Dynamics of Root Microorganisms in Closed Hydroponic Cropping Systems

Greenhouse production of vegetables in closed hydroponic cropping systems is a resource-efficient technique for the production of high-quality produce with a high yield per unit area. While this type of cropping system allows savings in terms of water and nutrient use, the recirculation of water increases the risk of pathogen dispersal. Oomycetous pathogens in particular, such as Pythium, thrive in aquatic environments and are among the most destructive pathogens. Root pathogen outbreaks could be controlled by sustaining a stable and high level of general microbiota. It is therefore of utmost importance to understand how biotic and abiotic factors affect the indigenous root-associated microbiota. In the present thesis, the impact of pathogen inoculation, plant age, nutrient availability and use of plant protection products on microbial communities associated with root and nutrient solution was investigated. It was demonstrated that all these factors had an effect on the indigenous microorganisms. Plant age in particular was a significant driver of microbial community structure due to its significant impact on root exudation patterns. Using 454-pyrosequencing, the effect on bacterial communities of the inoculation of P. ultimum was visualized. Although the effect was strongest at the fruit-bearing plant stage, clear differences were seen also in the seedling and flowering stages. Nutrient availability was revealed to have an impact on the number of colonyforming units of bacteria, fungi and fluorescent pseudomonads. Organic carbon specifically was shown to be the main driving force of microbial growth. However, not only the quantity, but also the quality and ratios between the different organic and inorganic nutrients had an effect on microbial numbers. Through DGGE analysis, a shift in the bacterial community structure was seen after the addition of a fungicide in the nutrient solution. The microorganisms present in the nutrient solution were not able to degrade the fungicide. This thesis also demonstrates the importance of combining culture-dependent and culture-independent methods in the analysis of microbial communities.