Dispersal modifies the responses of bacterioplankton to salinity disturbances

Dandan Izabel-Shen, Department of Ecology, Environment and Plant Sciences, Stockholm University, Stockholm, Sweden, Dandan Izabel-Shen and Klaus Jürgens, Section of Biological Oceanography, Leibniz Institute for Baltic Sea Research, Warnemünde, Germany, Silke Langenheder, Department of Ecology and Genetics/Limnology, Uppsala University, Uppsala, Sweden

Background/Question/Methods

Dispersal is increasingly recognized as an important factor influencing the assembly of microbial communities and the responses of microbes to environmental change. In aquatic systems, dispersal occurs as community coalescence, which involves the mixing of both cells from different communities and the environmental matrices of those cells. However, the extent to which community coalescence events contribute to a community’s response to changes in salinity as a function of its character and strength is unclear. Our study simulated a scenario that often occurs in estuaries: the mixing of marine and brackish communities by tides or aerosols. Mixing was reproduced in a transplant experiment in which dialysis bags containing bacterioplankton that had originated from brackish and marine regions of the Saint Lawrence Estuary were reciprocally incubated in the two environments for 5 days. Dispersal was mimicked in half of the microcosms from each environment by subjecting them to an exchange of cells from the marine and brackish assemblages at a daily exchange rate of 6% (v/v); the remaining microcosms served as non-dispersal treatments. This setup allowed comparisons between the mixed and non-mixed bacteria and thus an evaluation of both the community-level impact of exposure to new salinity environments as well as the contribution of dispersal to community assembly. The diversity and population variability of metabolically active bacterial communities were characterized, and taxa with a good dispersal capacity, based on occurrence and abundance, were identified.

Results/Conclusions
Although exposure of the cells to a new salinity level caused a loss of diversity, the new level of diversity was maintained or even increased by dispersal, with the extent depending on the initial diversity and on the metabolic plasticity of the community members. In addition, dispersal led to a greater resemblance between marine and brackish communities in both environments, especially in the case of communities incubated in the brackish environment. These results suggest that dispersal is followed by a taxonomic adjustment of the community composition such that, after their exposure to a salinity disturbance, good dispersers are able to thrive and subsequently colonize empty niches. Our study provides a better understanding of the fate of dispersed bacteria under saltwater intrusion scenarios and will allow more accurate predictions of bacterial responses to environmental changes.