A fast‐responding CO2 microelectrode for profiling sediments, microbial mats, and biofilms

Abstract

A new CO₂ microelectrode with a tip diameter of 10 µm and a response time (t₉₀) of ∼10 s is presented. The sensor allows CO₂ measurements with a detection limit of µM. The microsensor was tested in experimental systems of increasing complexity. A diffusion‐reaction simulation model was used to calculate CO₂ profiles in order to check the reliability of the measured profiles. Measured CO₂ and O₂ profiles showed that, in highly active layers with photosynthetic and respiratory organisms, local equilibrium of the carbonate system cannot be assumed. In such highly active systems, the CO₂ profiles were determined by the slow CO₂ hydration rate, the biological conversion rates, and the diffusion of all species of the carbonate system. We concluded that measured CO₂ profiles cannot easily be extrapolated to describe the total carbonate concentration profile, because CO₂ may not be in equilibrium with the rest of the carbonate system, and because a very accurate alignment of pH and CO₂ profiles is needed to calculate C_T. However, the new CO₂ microelectrode is useful in research involving biological processes directly producing or consuming CO₂ such as photosynthesis or respiration.