Invertebrate grazing determines enzyme production by basidiomycete fungi

Extracellular enzymes produced by heterotrophic microorganisms in the soil are responsible for the decomposition of organic compounds. Basidiomycete fungi are the primary decomposer agents in temperate wooded ecosystems and contribute extensively to extracellular enzyme activity and nutrient mineralisation within soils. Growth and development of basidiomycete mycelia is influenced by soil-dwelling invertebrate grazers with potential implications for fungal activity and ecosystem functioning. The impacts of four invertebrate taxa belonging to Isopoda, Myriapoda, Collembola and Nematoda on the production of eight hydrolytic enzymes by four saprotrophic basidiomycetes (Phanerochaete velutina, Resinicium bicolor and two strains of Hypholoma fasciculare) were compared in a factorial microcosm study. Grazing generally increased enzyme production but invertebrates had species-specific impacts on enzyme activity. The magnitude of grazing influenced enzyme activity; macrofauna (woodlice and millipedes) induced the greatest responses. Enzymatic responses varied markedly between fungi. Grazing enhanced enzyme activity in the exploitative mycelial networks of P. velutina and H. fasciculare, while the opposite effects were observed in the explorative R. bicolor networks. The impacts of soil fauna on nutrient mineralisation depend on fungal community composition. β-glucosidase, cellobiohydrolase, N-acetylglucosaminidase, acid phosphatase and phosphodiesterase activities were affected most frequently by grazing and invertebrate activity, and thus had direct consequences for carbon, nitrogen and phosphorous cycling. The results indicate that invertebrate diversity and community composition may influence the spatial distribution and activity of extracellular enzymes with direct implications for nutrient mineralisation and trunover in woodland soils.     

Invertebrate grazing affects nitrogen partitioning in the saprotrophic fungus Phanerochaete velutina

The heterogeneity of nutrients in forest soils is governed by many biotic and abiotic factors. The significance of nutrient patchiness in determining soil processes remains poorly understood. Some saprotrophic basidiomycete fungi influence nutrient heterogeneity by forming large mycelial networks that enable translocation of nutrients between colonized patches of dead organic matter. The effect of mycophagous soil fauna on these networks and subsequent nutrient redistribution has, however, been little studied. We used a soil microcosm system to investigate the potential effects of a mycophagous collembola, Protaphorura armata, on nutrient transfer within, and nutrient loss from, the mycelium of a saprotrophic basidiomycete fungus, Phanerochaete velutina. A ¹⁵N label, added to central mycelium, was used to track nitrogen movement within the microcosms across 32 days. Although collembola grazing had little impact on δ¹⁵N values, it did alter the partitioning of ¹⁵N between different regions of mycelia. Less ¹⁵N was transferred to new mycelial growth in grazed systems than in ungrazed systems, presumably because collembola reduced fungal growth rate and altered mycelial morphology. Surprisingly, collembola grazing did not increase the mineralization of N from mycelium into the bulk soil. Overall, our results suggest that mycophagous soil fauna can alter nutrient flux and partitioning within fungal mycelium; this has the potential to affect the dynamics and spatial heterogeneity of forest floor nutrients.     

Collembolan grazing affects the growth strategy of the cord-forming fungus Hypholoma fasciculare

