Abstract
Differences in salinity are boundaries that act as barriers for the dispersal of most aquatic organisms. This creates distinctive biota in freshwater and brackish water (mesohaline) environments. To test how saline boundaries influence the diversity and composition of host-associated microbiota, we analyzed the microbiome within the digestive tract of Theodoxus fluviatilis, an organism able to cross the freshwater and mesohaline boundary. Alpha-diversity measures of the microbiome in freshwater and brackish water were not significantly different. However, the composition of the bacterial community within freshwater T. fluviatilis differed significantly compared with mesohaline T. fluviatilis and typical bacteria could be determined for the freshwater and the mesohaline digestive tract microbiome. An artificial increase in salinity surrounding these freshwater snails resulted in a strong change in the bacterial community and typical marine bacteria became more pronounced in the digestive tract microbiome of freshwater T. fluviatilis. However, the composition of the digestive tract microbiome in freshwater snails did not converge to that found within mesohaline snails. Within mesohaline snails, no cardinal change was found after either an increase or decrease in salinity. In all samples, Pseudomonas, Pirellula, Flavobacterium, Limnohabitans, and Acinetobacter were among the most abundant bacteria. These bacterial genera were largely unaffected by changes in environmental conditions. As permanent residents in T. fluviatilis, they may support the digestion of the algal food in the digestive tract. Our results show that freshwater and mesohaline water host-associated microbiomes respond differently to changes in salinity. Therefore, the salinization of coastal freshwater environments due to a rise in sea level can influence the gut microbiome and its functions with currently unknown consequences for, e.g., nutritional physiology of the host.
Highlights
Salinity is among the most important environmental factors that determine the composition of aquatic microbial communities (Crump et al, 2004; Herlemann et al, 2011)
Significant changes in salinity are likely to result in the loss/change of key microorganism (Herlemann et al, 2011), changes in microbial metabolism (Neubauer, 2013), and nutrient cycling (Marton et al, 2012)
The aim of this study was to investigate the responses of a host-protected bacterial community to shifts in salinity using T. fluviatilis as a model system
Summary
Salinity is among the most important environmental factors that determine the composition of aquatic microbial communities (Crump et al, 2004; Herlemann et al, 2011). A global-scale metaanalysis of samples from different habitats suggested that salinity is the major determinant of bacterial communities (Lozupone and Knight, 2007). The gastropod Theodoxus fluviatilis is a Salinity Changes Microbiome in Host-Protected Environment widely distributed snail typically found in rivers and lakes as well as in brackish water in the coastal regions of Europe (Bunje, 2005). T. fluviatilis has the ability to live in fresh- and brackish waters up to a salinity 28 and are found in the western Baltic Sea (Bondesen, 1940; Zettler, 2008). The origin of the snail has been assumed to be the brackish Black Sea (Butenko, 2001)
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