Bacterial Diversity in a Dynamic and Extreme Sub-Arctic Watercourse (Pasvik River, Norwegian Arctic)

Abstract

Microbial communities promptly respond to the environmental perturbations, especially in the Arctic and sub-Arctic systems that are highly impacted by climate change, and fluctuations in the diversity level of microbial assemblages could give insights on their expected response. 16S rRNA gene amplicon sequencing was applied to describe the bacterial community composition in water and sediment through the sub-Arctic Pasvik River. Our results showed that river water and sediment harbored distinct communities in terms of diversity and composition at genus level. The distribution of the bacterial communities was mainly affected by both salinity and temperature in sediment samples, and by oxygen in water samples. Glacial meltwaters and runoff waters from melting ice probably influenced the composition of the bacterial community at upper and middle river sites. Interestingly, marine-derived bacteria consistently accounted for a small proportion of the total sequences and were also more prominent in the inner part of the river. Results evidenced that particular conditions occurring at sampling sites (such as algal blooms, heavy metal contamination and anaerobiosis) may select species at local scale from a shared bacterial pool, thus favoring certain bacterial taxa. Conversely, the few phylotypes specifically detected in some sites are probably due to localized external inputs introducing allochthonous microbial groups.

Ice Melt-Induced Variations of Structural and Functional Traits of the Aquatic Microbial Community along an Arctic River (Pasvik River, Norway)

“…A decrease in microbial diversity from the riverine to the most marine sites was recently reported by Papale et al [13] during an Ice-melt (+) sampling (June 2013) in the same area. In line with this, here the richness (in terms of OTU number) also showed a decrease from the riverine to the brackish stations in both periods, but a reduction of OTUs was also more evident in the Ice-melt (−) than in the Ice-melt (+) season, probably due to effects of runoff waters from melting.…”

“…The microbial community composition at the genus level also reflected the occurrence of taxonomic groups widely distributed in Arctic aquatic systems, e.g., the case of Polynucleobacter and Limnohabitans, but also the presence of taxonomic groups of more pronounced terrigenous origin, which suggest the influence of ice-melting processes. Polynucleobacter and Limnohabitans are two typically freshwater genera [53,54], also recently retrieved in the Pasvik river water and sediments, with higher abundances in water samples [13]. In support of this assumption, bacterial members of the genera Massilia, previously found in Arctic sediments and glacier permafrost [55,56], were detected at a higher percentage just at the riverine stations in the period of Ice-melt (+), which realistically had a more direct influence from the fluvial inputs.…”

“…In both cases, salinity and total dissolved solids values were respectively higher and lower in brackish stations than in riverine ones, probably due to a more direct effect of ice melting and terrestrial inputs on them (Tables 1 and 2). The lowest δ 13 POC values occurring in the samples collected in riverine stations (Table 2) seems to confirm a terrestrial origin of organic matter since, in general, terrestrial organic material is isotopically depleted in δ 13 POC values compared to marine organic matter [35]. According to the environmental assessment (Figure 6), the separation between brackish and riverine stations appeared more pronounced during the Ice-melt (−) period, while a more dispersed distribution occurred during the Ice-melt (+) period, by indicating a change in the physicalchemical configuration along the sites.…”

“…The PowerSoil kit (the MoBio Laboratories Inc., San Diego, CA, USA; Biogenerica, Catane, Italy) was used for DNA extraction. The PCR amplification of the 16S rRNA genes (V1-V2 region) was performed as described by Papale et al [13]. A two-step PCR protocol was applied to reduce biases in massive sequencing, i.e., a first step of 30 PCR cycles with conventional PCR primers was followed by 6 cycles PCR with barcoded primers for Ion Torrent sequencing using 0.5 µL of first reaction amplicon.…”

“…Prediction of how Arctic and sub-Arctic riverine ecosystems will respond to future climate change will derive from a better understanding of the ecological and biogeochemical traits of inhabiting microorganisms, which experience contrasting environmental conditions over the year. It is noteworthy that hydrological connections, local water residence time, and peculiar features (physical-chemical variables, influence of predation or competition) could strongly contribute in shaping the microbial community along the watercourse, by affecting the balance between the seeding of new microbial taxa from multiple sources (e.g., precipitation, soils, cryosphere) and local sorting [8][9][10][11][12][13].…”