Anaerobic bacteria in an aquarium: a horror or a normal thing?

If you ask a random aquarist if there are any places in their aquarium where there is no oxygen, most will answer “no” right away without giving it a second thought. Because it’s hard to imagine that in such a small space, where the water is constantly mixed by a filter and often also mechanically oxygenated, there could be a place where anaerobic bacteria are constantly fighting over territory. Why war? Why is it called this? Do we even need them? I think you should read the article about bacteria that don’t need oxygen and live in an aquarium.

Liquid oxygen distributor

We can find water everywhere, both on the whole planet and in the home tank. It is one of the things that you need to live. Not only because of the plants and animals that live there. Just as important are the properties of water itself. Water is a great solvent for many chemicals and gas molecules, like oxygen. It has a high specific heat, which means it stores energy well because of this.

Without oxygen, is there no way to live?

When a fan of science fiction movies or books thinks about life without oxygen, the first thing that comes to mind is his or her favorite hero dying because there isn’t enough oxygen on the space station. But there are living things that are much smaller than humans. You can’t see them with just your eyes. These organisms can live in places with no oxygen because they are smart enough to get it from the structures of chemical compounds.

An example of an environment without oxygen is Lake Tanganyika.

It may be the most well-known body of water in Africa. It is 677 km long (the distance from Zakopane to Gdask), 50 km wide on average, and covers 32,900 km2. With a depth of up to 1,433 meters (almost 5 Eiffel Towers) and a certain landscape (which affects the winds that blow there), it is impossible to keep the water moving. This means that there are parts of the lake that don’t have oxygen. During many studies of this environment, it has been shown that nitrogen does not build up. How can nature grow and thrive in a lake with so much wildlife and so much water?

Lake Tanganyika BCC research

A team of researchers decided to examine the composition of the so-called bacterial community in 2002. (BCC in short). Previously, shallow lakes, seas, and oceans were used for this type of research. Only Lake Baikal, the deepest lake on Earth, has been studied in terms of ancient deep lakes, though Lake Tanganyika only began to form about 12 million years ago.

According to research on BCC, there are vertical zones that are organized along temperature and oxygen concentration gradients. The graph below shows that this lake, which has an average depth of 150 meters, contains water devoid of oxygen. As a result, it is the largest anaerobic water reservoir in terms of volume. A balance has likely developed in the lake that ensures the stability of the ecosystem based on the diversity of species and consequent adaptations to the local environment among the bacteria at various depths. Stability is something that aquarists, as well as proponents of “low-tech” aquaria that use a layer of soil at the bottom of the tank, more or less consciously fight for.

What does it look like in Lake itself?

Because of the land around Lake Tanganyika, there are no strong winds that can mix the water layers together. So, Lake Baikal is the opposite of it. Because, even though it is deeper, this mixing and, as a result, this oxygenation happens everywhere in the volume. The best way to show the changes with depth is with the graph below, which comes from a 1999 paper called Nitrogen Dynamics in Northern Lake Tanganyika. Due to the poor quality of the original, it has been redrawn for this article.

By looking at just this one graph, we can draw a lot of conclusions that, on a small scale, will also work for a home aquarium.

1. We notice that there aren’t many ammonium ions (NH4+) and nitrate ions (NO3–) in the surface zone because phytoplankton eat so much of them.
2. Ammonium ions are low until 150 m deep (even in light-free conditions). This is because of the nitrification process, in which oxygen and little light turn ammonium ions into nitrate.
3. Because there is less light below 30 m, the amount of NO3– starts to rise. So, the phytoplankton stops doing what it was doing. Nitrification processes cause NO3– to build up in water.
4. Nitrates build up to a depth of about 75 m. Then, because there is less oxygen in the water, their concentration starts to drop. Bacteria that don’t need oxygen but can use the oxygen in the NO3– ion start to work.
5. When the amount of oxygen in the water drops to zero, the level of nitrate(V) also drops a lot. Because they are the only way for the bacteria in an anaerobic environment to get oxygen. NO3– is changed into one molecule of N2 gas.
6. As the concentration of NO3– goes down, the concentration of NH4+ goes up. At this depth, the water has no dissolved oxygen, so nitrification can’t happen. So, the amount of ammonium ions goes up. This is followed by a process called “mineralization of organic nitrogen,” in which organic nitrogen builds up in the sediments on the bottom.

Anaerobic bacteria in a home aquarium

There are bacteria that don’t need oxygen in every aquarium, which might surprise you. You can find them in a layer of gravel, sludge, a biological filter medium with the right amount of holes, or a filter that is clogged. Everywhere, small amounts of NO3– get into the layer and cause denitrification to happen. Because these bacteria are there, metal ions that had been trapped are also released. For example, iron, which I wrote about in the article Iron (Fe) in the aquarium, is one of the metal ions that is released.

Anaerobic bacteria under full control

So far, we’ve looked at examples of anaerobic bacteria that grow in nature without any help from humans. There is nothing stopping us from making a place where anaerobic bacteria can take care of removing NO3– from the aquarium all the time. We’ll need a nitrate reducer for this. It is a device that looks like a bucket filter and is usually used with a conductivity meter.

The conductivity of water is measured over a wide range, from negative to positive values. When things are happening that are related to aerobic conditions, the meter will always show a positive value, like +100 mV. When there is no oxygen, the conductivity is negative, like -100 mV. It is therefore a reliable way to find out right away what the situation is.

Through the reducer, water moves very slowly. If it is connected to a meter, the controller runs the pump based on what the meter says about how well the water flows. With such a small flow, oxygen can be used up in the reactions happening in the water per unit volume before it gets to the reactor, where it can’t get in.

For the denitrification process to work, a lot of protons and electrons are needed. So, to use a nitrate reducer, you have to “feed” the bacteria. Biodegradable polymers are used by professionals who reduce this. Both protons and electrons can be found in the salt of acetic acid. Here’s a summary of the reaction that was talked about:

5CH3COO– + 8NO3– + 3H+ → 10HCO3– + 4N2 + 4H2O

Anyone interested in marine aquariums can see from the diagram above that this is what the VM and VSVM methods are based on. I might talk more about this subject in the future

When a nitrate reducer is used, it is clear that there could be some technical problems. But this is a subject for a different article.

Let’s not bother them

We already know that there are anaerobic bacteria in a home aquarium and that they are very important. They lower the amount of NO3– in the air and let elements into the water that plants need to live. They also break down things that have died. In a way, they close the life cycle.

Beginner aquarists who think that frequent filter cleaning and bottom de-sludging will keep the water “sterile” are painfully sure that huge amounts of nitrate ions and other chemicals are released into the water when they do these things. These ions waited quietly in the substrate until it was their turn to be sacrificed to the anaerobic bacteria. This allowed the bacteria to live and keep our tank’s parameters stable, and the plants and animals enjoyed seeing them.

Literature:

Nitrogen Dynamics in Northern Lake Tanganyika by N. Brion, E. Nzeyimana, L. Goeyens, D. Nahimana, and W. Baeyens (1999)

1. De Wever, K. Muylaert, K. Van der Gucht, S. Pirlot, Ch. Cocquyt, Jean-Pierre Descy, Pierre-Denis Plisnier, W. Vyverman The Makeup of the Bacterial Community in Lake Tanganyika: Differences in the Vertical and Horizontal Directions (2002)