Graphical abstract Highlights • Respiratory CO 2 release/photosynthetic uptake rates are early indicators of toxicity • Repeatable, highly sensitive endpoint responds rapidly to environmental disturbances • Fast and non-destructive identification of potential threats in real time Abstract This paper presents an application of continuous monitoring of the emission and uptake rate of CO 2 in Lemna toxicity test. On a real-time basis, the CO 2 concentration data were collected by the Arduino platform-based respiratory activity measuring system (ResTox) and reported as CO 2 concentration dynamic curves. The results of CO 2 measurements demonstrated that tested metals (Co, Cu, Hg, and Cd), as well as herbicides (nicosulfuron, diquat, and tembotrione), stimulated the CO 2 exchange rates at low doses, while at high doses CO 2 exchange rates were inhibited. The addition of higher concentrations of clopyralid stimulated photosynthetic activity and caused a higher increase in respiration rates indicating its mode of action as auxin mimic herbicide. The results obtained underline the necessity of considering other biological endpoints like continuous measurements of gas exchange from the very beginning of exposure to toxicants. Simultaneous measurements of real-time CO 2 concentrations, as the primary effect of toxicant mode of action, and processes that are supported by carbon flux, as the secondary effect or endpoint, are needed to relate actual and substrate-induced or inhibited respiration and photosynthesis to those processes. Therefore, continuous measurements of CO 2 exchange rates can be implemented for the initial screening of potential toxicity to give valuable information that is needed for further examination of toxicity mechanisms and risk assessment. [ABSTRACT FROM AUTHOR]