The Effect of the Quantitative Protargol Stain and Lugol's and Bouin's Fixatives On Cell Size: A More Accurate Estimate of Ciliate Species Biomass

ABSTRACT. The quantitative protargol stain (QPS) is used to estimate ciliate biomass and species composition from mixed field samples. Length, width, breadth and volume of live Euplotes sp., Eutintinnus sp., Strobilidium spiralis, Strombidium acutum , and Gymnodinium sanguineum were compared with 0.6% acid Lugol's fixed, 5% Bouin's fixed, and QPS cells. Cells shrank due to treatments (ANOVA and Tukey's test, α= 0.05). Protistan post-fixation cell volume (as a percentage of live volume) was 55%-80% for acid Lugol's fixed, 40%-70% for Bouin's fixed, and 30%-65% for QPS. Each species shrank to a different extent; cytostructural elements apparently alter the effect of fixation. Egestion is likely not the main cause of shrinkage since the autotroph, G. sanguineum , shrank to the same extent as the heterotrophs when stained by QPS. If field studies do not consider fixation effects on cell size, biomass may be underestimated. We recommend, for studies on planktonic ciliates, either acid Lugol's and QPS be used concurrently or QPS be used alone and biovolume values divided by 0.4 to correct for shrinkage. We stress that this is a rough estimate as this value ranges from 0.3 to 0.45 for planktonic protists.