Probing the microenvironment of freshwater sediment macrofauna: Implications of deposit-feeding and bioirrigation for nitrogen cycling

The effect of sediment-dwelling macrofauna on nitrifying bacteria was investigated by direct probing in their overlapping habitat, i.e., the upper few millimeters of freshwater sediments. Microsensors (O2, NH þ 4 , NO 2 3 , and diffusivity) were used at the sediment surface and inside animal burrows to record steady-state and dynamic distributions of reactants, respectively. Short-term changes of metabolic activity (actual and potential nitrification rates) and long-term changes of abundance (fluorescence in situ hybridization) of nitrifying bacteria were determined. The presence of insect larvae (Chironomus riparius) increased the availability of O2 and NO 2 3 in the sediment pore water and inside animal burrows, suggesting promotion of nitrification and dissimilatory NO 3 reduction, particularly in the burrowing layer of C. riparius. At the sediment surface (i.e., in the feeding layer of C. riparius), however, nitrification was inhibited by low NH 4 availability and high macrofaunal grazing pressure. Consequently, both actual and potential nitrification rates decreased in the feeding layer. Inside burrows, no net nitrification was detected, despite high NH 4 availability and frequent O2 injections by larval ventilation activity. Conversely, burrows were sites of NH 4 production and NO 2 3 consumption. Nevertheless, the abundance of nitrifying bacteria increased measurably in the burrowing layer after prolonged incubation, but only in sediments in which the larvae were able to construct and ventilate stable burrows. Benthic aquatic macrofauna thoroughly alter the distribution of microbial populations in aquatic sediments. The animals excavate sediment for burrow construction and continuously redistribute particles because of locomotion and feeding (Krantzberg 1985). Thereby, particle-associated microbial populations become spatially arranged in a three-dimensional (3-D) mosaic rather than in a 1-D vertical sequence (Fenchel 1996b; Kristensen 2000). New associations arise that allow metabolic pathways to interact in a complex way. In addition, subsurface microbial populations become directly exposed to water and solutes from the water column because animals ventilate their burrows for oxygen acquisition (Riisgaard and Larsen 2005). Ventilation also enhances solute exchange between the water column and subsurface microbial populations through advective water transport through burrows (Aller and Aller 1998). Aside from these indirect effects, animals directly interact with microbes by feeding on them, which has a profound impact on microbial metabolic activity and biomass (Johnson et al. 1989, Van De Bund et al. 1994). Ingestion and egestion of bacteria often takes place at different sites in the sediment and thereby further promotes the spatial redistribution of microbes (Matisoff et al. 1999). Studies on bioturbation and bioirrigation in freshwater ecosystems are severely underrepresented relative to studies in coastal marine environments (Fenchel et al. 1998). Freshwater macrofauna have a smaller average body size and burrow depth than marine macrofauna. Consequently, immediate interactions of freshwater macrofauna and microbes are limited to the sediment surface and the relatively small burrows (Stief and de Beer 2002; Stief et al. 2004). Their habitat spatially coincides with the layer of the highest metabolic activities of bacteria in sediments (Aller and Aller 1986; Haglund et al. 2003). Inevitably, microbial populations close to the sediment surface are influenced by macrofaunal bioturbation, bioirrigation, and feeding. These animal activities affect the spatial distribution, the physical supply of reactants for microbially driven oxidation–reduction reactions, and the ultimate survival of microorganisms, respectively. The microbial responses range from short-term change of metabolic activity to long-term change of abundance and distribution. An appropriate investigation of animal–microbe interactions in freshwater sediments must therefore reconcile approaches on small spatial scales and broad temporal scales. We carried out laboratory experiments on the effects of Chironomus riparius larvae on the microbial nitrogen cycle (nitrification in particular) in freshwater sediments. C. riparius is a typical and often highly abundant inhabitant of soft sediments in lowland streams, ditches, and lake shores (Armitage et al. 1995). Because of its high tolerance to organic and inorganic pollution and O2 depletion, C. riparius is also widespread in saprobic streams and eutrophic lakes with intense nutrient turnover in the sediment. The larva constructs a U-shaped burrow that reaches approximately 10 mm into the sediment and is intermittently ventilated with oxygenated surface water (Leuchs 1986). C. riparius leaves the burrow only to feed on particulate organic matter that has settled onto the 1 To whom correspondence should be addressed. Present address: Department of Microbiology, Institute of Biological Sciences, Aarhus University, Ny Munkegade, Building 540, DK8000, Aarhus C, Denmark ([email protected]).