Sea levels rise as Earth warms, and with that comes a less obvious threat to freshwater ecosystems: saltier water. MIT researchers report that increasing salinity can reshape the microbial communities that drive key biogeochemical processes in rivers, estuaries, and coastal-adjacent waters.
Microbes are central to the carbon cycle, including the decomposition of organic matter such as algal biomass. In this study, the team asked what happens to these communities when they experience salt concentrations that mimic seawater intrusion.
Using microbial samples from three environments spanning a wide salinity gradient—including the Charles River (4 g/L), Boston Harbor (30 g/L), and a Massachusetts beach near Nahant (35 g/L)—the researchers cultivated each community across three new salinity conditions (16, 31, and 46 g/L) for two weeks.
Across all conditions, the communities sustained roughly the same overall growth rate, suggesting that total biomass accumulation can remain surprisingly resilient. However, community composition shifted sharply, with higher-salinity exposures reducing diversity as a subset of strains grew faster and took over.
The results indicate a classic ecological tradeoff: salinity stress may not immediately suppress growth, yet it can reorganize the ecosystem toward fewer, more competitive microbial types. “At higher salinity, you lose diversity,” the lead author Jana Huisman notes, while emphasizing the unexpected stability of growth and biomass production.
To test whether the laboratory pattern matches real-world systems, the researchers analyzed publicly available genomic datasets from aquatic environments such as the Chesapeake Bay, Gulf of Mexico, and Baltic Sea. They used the 16S rRNA gene copy number as a proxy for intrinsic maximum growth potential.
In these natural communities, higher-salinity habitats also tended to be dominated by faster-growing species, mirroring the lab findings and strengthening the case that salinity-driven selection is a recurring ecological force.
The study also raises concerns about downstream vulnerability. If diversity declines, microbial communities may become less able to withstand additional environmental pressures, even if their short-term growth rate appears intact.
Finally, the researchers did not map the specific functional roles of the strains that expand under salt stress. Some fast growers could enhance ecosystem performance, while others might include harmful pathogens—an open question for future work.
