4. Prof. Osnat Gillor

Water scarcity is an ever-growing, global problem especially in arid and semi-arid regions. Treated wastewater (TWW) irrigation is an effective alternative source that could alleviate the demand for potable water especially for agriculture. But TWW irrigation could contaminate irrigated soils and crops with emerging contaminants and enteric pathogens that may carry antibiotic resistance. Most studies on the effects of TWW irrigation have focused on possible contamination and the edaphic changes that shape the bacterial response. Yet, we know little about the role micropredators play in modulating the irrigated soil microbiome or the dissemination of pathogens and contaminants. In this framework, we investigated temporal and spatial succession of bacterial assemblages and pathogen survival in response to potential predator-prey interactions with native protists across soil types and agricultural practices. We hypothesised that the role of micropredators changed with irrigation water quality, soil and crop types, cultivation time and agricultural practices. To test our hypothesis we evaluated the fate of enteric pathogens in irrigation water, soils and crops samples collected in controlled and field environments. Concomitantly, we monitored the abundance, diversity, function, and interactions of the soil microbiome in the water and irrigated soils. In controlled environments, enteric pathogens introduced by irrigation quickly disappeared from the soil. This decrease has ceased once protists were inhibited by antibiotics, suggesting that the introduced bacteria were predated by phagocytic protists. This assumption was verified by analysing the protists communities in the soil samples showing an increase in the relative abundance of members of the Aveolata.  In the field samples, the bacterial communities significantly differed between TWW irrigated soil types independent of the quality of the irrigation water. Yet, the protist community was shaped by the water quality with members of the Aveolata and Cercocozoa taxa activated in the presence of the enteric bacteria contained in the TWW. These findings were further supported by network analyses, which indicated the intensity of bacteria-protist interactions upon irrigation with TWW. Our results reveal, for the first time, the non-negligible effects of TWW irrigation on the assembly soil protists communities across soil types and agricultural practices. The biological mechanisms underlying these effects can be well understood within a predator-prey relationship framework in which protists exert strong top-down control on introduced pathogens.