Novel microbial community composition and carbon biogeochemistry emerge over time following saltwater intrusion in wetlands

Abstract

Sea level rise and changes in precipitation can cause saltwater intrusion into historically freshwater wetlands, leading to shifts in microbial metabolism that alter greenhouse gas emissions and soil carbon sequestration. Saltwater intrusion modifies soil physicochemistry and can immediately affect microbial metabolism, but further alterations to biogeochemical processing can occur over time as microbial communities adapt to the changed environmental conditions. To assess temporal changes in microbial community composition and biogeochemical activity due to saltwater intrusion, soil cores were transplanted from a tidal freshwater marsh to a downstream mesohaline marsh and periodically sampled over 1 year. This experimental saltwater intrusion produced immediate changes in carbon mineralization rates, whereas shifts in the community composition developed more gradually. Salinity affected the composition of the prokaryotic community but did not exert a strong influence on the community composition of fungi. After only 1 week of saltwater exposure, carbon dioxide production doubled and methane production decreased by three orders of magnitude. By 1 month, carbon dioxide production in the transplant was comparable to the saltwater controls. Over time, we observed a partial recovery in methane production which strongly correlated with an increase in the relative abundance of three orders of hydrogenotrophic methanogens. Taken together, our results suggest that ecosystem responses to saltwater intrusion are dynamic over time as complex interactions develop between microbial communities and the soil organic carbon pool. The gradual changes in microbial community structure we observed suggest that previously freshwater wetlands may not experience an equilibration of ecosystem function until long after initial saltwater intrusion. Our results suggest that during this transitional period, likely lasting years to decades, these ecosystems may exhibit enhanced greenhouse gas production through greater soil respiration and continued methanogenesis.

Microbial community composition is affected by press, but not pulse, seawater intrusion

“…Consistent with our hypothesis, we found greater abundance of SRB in Press plots (Figure 4a), which had higher concentrations of porewater sulfide compared to other treatments (Figure 3) (Widney et al 2019). Our results are also consistent with other studies (Weston et al 2006, 2011; Dang et al 2019), including a transplant study of freshwater marsh soils into a brackish marsh that found a slight increase in abundance of sulfate reducers ( dsrA ) (Dang et al 2019). A shift in the microbial community towards SRB has the potential to negatively affect other members of the community because sulfate reduction can generate hydrogen sulfide that is toxic to more sensitive members of the community.…”

“…Relative to Control treatments, Pulse plots had a greater abundance of sulfate reducers (Figure 4a) but no difference in diversity (Figure 1). Other short-term studies have also reported no change in diversity in response to increased salinity (e.g., Edmonds et al 2009 after 3 weeks of dosing, and Dang et al 2019 after 1 year), similar to the response we observed in our Pulse plots. In response to Press seawater additions, SRB became even more common in the community (Figure 4a), and we were able to detect overall declines in microbial diversity (Figure 1) and more extreme shifts in community composition (Figure 2).…”

“…We did not observe this pattern in our data. As methanogenesis relies on decomposition of organic matter, other studies have indicated that methanogen abundances may be influenced by carbon availability (DOC and soil organic carbon) (Yuan et al 2016; Dang et al 2019). Although previous work from our experiment observed lower methane emissions in the Press treatment, there was little variation in methane production or DOC concentration across treatments during the month of October 2016 when the samples from the current study were collected (Herbert et al 2018) suggesting that C limitation constrained the abundances of methanogens across all treatments.…”

“…However, after 40 days, communities remained more phylogenetically related to their “home” environment compared to the “away” environment, suggesting that microorganisms are resilient to short-term perturbations (Morrissey and Franklin 2015). In a one-year study, transplanting TFM soils into a mesohaline environment led to an increased abundance of sulfate reducing bacteria (Dang et al 2019). Together, these studies demonstrate that microorganisms can be sensitive to changes in salinity, but that the responses may depend on the duration of the perturbation.…”

Rare microbial taxa emerge when communities collide: freshwater and marine microbiome responses to experimental mixing

“…Water column freshwater microbial taxa are particularly sensitive to the salt stress imposed by brackish conditions (Burke andBaird 1931, Ewert andDeming 2013), whereas marine microbial taxa are more robust to brackish waters, because of wider salinity tolerance (Herlemann et al 2011). Compositional turnover is high for water (Kisand et al 2005, Shen et al 2018) and soil (Dang et al 2019) microbial communities during brackish exposure, and we expect similar turnover of the water column community with this strong environmental filter. However, the impact of novel biotic interactions from a community blending freshwater and marine microbiomes is unknown.…”