Mycelia of cord-forming fungi show remarkable patterns of reallocation of biomass and nutrients indicating an important role of these, often extensive, organisms in the spatial translocation of energy and nutrients in forest soils. Despite the rich tradition of interaction studies between soil microarthropods and fungi, the spatial implications of these interactions, due to the potential growth responses of the fungi and to the translocation of energy and nutrients within the mycelial network, have been largely ignored. In this paper we analyse fungal growth responses in two-dimensional model systems composed of compressed soil, the cord-forming fungus Hypholoma fasciculare and three fungivorous Collembolan species. We hypothesised that (i) the highly co-ordinated nature of cord-forming fungi would lead to growth responses following collembolan grazing, and that, (ii) such changes are dependent on grazing intensity, and (iii) changes are dependent on the species grazing. Mycelial extent and hyphal cover decreased with increasing grazing density; at highest grazing density also the fractal dimension of the mycelial border decreased, indicating a less branched foraging front due to the regression of fine hyphae and the development of mycelial cords. Effects differed greatly between collembolan species although they exerted comparable grazing pressure (the smaller species were added in larger numbers according to their allometric size-metabolic rate relationships): while grazing by Folsomia candida resulted in less mycelial extension and hyphal cover, these variables were not affected when Proisotoma minuta and Hypogastrura cf. tullbergi grazed. The effects of a species mix suggested an additive effect of the component species. This shows that fungal mycelia may suffer from damage caused by few but large collembolans, affecting extension as well as coverage of the mycelium, but that fungi may compensate for the biomass loss caused by more but slightly smaller collembolans. In about 20% of the model systems H. fasciculare switched from a growth pattern with a broad contiguous foraging front and uniform growth in all directions to a pattern with fast growing sectors while other sectors stopped growth completely. The switch occurred in grazed systems exclusively; thus we interpret this observation as a fugitive response and as a strategy for quickly escaping from places where grazing pressure is experienced.     

Olfactory cues associated with fungal grazing intensity and secondary metabolite pathway modulate Collembola foraging behaviour

Fungal secondary compounds play an important role for springtail food choice and fitness. Little is known, however, on the role of olfactory cues for Collembola foraging behaviour and whether Collembola can olfactorily perceive volatiles associated with fungal secondary metabolite pathways. We investigated the ability of three species of Collembola (Folsomia candida, Heteromurus nitidus and Supraphorura furcifera) to use olfactory cues for discriminating between fungi of different phylogenetic affiliation (Aspergillus nidulans, Cladosporium cladosporioides, Ascomycota; Laccaria bicolor, Basidiomycota) and toxicity using fungal strains of A. nidulans with reduced secondary metabolite production. Furthermore, we studied if olfactory cues from hyphae injured by grazing affect the foraging behaviour of Collembola. We hypothesized that (i) Collembola are able to olfactorily perceive and respond to fungal species/strains with different secondary metabolite pathways and that (ii) Collembola are able to sense fungal mycelia injured by grazing. Each of the Collembola species studied preferentially oriented their foraging towards fungal strains of A. nidulans with suppressed secondary metabolites, and in particular towards the mutant where the global regulator for secondary metabolites (ΔlaeA) has been silenced. Two of the three Collembola species (H. nitidus and S. furcifera) sensed olfactory cues of previously grazed fungi and moved towards ungrazed fungi, however, the response of S. furcifera was restricted to fungi extensively exposed to grazing (5 days) suggesting that the response varies between Collembola species.Overall, the results support the first and second hypothesis indicating that Collembola (1) are able to olfactorily differentiate fungi of different toxicity and move towards more palatable fungi, and (2) showed a lesser preference for fungi previously exposed to grazing.     

Production of lignocellulose-degrading enzymes and degradation of leaf litter by saprotrophic basidiomycetes isolated from a Quercus petraea forest

