Biofilms As River Sentinels

Over 100 000 chemicals used by humans may find their way into aquatic ecosystems. As part of the application of the WFD1, monitoring of rivers has been stepped up to attain good ecological and chemical status of European rivers by 2015. The regulations require that the necessary means be available to measure pollution and assess its impact on environments and all life forms, in view of planning restoration efforts. Though it is possible to calculate the doses of chemicals at a given time or over a time span, there are few tools to evaluate the overall ecological status of a natural environment.

An original biological model

At Cemagref in Lyon, scientists are exploring a novel method to develop such bioindicators, i.e. the use of natural biofilms to detect pesticide pollution in rivers and understand the effects of such pollutants on the overall operation of aquatic ecosystems. Biofilms are groups of bacteria, algae and fungi that develop on immersed objects (see the box) and play a decisive role in the ecosystem. They produce organic matter through photosynthesis, thus making them one of the first links in the food chain, as well as break down organic matter and recycle nutrients. useful indicator of environmental quality

In addition to their rapid development, biofilms are useful due to their capacity to interact with dissolved substances, particularly pesticides, that they can break down and to their ability to adapt quickly to changes in the environment. Contamination of the aquatic environment by pesticides can modify their structure, their diversity (representation of the constituent species) and their operation, for example by changing their photosynthetic, respiratory or enzymatic activity, either definitively or temporarily. Biofilms are thus good indicators of environmental quality, capable of providing early warning of pollution by pesticides or any other toxic substance.

Research continues to characterise and distinguish the response of biofilms to environmental factors such as flow speeds, light conditions, physico-chemical composition, etc., on the one hand, and the effects of pesticides on the other. This work will, over time, make available functional tools for bioindication that complement the existing standardised tools that are suited essentially to organic pollution2. In parallel, the scientists are attempting to evaluate the capacity of biofilms to break down pollutants in situ. The double goal is to assess their natural capacity for biodegradation in order to take it into account for restoration work on contaminated rivers and to identify the species of bacteria and fungi best suited to break down contaminants.

 1 The Water framework directive (WFD), adopted by the European Union in 2000, sets good ecological status of European rivers as its main goal for 2015.

2 Notably the three specific indicators that meet AFNOR standards and WFD criteria, used in France, i.e. IBD (indice biologique diatomées / diatom biological index), IBGN (indice biologique global normalisé / biotic index) and IPR (indice poisson en rivière / river fish index), based on taxonomic criteria

Natural biofilms, a complex combination of micro-organisms

Everyone has felt, while walking barefoot over submerged rocks in a river, the slippery, viscous slime covering the rocks. This mucilaginous structure is a natural biofilm made up of a complex assembly of several species of bacteria, algae (green, blue-green, diatom), filamentous fungi, etc. This combination is capable of cellular exchanges and its structure and operation are modified by the environmental parameters such as the flow speed, the hydraulic regime, light conditions, the physico-chemical conditions of the environment, etc.

The photosynthetic algae, present in the biofilm when the environment is exposed to light, produce the organic matter that is eaten by browsing organisms (gastropods or certain fish). Bacteria and fungi, which are not photosynthetic, break down and recycle organic matter, thus actively participating in the carbon, nitrogen and phosphorus cycles. The biofilms found in sedimentary environments do not include algae. They contain exclusively bacteria and fungi. The latter are also particularly present in biofilms that colonise submerged leaves and wood.