Organic Electrochemical Transistors (OECTs) are now well-known, robust and efficient as amplification devices for redox reactions, typically biologically ones. In contrast, Electrolyte-Gated Organic Field Effect Transistors (EGOFETs) have never been described for that kind of application because field-effect transistors are known as capacitive coupled devices, i.e. driven by changes in capacitance at the electrolyte/gate or electrolyte/semiconductor interface. For such kind of transistor, any current flowing at the gate electrode is seen as a drawback. However, we demonstrate in this paper that not only the gate potential can trigger the source-drain current of EGOFETs, which is the generally accepted mode of operation, but that the current flowing at the gate can also be used. We propose a complete interpretation for this original behavior. Because EGOFETs can work directly in water, and as an example of application, we demonstrate here the possibility to monitor microalgae photosynthesis through the direct measurement of photosynthetic O₂ production within the transistor’s electrolyte, thanks to its electroreduction on the EGOFET’s gate. In a second step, microalgae were directly immobilized on the platinum gate, for a better sensitivity. As an example of application, monitoring the presence of pollutants such as weed killer in water is investigated.