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Quantitative fatty acid signature analysis (QFASA) as a biochemical tool to study the diet composition of predators is frequently used in marine ecology to infer trophic links in vertebrate consumers. However, the potential and challenges of this method in other ecosystems have only recently been studied. The application in soil ecosystems leads to hurdles not encountered in the marine, such as the low similarity of fatty acid signatures between resource and consumer. So far, diet estimation attempts have been semisuccessful, necessitating to adapt QFASA for use in soil food webs. Dietary fat content may play an important role, as it influences consumer metabolism, and thus calibration coefficients for fatty acid trophic transfer. A series of feeding trials with baker's yeast spiked with five different pure fatty acids at various concentrations was conducted with Collembola, and the changes in calibration coefficients were observed. From there, equations were gained through regression analysis and new sets of calibration coefficients were calculated. QFASA was applied on a range of basal resources and the results compared with previously defined calibration coefficients. Calibration coefficients changed with the proportion of fatty acids in the diet and differed between the three Collembolan species. The re‐estimation of diets showed an improvement of model performance by the new calibration coefficients and indicated several modes of fatty acid assimilation. These greatly influence the outcome of diet estimation, for example, algal and bacterial diets are likely underestimated due to high metabolic turnover rates. The application of QFASA in soil ecosystems remains challenging. The variation in calibration coefficients and the resulting decrease in estimation deviation indicate the merit of calculating calibration coefficients from consumer signatures through linear or exponential equations. Ideally, the method should, when extended to the entire fatty acid signature, allow correct determination of consumer diets in soil food webs. [ABSTRACT FROM AUTHOR]

Quantitative fatty acid signature analysis (QFASA) is widely used to investigate trophic interactions in marine ecosystems, as nutritionally important ω3 long-chain polyunsaturated fatty acids at the food web base allow tracing of their trophic transfer in the food chain. By contrast, the basal resources in soil food webs comprise a wider array of trophic markers, including branched-chain, cyclopropane as well as several mono- and polyunsaturated fatty acids. These diverse markers allow distinguishing between the three dominant soil carbon and energy channels, the root, bacterial and fungal pathway. QFASA has not been applied yet to soil ecosystems owing to the lack of a priori data to fit the model. The present work investigates the transfer of absolute and relative trophic marker fatty acids into Collembola as dominant representatives of the soil mesofauna. Three different specieswere fed on a variety of single diets characteristic for the green and brown food chain. Calibration coefficients were calculated and diet estimation trials for mixed diet set-upswere performed, using a library comprising 50 different resources. However, estimation of Collembola diet was only partially successful, identifying the main components, but not the correct relative proportions. Adjustments by fat content or diet group exclusion did not improve the results. Nonetheless, this work provides, to our knowledge, a first comprehensive dataset to translate the application of QFASA from marine to soil ecosystems. [ABSTRACT FROM AUTHOR]

Collembola grazing on arbuscular mycorrhizal fungi (AMF) may affect the transfer of mycorrhizosphere C to the soil food web and the regulation of soil mycorrhizal infectivity potential. We used a plant-free mesocosm experimental system with two different mycorrhizal substrates containing either Glomus deserticola or Gigaspora gigantea and three Collembola species (Heteromurus nitidus, Folsomia candida and Protaphorura fimata) to test the hypothesis that Collembola grazing can interfere directly with AMF feeding on the mycorrhizal propagules and/or indirectly changing structure of associated microbial populations. The fatty acid (FA) composition of Collembola grazers and of their mycorrhizal food substrates was assessed. The feeding strategies of H. nitidus and P. fimata varied distinctly between mycorrhizal substrates, whereas the diet of F. candida was invariant to the presence of mycorrhizal propagules. The FA marker for G. deserticola (16:1ω5) was not detected in Collembola lipids, indicating that this AMF was not consumed. In contrast, H. nitidus and P. fimata comprised the marker for feeding G. gigantea (20:1 ω9). Both mycorrhizal substrates were no profitable diet for H. nitidus, resulting in a decline of individuals. Collembola grazed on the bacterial and fungal populations in the mycorrhizal substrate, predominantly H. nitidus that even decreased microbial biomass, resulting in significant changes in the composition of the microbial community. In sum, grazing by microarthropods can directly and indirectly modulate AMF performance as well as interactions with microorganisms, which in turn can impose on the soil infection potential from AMF propagules. [ABSTRACT FROM AUTHOR]

