DISTRIBUTION OF BACTERIA, ESTIMATED BY A VIABLE COUNT METHOD, AND HETEROTROPHIC ACTIVITY IN DIFFERENT SIZE FRACTIONS OF AQUIFER SEDIMENT

This study investigated the relationship between aquifer sediment part icle fractions and bacterial counts and activity thereon. Samples of a quifer sediments were fractionated into different size fractions by fi ltering a sediment suspension successively through progressively small er mesh-sized filters (500, 100, 55, 20, 1.2, and 0.2 mu m). In all se diment samples, even though the abundance of the coarser particles was high, 91.9-100% of the viable number of bacteria in the fractionated sediment was found in the 1.2-100 mu m fraction, 40-96% in the 1.2-55 mu m fraction (silt-sized), and only 0.01-0.04% in the 0.2-1.2 mu m fr action. The microbial heterotrophic activity (measured as the (CO2)-C- 14 evolution from [C-14]acetate or [C-14]glucose) showed the same patt ern. After 2 days of incubation 73-99% of the total amount of (CO2)-C- 14 was evolved by the 1.2-100 mu m fraction, 46-68% by the 1.2-55 mu m fraction (silt-sized), but only 0.2-5.4% by the 0.2-1.2 mu m fraction . The finest fraction (0.2-1.2 mu m) also included bacteria in the por ewater, but the results showed that most of the bacteria and their act ivity were associated with small particles and thus were not free-livi ng in the pore-water. The viable counts were higher (3-6 times) on sma ll (1-2 mm) clay aggregates alone than for sediment including clay agg regates, indicating that even when clustered together, the clay partic les carried most of the viable bacteria. Good correlations were found between surface area and viable counts as well as heterotrophic activi ty in the particle fractions, except for the finest fraction. The acti vity measurements showed that at least some of the sediment-associated bacteria were active on the surfaces of the particles. The importance of small sediment particles as a carrier for microorganisms leads to the conclusion that quantitative sampling of microbial populations in aquifers must include sampling of the sediment, with sampling techniqu es retaining porewater with clay/silt particles.