I think everyone who uses aquarium substrates more complicated than turtle-bowl gravel will be interested in the workings of a Winogradsky Column. I've only recently discovered this elegant demonstration of the layered ecosystem that naturally gets under way in an undisturbed pond (or aquarium) substrate, powered by bacterial action. To many high school seniors it's probably a familiar classroom project.
A Winogradsky Column is named for the Russian microbiologist Sergei Winogradsky (1856-1953), who first studied mixed environments of microbes in natural conditions. He unraveled details of the nitrogen cycle and the physiology of the bacteria that oxidize hydrogen sulfide.
The column is a glass or clear plastic tube that you fill with water and aquarium sediments, seal at the top with plastic wrap and expose to light (in a window) for a few months. A Winogradsky Column is a simple model system that shows the metabolic diversity of bacteria in a self-ordering, self-regulating system. In layers inside the column, the various kinds of aerobic and anaerobic bacteria will develop, and those other bacteria that can switch metabolisms when oxygen gets scarce.
This experiment, which we could all set up ourselves, might relieve misconceptions about dangers of anaerobic zones and hydrogen sulfide production in the substrate.
Perhaps not everyone is ready to see how a Winogradsky column relates to aquaria. I think, as far as the relevance to aquariums is concerned, that you could begin by seeing a Winogradsky column as an aquarium itself. If you can accept a hexagonal tank as an aquarium, surely you can accept a cylindrical one. The Winogradsky column is a tall narrow cylinder, a small aquarium with a very deep substrate. Deep enough to demonstrate the bacterial zones that get established in our more conventional aquariums. The Winogradsky column also demonstrates why you don't want an aquarium substrate that is enriched with organic matter but too deep for full root-zone penetration by your plants.
Perhaps the relevance of the Winogradsky column to aquariums would be clearer if one understood it also as a model "core sample" of the undisturbed, natural aquarium substrate. Scientists commonly extract cores of sediments or of glacial ice because they offer a typical sampling of the layers. Extracting an unmixed core from the substrate of your aquarium might be very challenging; a Winogradsky column made from a siphoned out sample of a well-aged substrate would be easier to inspect, for one thing.
Exposure to light makes the sequences of strata visible because the various types of photosynthetic bacteria, both aerobic and anaerobic, are colored. All photosynthesis is carried out within light-reactive pigments invented by bacteria, though not all the photosynthetic pigments are the familiar green ones. As a result, the sequence of zones is clearly visible in the column. You could cover half the column with dark paper to give you a comparison/control more like conditions in your aquarium substrate. The species of bacteria are different where there is no light-- and they aren't colored-- but the zones are still present.
A self-stabilizing stratified ecology, delimited by opposing oxygen and alcohol gradients, also establishes itself in an undisturbed vinegar eelworm culture.
Links. An article by Brian Rogan "Investigating bacteria with the Winogradsky Column" describes the setup.
Detailed classroom instructions for a Winogradsky column use pond mud, but you'd substitute aquarium substrate that you siphon out from a single section, right down to the bottom glass.
John E. Lennox's briefer description is at his Penn State University personal page.
An article at the University of Edinburgh's site "The Microbial World" "Winogradsky column: perpetual life in a tube", is just one of ten "profiles" or articles you'll find there on the roles of bacteria and other microorganisms in environmental processes. Some of these features are also relevant to the ecology of aquarium setups: they cover cyanobacteria and nitrogen fixation and the fungi involved in wood decay.