Quantum Dots Reveal Shifts in Organic Nitrogen Uptake by Fungi Exposed to Long-Term Nitrogen Enrichment

RESEARCH ARTICLE Quantum Dots Reveal Shifts in Organic Nitrogen Uptake by Fungi Exposed to LongTerm Nitrogen Enrichment Nicole A. Hynson1*, Steven D. Allison2, Kathleen K. Treseder2 1 Department of Botany, University of Hawai‘i Manoa, 3190 Maile Way, Honolulu, HI, 96822, United States of America, 2 Department of Ecology and Evolutionary Biology, University of California Irvine, Irvine, CA, 92697, United States of America * a11111 Abstract Published: September 14, 2015 Anthropogenic nitrogen (N) enrichment can alter N dynamics associated with decomposing plant litter. However, it is unclear to what extent these alterations occur via microbial effects (e.g., changes in gene regulation, physiology, or community composition) versus plant litter effects (e.g., changes in composition of N and C compounds). To isolate microbial effects from plant litter effects, we collected plant litter from long-term N fertilized and control plots, reciprocally inoculated it with microbes from the two treatments, and incubated it in a common field setting for three months. We used quantum dots (QDs) to track fungal uptake of glycine and chitosan. Glycine is a relatively simple organic N compound; chitosan is more complex. We found that microbial and litter origins each contributed to a shift in fungal uptake capacities under N fertilization. Specifically, N fungi preferred glycine over chitosan, but control fungi did not. In comparison, litter effects were more subtle, and manifested as a three-way interaction between litter origin, microbial origin, and type of organic N (glycine versus chitosan). In particular, control fungi tended to target chitosan only when incubated with control litter, while N fungi targeted glycine regardless of litter type. Overall, microbial effects may mediate how N dynamics respond to anthropogenic N enrichment in ecosystems. Copyright: © 2015 Hynson et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Introduction OPEN ACCESS Citation: Hynson NA, Allison SD, Treseder KK (2015) Quantum Dots Reveal Shifts in Organic Nitrogen Uptake by Fungi Exposed to Long-Term Nitrogen Enrichment. PLoS ONE 10(9): e0138158. doi:10.1371/journal.pone.0138158 Editor: Xiangzhen Li, Chengdu Institute of Biology, CHINA Received: May 20, 2015 Accepted: August 25, 2015 Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: This work was supported by the Office of Science (BER), US Department of Energy (Program in Microbial Communities and Carbon Cycling), NSF DEB-1256896, and NSF EAR-1411942. Competing Interests: The authors have declared that no competing interests exist. Numerous laboratory and field studies have reported that environmental conditions can alter the composition and function of microbial communities [1–4]. For instance, changes in nitrogen availability, soil moisture, and temperature can each elicit shifts in microbial community structure in parallel with shifts in N dynamics, decomposition rates, or soil CO2 respiration [1– 4]. Moreover, a growing number of laboratory studies have demonstrated a link between community composition and function: direct manipulation of microbial community composition often alters N and C dynamics [5–10]. This link could occur because microbes vary in their physiological capacity to take up and transform various N compounds [11–16]. Thus, it is PLOS ONE | DOI:10.1371/journal.pone.0138158 September 14, 2015 1 / 13 Organic N Uptake by Fungi under N Enrichment possible that shifts in the microbial community can be partly responsible for alterations in ecosystem functions under environmental change, if these shifts alter the relative abundance of taxa with different physiological capabilities [17]. In addition, physiological shifts can occur within taxa via acclimation or adaptation [18, 19]. A “common garden” approach can be employed to isolate these microbial effects from the immediate influence of the environment. Indeed, several laboratory studies have found that microbial communities isolated from different environments can function differently from one another even when grown in a common environment [17, 20–24]. Nevertheless, when examining microbial contributions to ecosystem dynamics, field-based experiments are particularly worthwhile, because they can integrate complex environmental conditions that are often difficult to replicate in a laboratory. Only a minority of studies have compared the functions of different microbial communities by transplanting them into a common field setting [17, 25–28]. For the most part, they have done so by encasing microbes within “cages” of nylon mesh that has pores large enough to allow the passage of solutes and gases, but small enough to restrict the movement of fungi and bacteria [27]. These field studies have documented that ecosystemlevel dynamics like N mineralization, nitrification, and decomposition vary among communities transplanted to a common setting [17, 25–28]. However, the changes in microbial physiology that drive these differences remain largely unexamined, primarily because they are often difficult to assess in the field. In this study, we focus on fungal uptake of specific organic N compounds, because N uptake is a well-defined physiological process with a number of known consequences for ecosystem function [29]. For example, uptake of organic N can lead to immobilization of N in microbial biomass, which limits its availability for other organisms [30, 31]. Alternatively, microbes can mineralize their acquired organic N compounds and secrete excess N as ammonium, thereby augmenting N availability [32, 33]. Nitrogen availability is important, because net primary productivity of plants is N-limited in many ecosystems [34, 35]. We focused on N enrichment as an element of the environment that can alter microbial function. Anthropogenic N deposition is common in Southern California, with many natural ecosystems receiving more than 25 kg N ha-1 y-1 [36]. Nitrogen enrichment can affect microbes via numerous mechanisms, including alterations in nutrient contents of the plant litter they decompose [37]. For example, in a grassland in Southern California, N additions elicit increases in litter N [17]. In a common garden experiment in this ecosystem, microbes from the N-fertilized plots were associated with different decomposition rates [17] and enzymatic efficiencies [22] compared to those from the control plots. Specifically, after six months of decomposition, N microbes were associated with faster decomposition when they were incubated in the N-fertilized plots versus the control plots [17]. In addition, extracellular cellulases and hemicellulases tended to be more efficient when microbes were placed on litter from their home tre (...truncated)