Fungal importance extends beyond litter decomposition in experimental early-successional streams Aline Frossard, 1,2,4 * Linda Gerull, 3 Michael Mutz 3 and Mark O. Gessner 1,2,4,5 1 Department of Aquatic Ecology, Eawag: Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, 8600 Dübendorf, Switzerland. 2 Institute of Integrative Biology (IBZ), ETH Zurich, Universitätstrasse 16, 8092 Zurich, Switzerland. 3 Department of Freshwater Conservation, Brandenburg University of Technology, Seestrasse 45, 15526 Bad Saarow, Germany. 4 Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Alte Fischerhütte 2, 16775 Stechlin, Germany. 5 Department of Ecology, Berlin Institute of Technology (TU Berlin), Ernst-Reuter-Platz 1, 10587 Berlin, Germany. Summary Fungi are important decomposers of leaf litter in streams and may have knock-on effects on other microbes and carbon cycling. To elucidate such potential effects, we designed an experiment in outdoor experimental channels simulating sand- bottom streams in an early-successional state. We hypothesized that the presence of fungi would enhance overall microbial activity, accompanied by shifts in the microbial communities associated not only with leaf litter but also with sediments. Fifteen experimental channels received sterile sandy sedi- ment, minimal amounts of leaf litter, and one of four inocula containing either (i) fungi and bacteria, or (ii) bacteria only, or (iii) no microorganisms, or (iv) killed microorganisms. Subsequently, we let water from an early-successional catchment circulate through the channels for 5 weeks. Whole-stream metabolism and microbial respiration associated with leaf litter were higher in the channels inoculated with fungi, reflect- ing higher fungal activity on leaves. Bacterial commu- nities on leaves were also significantly affected. Similarly, increases in net primary production, sedi- ment microbial respiration and chlorophyll a content on the sediment surface were greatest in the channels receiving a fungal inoculum. These results point to a major role of fungal communities in stream ecosys- tems beyond the well-established direct involvement in leaf litter decomposition. Introduction Leaf litter decomposition is a fundamental and well- recognized function of fungi in stream ecosystems. A phy- logenetically heterogeneous group known as aquatic hyphomycetes generally dominates the communities in this habitat (Gessner et al., 2007; Krauss et al., 2011). When submerged, aquatic hyphomycetes produce large numbers of characteristic spores that disperse in flowing water to colonize leaf litter falling into streams from ripar- ian vegetation (Bärlocher, 2009). Multiple lines of evi- dence indicate that aquatic hyphomycetes play a much more prominent role in leaf decomposition than bacteria (Gessner et al., 2007; Krauss et al., 2011). This includes the observation that fungal biomass associated with decomposing leaves greatly exceeds bacterial biomass, typically accounting for more than 90% of the total het- erotrophic microbial biomass (Baldy et al., 2002; Gulis and Suberkropp, 2003a; Duarte et al., 2010). Although dominant on coarse organic substrates such as leaf litter, fungi are not universally important in stream habitats. In particular, their significance relative to that of bacteria varies with the type of organic matter colonized. Bacteria increase in relative importance as leaf decompo- sition proceeds (Suberkropp and Klug, 1976; Baldy et al., 1995) and the particle size of organic matter decreases (Findlay et al., 2002), which has been attributed to an increasing detrital surface-to-volume area that favours bacterial colonization and uptake of dissolved organic matter (Findlay et al., 2002; Docherty et al., 2006). In addition, bacteria effectively colonize surfaces in streams, including sandy sediments (Stevenson, 1996; Artigas et al., 2008), where they form extensive biofilms together with algae, whereas fungi tend to be of minor importance in this habitat (Harrop et al., 2009). Therefore, bacteria are thought to assume primary importance in driving the Received 5 February, 2012; revised 16 July, 2012; accepted 29 July, 2012. *For correspondence. E-mail aline.frossard@eawag.ch; Tel. (+49) 33082 6990; Fax (+49) 33082 69917. Environmental Microbiology (2012) 14(11), 2971–2983 doi:10.1111/j.1462-2920.2012.02865.x © 2012 Society for Applied Microbiology and Blackwell Publishing Ltd