Leaky Filters: A Warning to Aquatic Ecologists

Abstract

We examined the retention characteristics of commonly used 0.2-μm nucleation-track (Nuclepore^®, Poretics^®), polymer (Millipore^®, Sartorius^®) and inorganic membrane (Anopore^®) filters. Scanning electron micrographs of the filter surface showed many to contain large holes or pores, some 5 × larger than the manufacturer's stated nominal pore diameter. Electron micrographs of filtrates from both fresh and seawater samples contained a variety of organisms, including viruses, ultramicrobacteria, bacteria, phototrophic picoplankton, and larger nano- and microphytoplankton (e.g. diatoms), which we believe passed through the large openings. Total particles in the size range 1.0–12.8 μm passing through the various 0.2-μm filters tested ranged from 2.2 to 14.3%, with 'best' retention (>97%) by the Anopore^®. Average particle retention for all 0.2-μm filters tested was 92.5%. Higher passage rates (lower retention) are predicted for even smaller (<1.0 μm) particles (prochlorophytes, bacteria, viruses) and investigators are urged to use care when using polycarbonate or matrix type filters for partitioning size fractions for chemical, physiological, and ecological work, and caution when interpreting results, especially if separations have not been verified by microscopy, incubation, or culture.