Impact of nitrogen availability and nitrogen structural composition on fungal enzymatic activity and growth : how nutrient availability governs response and development of three saprotrophic Basidiomycetes

Sammanfattning: Fungi excrete a wide variety of extracellular enzymes to scavenge for nutrients, such as the often scarce yet essential nutrient nitrogen. All fungi produce highly specialized hydrolytic enzymes, e.g. peptidases, that depolymerize organic molecules. However, some organic matter such as lignin and tannins largely lack the nitrogen or oxygen bridges required for hydrolysis, and these recalcitrant structures thus require a different approach for decomposition in order to reach the nitrogen within. Certain fungal species have the capacity to produce highly reactive and unspecific peroxidases which free nutrients tied to these plant secondary metabolites. Peroxidases can break strong C-C bonds through electron transfer and efficiently catabolize unhydrolyzable compounds. Saprotrophic basidiomycetes are considered our main decomposers of recalcitrant organic matter in soils due to their ability to use manganese peroxidases and are key contributors to the cycling of carbon and nitrogen. These nutrient cycles are highly interlinked, which is why some forests are fertilized with nitrogen to boost plant biomass production and thus carbon sequestration, in attempts to decrease the greenhouse gas effect. As plants rely less on symbiosis with microorganisms while non-symbiotic microorganisms may benefit from increased nitrogen availability, fertilization is likely to have implications on fungal biodiversity. I investigated if and how three species of saprotrophic basidiomycetes adapt their peptidase and manganese peroxidase activity to varying sources of nitrogen and how it affects their biomass production. Laboratory experiments were conducted where the fungi were provided high and low concentrations of readily available mineral nitrogen, easily accessible organic nitrogen, recalcitrant nitrogen as tannin-protein complexes, or given no nitrogen at all. Manganese peroxidases and peptidases were sampled weekly for three weeks and analyzed colorimetrically and through fluorescence spectroscopy, respectively. Biomass was measured at the end of the experiment. Data was analyzed with Repeated Measures ANOVA and TukeyHSD. Recalcitrant nitrogen was expected to trigger high levels of manganese peroxidase activity, organic nitrogen to trigger increased peptidase, while mineral nitrogen was expected to cause no significant activity of either enzyme. All fungi were predicted to gain largest biomass when provided mineral nitrogen, and smallest biomass when provided no nitrogen at all. The three species differed in their responses, none of which fully met expectations. Generally, nutrient source did not affect enzymatic activity, but it was most often affected by what concentration of respective source it was given. Trends of enzyme activities over time was often similar for a fungus between concentrations of the same nitrogen source, but what these trends looked like varied between species. Growth seems in some cases to be correlating with levels of manganese peroxidase activity, where higher manganese peroxidase activity perhaps came at a cost of lower biomass production. Biomass responses varied between species where some benefitted from mineral nitrogen while others yielded greater biomass given recalcitrant organic matter. Such varying responses points to how challenging it is to forecast changes in community compositions and ecosystem function following anthropogenic interference.