Bacterial growth: what really happens

When people talk about bacterial growth in the marine aquarium, they tend to boil it all down to a sentence that simplifies things dangerously:

"we dose bacteria to lower nitrates and phosphates".

That is only the end result, not the process.

What are we really doing? We are deliberately pushing the microbial loop that, in nature, runs oligotrophic coral ecosystems.

How nutrients are taken up

Heterotrophic bacteria, stimulated by a source of bioavailable organic carbon, take up dissolved nitrogen and phosphorus to build their cell biomass.

This mechanism is widely documented in the marine literature (Azam et al., 1983, The ecological role of water-column microbes in the sea; Kirchman, 2008, Microbial ecology of the oceans).

There is only one key point to remember: nutrients do not disappear, they only change form. From dissolved, they become biological particles.

Carbonthe organic source you dose

NO₃ and PO₄dissolved in the water

Bacteriagrow

Skimmerpart is removed

Cell deathgoes back into the microbial web

Bacterioplanktonlive food for corals

Nutrients do not disappear: from dissolved they become biomass, that is, food for SPS.

The microbial loop: from carbon and dissolved nutrients to bacterioplankton. Azam et al. 1983 · Sorokin 1973 · Ferrier-Pagès et al. 1998.

Bacterial death and the birth of bacterioplankton

Once it has grown, the bacterial population does not just sit there, stable, for ever. Three things happen:

• part of it is removed by the skimmer;
• part of it meets cell death;
• part of it enters the food chain as bacterioplankton.

This is the key point in marine fishkeeping: in nature, bacterioplankton is a primary food source for corals, especially for SPS.

The key scientific references are Sorokin (1973, On the feeding of corals with bacteria and dissolved organic matter), Ferrier-Pagès et al. (1998, Feeding rates and assimilation efficiency of bacterioplankton in scleractinian corals) and Houlbrèque & Ferrier-Pagès (2009, Heterotrophy in tropical scleractinian corals).

Conclusion: corals capture, ingest and assimilate bacteria, getting carbon, nitrogen and phosphorus from them.

Bacteria are not foam

Thinking that bacteria only serve to "make darker foam to remove nutrients" is a technically wrong view.

The skimmer removes part of the bacterial biomass, but:

• another part stays in suspension;
• another is eaten directly by the corals;
• another feeds the microbial web of the substrate and the rocks.

In practice we are turning nutrients into live food, on a microscopic scale.

1. 1Skimmer removes part of the bacterial biomass
2. 2In suspension another part stays in the water
3. 3Eaten by corals another part is eaten directly
4. 4Microbial web another feeds the substrate and the rocks

Bacteria are not foam: we are turning nutrients into live food.

Sources: [1]

The role of the skimmer

Here too the point is clear, as we saw in the previous post: there is no such thing as "dry skimming" or "wet skimming" in absolute terms. There is skimming consistently with the organic load and with the level of bacterial stimulation.

Overly aggressive skimming does nothing but remove too much microbial biomass, and so deplete the food available.

Overly gentle skimming leads to bacterial build-up, higher oxygen consumption and redox instability.

References: Wild et al. (2004, Degradation and mineralization of coral mucus in reef ecosystems) and Rohwer et al. (2002, Diversity and distribution of coral-associated bacteria).

Too aggressive

• Removes too much biomass
• The corals are left without food

Consistent

• Follows the organic load
• And how much you are stimulating the bacteria

Too gentle

• Bacteria build up
• Less oxygen, unstable tank

"4 drops per 100 l"? No. You do not dose by the litre: you dose by watching how the tank responds.

The skimmer must follow the bacteria, and the dose is decided by watching the tank. Every tank has its own load, its own microbial community, its own flows.

Why standard doses make no sense

At this point it becomes very clear why the phrase we quoted in the previous post makes no sense at all:

"4 drops per 100 litres".

Every tank has a different organic load, a different microbial community, and different flows, oxygenation and skimming.

You do not dose by the litre. You dose by watching how the tank responds biologically.

In short

Bacterial growth is not a fashion, a recipe or a fixed test number: it is the conscious management of the microbial cycle, the same one that keeps coral reefs stable.

Sources cited by the author

• Azam, F., et al. (1983). The ecological role of water-column microbes in the sea. Marine Ecology Progress Series.
• Kirchman, D. L. (2008). Microbial Ecology of the Oceans. Wiley.
• Sorokin, Y. I. (1973). On the feeding of some scleractinian corals with bacteria and dissolved organic matter. Limnology and Oceanography.
• Ferrier-Pagès, C., et al. (1998). Feeding rates and assimilation efficiency of bacterioplankton in scleractinian corals.
• Houlbrèque, F., & Ferrier-Pagès, C. (2009). Heterotrophy in tropical scleractinian corals. Biological Reviews.
• Wild, C., et al. (2004). Degradation and mineralization of coral mucus in reef ecosystems.
• Rohwer, F., et al. (2002). Diversity and distribution of coral-associated bacteria. Marine Ecology Progress Series.

Who wrote this article

Sources

1. Houlbrèque F., Ferrier-Pagès C. (2009). Heterotrophy in tropical scleractinian corals. Biological Reviews 84(1): 1-17.