When bacteria are mentioned, they are often imagined as free-floating microorganisms suspended in water. In reality, this perception is far from accurate. Microbiological research has shown that approximately 95% of bacteria found in natural environments live as biofilms, meaning they are attached to solid surfaces and organized into structured microbial communities.

This lifestyle is not accidental. It is a highly effective evolutionary strategy that enables microorganisms to survive, grow, and perform their biological functions even under challenging environmental conditions. MBBR (Moving Bed Biofilm Reactor) biofilters take advantage of this natural ability by providing an ideal environment for biofilm development, ensuring efficient biological water treatment.

A much more efficient way of life

An isolated bacterium is extremely vulnerable. It is directly exposed to temperature variations, pH fluctuations, salinity changes, disinfectants, heavy metals, and variations in oxygen or nutrient concentrations.

In contrast, when a bacterium becomes part of a biofilm, it immediately benefits from the protection provided by the extracellular polymeric substances (EPS) matrix produced by the entire microbial community.

This matrix acts as a true protective barrier, limiting external stresses while maintaining a more stable environment around bacterial cells.

In an MBBR biofilter, this protection allows nitrifying bacteria, which are naturally fragile and slow-growing, to remain sustainably established on biological carriers despite the continuous movement of the media.

True cooperation between microorganisms

Contrary to what is often assumed, bacteria do not operate independently. Within a biofilm, they develop numerous interactions that improve the overall functioning of the microbial community.

The metabolic waste products produced by certain species become nutrients for other bacteria. This complementarity allows a much more efficient use of available resources.

In an MBBR biofilter, heterotrophic bacteria degrade part of the dissolved organic matter, while nitrifying bacteria use nitrogen compounds resulting from this degradation to carry out the nitrification process.

This organization creates a true microscopic ecosystem in which each group of microorganisms contributes to the overall balance of the biofilm.

Bacteria communicate with each other

One of the most fascinating aspects of biofilms is their ability to communicate.

Bacteria continuously exchange signaling molecules through a mechanism known as quorum sensing. This system allows them to evaluate population density and collectively adjust their behavior.

When the microbial community reaches a sufficient density, several functions can be activated simultaneously:

• EPS production;
• development of new colonies;
• modification of metabolic activity;
• adaptation to environmental conditions;
• strengthening of biofilm cohesion.

The biofilm therefore functions as a coordinated community rather than as a simple accumulation of independent cells.

A structure capable of constant adaptation

The environment of an RAS system is rarely perfectly stable. Flow rates change, fish grow, feeding rates evolve, and organic loading can increase rapidly.

Thanks to their collective organization, biofilms demonstrate remarkable adaptive capabilities.

Bacterial populations continuously evolve according to environmental conditions. Some species become more abundant while others decline, allowing the biofilm to maintain its efficiency despite operational changes.

This biological flexibility largely explains the robustness of properly designed MBBR biofilters.

A continuously active biological reservoir

Contrary to common belief, an efficient biofilm is not a static layer of bacteria.

Due to mixing and collisions between MBBR carriers, part of the biomass is regularly removed from the surface. This phenomenon is completely normal and even essential for proper biofilter operation.

The deeper layers remain firmly attached to the carrier, while the outer layers are continuously renewed.

This dynamic process allows:

• prevention of excessive biofilm ageing;
• maintenance of high biological activity;
• promotion of available surface recolonization;
• preservation of stable long-term nitrification performance.

The biofilm is therefore a living system in constant evolution rather than a simple bacterial deposit.

An ideal organization for nitrification

Nitrifying bacteria are among the most sensitive microorganisms present in a biofilter. Their growth rate is slow and their oxygen requirements are particularly high.

The biofilm provides them with a favorable environment where they can remain established over time, protected from hydraulic stresses while benefiting from a continuous supply of oxygen and ammonium.

This organization explains why MBBR biofilters are now considered one of the most reliable solutions for biological filtration in Recirculating Aquaculture Systems (RAS).

Key points

A biofilm is much more than a simple accumulation of bacteria on a plastic carrier. It is a true ecosystem capable of communication, adaptation, and continuous renewal.

This collective organization enables MBBR biofilters to provide efficient and sustainable nitrification, which is essential for maintaining excellent water quality in modern aquaculture facilities.