Hidden Decomposers: the Role of Bacteria and Fungi in Recently Intermittent Alpine Streams Functional Processes

Abstract The frequency of flow-intermittency and drying events in Alpine rivers is expected to increase due to climate change. These events can have significant consequences for stream ecological communities, though the effects of reduced flow conditions on the microbial communities of decomposing allochthonous leaf material require additional investigation. In this study, we investigated the bacterial and fungal communities associated with the decomposition of two common species of leaf litter, chestnut (Castanea sativa) and oak (Quercus robur). Sampling of experimentally placed leaf bags was spread over six collection points (up to 126 days after placement) at seven sites in the Western Italian Alps, which have historically differed in streamflow conditions. Bacterial and fungal communities from these leaf bags were identified using high-throughput amplicon sequencing. Chestnut and oak leaf bags harbored distinct bacterial and fungal communities, with a number of taxonomic groups differing in abundance, though bacterial communities began to converge at later sampling dates. Historical flow conditions (intermittent vs perennial rivers) and observed conditions (normal flow, low flow, ongoing drying event) had comparatively weak observed effects on bacterial and fungal communities to that of leaf type and collection date (i.e. length of decomposition). Our findings highlight the importance of leaf characteristics (e.g., C:N ratios, recalcitrance, etc.) to the in-stream conditioning of leaf litter and the need for additional investigations of drying events in Alpine streams. This study provides new information on the microbial role in leaf litter decomposition with expected flow changes associated with a global change scenario.