Microbes Are Trophic Analogs of Animals
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Microbes are trophic analogs of animals Shawn A. Steffana,b,1, Yoshito Chikaraishic, Cameron R. Curried, Heidi Hornd, Hannah R. Gaines-Daya, Jonathan N. Paulie, Juan E. Zalapab, and Naohiko Ohkouchic aDepartment of Entomology, University of Wisconsin, Madison, WI 53706; bUS Department of Agriculture-Agricultural Research Service, University of Wisconsin, Madison, WI 53706; cDepartment of Biogeochemistry, Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan; dDepartment of Bacteriology, University of Wisconsin, Madison, WI 53706; and eDepartment of Forest & Wildlife Ecology, University of Wisconsin, Madison, WI 53706 Edited by James M. Tiedje, Michigan State University, East Lansing, MI, and approved October 15, 2015 (received for review May 5, 2015) In most ecosystems, microbes are the dominant consumers, com- These organisms commandeer most of the heterotrophic biomass mandeering much of the heterotrophic biomass circulating through circulating through the food web (13). Indeed, in terrestrial systems, food webs. Characterizing functional diversity within the micro- the vast majority of primary production is not captured by herbi- biome, therefore, is critical to understanding ecosystem functioning, vores; rather, it falls to the ground and is consumed by microbes and particularly in an era of global biodiversity loss. Using isotopic fin- small invertebrate detritivores (7, 12, 14). Higher-order carnivores gerprinting, we investigated the trophic positions of a broad di- consume the detritivores, conjoining the upward flow of detritivore versity of heterotrophic organisms. Specifically, we examined the and herbivore biomass (9, 11, 15, 16), but if the trophic positions in naturally occurring stable isotopes of nitrogen (15N:14N) within amino the basal layers of the food web cannot be accurately measured, the acids extracted from proteobacteria, actinomycetes, ascomycetes, entire food web rests on a poorly known, tenuous platform (9). and basidiomycetes, as well as from vertebrate and invertebrate Here, we investigate whether the trophic positions of microor- macrofauna (crustaceans, fish, insects, and mammals). Here, we re- ganisms can be measured empirically, testing whether microbes are port that patterns of intertrophic 15N-discrimination were remark- trophically equivalent to the macrobiotic consumers in a food-chain. – ably similar among bacteria, fungi, and animals, which permitted Using amino acid stable isotope fingerprinting (17 20), we measure unambiguous measurement of consumer trophic position, indepen- with high accuracy the trophic positions (TPs) of cultured and free- dent of phylogeny or ecosystem type. The observed similarities roaming organisms. In past work, this method has produced accu- among bacterial, fungal, and animal consumers suggest that within rate TP estimates of zooplankton (18, 21), fish (17), gastropods a trophic hierarchy, microbiota are equivalent to, and can be inter- (17), amphibians (22), and insects (19, 20), ranging from simple digitated with, macrobiota. To further test the universality of this herbivores to higher-order carnivores. Although the utility of iso- finding, we examined Neotropical fungus gardens, communities in topic fingerprinting has been demonstrated in both aquatic (17, 21, which bacteria, fungi, and animals are entwined in an ancient, quad- 23, 24) and terrestrial ecosystems (19, 20), there remains an un- bridged knowledge gap between the macro- and microbiome. ripartite symbiosis. We reveal that this symbiosis is a discrete four- Recent studies have documented evidence of feeding guilds level food chain, wherein bacteria function as the apex carnivores, among the animals of brown food webs (11, 15, 25), and this animals and fungi are meso-consumers, and the sole herbivores are finding sets up questions as to the trophic positions of the resident fungi. Together, our findings demonstrate that bacteria, fungi, and microbiota. For example, when microbes feed on a resource, do animals can be integrated within a food chain, effectively uniting the they register trophically just as animals do? More specifically, what macro- and microbiome in food web ecology and facilitating greater is the trophic position of a fungus consuming a leaf, or an ar- inclusion of the microbiome in studies of functional diversity. thropod consuming that fungus? We address these questions, testing the hypothesis that fungi and bacteria are trophically compound specific | food chain | leaf-cutter ant | microbe | stable isotope equivalent to animals. We do so by measuring the degree of he trophic functions of species sculpt the unique character- Significance Tistics of the communities in which they are