AUTHOR=Zou Songbao , Zhang Qianqian , Zhang Xiaoli , Dupuy Christine , Gong Jun 

TITLE=Environmental Factors and Pollution Stresses Select Bacterial Populations in Association With Protists

JOURNAL=Frontiers in Marine Science

VOLUME=7

YEAR=2020

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2020.00659

DOI=10.3389/fmars.2020.00659

ISSN=2296-7745

ABSTRACT=<p>Digestion-resistant bacteria (DRB) refer to the ecological bacterial group that can be ingested, but not digested by protistan grazers, thus forming a specific type of bacteria-protist association. To test the hypothesis that the environment affects the assembly of DRB in protists, a mixotrophic ciliate, <italic>Paramecium bursaria</italic>, and a heterotrophic ciliate, <italic>Euplotes vannus</italic>, were reared at different temperatures, light conditions, and concentration gradients of antibiotic oxytetracycline and heavy metals. Community profiling indicated that the composition of DRB in both species varied significantly across the manipulated conditions, except for in <italic>P. bursaria</italic> under light/dark treatments. Clone library analysis of bacterial 16S rRNA genes showed that DRB were diverse. <italic>Pseudomonas</italic> became more abundant during the warmer treatment of <italic>P. bursaria</italic>, whereas the dominance of <italic>Pseudoalteromonas</italic> weakened and <italic>Vibrio</italic> became more abundant in <italic>E. vannus</italic> at a higher temperature. During the treatment of diel light:dark cycles, <italic>Aestuariibacter</italic> and <italic>Alteromonas</italic> were selected for in <italic>E. vannus</italic> but not <italic>Pseudoalteromonas</italic>, which was highly represented in the all-light and all-dark treatments. In contrast, <italic>P. bursaria</italic> consistently hosted <italic>Nevskia</italic>, <italic>Curvibacter</italic>, and <italic>Asticcacaulis</italic> under all light conditions. There were many bacterial species co-resistant to oxytetracycline and to protistan digestion, in which <italic>Sphingomonas, Alteromonas</italic>, <italic>Aestuariibacter</italic>, <italic>Puniceicoccaceae</italic> (Verrucomicrobia), <italic>Pseudomonas</italic>, and <italic>Sulfitobacter</italic> were frequently abundant. <italic>Flectobacillus</italic> and <italic>Aestuariibacter</italic> were major lead-resistant bacteria associated with the studied protists. <italic>Acinetobacter</italic> and <italic>Hydrogenophaga</italic> were abundant in the <italic>P. bursaria</italic> treated with a high dose of mercury. <italic>Aestuariibacter</italic> was found as a dominant group of DRB in <italic>E. vannus</italic> across all cadmium treatments. In summary, this study demonstrates for the first time that environmental stress selects for bacterial populations associated with protists and that there are diverse bacterial species that not only are resistant to pollution stresses but can also survive protistan predation. This work highlights that bacteria-protists associations need to be taken into account in understanding ecological and environmental issues, such as resilience of bacterial community and function, microbial co-occurrence, and quantity and distribution of antibiotic resistant bacteria and genes.</p>