Due to the production of lignocellulose-degrading enzymes, saprotrophic basidiomycetes can significantly contribute to the turnover of soil organic matter. The production of lignin- and polysaccharide-degrading enzymes and changes of the chemical composition of litter were studied with three isolates from a Quercus petraea forest. These isolates were capable of fresh litter degradation and were identified as Gymnopus sp., Hypholoma fasciculare and Rhodocollybia butyracea. Within 12 weeks of incubation, H. fasciculare decomposed 23%, R. butyracea 32% and Gymnopus sp. 38% of the substrate dry mass. All fungi produced laccase and Mn-peroxidase (MnP) and none of them produced lignin peroxidase or other Mn-independent peroxidases. There was a clear distinction in the enzyme production pattern between R. butyracea or H. fasciculare compared to Gymnopus sp. The two former species caused the fastest mass loss during the initial phase of litter degradation, accompanied by the temporary production of laccase (and MnP in H. fasciculare) and also high production of hydrolytic enzymes that later decreased. In contrast, Gymnopus sp. showed a stable rate of litter mass loss over the whole incubation period with a later onset of ligninolytic enzyme production and a longer lasting production of both lignin and cellulose-degrading enzymes. The activity of endo-cleaving polysaccharide hydrolases in this fungus was relatively low but it produced the most cellobiose hydrolase. All fungi decreased the C/N ratio of the litter from 24 to 15-19 and Gymnopus sp. also caused a substantial decrease in the lignin content. Analytical pyrolysis mass spectrometry of litter decomposed by this fungus showed changes in the litter composition similar to those caused by white-rot fungi during wood decay. These changes were less pronounced in the case of H. fasciculare and R. butyracea. All fungi also changed the mean masses of humic acid and fulvic acid fractions isolated from degraded litter. The humic acid fraction after degradation by all three fungi contained more lignin and less carbohydrates. Compared to the decomposition by saprotrophic basidiomycetes, litter degradation in situ on the site of fungal isolation resulted in the relative enrichment of lignin and differences in lignin composition revealed by analytical pyrolysis. It can most probably be explained by the participation of non-basidiomycetous fungi and bacteria during natural litter decomposition.     

Population performance of collembolans feeding on soil fungi from different ecological niches

The potential reproductive value of arbuscular mycorrhizal fungi (Glomus intraradices and Glomus invermaium), root pathogenic fungi (Rhizoctonia solani and Fusarium culmorum) and saprotrophic fungi (Penicillium hordei and Trichoderma harzianum) were examined for the collembolans Folsomia candida Willem and Folsomia fimetaria L. Dried baker's yeast (Saccharomyces cerevisiae) was used as a reference standard food in laboratory cultures. Collembolan performance was determined as final size, fecundity and population growth rate after when fed the fungal food sources for 31 days. The mycorrhizal fungi gave the least growth and fecundity compared with the other fungi, but G. intraradices gave good fecundity for F. candida. In terms of growth, Baker's yeast was a high-quality food for both adults and juveniles of both species, but it was a poorer food in terms of fecundity of F. candida. Preference of the fungi in all possible pairwise combinations showed that although F. fimetaria did not perform well on Glomus spp. and F. candida did not grow well on Glomus spp. their preference for these fungi did not reflect this. The highest fecundity was seen with the root pathogen F. culmorum. Different quality indicators such as the C:N ratio of the fungal food sources as well as other biological parameters are discussed in relation to their reproductive value and Collembola preferential feeding.     

Defensive strategies of soil fungi to prevent grazing by Folsomia candida (Collembola)

Fungi represent a major part of the living biomass in the upper soil horizon and serve as an important food source for many soil organisms. We hypothesized that certain mycelial characteristics may serve to protect fungi from grazing. Specifically, this study focused on the influence of poisonous or other repellent metabolites and crystalline structures at the hyphal surface on the feeding preference of the soil microarthropod Folsomia candida Willem. The formation of crystalline structures was studied microscopically and the content of certain metabolites such as amanitin and muscarin was investigated using analytical methods. The feeding preference of F. candida was studied in different in-vitro food choice experiments. Additionally, the palatability of the fungal isolates was estimated by the amount of egg clusters laid by F. candida and by analysing the carbon and nitrogen content of the mycelia. F. candida was repelled by fungal species with toxic metabolites or crystals on their hyphal surface, which indicates that these traits serve as feeding protection. F. candida preferred dark-pigmented fungi. Total number of egg clusters and feeding preference were not correlated. However, insects that fed on fungi without repellent characteristics laid the most eggs. The amount of carbon and nitrogen in the mycelium had no influence on feeding behaviour. We conclude that the content of repellent metabolites and crystalline structures at the hyphal surface are defensive strategies of soil fungi and strongly influence feeding preference of F. candida. Other traits such as palatability were less important. Our results help to explain collembolan feeding behaviour and interactions between soil fungi and Collembola.     

Size matters: What have we learnt from microcosm studies of decomposer fungus–invertebrate interactions?

