Preservation of marine planktonic ciliates: losses and cell shrinkage during fixation

For enumeration of microzooplankton (20–200 μm size fraction), including planktonic ciliates, water samples are usually fixed and preserved, then concentrated by sedimentation in Utermöhl chambers and examined with an inverted microscope. However, losses of ciliates may occur during fixation and handling, and fixation may shrink cells. Estimates of abundance with several commonly used fixatives were compared in samples from the North Atlantic and subarctic Pacific Oceans and in samples from cultures. Buffered formaldehyde has the advantage that it allows epifluorescence microscopy to be used, but, on average, ciliate counts from the North Atlantic were 56 % higher (95 % CI 30 to 82 %) in samples fixed with 10 % acid Lugol's solution than in samples fixed with 2 % formaldehyde. Ciliate counts from the subarctic Pacific were 23 to 49 % higher in samples fixed with 10 or 20 % acid Lugol's solution than in samples fixed with 5 % acid Lugol's solution. Fixation with 10 or 20 % acid Lugol's solution results in significantly higher cell counts than fixation with formaldehyde or dilute acid Lugol's solution, but shrinks cells severely and often distorts their morphology. Bouin's solution yields cell counts that are usually similar to those with 10 % acid Lugol's solution, but with less shrinkage. No single fixation method is ideal for all purposes; fixative-specific and assemblage- or taxon-specific correction factors are necessary for accurate estimates of cell numbers and cell volumes/biomass.