Use of microbial exoenzymatic activities in aquatic ecosystems studies
Exoenzymes play an important role in the transformation of organic compounds in aquatic environments : these biomolecules convert high molecular weight compounds by hydrolysis into monomeric or oligomeric compounds that are then assimilable by bacteria. This step is a key process in the microbial food web and microorganisms that produce exoenzymes are probably good competitors in aquatic environments. The different exoenzymes are located in different places on the cell membrane with respect to the bacterial cell wall type : exoenzymes of the gram negative bacteria are rather located on the outside of the cytoplasmic membrane or in the periplasm. Their regulation can be either at the level of exoenzyme synthesis or at the level of enzymatic expression. Their activity is generally described with a Michaelis-Menten equation. Most exoenzymes are inducible (phosphatase), but some are constitutive (aminopeptidase). The use of exoenzyme methods in the aquatic environment needs some care, both during application and interpretation. The intrinsic variability of enzyme activity differs with the type of exoenzyme : the more the activity is inducible, the higher is its variability : e. g. phosphatase (inducible : 16 %) in comparison with glucosidase (constitutive : 7 %). Experimental enzymatic substrates are in tact « model » molecules that are supposed to have the same behaviour as « natural » substrates : this is not always true. Spatial variability is also important in sedimentary environments : significant variations in activity exist, especially with depth and stratification of sediment, at a scale of 1 - 10 cm (± 50 %). A topographie knowledge of the studied environment and the definition of a suitable sampling strategy are thus very important. Sampling methodotogies in sediment, silt or coarse substrates are more difficult to set up than are those for the water column (heterogenous environment at the centimetre scale, variation of microbial population density, nutrient diffusion, gradients). The sampling of sediments with a corer, and the subsequent fractionation of the sediment into different layers for incubation in the laboratory, may modify the physicochemistry of the sediment and could influence bacterial activity (experimental artefact). A technique has been developed for incubation inside the corer in order to minimize perturbations but this technique is limited by the sediment granulometry. Conversely, homogenization of sediments allows a better standardization of experiments and yields more reproducible results. For studies on the assimilation capacity of aquatic ecosystems and biodegradation of organic matter, exoenzymatic activities data need to be associated with other biological or biochemical parameters : bacterial and/or phytoplanctonic biomass; precise analysis of assimilable organic matter in the particulate or dissolved phases; physicochemical data... Data concerning the modelling of exoenzyme activities in relation to parameters such as temperature or oxygen level are lacking. Integration of these data will afford a better overall understanding of the role of exoenzymes in the metabolism of aquatic environments, and will help establish the limits of the validity of this technique for global studies of the assimilation capacities and organic matter biodegradation in aquatic ecosystems.