The ongoing research ‘boom’ in soil ecology has been advanced by a widespread use of laboratory experiments to investigate mechanisms that could not be unravelled with field observations alone. Interactions between soil fungi and invertebrates have received considerable attention due to their trophic and functional importance in belowground systems. Saprotrophic cord-forming basidiomycete fungi are major agents of primary decomposition in woodland ecosystems, where they are also an important source of nutrition for fungal-feeding soil invertebrates. A plethora of microcosm experiments, with their main benefit being that they enable most variables to be kept constant while just a few are manipulated, have provided detailed insights into the ecology of fungus–invertebrate interactions. This review identifies important trends from this body of work (including a meta-analysis of grazing effects on fungal growth and wood decomposition) and explores the extent to which these patterns are supported by the few related experiments conducted in more complex mesocosm and field systems. Grazing in microcosms reduced fungal growth and increased decomposition, but with interaction-specific magnitude, reflecting invertebrate feeding preferences for different fungi. Macro-invertebrates (woodlice and millipedes) had stronger effects than micro- (e.g. nematodes) and meso- (e.g. collembola) invertebrates. This greater grazing pressure generally increased enzyme activities beneath mycelia during interactions in which wood decay was increased. Top-down effects of fungal-feeding can be extrapolated to more complex systems, but only for macro-invertebrates, particularly woodlice. Soil enzyme activity was stimulated, in microcosms and more complex systems, by short-term or low intensity grazing, but reduced when large areas of mycelium were removed by high-intensity grazing. Effects of differential fungal palatability on invertebrate populations are evident in microcosm studies of collembola. These bottom-up effects can be extrapolated more broadly than top-down effects; fungal community dominance determined collembola abundance and diversity, in mesocosms, and woodlouse abundance in the field. Using, as a case study, a series of experiments conducted at a range of scales, mechanisms underlying potential climate change effects on grazing interactions and decomposition are also explored. Biotic effects on decomposer community functioning are heterogeneous, depending on fungal dominance and the density of key macro-invertebrate taxa.     

Preference tests with collembolas on isogenic and Bt-maize

Collembolas are important members of belowground food webs. There is little information available on the effects of the plant residues of transgenic maize expressing Bacillus thuringiensis (Bt) toxin on soil animals, including collembola. This is why two questions were addressed in laboratory feeding experiments with three collembolan species: (i) Are collembola equally distributed on residues of isogenic and Bt-maize? and (ii) Do collembola show feeding preference to either of the maize types? Bt-maize (producing Cry1Ab toxin) proved to be a less preferred food source for Folsomia candida than the isogenic one. No similar phenomenon was found in the case of Heteromurus nitidus and Sinella coeca. F. candida reacted to as low as 3.45 (± 0.8 mg g⁻¹) Bt-toxin content of the maize. Our results show that the effect of the Bt-toxin producing maize on the collembolan is species specific.     

Colonization of nonmycorrhizal plants by mycorrhizal neighbours as influenced by the collembolan, Folsomia candida

 Soil microarthropods have been shown to stimulate or be detrimental to arbuscular mycorrhizal function by their grazing actions, but their role as dispersal agents has not been assessed. The ability of three species of arbuscular mycorrhizal (AM) fungi (Glomus etunicatum, Acaulospora denticulata, Scutellospora calospora) infecting Plantago lanceolata roots to further colonize neighbouring plants was measured in response to the distance between root systems and the presence of the collembolan, Folsomia candida. In the absence of collembola, all three fungal species infected neighbouring plants in two weeks or less (at short distances), but were not successful when neighbouring plants were placed 45 cm away or further. Colonization by G. etunicatum was the quickest at short distances, but S. calospora showed greatest ability to colonize at increasing distance,whereas A. denticulata was intermediate. In the presence of the collembolan, G. etunicatum took longer to colonize neighbouring plants, but was able to infect at least 30 cm further, illustrating the arthropod's ability to disperse the AM inoculum. A. denticulata increased its range by 10 cm in the presence of F. candida, but unlike G. etunicatum, there was no delay in the colonization. In contrast, colonization of neighbouring plants by S. calospora was negatively affected both in terms of overall distance and time. These data support the hypothesis that soil arthropods can act as dispersal agents for AM inoculum, but the extent of this is fungal species-specific. Received: 1 July 1998     

