Heterotrophic Bacteria

These are the recyclers — the unseen workforce that keeps a tank from slowly burying itself in its own waste. Every leaf that drops, every fish that dies, every smear of uneaten food and fish poop is, to a heterotrophic bacterium, lunch. They are the heterotrophs of the system: unlike the nitrifiers next door, they cannot build themselves out of thin chemistry, so they eat organic matter, burn most of it for energy, and turn the rest back into the dissolved nutrients that feed plants and algae. They are the heart of the microbial loop, and quietly one of the most consequential players in the whole tank.

What they eat, and how they choose

Heterotrophic bacteria graze four kinds of food at once and lean toward whichever is most plentiful:

Labile dissolved organic matter — fresh, easy stuff like amino acids and sugars leaking out of living cells. This is the bacteria's favourite by a wide margin, taken up fastest because it needs almost no breaking-down.
Suspended detritus — the drifting particulate debris of the tank, the everyday baseline of the bacterial diet.
Settled detritus — the same debris once it lands on the bottom. It is harder to get at, both because a settled lump offers less surface to attack and because only the fraction of the bacterial population actually living down in the sediment — roughly a third — can reach it.
Refractory dissolved organic matter — the tough, humic leftovers (the tea-coloured tannins of driftwood and aged water). Bacteria work on these only slowly, because they first have to secrete enzymes to cut the big molecules apart (hydrolysis).

The model blends these four preferences and the ease of feeding on each into a single shifting appetite (Monod kinetics with a half-saturation for each food), so the community automatically slides toward whatever is most available at the moment. Under ideal conditions their total feeding can run to several times their own body carbon per day — they are fast where the nitrifiers are slow. The exact preferences and half-saturations are tabulated in the Parameter Reference.

Of the carbon they eat, only a fraction — a bit over a quarter of the easy labile food, and a small sliver of the tough refractory stuff — becomes new bacterial cells; the rest is respired away as CO2. This conversion efficiency matters more than it looks, because it sets how much organic carbon ends up as grazeable bacterial biomass (food for the next link up the chain) versus simply vented back to the water as dissolved CO2.

Returning nutrients to the water

Detritus is usually richer in nitrogen and phosphorus than a bacterium needs for its own body. The surplus gets dumped straight back into the water as ammonium and phosphate — and this is the mineralization step that closes the nutrient loop, handing plants and algae the dissolved nutrients that were locked up in dead organic matter. Without it, a tank's nutrients would steadily disappear into an ever-growing pile of detritus. It is also, incidentally, a feed-in to the nitrogen cycle: the ammonia the nitrifiers oxidise comes in large part from this bacterial breakdown of organic matter.

Oxygen, and a vicious circle

Heterotrophic bacteria are obligate aerobes — they need oxygen to respire. They are good at scavenging it even when it runs low, but their growth still suffers under hypoxia, and the way it suffers sets up one of the more important feedbacks in the tank.

When settled detritus piles up thickly, oxygen can no longer diffuse to the bottom of the layer, and the deep part goes anoxic. The aerobic heterotrophs simply cede that zone — it becomes the territory of the denitrifiers and other anaerobes, who can respire without oxygen. The model works out how much of the layer is oxygen-free from the balance between oxygen supply and demand (Bouldin 1968; Cai & Sayles 1996), so the aerobic and anaerobic communities cleanly split the settled food by depth.

In the water column the same oxygen squeeze drives a self-reinforcing spiral. When there isn't enough oxygen to respire all the carbon they have taken in, bacteria make less new biomass and vent the leftover carbon back as fermentation waste (which keeps the carbon books balanced). Because less carbon is being built into cells, less nitrogen and phosphorus get locked into cells either — so more ammonium and phosphate spill back into the water on top of the usual surplus. The upshot is that a single oxygen shortage simultaneously slows bacterial growth, releases extra nutrients, and produces waste carbon. Those extra nutrients fertilise more algae, the algae respire harder at night, and oxygen drops further still: low oxygen begets nutrient release begets more algae begets lower oxygen. It is one of the engines behind a tank "tipping over."

Birth, death, and the brake on blooms

Bacteria could in principle bloom without limit on a rich food supply, but they don't — and the brake is viral. As bacterial density climbs, the viruses that infect them (phages) find hosts more easily, infections rise, and more cells burst. This density-dependent culling is a stabilising feedback that keeps runaway bacterial blooms in check. When the cells die, most of their contents leak out as fresh dissolved organic matter — promptly recycled by the survivors — and the rest settles or stays suspended as detritus.

They are hardy across a wide temperature range, comfortable in fresh water, and in turn they are a major food source for the ciliates and, to a lesser extent, the copepods that graze the microbial loop — the link that carries dissolved organic matter back up to animals large enough to see.

Key references

Bouldin, D.R. (1968). Models for describing the diffusion of oxygen and other mobile constituents across the mud–water interface. Journal of Ecology 56, 77–87.
Cai, W.-J. & Sayles, F.L. (1996). Oxygen penetration depths and fluxes in marine sediments. Marine Chemistry 52, 123–131.