Fluctuations in O2 and salinity influence bacterial dormancy in hypersaline lakes

Background/Question/Methods

Bacterial dormancy is a common life history strategy to weather temporal fluctuations of resources or stresses to wait until more “optimal” conditions are present for metabolic activity and growth. In extreme environments; however, the overriding effects of a constant stress constrains the need or benefit of bacteria entering dormancy. Bacteria are still entering and exiting dormancy in these stressful environments but it is unclear which environmental conditions are inducing dormancy. We measured seasonal fluctuations of microbial dormancy patterns over one year (i.e., January, April, July, October) in two extreme hypersaline lakes (i.e. in the northern ([NaCl]= 25825 mg/L) and southern ([NaCl]= 20896 mg/L) arms of the Great Salt Lake) and evaluated links between dormancy and salinity, dissolved oxygen, and temperature. We defined dormancy as the difference between DNA-based communities (i.e., all bacteria present in the community) and RNA-based communities (only the active bacteria) and used targeted metagenomics to analyze the 16S rDNA and rRNA extracted from the lake water samples. We sampled from the limnetic zone (i.e., approximately 200m from shore and 3m below the lake surface).

Results/Conclusions

We found that temporal fluctuations in salinity and dissolved O₂ drove changes in dormancy of specific bacterial taxa at both sample locations. Seasonal variation in salinity strongly related to microbial dormancy in both arms of the Great Salt Lake and demonstrated by a positive linear relationship (R² = 0.98 P <0.01) between dormancy and salinity. Additionally, as dissolved oxygen levels increased, dormancy declined (R² = 0.67 P =0.01), suggesting that the availability of O₂ drives the activity of specific lake bacteria taxa. Seasonal temperature fluctuations did not relate to dormancy patterns (R² = 0.01 P =0.54). We also observed that DNA-based communities fluctuated very little throughout the year, but RNA-based communities exhibited enormous shifts in response to seasonal fluctuations in both lakes.  For example, in DNA-based communities, the Bacteroidetes, Actinobacteria, and Gammaproteobacteria were always the most prominent taxa, but Bacteroidetes dominated RNA-based communities during the summer when Actinobacteria and Gammaproteobacteria were dormant. Further, Actinobacteria dominated RNA-based communities in the winter when Bacteroidetes was dormant. Our results suggest even in extreme environments, dormancy may be driven by more than one environmental variable and that seasonal variability induced phyla-specific metabolic changes.