Diversity of arbuscular mycorrhizal fungi associated with desert ephemerals in plant communities of Junggar Basin,northwest China

Colonization by and diversity of arbuscular mycorrhizal (AM) fungi associated with five common ephemerals, Eremopyrum orientale (L.) Jaub. et Spach., Gagea sacculifera Regel., Plantago minuta Pall., Tragopogon kasahstanicus S. Nikit., and Trigonella arcuata C. A. Mey. were investigated in four typical desert plant communities in Junggar Basin, northwest China. All five ephemerals examined were found to be colonized and formed typical arbuscules or vesicles. The proportion of root length colonized ranged from 2 to 85% with an average of 19%. Spore density in soil near the roots of different ephemerals varied from 1 to 120 spores per 20 ml soil, with an average value of 33 spores. Species richness averaged 8.8 AM fungal species in soil near the roots and ranged from 2 to 21. Fifty-four AM fungal taxa belonging to the genera Acaulospora, Archaeospora, Entrophospora, Glomus and Paraglomus were isolated and identified from soil around the roots. Glomus was the dominant AM fungal genus with a frequency of 100% and relative abundance of 82.6%. The AM fungal species with the highest frequency of occurrence was Glomus aggregatum with a frequency of 75%. G. microaggregatum was present in the highest relative abundance (16%). G. sacculifera, P. minuta and T. arcuata formed Arum-type mycorrhizas. Arbuscular mycorrhizal fungi are ubiquitous and Arum-type mycorrhizas are especially prevalent in these important desert communities used for grazing and traditional medicine.     

Do oribatid mites (Acari: Oribatida) show a higher preference for ubiquitous vs. specialized saprotrophic fungi from pine litter?

Previous studies of oribatid mite feeding preferences for different saprotrophic fungi were limited to ubiquitous fungal species, whereas saprophytes specialized to decompose particular substrates have been neglected. We examined the preference of seven oribatid mite species (Adoristes ovatus, Eniochthonius minutissimus, Eueremaeus silvestris, Nothrus silvestris, Oppiella subpectinata, Porobelba spinosa and Spatiodamaeus verticillipes) for nine autochthonous saprotrophic fungi from Scots pine litter (Pinus sylvestris). Among the fungal species offered were specific coniferous litter colonizers (Allantophomopsis lycopodina, Ceuthospora pinastri, Hormonema dematioides, Scleroconidioma sphagnicola, Verticicladium trifidum, Marasmius androsaceus and Sympodiella acicola) and two ubiquitous species (Cladosporium herbarum and Oidiodendron griseum). The fungi were inoculated on fragments of pine needles and offered simultaneously and separately to the mites. Our main hypothesis, that oribatid mites (usually occurring in more than one type of ecosystems) would prefer the ubiquitous fungal species rather than those specific to pine litter, was supported only partly. The ubiquitous C. herbarum was highly preferred by all studied mites, but most of them preferred one or more of the specialized fungi with similar intensity. The basidiomycete M. androsaceus along with sterile needles were consistently avoided by all mites in all experiments. Our results do not support the hypothesis, that the “true” fungivorous oribatid mites in traditional sense are more selective fungal feeders than are the “unspecialized” panphytophagous ones. We observed no gradation in preference of fungi for oribatid mites as a group, but rather a discontinuous and dynamic mosaic with particular mites preferring particular fungal species. This heterogeneous mosaic shapes the feeding niches occupied by particular oribatid mite species and probably reduces competition for food source among numerous species coexisting in a given habitat and time.     

