What is a Bacterial Bloom?There are 2 types of bacteria at work in our tanks:-
Autotrophic Bacteria - Bacteria capable of synthesizing its own food from inorganic substances, using light or chemical energy. Our beneficial filter bacteria are autotrophs.
Heterotrophic Bacteria - Bacteria that cannot synthesize its own food and is dependent on complex organic substances for nutrition. The heterotrophs in our aquariums mineralise the organic waste (break down the uneaten food, fish waste, dead plant matter etc into ammonia).
Contrary to popular belief, it is commonly the heterotrophs which are seen in our bacterial blooms, not our trusted autotroph nitrifiers.
It is the heterotrophs which are primarily responsible for creating the "bio-film" (slimy residue found on the tank walls and ornaments) which builds up in our aquariums.
The heterotrophs are generally bigger than the autotrophs and therefore don't attach themselves to surfaces with the same ease. They also reproduce much more quickly. Heterotrophs can reproduce in around 15 - 20 minutes, whereas autotrophs can take up to 24 hours to reproduce.
In a newly set-up aquarium, the heterotrophs get to work quicker than the autotrophs, causing the 'cycling bloom' we so often see. Blooms are almost certainly heterotrophic if they are caused by a build up of organic waste in the substrate, which most, if not all, are.
Bacterial blooms are common in tanks with apparently no organics present (for example, where all that is in the tank is water and ammonia for a fishless cycle). This is caused by the dechlorination of the water suddenly enabling the water to support bacterial populations. The heterotrophs immediately get to work on the organics in the water itself. The severity of the bloom and even whether a bloom happens at all is dependant upon the level of organics contained in the water supply.
Our autotroph nitrifiers are strictly aerobic (require oxygen), but the heterotrophs can be facultative anaerobic (they can switch between aerobic and anaerobic function depending on their environment). Therefore the heterotrophs in the substrate will be in their anaerobic state and breaking down the organic waste into ammonia, but if they bloom up into the water column, they will switch to their aerobic form and will start to convert the ammonia back to nitrite, although very inefficiently. The heterotrophs are around 1,000,000 times less efficient at ammonia oxidisation than our beneficial autotrophs as the heterotrophs are not true nitrifiers.