A FAST-RESPONDING CO2 MICROELECTRODE FOR PROFILING SEDIMENTS, MICROBIAL MATS, AND BIOFILMS

A new CO2 microelectrode with a tip diameter of 10 mu m and a response time (t(90)) of similar to 10 s is presented. The sensor allows CO2 m easurements with a detection limit of <3 mu M. The microsensor was tes ted in experimental systems of increasing complexity. A diffusion-reac tion simulation model was used to calculate CO2 profiles in order to c heck the reliability of the measured profiles. Measured CO2 and O-2 pr ofiles showed that, in highly active layers with photosynthetic and re spiratory organisms, local equilibrium of the carbonate system cannot be assumed. In such highly active systems, the CO2 profiles were deter mined by the slow CO2 hydration rate, the biological conversion rates, and the diffusion of all species of the carbonate system. We conclude d 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 bi ological processes directly producing or consuming CO2 such as photosy nthesis or respiration.