Distribution of bacteria, estimated by a viable count method, and heterotrophic activity in different size fractions of aquifer sediment

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Abstract
This study investigated the relationship between aquifer sediment particle fractions and bacterial counts and activity thereon. Samples of aquifer sediments were fractionated into different size fractions by filtering a sediment suspension successively through progressively smaller mesh‐sized filters (500, 100, 55, 20, 1.2, and 0.2 μm). In all sediment 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 μm fraction, 40–96% in the 1.2–55 fan fraction (silt‐sized), and only 0.01–0.04% in the 0.2–1.2 μm fraction. The microbial heterotrophic activity (measured as the 14CO2 evolution from [14C]acetate or [I4C]glucose) showed the same pattern. After 2 days of incubation, 73–99% of the total amount of 14CO2 was evolved by the 1.2–100 μm fraction, 46–68% by the 1.2–55 fan fraction (silt‐sized), but only 0.2–5.4% by the 0.2–1.2 fan fraction. The finest fraction (0.2–1.2 fan) also included bacteria in the porewater, but the results showed that most of the bacteria and their activity were associated with small particles and thus were not free‐living in the pore‐water. The viable counts were higher (3–6 times) on small (1–2 mm) clay aggregates alone than for sediment including clay aggregates, indicating that even when clustered together, the clay particles carried most of the viable bacteria. Good correlations were found between surface area and viable counts as well as heterotrophic activity in the particle fractions, except for the finest fraction. The activity 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 techniques retaining porewater with clay/silt particles.