Abstract: We used fatty acid (FA) analysis to investigate green algae and cyanobacteria as food sources for Collembola. We studied the effects of food quality on body mass and on neutral lipid (NLFA) and phospholipid (PLFA) fatty acid patterns of Collembola. Folsomia candida, Heteromurus nitidus and Protaphorura fimata were fed with common green algae (Chlorella vulgaris), filamentous soil algae (Klebsormidium flaccidum), cyanobacteria (Nostoc commune) and baker's yeast (Saccharomyces cerevisiae). Body mass of F. candida and H. nitidus was highest when reared on C. vulgaris and S. cerevisiae. P. fimata gained the most weight when fed baker's yeast. K. flaccidum and N. commune as resources resulted to low biomass in all Collembola. The four diets caused significant differences in the NLFA and PLFA composition of Collembola after six weeks of feeding. Two new trophic biomarker FAs indicating algal diets were assigned with 16:3ω3,6,9 and 16:2ω6,9, which were only present in NLFAs of Collembola consuming C. vulgaris and K. flaccidum. The amount of FAs from the ω7 family was high in Collembola lipids with cyanobacteria and yeast as food sources, whereas only trace amounts occurred in the NLFA fraction with algae as the resource. In summary, common soil algae and cyanobacteria differed in food quality for Collembola, depending on their growth form (unicellular versus filamentous) and/or secondary metabolites (e.g. cyanobacteria). The new FA biomarkers detected will allow further investigation of these trophic interactions under field conditions; for example, assessing the role of collembolan grazers in the formation of biological soil crusts. [Copyright &y& Elsevier]

Abstract: Collembola are abundant and ubiquitous soil decomposers, being particularly active in the rhizosphere of plants where they are assumed to be attracted by high microbial activity and biomass. While feeding on root associated microorganisms or organic matter they may also ingest plant roots, e.g. particularly root hairs and fine roots. Employing stable isotope analysis we investigated Collembola (Protaphorura fimata Gisin) feeding preferences and types of ingested resources. We offered Collembola two resources with distinct isotope signatures: a C4 plant (Zea mays L.) planted in soil mixed with 15N labelled litter of Lolium perenne L. (C3 plant). We hypothesised that Collembola obtain their nutrients (C and N) from different resources, with their carbon being mainly derived from resources that are closely associated to the plant root, e.g. root exudates, causing enrichment in 13C in Collembola tissue, while the incorporated nitrogen originating from litter resources. In contrast to our hypothesis, stable isotope analysis suggests that in absence of plant roots Collembola derived both the incorporated C and N predominantly from litter whereas in presence of plant roots they switched diet and obtained both C and N almost exclusively from plant roots. The results indicate that Collembola in the rhizosphere of plants, being assumed to be mainly decomposers, in fact predominately live on plant resources, presumably fine roots or root hairs, i.e. are herbivorous rather than detritivorous or fungivorous. These findings have major implications on the view how plants respond to decomposers in the rhizosphere. [Copyright &y& Elsevier]

Abstract: The lipid composition of Collembola and their potential food resources was assessed in three deciduous forest stands, in order to gain insight into food web linkages under field conditions. Fatty acids (FAs) previously assigned as trophic markers in laboratory experiments were used to investigate feeding strategies in situ. As potential food sources soil microbiota and plant debris were characterised by their phospholipid fatty acid (PLFA) composition. Both the amount and the pattern of PLFAs differed between sites and soil depth, in particular the bacterial and plant marker FAs in the upper soil layer. Thus, the availability of resources for micro-detritivores varied due to forest and soil layer. The lipid composition of vital and senescent beech leaves was predominantly influenced by metabolic status and represented a quite homogenous FA resource across forest stands. Comparing Folsomia quadrioculata, Lepidocyrtus lignorum, Neanura muscorum and Pogonognathellus longicornis between the different forests revealed FA profiles to be predominantly affected by site, suggesting a diet shift depending on resources at hand. However, species-specific differences in individual FAs occurred, likely related to feeding strategy and physiological activity. Lipids of Collembola comprised low amounts of bacterial marker FAs, and bacterial consumption may occur to some extent, particularly on Gram-positives. The marker FA for predatory feeding, , was found in several species, although in low amounts. This contradicts known feeding habits and caution is advisable in using as trophic marker. Overall, as indicated by high proportions of oleic or linoleic acid, most species were either plant litter or fungal feeders, with some transitions. The ratio ,9 is proposed as a tool to distinguish between these two major feeding strategies in Collembola. [Copyright &y& Elsevier]

