Size distribution and activity of marine microheterotrophs1

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Abstract
Natural populations of marine heterotrophic microorganisms in a variety of water masses were size fractionated with Nuclepore filters in a range of pore diameters of 0.1–8 µm. About 90% of the heterotrophic assimilation of 3H‐labeled substrates (d‐glucose, acetate, and l‐serine) was by organisms that pass through 1‐µm filters; essentially all heterotrophic activity was by organisms passing through 5‐µm filters. The 1‐µm filterable fraction contained bacteria and, in some seawater samples, microflagellates. About 90% of the microbial heterotrophic activity in the seawater samples examined was due to those microorganisms (presumably bacteria) that arc unattached to larger particulate material.