Global change is occurring at an ever increasing rate, and there is a pressing need to understand how ecosystems will respond across a variety of scales. Microbial diversity and ecosystem function are inextricably linked, therefore the study of how both taxonomic and functional properties of microbial communities change along environmental gradients may help us better understand the underlying mechanisms that drive this relationship. This is because ecosystem function is driven by biogeochemical processes that are regulated by specific microbial functional groups (e.g., nitrate-, sulfate- reducers and methanogens). The above approach was applied in tidal wetlands that are prone to salinization due to sea-level rise.
Soil microbial communities were sampled from five tidal wetlands in Pamunkey River, a tributary to the York River Estuary in lower Chesapeake Bay (Virginia, USA), along an estuarine salinity gradient (0.1-10 ppt). Basic environmental data were collected (e.g., soil organic matter content, porewater chemistry, pH, etc.) and communities were assessed using 16S tag sequencing. Functional profiles were generated using PICRUSt modeling and analyzed using a distance-decay approach. The relative importance of spatial separation distance compared to environmental heterogeneity was assessed using Mantel tests.
Our results show that both 16S and functional genes are likely dispersal limited, but that environmental conditions can have an additional selective effect. Salinity was an especially strong driver, and correlated with a decrease in richness within all functional groups. Decay rates (β-diversity) were significantly greater for phylotypes associated with DNRA (nrfA), sulfate reduction (dsrA), and methanogenesis (ascD & mrcA), compared to denitrification (nirK) and 16S. Overall changes in phylogenetic community structure across sites were roughly concordant with changes in the composition of each functional group.
The observed changes in functional group diversity and abundance suggest that increased salinity in these soils may lead to significant pathway shifts in ecosystem process rates (e.g., organic matter mineralization). For some functional groups, a modest increase in salinity of ≤ 3 ppt was enough to reduce diversity by 50%. In contrast, similar reductions in 16S diversity required a salinity change of ~13 ppt. These results highlight the importance of considering both taxonomic and functional group diversity for understanding the effects of environmental change in ecosystems.