Salinity as a structuring factor for the composition and performance of bacterioplankton degrading riverine DOC

Abstract

The impact of salinity on the composition and functional performance (biomass production, growth efficiency and growth rates) of bacterial communities was investigated using batch cultures growing on dissolved organic carbon from a river draining into the Northern Baltic Sea. The cultures were adjusted to riverine or estuarine salinity levels and inoculated with bacteria from these two environments. Bacterial growth efficiencies differed in response to salinity and the origin of the inoculum. When salinity was adjusted to correspond to the salinity at the site where the inoculum was retrieved, growth efficiency was relatively high (11.5+/-2.6%). However, when bacteria were confronted with a shift in salinity, growth efficiency was lower (7.5+/-2.0%) and more of the utilized carbon was respired. In contrast, growth rates were higher when bacteria were exposed to a change in salinity. The composition of the bacterial communities developing in the batch cultures differed, as shown by 16S rDNA DGGE, depending on the origin of the inoculum and salinity. Reverse and direct DNA-DNA hybridization revealed salinity optima in the growth of specific bacterial strains as well as broader phylogenetic groups. Strains belonging to the alpha- and beta-Proteobacteria, Actinobacteria and gamma-Proteobacteria other than the genus Pseudomonas showed higher relative abundance under freshwater conditions, whereas strains of the genus Pseudomonas and the Cytophaga-Flavobacterium-Bacteroides group were favored by estuarine conditions. Generally, our results demonstrate functional changes associated with changes in community composition. We suggest that even moderate changes in salinity affect bacterial community composition, which subsequently leads to altered growth characteristics.

Optical properties and bioavailability of dissolved organic matter along a flow-path continuum from soil pore waters to the Kolyma River mainstem, East Siberia

“…Previous studies in the Arctic underscore the importance of residence times as well as a significant combined role for photo-and biological degradation along the flow path in Arctic watersheds (Cory et al, 2007(Cory et al, , 2013Merck et al, 2012;Laurion and Mladenov, 2013). These previous results show that the photochemical "pretreatment" of stream DOM that occurs during export into lakes and coastal zones may impact the ability of microorganisms to mineralize DOM.…”

“…Further downstream in larger tributary and Kolyma mainstem waters, it has been shown that lower total amounts of bioavailable DOC is supported almost entirely from predominantly modern radiocarbon aged surface soils and vegetation sources . Aquatic microorganisms may therefore be readily acclimating to significant shifts in DOM composition caused by selective losses of unique DOM fractions (e.g., Kaplan and Bott, 1983;Spencer et al, 2015) alongside high internal demand for labile DOM by stream communities in lower-order streams, which would otherwise generally be expected to result in decreased DOM lability with increasing water residence time (Stepanauskas et al, 1999a, b;Wikner et al, 1999;Langenheder et al, 2003;Sondergaard et al, 2003;Fellman et al, 2010Fellman et al, , 2014.…”

“…umn: (i) photochemical reactions, where DOM is degraded to CO 2 or to compounds bioavailable for bacterial uptake (Moran and Zepp, 1997;Laurion and Mladenov, 2013;Cory et al, 2014); (ii) flocculation of terrestrial DOM resulting in the settling of particulate organic matter (Wachenfeldt et al, 2009); (iii) loss via aggregation of DOM owing to changes in ionic strength when freshwater mixes with sea water (Sholkovitz, 1976); (iv) DOM sorption to particles and sedimentation (Chin et al, 1998); bacterial uptake and utilization of the bioavailable fraction (Bronk, 2002;Karl and Björkman, 2002;Mann et al, 2014;Spencer et al, 2015). Measurements of waters along a hydrologic flow path may provide unique insights into the characteristics of DOM as it is modified through these various processes along the soilstream-river continuum.…”

Mechanisms determining the fate of dispersed bacterial communities in new environments

“…Possible reasons for the observed overall functional equivalence of both communities in both incubation environments, respectively, could be, for example, that the salinity differences between the samples were not high enough to significantly stress the communities (0 vs 4.5 psu). However, it has previously been shown that similar 'mild' salinity differences can affect microbial communities both in terms of function and composition (for example, Langenheder et al, 2003). Another possibility could therefore be that the rock pool communities were already 'pre-conditioned' to environmental fluctuations, as they regularly experience quite drastic differences in environmental conditions (Jocque et al, 2010).…”

Bacterial Community Structure Shift Driven by Salinity: Analysis of DGGE Band Patterns from Freshwater to Seawater of Hyeongsan River, Korea

“…On a larger scale, spatial differences -water depth for an example -and seasonal changes can also be factors that shift structures of bacterial communities [2,16]. Among those factors affecting bacterial community structures, the salinity gradient is a factor of dramatic impact within aquatic environments [9]. Compared to factors such as temperature, pH or nutrition which may change inconsistently by seasonal shift, pollution or any diverse activity that may modify conditions of aquatic environments, salinity is a definite factor that continuously increases upstream as freshwater streams flow toward the open ocean.…”