Preservation-induced distortions in aquatic protists: mechanistic biases, quantitative uncertainties and ecological consequences.

Aquatic protist quantification underpins our understanding of microbial food webs, biogeochemical cycling and ecosystem functioning. Yet, most estimates of abundance, biomass, size structure and community composition still rely on preserved samples, even though fixation alters the organisms it is meant to preserve. Here, I synthesize experimental, methodological and comparative studies spanning several decades to show that fixation is not neutral, but a systematic, taxon-dependent source of distortion. Across common fixatives, including Lugol's iodine, formaldehyde, protargol and glutaraldehyde, cell volume may shrink by >80% or swell by 30-55%, depending on taxon and protocol. Abundance estimates are likewise biased: aldehyde-preserved samples often yield ciliate counts 10-50% lower than Lugol-preserved samples, with even greater losses in fragile naked taxa, while some flagellate staining workflows recover only 20-30% of live abundance. These effects are further modified by storage time, handling and analytical method, generating error cascades that affect biomass estimates, trophic interpretation and carbon flux calculations. Because these artifacts vary across taxa and workflows, universal correction factors are not appropriate. Instead, fixation acts as a selective ecological filter, reshaping plankton communities before observation.