High Salinity Stabilizes Bacterial Community Composition and Activity

Abstract

Dormancy is offered as an explanation for how bacteria are able to survive temporal fluctuations in resource availability or adverse environmental conditions. Bacteria may exit this reversible, low-metabolic state and resuscitate, becoming metabolically active in response to favorable environmental cues. In extreme environments, the prevalence of dormancy is only expected to rise as stressful conditions intensify, however, the overriding effects of a constant stress in extreme environments may overshadow other environmental cues from influencing activity. We evaluated shifts in microbial community composition and dormancy and related these variables to water chemistry over one year in six lakes in UT, USA. The lakes represented a salinity gradient ranging from 303.22 PSU in the North Arm of the Great Salt Lake to 0.52 PSU in Deer Creek Reservoir. We analyzed 16S rDNA-based communities (i.e., all bacteria present in the community) and 16S rRNA-based communities (i.e., only active bacteria) with target metagenomics, observed and analyzed changes in bacterial community composition over time, and calculated dormancy based on rRNA to rDNA ratios of the relative recovery of individual operational taxonomic units. We measured changes in lake chemistry (i.e., temperature, pH, dissolved oxygen, total nitrogen, total phosphorus, dissolved organic carbon) and linked them to dormancy and community composition with multiple regression models and metric distance matrices in R. We found that rDNA-based community composition was driven primarily by salinity and dissolved oxygen concentrations (P \u3c .001); that is, where these variables were most extreme, very little change in community composition occurred. rRNA-based community composition, however, was additionally highly associated with fluctuations in phosphorus concentrations (P \u3c .001). Furthermore, the prevalence of total dormancy in a lake was positively associated with the proportion of rare taxa (OTU’s representing \u3c0.1% of the total bacteria) in the system. These findings suggest that high salinity and low oxygen prevent the introduction and prosperity of taxa that are not predisposed to such extreme conditions. Without a stabilizing extreme variable, the freshwater lakes catered to a more diverse range of bacteria, which were then subject to changes in multiple variables (i.e., phosphorus availability), high levels of competition, and increased use of dormancy as a life history strategy