Abstract: Stable isotope analysis has been used as a powerful tool in food web studies in terrestrial ecosystems. In addition the occurrence and abundance of fatty acids may serve as indicator for feeding strategies of soil animals. Here we combine both approaches and investigate the fatty acid composition, δ 13C values of bulk tissues and individual fatty acids in soil organisms. The fungi Chaetomium globosum and Cladosporium cladosporioides were isotopically labelled by fructose derived from either C3 or C4 plants, and the fungal-feeding nematode Aphelenchoides sp. was reared on C. globosum. Fungi and nematodes were used as diet for the Collembolan Protaphorura fimata. The sugar source was fractionated differently by fungal lipid metabolism in a species-specific manner that points to a sensitivity of physiological processing to the non-random distribution of 13C/12C isotopes in the molecule. As a general trend stearic acid (18:0) was depleted in 13C compared to the precursor palmitic acid (16:0), whereas its desaturation to oleic acid (18:1 ω9) favoured the 13C-rich substrate. Fatty acid profiles of P. fimata varied due to food source, indicating incorporation of dietary fatty acids into Collembolan tissue. Individuals feeding on fungi had lower amounts in C20 fatty acids, with monoenoic C20 forms not present. This pattern likely separates primary consumers (fungivores) from predators (nematode feeders). The isotopic discrimination in 13C for bulk Collembola ranged between −2.6 and 1.4‰ and was dependent on fungal species and C3/C4 system, suggesting differences at metabolic branch points and/or isotope discrimination of enzymes. Comparison of δ 13C values in individual fatty acids between consumer and diet generally showed depletion (i.e. de novo synthesis) or no changes (i.e. dietary routing), but the fractionation was not uniform and affected by the type of ingested food. Fatty acid carbon isotopes were more variable than those of bulk tissues, likely due to both the distrimination by enzymes and the different lipid origin (i.e. neutral or polar fraction). [Copyright &y& Elsevier]

The effect of food resources, varying in C/N ratio, lipid and ω3 polyunsaturated fatty acid (ω3-PUFA) content, on the fitness of three Collembola species (the euedaphic Protaphorura fimata , hemiedpahic Folsomia candida and epedaphic Lepidocyrtus violaceus) was investigated. Dietary routing of fatty acids served as an indicator for resource consumption and assimilation. Laboratory feeding experiments were performed with single and mixed diets made from algae, cyanobacteria, fungi, soil bacteria or plant roots and leaves. Collembola exhibited species-specific responses in biomass and survival rate to the diets differing in quality, reflecting adaptations to their soil environment. Generally, wide dietary C/N ratios had a negative impact, with roots especially unfavorable, while the dependence on ω3-PUFAs varied. Mixing diets did mostly result in intermediate results where high-quality resources compensate for low-quality resources, with only mixes of resources favoured by Collembola in the wild showing synergistic positive effects. The results suggest that most resources of soil detritivores, irrespective of green or brown food chain, represent a non-optimal diet, with clear adaptations depending on habitat and the potential to truly exploit any resource mostly exhibited in euedaphic species. • Trophic shift of marker fatty acid can assess assimilation into consumer tissue. • Dietary ω3-fatty acid content and C/N ratio influence Collembola biomass/survival. • Effects of low-quality diet aren't compensated by mixing with high-quality diets. [ABSTRACT FROM AUTHOR]

