Acclimation to sublethal exposures to a model nonpolar narcotic: population growth kinetics and membrane lipid alterations in Tetrahymena pyriformis

Abstract

Tetrahymena pyriformis has been shown to acclimate (i.e. phenotypic change) to the presence of sublethal levels of hydrophobic organic chemicals considered to act via nonpolar narcosis mode of toxic action. The theoretical site of action for narcosis is the cellular membrane. The objective of this work was to parallel population growth kinetics with molecular toxicology to investigate acclimation. To this end naive and pre-exposed populations of ciliates were exposed to sublethal concentrations of the model nonpolar narcotic, 1-octanol. A control, solvent control, and three sublethal concentrations of 1-octanol were tested. Naive populations exposed to 1-octanol exhibited a concentration dependent lag phase in growth where there was no growth followed by growth at rates similar to control populations. Pre-exposed populations transferred to the same or a lower concentration did not exhibit a lag phase and grew at rates equal to control populations. Pre-exposed populations transferred to a higher concentration exhibited a lag phase in growth. This lag phase was shorter for pre-exposed populations than for naı̈ve populations. The relative percent of the following fatty acid methyl esters (FAMEs) did not change with 1-octanol exposure: 12:0, 14:0, 15:0, iso15:0, and 17:0. However, an increase was observed for FAMEs 16:0 and 18:0 with exposure to 1-octanol. Conversely, with exposure there was a decrease for the following FAMEs: 16:1, 18:1, and 18:2 Δ6,11. The overall decrease in the number of π-bonds is thought to be related to a net decrease in overall fluidity. Additionally, there was an increase in the ratio of 16/18 carbon FAMEs implicating physical accommodation of the compound within the membrane. Molecular changes are directly correlated with the population growth trends.