Bacterial and fungal responses to drying and rewetting using bronopol or cycloheximide as inhibitors

Sammanfattning: Microbial decomposers breakdown soil organic matter, releasing carbon dioxide (CO2) to the atmosphere. This release of CO2 could therefore contribute to future climate change. However, microbial decomposers might also be affected by climate change, as for example the intensity and frequency of drought and rainfall events are expected to increase, resulting in more drying-rewetting (D/RW) events in soils. In this study I investigated the roles of bacteria and fungi as the main decomposers in soil and their interactions and contribution to CO2 release upon D/RW. I also tested whether pre-treating soils with litter would select for microbes quick at colonizing new carbon resources, and hence if this would make microbes quicker at responding to D/RW. To examine the interaction between fungal- and bacterial growth rates, the bactericide bronopol and fungicide cycloheximide was used to experimentally inhibit either bacterial- or fungal growth following D/RW. A dose-response relationship for bronopol and cycloheximide was first found, showing how these inhibitors affect microbial growth in soil. Bacterial growth rates were measured using 3H-Leucine (Leu) incorporation, fungal growth rates were measured using 14C-acetate-in-ergosterol incorporation . The D/RW experiment was conducted by first airdrying the soils, the soils were then divided into the numbers of samples we are interested in measuring. The samples were then rewetted accordingly with water or water containing inhibitors, the respiration (0 < 120h) and growth rates (0 < 150h) was then measured. The results showed bacterial and fungal interactions and their responses upon D/RW, identifying correlations between respiration- and growth rates during this period . CO2 have shown to respond very well to the fungal- and bacterial growth telling us that they both are major contributors to CO2 releases during the period following D/RW. There was a clear competitive release with the inhibition of the targeted microbial group (i.e. more bacterial growth when fungi were suppressed, and vice versa). There was no clear indication of a shifted response in soil with litter treatment, however more test and replicates using other carbon sources is needed to falsify our hypothesis. As a whole this thesis confirmed the results of some previous studies within this topic. That aside this thesis also found different bacterial and fungal interactions which could contribute to environmental applications or implementations to fight the global warming by regulating soil microbes. By promoting soil as a carbon sink, it would be possible to reduce the total amount of CO2 in the atmosphere. This work was conducted at the Rousk Lab, section for microbial ecology at Lund University.