Bioaccumulation of toxicants is one of the key functions of microbial communities, including microalgae and bacteria, in the environment. Micro-benthic algae and bacteria are often organised in a multi-layers biofilm at the interface sediment/water, the surface of a sediment particle, or a plant. These biofilms are often limited in space by a matrix of extra-cellular polymeric secretions (EPS) which is known to fix and accumulate metal ions, metalloids and molecules. The organisation in biofilm is supposed to be a resistance state for micro-organisms during stress periods and may be dependent on density-dependent signal released by bacteria (autoinducers). Interactions between the micro-organisms composing the biofilms are a challenging research area (e.g., quorum sensing and biofilm community development). Multiple chemical equilibria are occurring, from anoxic dependent to oxygen limited reactions, which mean a coexistence of very diverse heterotrophic and autotrophic micro-organisms.
  
  Ecotoxicological studies or risk assessment of chemicals are a major duty of many international projects. But in most of the assessment models the functions performed by the micro-organisms are not linked to the microbial diversity.
 
 This project sets out to assess microbial community effects of diffuse toxicant stress in three different locations in The Netherlands (Biesbosch, Vechtplassen and floodplains of the river Waal). In laboratory experiments we are focusing on some of the toxicants, mostly heavy metal like Zn or Cu. How are communities affected by differences in toxicant concentrations? Which ecosystem function(s) is (are) influenced, if any? Micro-organisms sensitivity is known to be variable depending on many other abiotic parameters and for certain species varying from strain to strain.
  
  New techniques are giving numerous insights on the ecology and diversity of the communities forming biofilms with high resolution and sensitivity. Most of the studies involve the wild range of microscopes (atomic-force microscopy, confocal scanning laser microscopy, and low-temperature scanning electron microscopy). Few investigations have been undertaken to assess exact species composition, diversity and correlation of the biofilm community with help of the new genetic techniques such has the Polymerase Chain Reaction (PCR), the Degrading Gradient Gel Electrophoresis (DGGE) or sequencing reaction. A library of DGGE patterns is undertaken for the three different sites over one year. Communities used in laboratory for PICT and PAM experiments have also been investigated with help of genetic tools. DGGE fingerprinting of eukaryotic and prokaryotic communities suggests that a fixed fingerprinting for eukaryotes (or prokaryotes) over different sites (or over time) does not obligatory imply stability for the whole community.