Microbial Origin of Aquaponic Water Suppressiveness against Pythium aphanidermatum Lettuce Root Rot Disease

Aquaponic systems are an integrated way to produce fish and plants together with mutual benefices. In this kind of system, the use of phytosanitary treatments to control plant pathogens is sensitive. The reason is the risk of toxicity for fish present in the same water loop especially in case of a couple aquaponics. Among plant pathogens, Pythium aphanidermatum is problematics due to the Oomycetes capacity to produce mobile form of dispersion in the recirculated water. However, this study has for aim to elucidate the potential antagonistic capacity of aquaponic water against P. aphanidermatum diseases. It was shown that aquaponic water owned an inhibitory effect of P. aphanidermatum mycelial growth in in vitro conditions. The same result was shown when lettuces growing in aquaponic water were inoculated by the pseudo-fungus. The disease was thus suppressed in presence of aquaponic water, contrary to lettuces grown in hydroponic water or complemented aquaponic water (aquaponic water plus mineral nutrients). Root microbiota were analysed by 16S rDNA and ITS Illumina sequencing to determine the origin of this aquaponic suppressive action. It was determined that diversity and composition of the root microbiota was significantly correlated with the suppressive effect of AP water. Several taxa were identified to be involved in this effect. Moreover, few of these microorganisms, at the genus level, were known to have an antagonistic effect against P. aphanidermatum. These innovating results indicated that AP water could be an interesting and novel source of antagonistic agents adapted to control P. aphanidermatum diseases in soilless culture.