Inter- and intraspecific feeding selectivity of Folsomia candida (Willem) (Collembola,Isotomidae) on fungi

Summary Selective grazing of fungi by soil microarthropods may affect decomposition rates of litter materials and the structure of microarthropod and fungal communities. We developed laboratory methods to assay feeding selectivity and investigated the preferences of the collembolan Folsomia candida on three fungi: Acremonium sp., Paecilomyces varioti, and Penicillium citrinum. F. candida showed stronger preference for Acremonium sp. than for P. varioti and P. citrinum. Oviposition site selection followed the same pattern. Actively metabolizing hyphae of Acremonium sp. and P. varioti were preferred over senescent hyphae, while spores of P. citrinum were preferred over active hyphae. If microarthropod preference for active hyphae is extensive, microarthropod regulation of decomposition could be more important than their biomass indicates. Furthermore, as the P. citrinum results indicate, mechanisms of microbial dissemination may include selective grazing.Contribution in part to International Symposium on Faunal Influences on Soil Structure, Edmonton, Alberta, Canada (11–13 June 1984), organized by W. B. McGill; and the annual meeting of the Ecological Society of America, Minneapolis, Minnesota (17–21 June 1985)     

The effect of diet on isotopic turnover in Collembola examined using the stable carbon isotopic compositions of lipids

Combined compound-specific stable carbon isotopic methods and fatty acid abundance determinations have been used to examine feeding preferences and C allocation in organisms where direct observation of feeding is difficult. In order to examine the effect of differing diets on the δ¹³C values of fatty acids and sterols of Collembola, the diets of two collembolan species, Folsomia candida and Proisotoma minuta, were switched from a yeast diet to one of four isotopically distinct diets, and the δ¹³C values of the lipids monitored over the next 39 d. Cholesterol remained the only sterol detected in both collembolan species, despite the diets containing widely differing sterol compositions. The δ¹³C values of collembolan lipids recorded after long term feeding were often different to those of the same components in the diet, indicating that fractionation or partitioning occurs during digestion, assimilation and biosynthesis within the Collembola, thereby shifting consumer lipid δ¹³C values away from those of the corresponding dietary components. The rates of change of δ¹³C values differed among compounds, with half-lives ranging between 29 min and 14 d. Some of these differences appear to be related to the abundance of dietary components, such that fatty acids present in high abundance in the diet (e.g. 18:2(n−6)) were rapidly assimilated in high proportions into collembolan lipids, leading to a rapid change in δ¹³C values. Similarly, isotopic turnover in the 16:1(n−7) fatty acid, present in the newly presented diets in only low abundances, was significantly correlated to the rate of removal of this component from the consumer fatty acid pool. The rates of change of δ¹³C values in P. minuta lipids did not vary significantly with diet, whilst the rates of change of δ¹³C values of lipids in F. candida were affected by the diets the Collembola consumed. Results of an experiment providing F. candida and P. minuta with two diets of different quality demonstrated that F. candida responded to the high quality diet with increased growth and fecundity, whilst P. minuta responded with increased fecundity only. Thus, the abilities of the two species to respond to diets of varying quality, amongst other factors, is concluded to lead to differences in the rates of change of δ¹³C values reflecting differences in lipid turnover.     

Contrasting effects of nitrogen limitation and amino acid imbalance on carbon and nitrogen turnover in three species of Collembola

Soil animal detritivores play an important role in facilitating decomposition processes but little information is available on how the quality of dietary resources affects their stoichiometry of carbon (C) nitrogen (N) and phosphorus (P), and turnover of C and N. This study investigated how a fungal diet, Fusarium culmorum, with a low N content and imbalanced amino acid (AA) composition affected the physiology of three soil-dwelling collembolans (Folsomia candida, Protaphorura fimata and Proisotoma minuta) in comparison to a control diet, Saccharomyces cerevisiae, with a high N content and balanced AA composition. We compared the elemental composition of animals, their growth rates and tissue replacement of C and N. We also measured the individual AA δ¹³C to investigate the extent that Collembola may rely on endogenous sources to compensate for scarcity of essential AAs. The results showed that animal's N content tracked closely the composition of their diets, decreasing from around 10 to 7% N from the high to low N diet. They also had a significant increase of C and a decrease of P. P. fimata was less affected than F. candida and P. minuta. The total incorporation of C and N in the animals due to growth and tissue replacement decreased from 11-17 to 6-12% DM d⁻¹ on the high and low N diet respectively with P. fimata experiencing the smallest change. Essential AAs δ¹³C did not always match perfectly between Collembola species and their diets; particularly on the low N diet. Isotope patterns of AAs indicate that bacteria may have been the alternative source of essential AAs. While the results of this study cannot be extrapolated directly to the dynamics of Collembola populations in the field, they serve to demonstrate their flexibility in adapting physiologically to the temporal and spatial patchiness of the soil environment.     

