Isotopic evidence of biotrophy and unusual nitrogen nutrition in soil-dwelling Hygrophoraceae.

Several lines of evidence suggest that the agaricoid, non-ectomycorrhizal members of the family Hygrophoraceae (waxcaps) are biotrophic with unusual nitrogen nutrition. However, methods for the axenic culture and lab-based study of these organisms remain to be developed, so our current knowledge is limited to field-based investigations. Addition of nitrogen, lime or organophosphate pesticide at an experimental field site (Sourhope) suppressed fruiting of waxcap basidiocarps. Furthermore, stable isotope natural abundance in basidiocarps were unusually high in 15 N and low in 13 C, the latter consistent with mycorrhizal nutritional status. Similar patterns were found in waxcap basidiocarps from diverse habitats across four continents. Additional data from 14 C analysis of basidiocarps and 13 C pulse label experiments suggest that these fungi are not saprotrophs but rather biotrophic endophytes and possibly mycorrhizal. The consistently high but variable δ15 N values (10-20‰) of basidiocarps further indicate that N acquisition or processing differ from other fungi; we suggest that N may be derived from acquisition of N via soil fauna high in the food chain.

You are what you get from your fungi: nitrogen stable isotope patterns in Epipactis species.

Background and Aims: Partially mycoheterotrophic plants are enriched in 13 C and 15 N compared to autotrophic plants. Here, it is hypothesized that the type of mycorrhizal fungi found in orchid roots is responsible for variation in 15 N enrichment of leaf tissue in partially mycoheterotrophic orchids. Methods: The genus Epipactis was used as a case study and carbon and nitrogen isotope abundances of eight Epipactis species, fungal sporocarps of four Tuber species and autotrophic references were measured. Mycorrhizal fungi were identified using molecular methods. Stable isotope data of six additional Epipactis taxa and ectomycorrhizal and saprotrophic basidiomycetes were compiled from the literature. Key Results: The 15 N enrichment of Epipactis species varied between 3·2 ± 0·8 ‰ ( E. gigantea ; rhizoctonia-associated) and 24·6 ± 1·6 ‰ ( E. neglecta ; associated with ectomycorrhizal ascomycetes). Sporocarps of ectomycorrhizal ascomycetes (10·7 ± 2·2 ‰) were significantly more enriched in 15 N than ectomycorrhizal (5·2 ± 4·0 ‰) and saprotrophic basidiomycetes (3·3 ± 2·1 ‰). Conclusions: As hypothesized, it is suggested that the observed gradient in 15 N enrichment of Epipactis species is strongly driven by 15 N abundance of their mycorrhizal fungi; i.e. ɛ 15 N in Epipactis spp. associated with rhizoctonias < ɛ 15 N in Epipactis spp. with ectomycorrhizal basidiomycetes < ɛ 15 N in Epipactis spp. with ectomycorrhizal ascomycetes and basidiomycetes < ɛ 15 N in Epipactis spp. with ectomycorrhizal ascomycetes.

Mean reproductive traits of fungal assemblages are correlated with resource availability.

Organisms have evolved a fascinating variety of strategies and organs for successful reproduction. Fruit bodies are the reproductive organ of fungi and vary considerably in size and shape among species. Our understanding of the mechanisms underlying the differences in fruit body size among species is still limited. Fruit bodies of saprotrophic fungi are smaller than those of mutualistic ectomycorrhizal fungi. If differences in fruit body size are determined by carbon acquisition, then mean reproductive traits of saprotrophic and ectomycorrhizal fungi assemblages should vary differently along gradients of resource availability as carbon acquisition seems more unpredictable and costly for saprotrophs than for ectomycorrhizal fungi. Here, we used 48 local inventories of fungal fruit bodies (plot size: 0.02 ha each) sampled along a gradient of resource availability (growing stock) across 3 years in the Bavarian Forest National Park in Germany to investigate regional and local factors that might influence the distribution of species with different reproductive traits, particularly fruit body size. As predicted, mean fruit body size of local assemblages of saprotrophic fungi was smaller than expected from the distribution of traits of the regional species pool across central and northern Europe, whereas that of ectomycorrhizal fungi did not differ from random expectation. Furthermore and also as expected, mean fruit body size of assemblages of saprotrophic fungi was significantly smaller than for assemblages of ectomycorrhizal species. However, mean fruit body sizes of not only saprotrophic species but also ectomycorrhizal species increased with resource availability, and the mean number of fruit bodies of both assemblages decreased. Our results indicate that the differences in carbon acquisition between saprotrophs and ectomycorrhizal species lead to differences in basic reproductive strategies, with implications for the breadth of their distribution. However, the differences in resource acquisition cannot explain detailed species distribution patterns at a finer, local scale based on their reproductive traits.

Network analysis reveals ecological links between N-fixing bacteria and wood-decaying fungi.

Nitrogen availability in dead wood is highly restricted and associations with N-fixing bacteria are thought to enable wood-decaying fungi to meet their nitrogen requirements for vegetative and generative growth. We assessed the diversity of nifH (dinitrogenase reductase) genes in dead wood of the common temperate tree species Fagus sylvatica and Picea abies from differently managed forest plots in Germany using molecular tools. By incorporating these genes into a large compilation of published nifH sequences and subsequent phylogenetic analyses of deduced proteins we verified the presence of diverse pools corresponding to functional nifH, almost all of which are new to science. The distribution of nifH genes strongly correlated with tree species and decay class, but not with forest management, while higher fungal fructification was correlated with decreasing nitrogen content of the dead wood and positively correlated with nifH diversity, especially during the intermediate stage of wood decay. Network analyses based on non-random species co-occurrence patterns revealed interactions among fungi and N-fixing bacteria in the dead wood and strongly indicate the occurrence of at least commensal relationships between these taxa.