Sharp contrasts between freshwater and marine microbial enzymatic capabilities, community composition, and DOM pools in a NE Greenland fjord

Abstract

Increasing glacial discharge can lower salinity and alter organic matter (OM) supply in fjords, but assessing the biogeochemical effects of enhanced freshwater fluxes requires understanding of microbial interactions with OM across salinity gradients. Here, we examined microbial enzymatic capabilities—in bulk waters (nonsize‐fractionated) and on particles (≥ 1.6 μm)—to hydrolyze common OM constituents (peptides, glucose, polysaccharides) along a freshwater–marine continuum within Tyrolerfjord‐Young Sound. Bulk peptidase activities were up to 15‐fold higher in the fjord than in glacial rivers, whereas bulk glucosidase activities in rivers were twofold greater, despite fourfold lower cell counts. Particle‐associated glucosidase activities showed similar trends by salinity, but particle‐associated peptidase activities were up to fivefold higher—or, for several peptidases, only detectable—in the fjord. Bulk polysaccharide hydrolase activities also exhibited freshwater–marine contrasts: xylan hydrolysis rates were fivefold higher in rivers, while chondroitin hydrolysis rates were 30‐fold greater in the fjord. Contrasting enzymatic patterns paralleled variations in bacterial community structure, with most robust compositional shifts in river‐to‐fjord transitions, signifying a taxonomic and genetic basis for functional differences in freshwater and marine waters. However, distinct dissolved organic matter (DOM) pools across the salinity gradient, as well as a positive relationship between several enzymatic activities and DOM compounds, indicate that DOM supply exerts a more proximate control on microbial activities. Thus, differing microbial enzymatic capabilities, community structure, and DOM composition—interwoven with salinity and water mass origins—suggest that increased meltwater may alter OM retention and processing in fjords, changing the pool of OM supplied to coastal Arctic microbial communities.

A Sea Change in Microbial Enzymes: Heterogeneous latitudinal and depth-related gradients in bulk water and particle-associated enzymatic activities from 30°S to 59°N in the Pacific Ocean

“…3c). Endo-acting enzymes thus likely play a critical role in degradation of particles, as previously observed among polysaccharide hydrolases in the North Atlantic (D'Ambrosio et al 2014), the central Arctic (Balmonte et al 2018), and a northeast Greenland fjord (Balmonte et al 2020). Although their activities vary regionally, endo-acting enzymes produced by particle-associated microbial communities likely play a widespread role in efficient degradation of protein and carbohydrate constituents of particulate matter (Obayashi and Suzuki, 2005) from surface to deep waters (Fig.…”

“…Glucosidase activities were measured using the following methylumbelliferyl (MUF)-labelled compounds: α-glucopyranoside (α-glu) and β-glucopyranoside (β-glu). Whereas Leu, α-glu, and β-glu have been used in a wide range of environmental settings, substrate proxies measuring chymotrypsin and trypsin activities have been used only in a limited number of systems (e.g., Arnosti, 2015;Balmonte et al 2019;Balmonte et al 2020;Bong et al 2013;Obayashi and Suzuki, 2005;Steen and Arnosti, 2013).…”

“…These findings suggest the 64 importance of variations in microbial community structure in shaping differences in microbial Vetter et al 1998, Zhao et al 2020. A wider spectrum of enzyme activities has been detected on particles than either the free-living fraction (D'Ambrosio et al 2014) or in bulk waters 90 (Balmonte et al 2018;Balmonte et al 2020), observations that highlight the broad range of 91 enzymes required to degrade POM. However, these investigations have only been carried out in 92 a limited range of settings.…”

Microbial Abundance and Enzyme Activity Patterns: Response to Changing Environmental Characteristics along a Transect in Kongsfjorden (Svalbard Islands)

“…The cumulative effects of increased temperatures and UV radiation on aquatic and terrestrial domains as well as on living organisms have been widely studied [1]. At the microbial level, a significant contribution to current knowledge of the distribution, community dynamics, and metabolic activities of marine microorganisms inhabiting Arctic marine ecosystems has been provided by a number of previous investigations [2][3][4][5][6][7][8][9]. Higher phytoplankton growth, bacterial production, and grazing pressure on bacteria were previously observed in sites directly affected by sea-ice melting compared to others that were unaffected, suggesting a substantial influence of the ice melting process on the productivity and the biogeochemistry of Arctic waters [10].…”

Greenland liquid water runoff from 1979 through 2017

“…Over the past decades, liquid runoff from Greenland has increased (Mernild and Liston, 2012;Bamber et al, 2018;Trusel et al, 2018;Perner et al, 2019). When that runoff leaves the ice sheet and enters fjords and coastal seas, it influences a wide range of physical (Straneo et al, 2011;An et al, 2012;Mortensen et al, 2013;Bendtsen et al, 2015;Cowton et al, 2015;Mankoff et al, 2016;Fried et al, 2019;Cowton et al, 2019;Beckmann et al, 2019), chemical (Kanna et al, 2018;Balmonte et al, 2019), and biological (Kamenos et al, 2012;Kanna et al, 2018;Balmonte et al, 2019) systems (Catania et al, 2019). The scale of impacts ranges from the ice-ocean boundary to the distal open ocean (Gillard et al, 2016).…”