Download Abstract Mac Rudnick - NIOO-KNAW

ABSTRACT
Soil microorganisms evolved several strategies to compete for limited nutrients in soil. Bacteria of the genus Collimonas
developed a way to exploit fungi as a source of organic nutrients. This strategy has been termed “mycophagy”. In this
thesis, research is presented with a focus on two aspects of bacterial mycophagy: 1) Investigation of strategies and traits
that are important for Collimonas bacteria to enable a mycophagous lifestyle, 2) Investigation of occurrence of mycophagy
among other soil bacteria.
Focusing on Collimonas bacteria, we find that several traits related to the mycophagous interaction with the fungal hosts,
such as production of fungal inhibitors, are phylogenetically conserved. This implies that differentiation in lifestyles of
Collimonas strains, is corresponding with phylogenetic distance. Furthermore, we show that collimonads are very motile
in a soil-like matrix, especially when being confronted with low nutrient concentrations. This high motility can be used in
order to effectively move towards oxalic acid (a metabolite exuded by a range of fungi for different purposes) in a
concentration depended manner. Our results suggest that directed motility is an important trait, characterizing the
mycophagous lifestyle of collimonads.
In order to screen for other mycophagous bacteria besides collimonads, two baiting approaches (long- and short-term)
were developed. With both approaches, we find fungal hyphae to be commonly colonized by specific communities of
rhizosphere mycophagous bacteria. Furthermore, mycophagous colonizers show clear feeding preferences for fungal
hosts. Interestingly, a surprisingly high amount of mycophagous bacteria belong to genera well known to harbor plant
pathogenic strains. Considering the importance of mycophagous bacteria in the rhizosphere, we finally propose the
“Sapro-Rhizosphere” concept. This concept states that a substantial amount of plant derived carbon that is channeled
through rhizosphere fungi (primary consumers) might be finally consumed by mycophagous bacteria (secondary
consumers).
Taken together, by using molecular biological as well as microbiological methods, this thesis further extends our
knowledge on the ecology of mycophagous Collimonas bacteria and highlights the importance of mycophagous bacteria in
the rhizosphere.