Two broad groups, five classes
By their relationship to oxygen, microorganisms divide into aerobic and anaerobic types, further split into five classes.
The five classes
Obligate aerobes (e.g. Bacillus, nitrifying bacteria, most fungi and actinomycetes); facultative anaerobes (grow mainly aerobically, also without oxygen — many yeasts); microaerophiles (e.g. Vibrio — grow only at low oxygen partial pressure); aerotolerant anaerobes (no oxygen needed, but molecular oxygen is not harmful); and obligate anaerobes, which are inhibited or killed even by brief oxygen contact and obtain energy through fermentation and anaerobic respiration (e.g. bifidobacteria, photosynthetic bacteria).
Why the distinction matters in a pond
In a pond the two groups compete for the same organic material, and oxygen decides the winner. While dissolved oxygen at the bottom stays above roughly 2 mg/L, aerobic bacteria and fungi oxidise feed residue, faeces and dead algae into carbon dioxide, water and nitrate — end products that stay in the water column and can be taken up again. Once the bottom layer loses its oxygen, the same material is decomposed anaerobically instead, and the end products change: hydrogen sulphide, ammonia and nitrite accumulate in the sediment and diffuse upward into the water.
What an anaerobic bottom looks like
An anaerobic sediment announces itself before any meter does. The mud turns black a centimetre or two below the surface, it smells of rotten eggs when disturbed, and gas bubbles rise when a pole is pushed into it. Shrimp and fish avoid the area, and feed conversion worsens even though the ration has not changed. In shrimp ponds this is the condition that most often precedes a bottom-related oxygen crash.
Keeping the sediment aerobic
Three levers keep the bottom oxidised. First, deliver air where the decomposition actually happens: microporous tubing laid on the pond floor holds the boundary layer above 4 mg/L, whereas a surface paddle wheel only mixes the top metre. Second, do not overload the system — every kilogram of uneaten feed becomes oxygen demand on the bottom, so feed to appetite and check the feeding trays. Third, remove what has already accumulated: dredging surplus silt between cycles stops the demand from compounding season after season.
A note on probiotics
Commercial bacterial preparations work only where the environment suits them. Dosing aerobic bacilli into a pond whose bottom is already anaerobic wastes the product, because the organisms cannot establish themselves without oxygen. Restore the oxygen first, then dose — the two measures are sequential, not alternatives.
Frequently asked questions
Q: Into how many classes do microorganisms divide by their relationship to oxygen? A: Five, grouped under two broad types: aerobic and anaerobic.
Q: Which bacteria are obligate aerobes? A: Bacillus, the nitrifying bacteria, and most fungi and actinomycetes.
Q: What is a facultative anaerobe? A: An organism that grows mainly aerobically but can also live without oxygen. Many yeasts are facultative anaerobes.
Q: What is a microaerophile? A: An organism that grows only at low oxygen partial pressure, such as Vibrio.
Q: What separates aerotolerant anaerobes from obligate anaerobes? A: Aerotolerant anaerobes need no oxygen but are not harmed by molecular oxygen. Obligate anaerobes are inhibited or killed even by brief oxygen contact, and obtain energy through fermentation and anaerobic respiration — bifidobacteria and photosynthetic bacteria are examples.
