Effects of Fixation on Cell Volume of Marine Planktonic Protozoa

Abstract

The effects of fixation on the cell volume of marine heterotrophic nanoflagellates and planktonic ciliates were investigated. Decreases in cell volume depended on the combination of the protozoan taxa and the particular fixative. For a particular fixative and protozoan species, degree of shrinkage was independent of physiological state. The volume of fixed cells was found to be approximately 20 to 55% lower than the cell volume of live organisms. For the heterotrophic microflagellates, the fixatives ranked, in order of decreasing effect on cell volume, as glutarialdehyde, formaldehyde, acid Lugol's solution, and modified van der Veer solution. With oligotrichous ciliates and a tintinnid ciliate, formaldehyde caused less shrinkage than glutaraldehyde or acid Lugol's solution. With the aldehyde fixatives, the microflagellates were found to shrink more than the ciliates. Differential effects of fixation on cell volumes may result in an underestimation of the biomass of certain protozoan taxa in natural samples. Protozoa are major consumers of bacterioplankton (14, 31, 36) and phytoplankton (11, 19; D. A. Caron, Ph.D. thesis, Massachusetts Institute of Technology, Cambridge, 1984) in marine planktonic ecosystems. In addition to their role as grazers, heterotrophic protozoa, rather than bacteria, may also be the major nutrient remineralizers in open ocean (17, 45). The microbial food web is considered to be an important source of metabolic activity in open ocean (30, 35, 46); thus, data on the abundance and activity of heterotrophic protozoa are crucial to understanding the Plow of energy and material in the ocean. Heterotrophic microflagellates and ciliates usually dominate the protozoan biomass; however, because they overlap

Bacterivory by sestonic protists in a southeastern blackwater river

“…Protist biovolumes were calculated as prolate spheroids from length and width measurements, converted to equivalent spherical diameters, and separated into size classes. Because shrinking of the cells occurs in fixation, the volume estimate is probably 20-55% conservative (Choi and Stoecker 1989). Carbon content was estimated from biovolume assuming that protist specific gravity equals 1.0 (Sherr and Sherr 1984), dry mass is 20% of wet mass, and C is 40% of the dry mass fraction (Beers and Stewart 1979).…”

Cultivation-independent analysis reveals a shift in ciliate 18S rRNA gene diversity in a polycyclic aromatic hydrocarbon-polluted soil

“…Observation of ciliate populations in fresh samples may be hampered by the fact that e.g. many ciliates are dying within a short time when observed under a cover slip, whereas fixation of environmental samples often results in drastic changes of cell shapes (Choi & Stoecker, 1989).…”

CARBON PARTITIONING WITHINPHAEOCYSTIS ANTARCTICA(PRYMNESIOPHYCEAE) COLONIES IN THE ROSS SEA, ANTARCTICA

“…Alternatively, as filters clog, a portion of the POC may be drawn through filters, resulting in an underestimate of POC (Carlson et al 1999). Microscopic methods can be biased because of disruption or shrinkage of cells upon preservation (Choi and Stoecker 1989), and formulae to convert cell volumes to carbon units have errors associated with them (Verity et al 1992). For taxa like Phaeocystis that have extracellular mucilage, microscopic observations are the only means presently available to quantify the relative proportions of carbon contained in mucilage and cells (Rousseau et al 1990).…”