankton. Not in the average home aquarium, apparently. Without enough light for photosynthesis, say during an Arctic winter, some of these freshwater chrysophytes turn nasty, abandoning photosynthesis to gobble bacteria or diatoms.
 
Links. To appreciate how beautiful diatoms are, you need to have an electron microscope. Failing that, you might want to see the diatom stuff at Bowling Green State University's archive of micropix and electron microscopy of a few freshwater diatoms (loosely characterized as "algae") but many marine forms, and a generous set of links to other diatom, algae, cyanobacteria and microscopy sites.
 
A central resource is the Diatom Homepage hosted by U. of Indiana, with more diatom information than you need to know today, since the emphasis in diatoms tends to be on the marine forms, but there are links to all kinds of other algae and microbe pages. Our freshwater diatoms are just a sideshow. The marine diatoms take the center ring, and paleolimnologists assess ancient climates and date stratified rock layers by the fossil diatoms they contain. There are lots of links here also to the other algae, or the real algae, depending on your viewpoint. 
 
The California Academy of Sciences collects North American freshwater diatoms and offers an introduction to diatoms and an identification resource (if you're looking at your diatoms through a microscope) and even a glossary of the terms diatomists need to describe the complicated diatom frustules and other physical characteristics that are unique to diatoms— or to win at Scrabble: "striae: rows of puncta along a transapical axis."
 
But the UCal Berkeley Chrysophyta homepage gives the quickest introduction. You won't need all this, unless you get sucked into a disagreement over whether diatoms are algae. ("That depends on your definition of 'algae'" is the correct riposte.)