A low-cost dissolved organic matter probe for aquatic ecosystems

Naturally occurring dissolved organic matter (NDOM) is a complex, heterogeneous mixture of carbonaceous substances derived either from the excretions of living organisms or from the natural processes of decomposition and transformation of organic residues (plants, animals, microorganisms). It plays a crucial role in the sequestration of atmospheric carbon and in the transport of metals. Climate change, decrease in the acidification of rivers and lakes (Herreid et al. 2024), land-use change and discharge of anthropogenic organic matter are among the factors leading to a continuous evolution of DOM in water bodies and thus modifying the global cycle of carbon and directly impacting water resources such as those used for drinking water production (Chen et al. 2025). DOM cannot be defined through specific chemical compounds and is considered as a supramolecular assembly of small molecules such as sugars, tannins, proteins, lipids, etc. Optical methods such as UV-visible spectroscopy and fluorescence spectroscopy are commonly used to investigate DOM molecular and structural properties in laboratories using benchtop devices. Our goal within the TERRA FORMA project has been to go further by developing an in-situ and low-cost spectrometer that combines UV-Visible light absorption and fluorescence measurements for the analysis of DOM, focusing on protein-like (a potential marker for faecal contamination, Kim et al. 2024) and humic-like characteristics. The system should be efficient and easy to use for researchers as well as for participants in citizen science programs. The selection of the LEDs and photodiodes has been optimized in terms of wavelength range and power. A 275 nm emitting LED is used for DOM absorbance (measurement with a 220-280 nm working range photodiode) and protein-like fluorescence (measurement with a 345-450 nm working range photodiode). A 370 emitting LED is used for humic-like fluorescence (400-550 nm working range photodiode). The optical devices are driven with an Arduino Nano. The final probe (Fig. 1) is fitted with quartz windows and protected by a copper-based wire mesh. It will be used in a stand-alone mode (for long-term deployment) or in a nomadic mode (connected to a case fitted with pH, conductivity and temperature probes). Before finalizing the 3D printing of the final probe (Fig. 1), a test set-up has been produced, which can be used to optimize the light pulse regime necessary to accommodate the heat production by the LEDs, as well as to test the sensitivity of the device. The test set-up is controlled from a PC using a LabView program. Various model substances such as tryptophan and other indole-based compounds, quinine sulfate, quinones were tested (pure and in mixtures) as well as samples from different aquatic ecosystems (surface water impacted by natural and anthropogenic DOM sources, effluents from constructed wetlands, etc.). The system is sensitive to tryptophan (fluorescence with excitation at 270 nm), quinine (fluorescence with excitation at 370 nm) and humic acid (absorbance at 270 nm and fluorescence with excitation at 370 nm) (Fig. 2 and Table 1). The responses obtained on the test set-up are compared to data provided on the same samples by benchtop devices.