Microbes

Microbes are organisms which are too small to be seen with the naked eye. In this article microbes refers to bacteria, fungi and algae which comprise the bottom of the food chain. In the terrarium, as well as in nature, microbes are the first organisms to break down rich organic matter. They quickly grow around the detritus absorb the easily digested nutrients from it, leaving the difficult to digest remains which will eventually processed by small animals and fungi. In addition to their role in breaking down dead matter, microbes can also act to help plants grow (bacteria and fungi) or directly to photosynthesize and generate nutrients (algae).

Bacteria

Bacteria represent an incredibly diverse group of organisms. Some are efficient at using oxygen to break down sugars into CO2 and water, while others are killed by the presence of oxygen. They generally break complex molecules down into simpler compound which can be used as nutrients by plants, such as carbon dioxide and ammonia. The nutrient cycling of the bacteria is what allows the nitrogen and carbon cycles to turn.

A subclass of bacteria, called actinomycetes are especially useful in the soil. They form filamentous networks, similar to fungal hyphae, which can link up with plant root as a symbiosis. Some actinometes can ever perform the difficult task of converting elemental nitrogen (an inert gas which makes up 78% of the earths atmosphere) into ammonia, an essential fertilizer for all plants.

Fungi

With bacteria, fungi make up the most productive decomposers in the soil, liberating organic molecules so that plants can recycle them back into the food web. This is also true for your vivarium. Leaf litter, dead bugs and feces are all quickly colonized by microbes, making the nutrients available to the ecosystem. Molds and simple fungi will quickly colonize nutrient rich organic material. After the initial population boom, these simple fungi sporulate and die, where they are eaten by small invertebrates. With all of the simple nutrients consumed, slower growing fungi start to colonize and break down the more difficult to digest remains. Fungi have cell walls made from chitin (poly-N-acetylglucosamine), which is sufficiently different from the tough plant material made of cellulose, hemicellulose and lignin that the fungi can secrete digestive enzymes without themselves being effected.

Like actinomycetes, some fungi can form symbiotic relationships with plants, using a filamentous network of hyphae called mycorrhyzae. The mycelium of the fungus can act as an extended root network, trading nutrients with the plant. One researcher showed that a fungus is able to capture bugs and transport the nutrients from the bug to the roots of the symbiotic plant. In such an exchange, the fungus generally provides nitrogen to the plant and the plants provides sugar to the fungus. Many plants will grow more robust in the presence of mycorhizal fungi. This is a case where a balanced ecosystem makes allows multiple different organisms to benefit together, and shows how a complex picosystem can provide positive results impossible in a more controlled terrarium.

Algae

Algae is surprisingly common on the ground. You might think of it as only occurring underwater, but it actually grows on moist surfaces such as wood and leaf litter where it is an important source of food for springtails and other small invertebrates. Algaes can be either simple plants or photosynthetic bacteria, which input energy into the food chain by turning CO2 and water into sugar in presence of light.

In your terrarium, algae can become a nuisance. It will grow as a film on wet glass, obstructing your view. The best way to prevent this is to either keep the glass dry or to frequently wipe it off with a paper towel. Sometimes a thick layer of algae will form as green spherical bubbles, up to 1cm in diameter. This can be a pain, as it will eventually suffocate the plants underneath. In my experience, cuttign down on the light levels will prevent this algae from growing so fast.