Key role of streambed moisture and flash storms for microbial resistance and resilience to long‐term drought

Abstract

Spatial and temporal widening of drought periods together with occurrence of flash storm events are consequences of global change affecting temporary stream ecosystems. Streambed heterotrophic microbes and the key biogeochemical processes they carry on could be endangered by the strengthening of drought episodes. Here, we performed a 165‐day experiment through 12 streambed sediment columns to study heterotrophic microbial functional and structural responses to long‐term drought. Two sediment depths (surface and hyporheic) and leaf litter were monitored under three treatments: control (maintained in wet, pool‐like conditions), dry (5 months of drought), and dry–storm (5 months of drought including two flash storms). All treatments were then followed by rewetting. Surface sediment followed by leaf litter was the most affected by the long‐term drought as shown by the reduction of polysaccharidic enzyme activities and litter decomposition rate, although lignin decomposition was not affected. This resulted in a greater use of recalcitrant compounds which might be due to reduced labile organic matter sources and the greater resistance of fungi than bacteria to drought. Moreover, in surface sediment, bacterial viability and algal biomass were reduced while the production of extracellular polymeric substances was intensified with dryness, suggesting its potential role as survival strategy. Hyporheic sediments appeared more resistant to long‐term drought, and this might be linked to its slightly higher water content (2.5%) than the surface (0.5%) during drying together with a greater content of fine material that allowed fungi and bacteria to survive and extracellular enzyme activities to be maintained. Microbial resilience to long‐term drought in sediment and leaf litter was promoted by the flash storms as shown by the fast recovery of enzyme activities, bacterial viability and leaf litter decomposition rate in the dry–storm treatment, once rewetted. Storms might liberate previously occluded carbon sources that fuel the fast microbial reactivation as well as providing sediment moisture for microbial survival. Our results highlight that long‐term drought may compromise stream biogeochemical processes linked to organic matter decomposition and that microbes are highly sensitive to minimal water content changes. Thus, long‐term drought consequences could be mitigated by occasional precipitations or by natural streambed moisture.

Multi‐model assessment of hydrological and environmental impacts on streambed microbes in Mediterranean catchments

“…Similar to previous studies, we found that the presence of specific extracellular enzyme activities (EEA) is influenced by the extended duration of the dry phase (Zoppini et al ., ; Freixa et al ., ; Gionchetta et al ., ). With increasing dryness, PHE activity is enhanced indicating a greater capability to degrade lignin compounds, while the tendency of β‐glucosidase and β‐xylosidase to decrease suggested a reduced capability to degrade simple polysaccharides.…”

“…Fresh sediment subsamples (1 ml) were analysed for the activity of two hydrolytic enzymes, β‐ d ‐1, 4‐glucosidase (EC 3.2.1.21, GLU), β‐xylosidase (EC 3.2.1.37, XYL), as well as for the oxidative enzyme PHE (EC 1.14.18.1, PHE), as indicators of the cellulose, hemicellulose and lignin compound decomposition capacity respectively. For the EEA, we followed the procedure from Gionchetta and colleagues (). The hydrolytic enzyme (GLU and XYL) values were expressed as nmol MUF gDW −1 h − 1, and the PHE enzyme values were expressed as nmol 2,3‐dihydroindole‐5,6‐quinone‐2‐carboxilate (DIQC) gDW −1 h −1 .…”

“…The increase of streambed aridity in intermittent streams and the consecutive reduction of sediment-water content due to extended exposure to the air may affect the hosted microbiota and compromise microbial metabolism (Rees et al, 2006;Marxsen et al, 2010;Febria et al, 2015;Gionchetta et al, 2019). Microbial streambed communities are responsible for key ecosystem processes such as nutrient cycling and biogeochemical processes, and their metabolic capabilities are fundamental for maintaining fluvial ecosystem functioning (Datry et al, 2017).…”

“…In headwater sections, a greater accumulation of leaf litter combined with a reduced percentage of sand and gravel sediment particles could lessen the loss of organic matter and water content during dry periods, limiting the reduction of microbial activity (Freixa et al, 2016). Other studies have reported that leaf packs buried in the sediment maintain high humidity under extreme dry conditions, becoming temporal shelters for microbial survival (Gionchetta et al, 2019). Similarly, the presence of riparian vegetation, able to curtail water loss from sediments under dry conditions, has been considered a hydrological refuge for microbiota inhabiting the streambed (Mclaughlin et al, 2017).…”

Root Fungal Endophytes and Microbial Extracellular Enzyme Activities Show Patterned Responses in Tall Fescues under Drought Conditions

“…Bacteria and fungi both produce extracellular enzymes, and the increase in chitinase and phenol oxidase activities may signal a role in drought tolerance or recovery. Previous studies have shown that fungal and bacterial biomass and extracellular enzyme production often respond differently to drought and that they show distinct enzyme partitioning [50][51][52][53][54]. The interest in the role of drought in ecosystem functioning has not revealed a consistent response of microbial communities and extracellular enzyme activities to drought and recovery.…”