COS 292-4 - Moisture induced soil community differences impact chitin degradation

Abstract: Chitin is the second most abundant polymer in the environment, and due to its composition, its degradation couples the carbon and nitrogen cycles. Complete decomposition of chitin is a community process requiring multiple extracellular enzymes. Yet it’s not clear how enzyme production is linked to substrate acquisition and growth by community members. Further, physical access to substrates in soil and organismal interactions are impacted by moisture, which controls connectivity through pore spaces. We carried out laboratory experiments to explore the effects of moisture on chitin decomposition. Soil samples were collected from high (100%), and low (25%) field capacity irrigation treatments from a 4-year field experiment in Prosser, WA. Half the jars were amended with chitin and maintained at field moisture for seven days in order to study chitin decomposition in soils acclimated to high or low moisture.

In kinetic assays, on nycodenz extracted cells, exochitinase and endochitinase activities respond earlier for high moisture +chitin samples than low moisture +chitin samples, and activity in -chitin control soils responded last with no differences due to moisture. Exochitinase in high moisture +chitin samples responded immediately, but low moisture +chitin treatments were delayed. After labelling cells with activity-based probes, fluorescence-activated cell sorting revealed a larger population of cells with exochitinase activity than endochitinase activity, indicating this step in chitin degradation is more broadly expressed in the microbial community. Sequencing of 16S indicates differences between bacterial communities in low and high moisture treatments. However, we saw no differences in +chitin vs -chitin samples within moisture levels. Chitin addition caused a significant drop in Shannon diversity in +chitin samples, caused by declines in the evenness of the communities, suggesting chitin addition may enrich chitin degraders in the community. In contrast to differences between moisture levels seen in kinetic assays and community structure, we found that cumulative respiration was significantly higher in +chitin soils than -chitin soils, but there was no significant difference between moisture treatments. Further, respiration rates did not differ between +chitin and -chitin soils for the first two days, suggesting a delay in metabolism of chitin derived carbon despite early expression of endochitinase in high moisture + chitin samples.

Our results indicate differential responses to chitin amendment and moisture, providing evidence for temporal dynamics of microbial responses. Ongoing works seeks to identify specific taxa expressing chitin degradation enzymes to understand how soil microbiomes cooperate to use chitin as a carbon and nitrogen resource.