The relation between concentration of elements and microbial activity in the water samples of Higashi-Hiroshima Campus, Hiroshima University was investigated. Energy dispersive X-ray spectroscopy revealed that microbial mat contains iron, aluminium, silicon and phosphorus. Model experiment revealed that the potassium was adsorbed by living microorganism in the microbial mats, while it was not adsorbed by dead microbial mat. Iron was adsorbed by both living and dead microbial mats. The present results explain the increase in the total β-radioactivity of water sample in summer and the decrease in winter.