Size distribution and activity of marine microheterotrophs1

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 µ m. About 90% of the heterotrophic assimilation of 3 H‐labeled substrates ( d ‐glucose, acetate, and l ‐serine) was by organisms that pass through 1‐ µ m filters; essentially all heterotrophic activity was by organisms passing through 5‐ µ m filters. The 1‐ µ m 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.