Effects of Geosmin on the Behavior of Soil Protists

Geosmin is a volatile organic compound (VOC) produced by a range of different soil microorganisms, and is most commonly recognized for its characteristic "earthy" scent evident after rainfall. Though it remains unclear why microorganisms produce geosmin, we know that exposure to geosmin can influence behaviors across a wide range of organisms, serving as both an attractant and a repellant, but geosmin effects on soil protists remain largely unstudied. We investigated how soil protists respond to geosmin exposures, focusing on representatives of three morphological groups of protists, Colpoda sp. (ciliate), Cercomonas sp. (flagellate), and Acanthamoeba castellanii (naked amoeba), testing the hypothesis that geosmin production by bacteria influences soil protist behavior. We conducted experiments to evaluate protist excystment (waking up) and predation responses to geosmin-producing (Streptomyces coelicolor M145) and non-producing (S. coelicolor J3003) bacteria, as well as synthetic geosmin. All three protists excysted at higher rates when exposed to geosmin-producing bacteria or synthetic geosmin, while no significant excystment occurred with the non-producing strains or in the absence of synthetic geosmin. Protist feeding preferences were also affected, with two of the three protists (Cercomonas sp. and A. castellanii) less likely to predate geosmin-producing versus non-producing bacterial strains. Our findings suggest that soil protists can detect geosmin as a signal indicating favorable soil conditions and geosmin production by bacteria may serve as a deterrent to predation by protists. More generally, our results highlight the ecological significance of geosmin in the soil food web and its role in mediating bacteria-protist interactions.