Abstract: Arbuscular mycorrhiza (AM) mycelia networks are important for nutrient allocation in many plants, but fungivorous soil invertebrates such as Collembola can modulate the symbiosis by grazing on the extra-radical mycelium (ERM). This study employs a dual biomarker approach with stable isotopes and fatty acids to disentangle trophic interactions of Collembola in a plant-fungal soil system with maize (Zea mays) and the AM fungus Glomus mosseae. To separate ERM and root mediated effects, root (RC) and hyphal compartments (HC) were used, and the latter was spiked with labeled 15N substrate. The euedaphic Collembola species Protaphorura fimata was introduced as the fungal and root grazer. Generally, the presence of Collembola in RC fostered biomass and phosphorous uptake in roots colonized with AM. Nitrogen transport from HC to RC was not altered, indicating that Collembola did not disrupt the ERM network via grazing. Collembola–fungus interactions fostered AM hyphal proliferation in HC, whereas in RC it induced a change from fungal senescence with build-up of storage reserves, to an active foraging phase. A distinct diet switch by Collembola between HC and RC indicated different ERM palatability meditated by the presence or absence of the host plant. Overall, Collembola grazing increased ERM nutrient sequestration, particularly phosphorus, and in turn plant performance. Collembola modified fungal phenology, favoring fungal colonization over reproductive phases. These trophic interactions were strongly determined by fungal life stage, with the establishment of a functional mycorrhiza as a crucial factor. [Copyright &y& Elsevier]

Summary: Sex ratios of 31 species of Collembola of a temperate oak-beech forest were investigated in 2 months intervals during 1 year. For the estimation of population dynamics, the abundance and dominance of taxa in the litter (O L,F,H), the 0–3 and 3–6cm soil layer (A h) were assessed. A total of six species, including the abundant Mesaphorura machrochaeta, Parisotoma notabilis, Neanura muscorum and Isotomiella minor formed pure female populations suggesting that they reproduced by parthenogenesis. In three species, Lepidocyrtus cyaneus, Orchesella flavescens and Tomocerus longicornis, the mode of reproduction remained uncertain. The remaining 22 species including the widespread Folsomia quadrioculata, Protaphorura fimata and Lepidocyrtus lignorum formed bisexual populations, suggesting that sexual reproduction predominates in Collembola of the studied forest. Sex ratios of sexual species ranged between 33% and 90%. In five species the sex ratio was balanced (ca. 50%), in eight species males made up about one third of the population and in seven species males were rare constituting less than 25% of adults. Females generally comprised 77% of adult Collembola. Densities were highest in late summer and autumn. Except for M. machrochaeta, the density of Collembola deeper in soil (3–6cm) was low. Consistent with earlier studies parthenogenetic species predominated deeper in the soil (euedaphic species), but some parthenogenetic species were hemiedaphic. The sex ratio of bisexual Collembola species in the litter layer generally was more female biased than that in 0–3cm of the mineral soil. Presumably, females concentrate at sites with high density of resources whereas males are relatively more abundant at sites more favourable for spermatophore placement. Neither climatic factors (i.e. season) nor population parameters (i.e. density) correlated with the sex ratio of Collembola. [Copyright &y& Elsevier]

Abstract: Compound specific stable isotope analysis (13C/12C ratio of fatty acids) was used to assess the allocation of plant carbon in soil microbiota, and to identify the trophic links to microbial grazers in an arable field with long-term mineral and organic fertilizer amendments. The feeding strategy of two dominant Collembola species, epedaphic Isotoma viridis and euedaphic Willemia anophthalma was determined. The investigation was conducted following a shift to amaranth, a C4 plant, after 27years of continuous C3 crop rotation. The influence of new C4 plant carbon was observed in microbial phospholipids (PLFAs) with higher δ13C recorded in C4 amaranth than in C3 clover soils. The strongest enrichment occurred in the fungal PLFA 18:2ω6,9c and bacterial PLFA 18:1ω9t with 11.2‰ and 6.6‰, respectively. However, other bacterial PLFAs showed no isotopic change, suggesting that the microbial community simultaneously utilized “new” and “old” plant carbon. The δ13C of Collembola fatty acids displayed species specific lipid pattern, which was affected by crop type, but not fertilizer amendments. Isotopic separation of Collembola lipids from amaranth and clover plots was more distinct in I. viridis than W. anophthalma. With up to 18‰, the enrichment in Collembola lipids was stronger than in microbial PLFAs, pointing to a distinct incorporation of carbon resources originating from the actual plant residues. The δ13C pattern in I. viridis indicated trophic links with bacteria, saprotrophic fungi and plant tissues, while saprotrophic fungi and plant tissues were accountable for the patterns observed in W. anophthalma. [Copyright &y& Elsevier]

