To What Extent Do Food Preferences Explain the Trophic Position of Heterotrophic and Mixotrophic Microbial Consumers in a Sphagnum Peatland?

Abstract

Although microorganisms are the primary drivers of biogeochemical cycles, the structure and functioning of microbial food webs are poorly studied. This is the case in Sphagnum peatlands, where microbial communities play a key role in the global carbon cycle. Here, we explored the structure of the microbial food web from a Sphagnum peatland by analyzing (1) the density and biomass of different microbial functional groups, (2) the natural stable isotope (δ 13 C and δ 15 N) signatures of key microbial consumers (testate amoebae), and (3) the digestive vacuole contents of Hyalosphenia papilio, the dominant testate amoeba species in our system. Our results showed that the feeding type of testate amoeba species (bacterivory, algivory, or both) translates into their trophic position as assessed by isotopic signatures. Our study further demonstrates, for H. papilio, the energetic benefits of mixotrophy when the density of its preferential prey is low. Overall, our results show that testate amoebae occupy different trophic levels within the microbial food web, depending on their feeding behavior, the density of their food resources, and their metabolism (i.e., mixotrophy vs. heterotrophy). Combined analyses of predation, community structure, and stable isotopes now allow the structure of microbial food webs to be more completely described, which should lead to improved models of microbial community function.

Effects of rotifers, copepods and chironomid larvae on microbial communities in peatlands

“…This effect is particularly important in the spring and summer, when chironomid larvae attain their greatest abundance. Future research should also take into account the effect of meiofauna on the microorganism structure of the food web in peatland ecosystems, as literature data show that in some types of peatland this group can attain substantial numbers (Jassey et al 2013).…”

“…The guts of chironomid larvae were mainly dominated by amoebae H. elegans and Nebela sp. As shown in a study by Jassey et al (2013), these species feed mainly on bacteria, ciliates, and other amoebae. Gilbert et al (2003) showed that Nebela tincta-type amoebae feed on a wide range of living, senescent, or dead microorganisms (fungi, microalgae, and other testate amoebae) and on organic remains.…”

“…A substantial decrease in the abundance of testate amoebae in the control sample at the end of the experiment may have affected the other components of the microbial communities. According to the literature data (Mieczan et al 2012;Jassey et al 2013;Marcisz et al 2014), testate amoebae are considered 'top predators' within the microbial food web in peatland ecosystems. The guts of chironomid larvae were mainly dominated by amoebae H. elegans and Nebela sp.…”

“…The species can feed on ciliates, rotifers, and even small nematodes, i.e. organisms from higher trophic levels (Jassey et al 2013). Therefore, they could contribute substantially to controlling the abundance of potential consumers of bacteria and ciliates.…”

“…Microorganisms were extracted from samples using the following extraction method (Jassey et al 2013). To extract the different groups of micro-and macroorganisms from the moss, each sample was first shaken for 1 min with a vortex and then filtered through a 250-m mesh filter.…”