USE OF AN OXYGEN-INSENSITIVE MICROSCALE BIOSENSOR FOR METHANE TO MEASURE METHANE CONCENTRATION PROFILES IN A RICE PADDY

An oxygen-insensitive microscale biosensor for methane was constructed by furnishing a previously described biosensor with an oxygen guard, The guard consisted of a glass capillary containing heterotrophic bact eria, which consumed oxygen diffusing through the tip membrane, thus p reventing it from diffusing into the methane-sensing unit, Oxygen micr oprofiles were measured through the oxygen guard capillary, demonstrat ing the principle and limitations of the method, When the tip of the g uard capillary was exposed to 100% oxygen at 21 degrees C, heterotroph ic oxygen consumption prevented oxygen from diffusing further than 170 mu m into the capillary, whereas atmospheric levels of oxygen were co nsumed within 50 mu m, The capacity of the oxygen guard for scavenging oxygen decreased with decreasing temperature, and atmospheric levels of oxygen caused oxygen penetration to 200 mu m at 5 degrees C, The se nsors could be manufactured with tip diameters as small as 25 mu m, an d response times were about 1 min at room temperature, Pore water prof iles of methane concentrations in a rice paddy soil were measured, and a strong correlation between the depths of oxygen penetration and met hane appearance was observed as a function of the light regimen; this finding confirmed the role of microbenthic photosynthesis in limiting methane emissions from surfaces of waterlogged sediments and soils.