Negative interactions override the benefits of abundant nutrients in experimental bacterial communities

Unravelling how nutrient availability and resource-mediated interactions shape bacterial communities is a central goal of microbial ecology. Recent ecological theory has proposed that more abundant nutrients, by allowing for longer cross-feeding chains, can sustain richer communities. Here we tested this hypothesis by exploring how hundreds of soil-derived bacterial communities respond to an increase over three orders of magnitude in the concentration of a variety of single sources carbon, including sugars and organic acids. Contrary to theoretical expectations, we observed reduced diversity and indications of alternative stable states at higher carbon levels, suggesting that negative interactions override the benefits of nutrient abundance. Pairwise competition experiments with isolates from experimental communities recapitulated both the reduced diversity and the alternative stable states at higher carbon concentration, and spent media assays suggest that they are driven by increased toxin secretion. A consumer-resource model with nutrient- and toxin- mediated interactions further bolsters our conclusions.