Microsensors for Sediments, Microbial Mats, and Biofilms

In sediments, microbial mats, and biofilms steep gradients of substrates and products develop, due to high metabolic activities and mass transfer limitations (see Chapters Biofilms ; Microbial Mats ; Sediment Diagenesis – Biologically Controlled ). Stratifications of microenvironments develop that determine the existence and activity of microbial consortia. For example, in active biofilms oxygen can penetrate less than 50 µm (de Beer et al., 1994c; de Beer et al., 1993), in sediments oxygen penetrates typically less than 2 mm (de Beer, 2001; Jørgensen and Revsbech, 1985; Meyers et al., 1987; Revsbech, 1983; Sorensen et al., 1981; Sweerts and de Beer, 1989). Below the oxic zone anaerobic microbial processes can occur, such as denitrification and sulfate reduction (see Chapter Sulfur Cycle ), that determine the element cycling (the coupled degradative and chemolithotrophic processes) in the system as a whole. Stratifications on such a small scale cannot be studied by more classical techniques such as pore-water extraction and chemical analyzes. Extraction is impossible on such small scales, as it is highly destructive for the system and concentrations will have changed during the extraction procedure. To study such stratified systems we need microsensors that are minimally invasive and directly measure the concentrations without disturbing the system in which we measure. Moreover, as the system is not disturbed by microsensor analyses, we can perform sequences of measurements and, for example, study how the system responds to environmental changes. Microsensor measurements have enormously enhanced the insight in the vertical distribution of microbial processes in sediments. Microsensors were originally developed for physiological studies in animal and plant tissues, and even for intracellular measurements (Ammann, 1986; Hinke, 1969; Thomas, 1978), but soon applied in environmental studies (Bungay et al., 1969). They became an established technique for ecological studies by the developing work of Revsbech, who made them more robust and invented a large diversity of sensors (Kühl and Revsbech, 2000; Revsbech and Jørgensen, 1986)