Protists’ selective predation of bacterial cells is an important regulator of soil microbiomes, which might influence the success of bacterial releases in soils. For instance, the survival and activity of introduced bacteria can be affected by selective grazing on resident communities or the inoculant, but this remains poorly understood. Here, we investigated the impact of the introduction in the soil of two protozoa species, Rosculus terrestris ECOP02 and/or Cerocomonas lenta ECOP01, on the survival of the inoculants Bacillus mycoides M2E15 (BM) or B. pumilus ECOB02 (BP). We also evaluated the impact of bacterial inoculation with or without protozoan addition on the abundance and diversity of native soil bacterial and protist communities. While the addition of both protozoa decreased the survival of BM, their presence contrarily increased the BP abundance. Protists’ selective predation governs the establishment of these bacterial inoculants via modifying the soil microbiome structure and the total bacterial abundance. In the BP experiment, the presence of the introduced protozoa altered the soil community structures and decreased soil bacterial abundance at the end of the experiment, favouring the invader survival. Meanwhile, the introduced protozoa did not modify the soil community structures in the BM experiment and reduced the BM + Protozoa inoculants’ effect on total soil bacterial abundance. Our study reinforces the view that, provided added protozoa do not feed preferentially on bacterial inoculants, their predatory behaviour can be used to steer the soil microbiome to improve the success of bacterial inoculations by reducing resource competition with the resident soil microbial communities.

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Polyhydroxyalkanoates (PHA) are synthesised by bacteria as carbon storage material. The protein PhaG directs carbon from non-related carbon sources such as glycerol, metabolised through fatty acid de novo synthesis (FAS) pathway, with PHA synthesis. The gene that codifies for this protein has not yet been found in the genome of Ralstonia eutropha H16, a model organism. By bioinformatic comparison to already known PhaG proteins, a PhaG-like protein was found codified by gene H16_A0147 and presence of the gene was preliminary confirmed by PCR. This is the first study that shows the presence and characteristics of a PhaG-like protein in R. eutropha H16 and represents the first step for the identification of a connection between FAS and PHA pathways in this model bacterium. Further gene deletion and enzymatic activity studies are necessary to confirm this potential relationship, which could improve industrial PHA production and utilisation of agro-industrial residues such as glycerol.

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