Changes in Microbial Community Structure and Function Following Salinization of Tidal Freshwater Wetlands

Abstract:

Tidal freshwater wetlands are subject to multiple stressors derived from the watershed as well as ones determined by oceanic processes. Saltwater intrusion, which is occurring due to both changes in precipitation and rising sea levels, is of particular concern because elevated salinity impacts a wide range of abiotic and biotic processes. We have been investigating microbial community responses to salinization using an integrated multi-scale approach that includes a three-year in situ field manipulation (repeated additions of brackish water to plots in a tidal freshwater wetland), transplant experiments of freshwater soils to more saline sites, and long-term studies of wetlands along an existing estuarine salinity gradient. Results demonstrate that salinity is a strong driver of microbial community composition and can induce changes in both phylogenetic community structure and functional gene abundance. However, these effects were manifest at different time scales and rarely synchronized with biogeochemical responses. In general, effects on soil carbon cycling (extracellular enzyme activity and CO₂/CH₄ production) appeared quickly, whereas microbial communities changed more slowly over time. Response patterns were similar when transplant experiments and in situ manipulations were of similar duration, but the effect was consistently subdued for the in situ manipulations. This suggests that the intact soil-plant-ecosystem may help buffer salinization effects, at least in the short term (i.e., weeks to months), compared to soil-only responses. Alternately, this result could illustrate the importance of immigration/colonization in determining community dynamics, since transplanted soils were exposed to a larger pool of saline-adapted microorganisms. The gradual changes in microbial community structure that we observed during these manipulative studies were mirrored in our field survey, which suggests that salinity gradients can be useful space-for-time proxies to study the effects of long-term salinity exposure. Understanding how microbial communities adapt to salinity stress over varying time scales is essential for developing robust predictive models of how ecosystems will respond to global change.