Bacterial invasion and protozoan grazing increase transfer efficiencies of polymeric organic matter in microbial model systems

Refractory polymeric matter contributes the majority to organic carbon in freshwater, but their bacterial utilization remains unresolved. Our lack in knowledge is mainly based on the fact that degradation experiments predominantly use bacterial pure cultures and thereby neglect the multitude of organismic interactions and feedback mechanisms of complex, natural systems. By using simplified aquatic bacterial communities, we tested whether invasions of a competitor, a nanoflagellate predator, or both affect microbial utilization of refractory substrates. Whereas the relative abundance of resident bacterial strains did not change after invasion, the productivity of the systems greatly increased. Introduction of predators resulted in a 5-10 fold increase in bacterial abundances in chitin and cellulose treatments. Concomitantly, bacteria shifted from free-living to an aggregate ecotype which could promote utilization of refractory substrates by syntrophic interactions between bacterial cells. Higher microbial utilization of refractory substrates in an "aggregate meta-metabolism" may occur via local bacterial growth induced by a spatially concentrated release of readily available labile substrates by protozoan grazing. Our model shows that complexity of microbial interactions increases productivity and C transfer efficiency of aquatic ecosystems. The interaction between microbes of different ecological status in a spatially reduced environment (the co-aggregate) could thus be the key factor for chitin and cellulose degradation in waters, as confirmed by our extrapolations on a world scale. Hence, to better understand turnover of the vast pool of refractory polymeric organic matter in natural ecosystems organismic interactions and their feedback mechanisms need to be taken into account.