Potential for monoterpenes to affect ectomycorrhizal and saprotrophic fungal activity in coniferous forests is revealed by novel experimental system

The fungal community in coniferous forest soils plays a pivotal role in ecosystem processes such as decomposition and carbon and nutrient cycling. Both saprotrophic (SP) and ectomycorrhizal (ECM) fungi occur throughout the upper soil horizons in coniferous forests and could therefore be exposed to high concentrations of monoterpenes occurring in the needle litter and roots of some tree species. Previous work has noted the differential effects of monoterpenes on the mycelial growth of a range of both SP and ECM fungi when grown in artificial nutrient media. This study used a novel experimental system to assess the effect of environmentally relevant concentrations of monoterpenes on the activity of ECM and SP fungi grown on more natural substrata. Exposure of the ECM fungus Paxillus involutus (Batsch) Fr. to vapours of either α-pinene or β-pinene resulted in a significantly greater proportion of root tips being colonised by the fungus when it was grown with seedlings of Picea abies (L.) Karst. Exposure to monoterpenes resulted in a significant decrease in respiration rate of two species of litter degrading SP fungi, Mycena galopus var. candida J. E. Lange and Collybia butyracea (Bull.) P. Kumm. There was no difference in response between the two SP species, despite the fact that previous tests in liquid nutrient media, with monoterpenes at higher concentrations, indicated that one species was sensitive and one was not. The high volatility and low solubility of monoterpenes in water make them challenging to work with. The experimental system developed here, although still artificial, provides a bridge between pure culture studies in defined media and all the complexities of forest soils in the field, by allowing the exposure of fungi to environmentally relevant monoterpene concentrations in more natural substrata.     

Differential response of ectomycorrhizal and saprotrophic fungal mycelium from coniferous forest soils to selected monoterpenes

The mycelia of saprotrophic (SP) and ectomycorrhizal (ECM) fungi occur throughout the upper soil horizons in coniferous forests and could therefore be exposed to high concentrations of monoterpenes occurring in the needle litter of some tree species.Monoterpenes are mycotoxic and could potentially affect fungi that are exposed to them in the litter layers. In order to investigate whether monoterpenes typical of coniferous litters could influence fungal communities, we analysed the monoterpene content of freshly fallen needles of Pinus sylvestris, Picea abies and Picea sitchensis. The most abundant monoterpenes were found to be α-pinene, β-pinene and 3-carene. We evaluated the effects of these three monoterpene vapours on the biomass production of 23 SP isolates and 16 ECM isolates. Overall, 75% of ECM isolates and 26% of SP isolates were significantly inhibited by at least one of the monoterpene treatments and both intra- and inter-specific variations in response were observed.Monoterpene concentrations are highest in surface litters. The differential effects on fungal taxa may influence the spatial and temporal distribution of fungal community composition, indirectly affecting decomposition and nutrient cycling, the fundamental ecosystem processes in which fungi have a key role in coniferous forest soils.     

Effects of temperature and life stage on the fatty acid composition of Collembola

