The distribution of chemical compounds (e.g., oxygen, sulfide, pH) and physical parameters (e.g., light, temperature) in microbial systems such as sediments or biofilms is the result of (biological and chemical) activity and transport.

• Biological and chemical activity is a transformation of chemical compound(s) from one type to another, e.g., A + B → C, the former being performed or mediated by microorganisms.

• Transport is a transformation of these compounds in time and space (e.g., by diffusion or advection).

In research fields like microbial ecology or biogeochemistry, one is interested in the rates of activities and transport of chemical compounds and physical parameters. This helps understand how the the studied system functions and how it is regulated.

Microsensors, due to their very small size, enable minimally invasive measurements of the spatial distributions of the physico-chemical parameters and their temporal dynamics. Using simple theoretical concepts, one can calculate the transformation rates (i.e., of biological/chemical activity and transport) from these measurements, and thus couple 'what I want' with 'what I can measure'.