In detritus-based ecosystems, autochthonous primary production contributes very little to the detritus pool. Yet primary producers may still influence the functioning of these ecosystems through complex interactions with decomposers and detritivores. Recent studies have suggested that, in aquatic systems, small amounts of labile carbon (C) (e.g., producer exudates), could increase the mineralization of more recalcitrant organic-matter pools (e.g., leaf litter). This process, called priming effect, should be exacerbated under low-nutrient conditions and may alter the nature of interactions among microbial groups, from competition under low-nutrient conditions to indirect mutualism under high-nutrient conditions. Theoretical models further predict that primary producers may be competitively excluded when allochthonous C sources enter an ecosystem. In this study, the effects of a benthic diatom on aquatic hyphomycetes, bacteria, and leaf litter decomposition were investigated under two nutrient levels in a factorial microcosm experiment simulating detritus-based, headwater stream ecosystems. Contrary to theoretical expectations, diatoms and decomposers were able to coexist under both nutrient conditions. Under low-nutrient conditions, diatoms increased leaf litter decomposition rate by 20% compared to treatments where they were absent. No effect was observed under high-nutrient conditions. The increase in leaf litter mineralization rate induced a positive feedback on diatom densities. We attribute these results to the priming effect of labile C exudates from primary producers. The presence of diatoms in combination with fungal decomposers also promoted decomposer diversity and, under low-nutrient conditions, led to a significant decrease in leaf litter C:P ratio that could improve secondary production. Results from our microcosm experiment suggest new mechanisms by which primary producers may influence organic matter dynamics even in ecosystems where autochthonous primary production is low.

The assembly of large multimeric complexes in the crowded cytoplasm is challenging. Here we reveal a mechanism that ensures accurate production of the yeast proteasome, involving ribosome pausing and cotranslational assembly of Rpt1 and Rpt2. Interaction of nascent Rpt1 and Rpt2 then lifts ribosome pausing. We show that the N-terminal disordered domain of Rpt1 is required to ensure efficient ribosome pausing and association of nascent Rpt1 protein complexes into heavy particles, wherein the nascent protein complexes escape ribosome quality control.Immunofluorescence and in situ hybridization studies indicate that Rpt1-and Rpt2encoding mRNAs colocalize in these particles that contain and depend upon Not1, the scaffold of the Ccr4-Not complex. We refer to these particles as Not1-Containing Assemblysomes (NCA), as they are smaller and distinct from other RNA granules such as stress granules, GW-or P-bodies. Synthesis of Rpt1 with ribosome pausing and NCA induction is conserved from yeast to human cells.