Contribution of Free-Living and Attached Bacteria to Denitrification in the Hypolimnion of a Mesotrophic Japanese Lake

Abstract

Denitrification by both free-living bacteria and those attached to particles larger than 1μm was examined by the acetylene blockage technique in the hypolimnion of Lake Kizaki, Japan, from August to November, 1995.Bottom sediment gases(Ar, O₂, N₂, CH₄ and CO₂)were also measured at the beginning of June.Denitrification activity was detected from September to November, and ranged from 0.026 to 0.238μM-N₂O day^-1.During this period, free-living bacteria accounted for 31 to 57% of total denitrification.The total number of bacterial cells ranged from 2.34×10⁶ to 1.24×10⁷ml^-1, of which free-living bacteria accounted for 46 to 87%.High levels of N₂ in the surface sediments from both littoral and deep zones of the lake were observed, suggesting high rates of denitrification.These results show that both attached and free-living bacteria showed significant denitrification activity, with attached bacteria composing a higher proportion of the population in September in the deepest layer(28m).Free-living denitrifiers seemed to increase from September to November with expansion upwards of the low oxygen layer.