The Use of Colloidal Silica To Extract Nematodes From Small Samples of Soil or Sediment

Abstract

The use of colloidal silica as a medium for the extraction of nematodes by flotation was examined in order to rationalise the diverse range of methods currently in use. Significantly more nematodes were extracted from small samples of rhizosphere soil and estuarine sediment by using colloidal silica (Ludox HS30 diluted to 1.10 g cm -3 with water) than by density-gradient centrifugal flotation in sucrose, a modified Baermann funnel extraction or by repeated decantation. No benefit was gained by using polyvinylpyrrolidone coated silica (Percoll), by initial mixing of the sample with kaolin or by using density-gradient centrifugation rather than centrifugation of a sample/Ludox mixture. Increasing the amount of sample mixed with a given amount of Ludox gave a linear increase in nematodes recovered up to a critical point (0.036 g rhizosphere sample and 0.221 g estuarine sample ml -1 Ludox). Rhizosphere nematodes passed through a 53 μm mesh and the sample/Ludox mixture required centrifugation so that the sample could be filtered through a 15 μm mesh. Estuarine nematodes were retained by a 53 μm mesh and simple sedimentation of the sample/Ludox mixture was sufficient for extraction.