Water is an essential component of our lives. To ensure its quality, the development of instruments for biological monitoring is crucial. In this context, biosensors provide a solution that encompasses both chemical and toxicological detection aspects. The technology presented in this document is a biosensor based on the assessment of the activity of photosynthetic systems. The scientific foundations underlying this concept, both in terms of the materials used and the measured reactions, are described in detail. This document aims to encourage the use of simple, effective, and cost-efficient tools for large-scale biological monitoring of water.