Leaky Filters: A Warning to Aquatic Ecologists

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...

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