embedded (1, 2). It is not surprising, then, that sustained biodiversity losses (3, 4) are “ ” We report evidence that microbes are trophically equivalent to fueling the trophic downgrading of Earth (5) and altering the animals. When bacteria or fungi are fed the same diets as an- functioning of all major ecosystem types (6). The most abundant, — — imals, the microbes register the same trophic position as ani- ubiquitous organisms in most ecosystems the microbiota are mals. This discovery reframes how microbes can be viewed also likely being impacted, and these organisms are often the least within food chains and facilitates the inclusion of the micro- understood, particularly in terms of functional diversity (7, 8). If we biome in functional diversity studies. To demonstrate the are to better understand the impacts of biodiversity loss on eco- broad applicability of our approach, we investigated the an- ECOLOGY system functioning, we will need to have a means to more com- cient symbioses represented by leaf-cutter ant fungus gardens, prehensively measure the trophic niches of both the macro- and revealing four discrete trophic levels within this community microbiome. Further, such organisms must be examined while in- and providing evidence that fungi, not ants, are the dominant tegrated within their respective communities (1). Assessing the herbivores of the Neotropics. Altogether, we show that mi- trophic niches of microbial and animal species has too often been a crobes can be integrated with plants and animals in a food theoretical endeavor (9), with macrofauna generally associated “ ” chain, thereby unifying the macro- and microbiome in studies with plant-based food webs ( green food webs )andthemicro- of trophic ecology. fauna relegated to the sphere of decomposition (“brown food ” – webs )(911). It has proven difficult to merge these two spheres Author contributions: S.A.S., Y.C., C.R.C., and J.N.P. designed research; S.A.S., Y.C., H.H., and using a shared trophic metric, not only because of the difficulties in J.N.P. performed research; Y.C. and N.O. contributed new reagents/analytic tools; S.A.S., identifying microbial diversity (8), but also because of methodo- H.R.G.-D., and J.E.Z. analyzed data; S.A.S., Y.C., C.R.C., H.R.G.-D., and J.E.Z. wrote the paper; logical obstacles in measuring the trophic position of a microbe. S.A.S. provided project funding; and C.R.C., J.N.P., J.E.Z., and N.O. provided project support. Consequently, trophic ecology has tended to focus on green food The authors declare no conflict of interest. webs, often leaving the trophic hierarchies of brown webs This article is a PNAS Direct Submission. less resolved. Freely available online through the PNAS open access option. Uniting green and brown food webs is critically important to 1To whom correspondence should be addressed. Email: [email protected]. studies of biodiversity and ecosystem functioning because in most This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. ecosystems, microbes are the dominant heterotrophs (3, 7, 12). 1073/pnas.1508782112/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1508782112 PNAS | December 8, 2015 | vol. 112 | no. 49 | 15119–15124 Downloaded by guest on October 4, 2021 intertrophic 15N-discrimination within amino acids extracted from consumer taxa, focusing on two key compounds: glutamic acid and phenylalanine (17, 19, 20, 26). For microbial organisms to be trophically equivalent to animals, their respective patterns of intertrophic 15N-discrimination in glutamic acid and phenyl- alanine must mirror those of animals. In multiple controlled- feeding studies involving a diversity of vertebrate and invertebrate consumers, net 15N-discrimination between glutamic acid (glu) and phenylalanine (phe), commonly referred to as the trophic discrimination factor (TDFglu-phe) (27), has tended to be centered near +7.64 ± 0.60‰ (17, 20, 21). Although centered at 7.64, variability in the TDFglu-phe can generate “noise” around the trophic position (TPglu-phe) estimate for any given consumer (Eq. 1). Variability in the TDFglu-phe among consumer groups, therefore, must be assessed using known, homogeneous diets. Ultimately, the degree to which the TDFglu-phe is consistent among microbial and animal consumers will dictate whether macro- and microbiota can be considered trophically equivalent. To assess the universality of our approach, we examine the trophic identities of the organisms in leaf-cutter ant fungus gar- dens. Fungus gardens represent discrete community modules in which animals (ants), fungi, and bacteria have coevolved within an ancient symbiosis (28, 29) (Fig. 1). Leaf-cutter ants (Acromyrmex or Atta) are prodigious harvesters of leaf material and have been Fig. 1. Denizens of a leaf-cutter ant fungus garden.