Quantitative centrifugation to extract benthic protozoa from freshwater sediments

TWO METHODS FOR EXTRACTING PROTISTS FROM FRESHWATER SEDIMENT ARE DESCRIBED: (i) an adapted isopycnic centrifugation technique for sandy and gyttja-like sediments and (ii) a rate zonal centrifugation technique for sediments rich in particulate organic material (litter-like sediments). The recoveries of protists during isopycnic centrifugation in media of several densities were compared. No significant losses in sodium diatrizoate and Percoll were recorded. After known amounts of nanoflagellates were added to azoic sediments, the protists were extracted and counted. For sandy sediments, we found 100% recovery, and for the gyttja-like sediments we found a maximum recovery of 94%. The recovery of protozoa extracted from litter-like sediments, characteristic of littoral systems, depends on a given centrifugal force, on time, and on the dimensions of the flagellates. A recovery model which takes into account cell dimensions and centrifugation characteristics gives the minimum expected recovery.