Abstract: We investigated the Collembola community at an arable field where mineral and organic fertilizers have been applied at low and high rates for 27 years. As food resources for Collembola, the soil microbial community was analyzed using phospholipid fatty acids (PLFAs). A special focus was put on AM fungi, which were estimated by the marker 16:1ω5 in PLFA (viable hyphae) and neutral lipid fatty acid (NLFA – storage fat in spores) fractions. Additionally, whole cellular lipids in crop plant tissues and manure were assessed. Greater Collembola species richness occurred in plots where mineral fertilizer was added. In contrast, soil microbial biomass including AM fungal hyphae increased with addition of organic fertilizer, while the amount of AM fungal spores and biomass of saprotrophic fungi were not affected by fertilizer type. The lipid pattern in wheat roots was altered by fertilizer type, application rate and their interaction, indicating different rhizosphere communities. In sum, the availability and composition of food resources for Collembola changed considerably due to farm management practice. The major diet of three dominant Collembola species, Isotoma viridis, Willemia anophthalma and Polyacanthella schäffer was determined by lipid profiling. Multivariate analysis demonstrated species specific lipid patterns, suggesting greater importance of species than management practice on the diet choice. Nevertheless, feeding strategy was affected by fertilizer type and availability of resources, as trophic biomarker fatty acids indicated feeding on wheat roots (and to some extent saprotrophic fungi) with mineral and a shift to soil organic matter (litter, detritus) with organic fertilization. Although AM fungi dominated the soil fungal community, the AMF marker 16:1ω5 was not detected in Collembola lipids, indicating that these were not consumed. The very low amount of saprotrophic fungi in the soil and the fact that Collembola as major fungal grazers did not feed on AM fungi indicates that the fungal energy channel in the investigated arable field is of little importance to the faunal food web. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: Fatty acid (FA) analysis is a promising tool to study trophic relationships in soil food webs. We determined FA biomarkers to trace bacterial food sources (Bacillus megaterium, Pseudomonas putida, Enterobacter aerogenes) of Collembola (Heteromurus nitidus, Protaphorura fimata, Folsomia candida). In addition, δ15N, δ13C, C/N ratio, body weight and NLFA/PLFA ratio (neutral lipid/phospholipid fatty acids) of Collembola were assessed. These measures indicated that P. putida ranked first, B. megaterium second and E. aerogenes third in food quality. FAs specific for bacteria were found in the NLFAs of the Collembola reflecting the respective bacterial diet. Biomarker FAs for gram-positive bacteria were methyl branched i14:0, i15:0, a15:0 and i17:0. Consumption of gram-negative bacteria was reflected by the cyclic form cy17:0 (E. aerogenes, P. putida) and by 16:1ω5 (P. putida). [Copyright &y& Elsevier]

Abstract: In soil a high number of species co-exist without extensive niche differentiation, which was assigned as ‘the enigma of soil animal species diversity’. In particular, the detritivores are regarded as food generalists. We have investigated nitrogen stable isotope ratios (15N/14N) of a major decomposer group, the Collembola, to evaluate trophic relationship and determine feeding guilds. Additionally, the δ 15N values of potential food sources such as mosses, lichens and other plant derived material (bark, nuts, leaves) were analysed. The natural variation in nitrogen isotopes was assessed in 20 Collembola taxa from three deciduous forest stands. The δ 15N signature formed a continuum from phycophages/herbivores to primary and secondary decomposers, reflecting a gradual shift from more detrital to more microbial diets. The δ 15N gradient spanned over 9 δ units, which implies a wide range in food sources used. Assuming a shift in 15N of about 3 ‰ per trophic level, the results indicate a range of three trophic levels. These variations in 15N/14N ratios suggest that trophic niches of Collembola species differ and this likely contributes to Collembola species diversity. [Copyright &y& Elsevier]