Substrate complexity buffers negative interactions in a synthetic microbial community of leaf litter degraders

Microbes associated with leaf litter, the top layer of soil, collectively decompose organic matter such as plant polysaccharides, and respire carbon dioxide, regulating the land-atmosphere fluxes of carbon. Therefore, it is crucial to understand the processes limiting biopolymer degradation and their influences on soil community properties. For example, it is still unclear how substrate complexity – defined as the structure of the saccharide and the amount of external processing by extracellular enzymes – influences species interactions and species coexistence. Here, we tested the hypothesis that growth on monosaccharides (i.e., xylose) promotes negative interactions through resource competition, and growth on polysaccharides (i.e., xylan) promotes neutral or positive interactions through resource partitioning or synergism among extracellular enzymes. We assembled a three-species community of leaf litter-degrading bacteria isolated from a grassland site in Southern California. In the polysaccharide xylan, pairs of species stably coexisted and grew equally in co-culture and in monoculture. Conversely, in the monosaccharide xylose, competitive exclusion and negative interactions prevailed. These pairwise dynamics remained consistent in a three-species community: all three species coexisted in xylan, while only two species coexisted in xylose. A mathematical model parameterized from single-species growth behaviors showed that in xylose these dynamics could be explained by resource competition. Instead, the resource competition model could not predict the coexistence patterns in xylan. Overall, our study shows that substrate complexity influences species interactions and patterns of coexistence in a synthetic microbial community of leaf litter degraders that can serve as a model for studying carbon cycling and climate change. ![Figure][1] ### Competing Interest Statement The authors have declared no competing interest. The genome sequences generated during the current study are available in the GenBank repository under accession numbers SAMN38146147 and SAMN38146146. All the other data generated or analyzed during this study are included in this published article and its supplementary information files. [1]: pending:yes