Stream Biofilm Responses to Flow Intermittency: From Cells to Ecosystems

Abstract

Temporary streams are characterized by the alternation of dry and wet hydrological phases, creating both a harsh environment for the biota as well as a high diversity of opportunities for adaptation. These systems are mainly microbial-based during several of these hydrological phases, and those growing on all solid substrata (biofilms) accordingly change their physical structure and community composition. Biofilms experience large decreases in cell densities and biomass, both of bacteria and algae, during dryness. Algal and bacterial communities show remarkable decreases in their diversity, at least locally (at the habitat scale). Biofilms also respond with significant physiological plasticity to each of the hydrological changes. The decreasing humidity of the substrata through the drying process, and the changing quantity and quality of organic matter and nutrients available in the stream during that process, causes unequal responses on the biofilm bacteria and algae. Biofilm algae are affected faster than bacteria by the hydric stress, and as a result the ecosystem respiration resists longer than gross primary production to the increasing duration of flow intermittency. This response implies enhancing ecosystem heterotrophy, a pattern that can be exacerbated in temporary streams suffering of longer dry periods under global change.Keywords: biofilm, dry-rewetting cycle, temporary, intermittency, bacteria, algae, Mediterranean THE RELEVANCE OF TEMPORARY STREAMS IN THE WORLDFlow intermittency is part of the natural hydrology for streams and rivers, especially in arid and semi-arid landscapes. Temporary streams, or those that cease to flow at some points in space and time along their course, are a large fraction of many river networks, basically in tributaries of small order, but also in segments in the middle and lower parts of river networks. The number of temporary streams has been probably underestimated , as it appears from the application of novel on-site sensors, advances in remote sensing, and new modeling approaches. Flow intermittency has been estimated to account for 69% of first-order streams below 60 • , and up to 34% of larger order rivers (Raymond et al., 2013). Flow intermittency produces periodic or eventrelated loss of hydrologic connectivity between stream compartments, with consequences for all the processes ongoing in a waterway. Thus, flow intermittency triggers a chain of cascading effects eventually affecting water chemistry and biogeochemistry, as well as biodiversity, and ultimately community structure and ecosystem functioning.The direct implications of climate change and anomalous climatic events on flow regime are behind of the increasing temporality of many streams and rivers. Complex planetary scale processes, as well as local processes, may justify these situations. Anomalies such as the El Niño

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

“…Although certain bacterial cells can resist desiccation thanks to different resistance strategies (e.g. entering dormancy, maintaining osmotic equilibrium, reducing their growth and respiration rates, or finding refuge in deeper sediment; Orchard & Cook, 1983;Moyano, Manzoni, & Chenu, 2013;Sabater, Timoner, Borrego, & Acuña, 2016), bacteria are generally less tolerant than aquatic fungi, which can move more easily across the dry habitat. Aquatic fungi also resist drying better than bacteria because of specific traits such as mycelial growth and spore motility (Manzoni, Schaeffer, Katul, Porporato, & Schimel, 2014;Mora-Gomez et al, 2016).…”

Fishpond dams affect leaf‐litter processing and associated detritivore communities along intermittent low‐order streams

“…However, changes in water flow regime, chemical properties and/or food resources due to the fishponds may affect bacteria, fungi and shredder communities that break down leaflitter in stream. This in turn may affect organic-matter processing and thus stream ecosystem functioning, and perhaps especially so in streams with naturally intermittent flow regimes Datry, Corti, & Philippe, 2012;Datry et al, 2011;Sabater, Timoner, Borrego, & Acuña, 2016). For evaluating the adverse effects of fishpond dam perturbations on intermittent stream functioning, we chose to evaluate leaf-litter breakdown (LLB) and associated microbial communities.…”

Simulating rewetting events in intermittent rivers and ephemeral streams: A global analysis of leached nutrients and organic matter

“…Arce, Sánchez‐Montoya, Vidal‐Abarca, Suárez, & Gómez, ; Merbt, Proia, Prosser, Casamayor, & von Schiller, ; Ostojic, Rosado, Miliša, Morais, & Tockner, ; Tzoraki, Nikolaidis, Amaxidis, & Skoulikidis, ). Furthermore, riverbeds can be covered by biofilm mats (hereafter referred to as “biofilm”), composed of microorganisms (algae, bacteria, fungi) embedded in a matrix of extracellular polymeric substances (Sabater, Timoner, Borrego, & Acuña, ), whose remnants can often be seen even during the dry phase. Biofilm's leachate may contain highly bioavailable organic carbon and nitrogen due to the accumulation of exudates and products of cell lysis (Romaní et al, 2017; Schimel, Balser, & Wallenstein, ).…”