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." Diatoms (Bacillariophyta) are photosynthesizing single-celled organisms. Their golden brown color comes from a carotenoid, fucoxanthan, unique to the Chrysophytes, which masks their chlorophylls' green. 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. If you adjust your pH, or reduce the dissolved organics in your water, you may temporarily knock back the diatom population, but other species of diatoms that are more suited to the new conditions are quite likely to replace them.

Diatoms form an exoskeleton in two parts, 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. In life the silica shell is covered with an organic skin; in death fossilized diatom skeletons form the diatomaceous earth used in water-polishing 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 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.

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-- or to win at Scrabble: "striae: rows of puncta along a transapical axis."

The Diatom Home Page at Indiana U. is a hub of diatom links. There 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 Diatom collection website introduces diatoms, along with some of the specialized vocabulary that may make your eyes mist over momentarily, viz-- "now, do these frustules exhibit heterovalvy?"-- that diatomists have evolved for dealing with physical characteristics that are unique to diatoms.

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.)

Diatom control. Low light levels don't discourage diatoms. They can photosynthesize at dim light intensities even algae can't use. Often diatoms become less of an issue as an aquarium matures. It may be that higher light levels directly inhibit diatoms, or it may be that stabilized conditions ordinarily favor green algae, which overgrow the diatoms, as long as they get enough light to prosper.

Reducing the concentration of dissolved silica is an approach that's often talked about: tap water may be rich in dissolved silica. A Russian study suggested that high ratios of silica to phosphorus (as phosphate) encouraged the diatoms in the algae/cyanobacteria/diatom community, but that lower ratios of silica to phosphorus found planktonic green algae displacing diatoms. If such studies of "green water" algae grown in an illuminated lab flask are relevant to aquarium experiences (why would they not be?), they suggest that as phosphates build up in a maturing system, and as initial dissolved silica is scavenged by diatoms, the changing ratio Si:P encourages green algae to displace diatoms.

The phosphate-adsorbing pillows of aluminum oxide you can put in your filter will also lower silica levels in aquarium water; sometimes they have been represented as an option in diatom control. But according to biologist Lynn Margulis, diatoms are so competent at removing silica from the water to form their lacy tests that they can reduce the silica concentration to 1 ppm, which is below the value that a hobbyist's chemical testing technique can even detect. So diatoms can never effectively be reduced in number by trying to control silica in the water, though you'll often hear this attempt recommended.

Besides, as Craig Bingaman in an article on marine diatoms for Aquarium Frontiers, Feb 2000, said, "It takes more than silicate to grow diatoms, and if diatoms are growing, they are growing by using nutrients that might otherwise fuel the growth of other types of algae."

Instead, try more intense lighting, though not a longer photoperiod. Otocinclus, the miniature Loricariid catfish, are enthusiastic diatom eaters. Try them. More industrious glass-cleaning on your part may help. And patience.

Dinoflagellates. Dinoflagellates also often get classed among algae, under the misleading impression that any single-celled photosynthesizer must be some kind of alga. They are unicellular organisms, sometimes bound in armor plates of cellulose that have fantastic and beautiful shapes. They move by vibrating their two whiplike flagella, one of which lies in a crosswise groove and imparts a revolving twist to dinoflagellate movement. In lakes, photosynthesizing dinoflagellates form a significant part of the green trophic base, though not all dinoflagellates are capable of photosynthesis. Some dinoflagellates hitch rides on larger creatures, such as copepods; others are actively parasitic. Dinoflagellates have complicated life cycles that may include a resistant cyst that helps them disperse even into isolated or temporary waters. They are certain to be in the aquarium, though you'd need a pretty good microscope to catch a glimpse, they are so small. Not all dinoflagellates are microscopic, however; Noctiluca, the "nightlight," a phosphorescent marine dinoflagellate that may coat every dip of the midnight oar with a shower of greenish sparkles, can get to be a couple of millimeters across and attack fish eggs. Dinoflagellates most often come to public notice in connection with notorious "red tides" in enriched coastal saltwater. (The red color actually comes from an alga that blooms with the fish-killing dinoflagellate in those unnaturally enriched waters.)

Though 90% of the world's dinoflagellates are marine, there is one parasitic freshwater form, Oodinium, technically Piscinoodinium, that sometimes troubles us. When you hear about a "parasitic alga," it's Piscinoodinium they're talking about. It can get some energy from photosynthesis while it waits for a host fish to settle on. The combination of chlorophyll and carotenoids in individual Oodinium's chloroplasts gives the parasite colony its familiar golden color.

Links. Andrew MacRae's good brief introduction to dinoflagellates, with some electron micrographs, is at the Dinoflagellate Page, hosted by U. of Calgary.

The U of C Berkeley site has good material on dinoflagellate roles in the ecology of marine waters.

The "Red Tide" dinoflagellate is named Pfeisteria to honor Dr Lois Pfeister, University of Oklahoma, a pioneer in studying freshwater dinoflagellates. An homage by one of her students describes her detective work that identified a dinoflagellate that was responsible for pond-raised catfish deaths.

Yellowing green water. If your "green water" develops a yellowish tinge, it's unlikely to be caused by a bloom of free-swimming diatoms or, even less likely, by dinoflagellates. It's more likely that many green algal cells are dying. Do an emergency water change immediately, to dilute the toxins they are exuding.