| Abstract [eng] |
Surface sediments are interesting spots to analyze the paradigm of biodiversity and ecosystem functioning due to the multiple physic and chemical gradients that shape the interactions among microbial communities, macrofauna and primary producers. Sediments receive large inputs of organic matter and are sites of intense biogeochemical processes, mediated by microbial communities and facilitated by macrofauna, ultimately resulting in nutrients uptake by benthic primary producers or their recycling to the water column. The relationship between diversity and ecosystem functioning was analyzed at different spatial scales in the benthic compartment of two shallow eutrophic lagoons, the Curonian Lagoon (Lithuania) and the Sacca di Goro (Italy). Special attention was given to the benthic nitrogen (N) cycle, due to the critical role of this element in aquatic ecosystem functioning and to the complex regulation of its various oxic and anoxic reactions, carried out by diverse microbes and strongly influenced by macrofauna and primary producers. Investigations were carried out at different spatial scales included whole lagoon (macro-scale) as well as single macrofauna individuals and holobionts’ microbiomes (micro-scale). At the two lagoons the benthic functioning was evaluated by quantifying rates of whole system respiration and production via gas exchange, nutrient cycling and exchange at the sediment-water interface. Multivariate statistical analyses were used to reveal the interactions between the dominant macrofauna species and net solute fluxes and speculate about underlying processes. Such approach allowed to reconstruct how different macrofauna functional groups shape benthic N cycling in different macro areas of the Sacca di Goro, and determine net loss, net recycling or different level of coupling between processes (e.g., ammonification and nitrification, or nitrification and denitrification). In the Curonian Lagoon the whole scale approach was used to verify whether macrofauna act as a natural buffer against redox-dependent phosphorus recycling during short-term events of oxygen shortage. Manipulative experimental approaches addressed specific processes at the microscale, in sediments colonized by different macrofauna functional groups, and along gradients of density. Such approaches included intact or reconstructed sediment incubation, metabolic measurements of single macrofauna individuals, and the use of 15N-labeled inorganic N forms to measure specific microbial transformations (denitrification, anammox, nitrate ammonification, N-fixation) in sediments or in macrofauna microbiota. Moreover, molecular tools were used to analyze microbial diversity (16S rRNA metabarcoding) and activity (marker genes and transcripts) in holobionts. Three organisms that are abundant in the Curonian lagoon were considered: the burrowing larvae of Chironomus plumosus, the filter feeder bivalve Dreissena polymorpha and the phytophagous gammarid Pontogammarus robustoides. Results suggest that in the Sacca di Goro lagoon macrofauna play an important role, in regulating N transformations. However, its importance also depends on the prevailing environmental factors (i.e., salinity, hydrodynamics and background nutrient concentrations). Whereas in the Curonian Lagoon bioturbation did not significantly affect the nutrient metabolism and the stability of reductive-oxidative reactions during anoxia events. Molecular studies revealed that Chironomid larvae burrows are hot-spots of microbial communities involved in N cycling and that these organisms, via bioirrigation, significantly enhance both the recycling of ammonium and N removal via denitrification. Mussels primarily enhance the recycling of N to the water column, both via direct excretion and by stimulating dissimilatory nitrate reduction to ammonium. The latter is likely an effect of mussel’s biodeposits. For these two organisms the quantification of functional genes showed a significantly higher potential for microbial denitrification, nitrate ammonification and N2-fixation in macrofauna as compared to the surrounding environment. As chironomid and dreissenid densities in eutrophic lagoons are large, animals-associated microbes may account for a substantial (and so far, overlooked) N import and recycling. P. robustoides was finally demonstrated to have an important role in the survival of Chara contraria in the eutrophic Curonian Lagoon. The gammarid facilitates C. contraria via active grazing on the macroalgaeassociated epiphytes combined with ammonium excretion, thus supporting the growth of the characeans. |
