Water is an essential element for our life. In order to ensure its quality, the development of biomonitoring tools has become essential. In this context, biosensors provide an answer that includes both chemical and toxicological detection. The technology presented in this paper is a biosensor based on the evaluation of the activity of photosynthetic systems. The scientific basis underlying this concept, both in terms of the material used and the response measured, is detailed. This document is intended as an incentive for the large-scale use of simple, effective and economical tools for water biomonitoring.