Fatty acid (FA) analysis is used as a promising tool to investigate trophic interactions in soil food webs. The FA profile of neutral lipids in consumers is affected by the diet, and the occurrence and amount of certain FAs can reflect feeding strategies. We investigated the lipid composition of the Collembola Folsomia candida, Heteromurus nitidus and Protaphorura fimata with the fungus Chaetomium globosum as food source. The impact of environmental temperature and life stage was assessed, with special respect to linoleic acid (18:2ω6,9) as a marker FA for fungal feeding. In all Collembola species the ratio of C16/C18 in neutral lipid fatty acids (NLFAs) increased with decreasing temperature. In the NLFAs of F. candida and H. nitidus the Unsaturation Index and the amount of 18:2ω6,9 decreased with temperature, whereas in P. fimata effects were the opposite. The composition of phospholipid fatty acids (PLFAs) differed between species, but was little affected by temperature. The degree of unsaturation in NLFAs increased with the age of Collembola, mainly due to higher amounts of 18:2ω6,9 and a lower proportion of 18:1ω9. The biomarker linoleic acid represented over 20% of FAs in all fungal feeding Collembola. Despite considerable influence of temperature and life stage on its proportion, the amount was always higher than in individuals reared on other diets. This suggests that linoleic acid can serve as marker for fungal feeding independent of such physiological variations in Collembola.     

A possible role for saprotrophic microfungi in the N nutrition of ectomycorrhizal Pinus resinosa

We determined whether Pinus resinosa, selected ectomycorrhizal and saprotrophic microfungi have access to various organic nitrogen sources commonly found in the forest. Vector analysis demonstrated nitrogen limitation of the P. resinosa in the plantation from which most of the fungi were isolated, establishing this study's relevance. Nonmycorrhizal P. resinosa seedlings did not absorb significant N from amino acids. The ectomycorrhizal fungi, including Pisolithus tinctorius, Suillus intermedius and Tylopilus felleus, obtained substantial N from amino acids, a limited amount of N from glucosamine, and essentially no N from protein-tannin complex. In contrast, Penicillium and Trichoderma readily acquired N from protein-tannin and glucosamine. Thus, there was an increasing ability to obtain N from complex organic N sources from plant to ectomycorrhizal fungi to saprotrophic fungi. Furthermore, N mineralization from an organic N source by Penicillium depended on the C:N ratio. We conclude that acquisition of relatively simple organic N sources by P. resinosa is likely to be largely indirect via ectomycorrhizal fungi, and that more complex organic N sources may become accessible to ectomycorrhizal fungi (and thus possibly their host plants) following mineralization by saprotrophic fungi such as Penicillium or Trichoderma when C:N ratios are sufficiently low.     

Collembolan trophic preferences determined using fatty acid distributions and compound-specific stable carbon isotope values

The trophic preferences of soil invertebrates such as Collembola are often determined by the analysis of gut contents, or through visual observations of the location of individuals. As an alternative approach, two species of Collembola, Folsomia candida and Proisotoma minuta, were offered a choice of the soil fungus Cladosporium cladosporioides or the bacterial feeding nematode Panagrellus redivivus; each exhibited distinct fatty acid profiles and stable carbon isotopic compositions. Over 21 days, the fatty acids i15:0, i17:0, 18:1(n-7) and 18:2(n-6) all increased in abundance in both collembolan species consistent with direct routing from the nematode dietary choice which contained a high concentration of these components. Collembolan fatty acid δ¹³C values increased by between 5.7 and 21.6‰ over 21 days reflecting those of the nematode diet. Therefore, both fatty acid profiles and δ¹³C values were consistent with a strong feeding preference of F. candida and P. minuta for the nematodes over the offered fungi. In fact, neither collembolan species consumed any detectable amount of C. cladosporioides. Comparison of the δ¹³C values of the 16:0 and 18:0 fatty acids (which are biosynthesised by the Collembola as well as directly incorporated from the diet) and the 16:1(n-7) and 18:2(n-6) components (which are not biosynthesised by the Collembola) demonstrated that the input of distinct pools of C can lead to large shifts in δ¹³C values between diet and consumer. The fatty acids that were not biosynthesised by Collembola better reflected the δ¹³C values of the diet helping to differentiate between biosynthesised and directly incorporated compounds; an important prerequisite in the interpretation of compound-specific δ¹³C values in trophic behaviour tests. The combination of fatty acid distributions and δ¹³C values is a significant improvement on traditional methods of examining feeding preferences, since it determines directly the assimilated dietary carbon rather than relying on indirect observations, such as the proximity of individuals to a defined food source.