Light over mechanics: microbial community structure and activity in simulated migrating bedforms are controlled by oscillating light rather than by mechanical forces

Sandy sediments of lowland streams are transported as migrating ripples. Benthic microorganisms colonizing sandy grains are exposed to frequent moving-resting cycles and are believed to be shaped by two dominant environmental factors: mechanical stress during the moving phase causing biofilm abrasion, and alternating light-dark cycles during the resting phase. Our study consisted of two laboratory experiments and aimed to decipher which environmental factor causes the previously observed hampered sediment-associated microbial activity and altered community structure during ripple migration. The first experiment tested the effect of three different migration velocities under comparable light conditions. The second experiment compared migrating and stationary sediments under either constant light exposure or light oscillation. We hypothesized that microbial activity and community structure would be more strongly affected by (1) higher compared to lower migration velocities, and by (2) light oscillation compared to mechanical stress. Combining the results from both experiments, we observed lower microbial activity and an altered community structure in sediments exposed to light oscillation, whereas migration velocity had less impact on community activity and structure. Our findings indicate that light oscillation is the predominating environmental factor acting during ripple migration, resulting in an increased vulnerability of light-dependent photoautotrophs and a possible shift toward heterotrophy.

When water returns: Drying history shapes respiration and nutrients release of intermittent river sediment

Climate change and anthropogenic water demand have increased the frequency and duration of drying periods across rivers and streams worldwide. However, the biogeochemical processes during the water return in desiccated riverbeds are still unclear. Drying is a complex and diverse process and biogeochemical implications upon flow resumption may depend on attributes of the drying and river sediment characteristics (i.e., organic matter content [OM]). In order to understand the effect of drying duration and intensity on the biogeochemical dynamics following flow resumption, we exposed OM- and non-enriched river sediment from an intermittent river section to three different drying intensities (low: shade and rain; moderate: no shade and rain; high: no shade and no rain), each for three drying durations (10, 30 and 90 days). We determined the sediment-associated microbial respiration and dissolved organic carbon (DOC), ammonium‑nitrogen (NH₄-N), nitrate‑nitrogen (NO₃-N) and soluble reactive phosphorus (SRP) net release/retention rates of the nine drying treatments in flow-through microcosms over four days past flow resumption. Under the most intense and prolonged drying, non-enriched sediments showed a lag response in respiration on the first day after flow resumption, while all other treatments had either a linear increase or an early pulse in respiration. After 48 h, respiration remained constant, with minor changes in respiration dynamics regardless of the OM content of the sediment and drying attributes. The drying duration and intensity had greater effects on SRP release/retention soon after the flow resumption, while NH₄-N and NO₃-N release/retention rates were more strongly affected four days later. Our results suggest that drying attributes influence the biogeochemical dynamics more strongly during the first 24 h upon flow resumption. However, neither respiration nor nutrient dynamics recovered within four days to levels of the sediments before drying for any drying treatments. Hence, the atrributes of the drying have considerable implications in rivers biogeochemistry upon flow resumption.

Attributes of Drying Define the Structure and Functioning of Microbial Communities in Temperate Riverbed Sediment

Combined effects of climate change and increasing anthropogenic water demand have increased and extended dry period occurrences in rivers worldwide. Riverbed drying can significantly affect sediment microorganisms, crucial drivers of biogeochemical processes in lotic systems. In this study, we evaluated how sediment bacterial and fungal community structure and composition (based on 16S rRNA gene and ITS metabarcoding) and microbial functions (community respiration and extracellular enzymatic activities) respond to different riverbed drying intensities over 90 days. Riverbed sediment collected in a flowing reach of the Spree river in northeastern Germany was dried under different rates in outdoor mesocosms during the summer months of 2018. Our results demonstrate that drying attributes (duration and intensity) and sediment organic matter (OM) content play a crucial role in sediment microbial community assembly and functioning throughout drying. Milder drying surprisingly triggered a more rapid and drastic change in the microbial community composition and diversity. After 90 days of drying, Bacilli (Firmicutes) became the dominant bacterial class in most treatments, except in sediments with low OM content under the most severe drying treatment. Fungal amplicon sequence variants (ASVs) from Dothideomycetes (Ascomycota) had by far the highest relative abundance in all our treatments at the end of the drying experiment, making up 65.1% to 94.0% of the fungal reads. CO₂ fluxes, a proxy for sediment community respiration, were rapidly and strongly affected by drying in all treatments. Our results imply that even short riverbed drying periods are likely to have significant consequences for the biogeochemical dynamics in recently formed non-perennial temperate rivers.