Spatial Partitioning of Microbial Functional Potential Suggests Complex Nitrogen Cycling in a Subterranean Estuary

Area of Science:

• Environmental Microbiology
• Geochemistry
• Coastal Ecology

Background:

• Subterranean estuaries (STEs) act as critical bioreactors for groundwater-derived nitrogen (N) entering coastal zones.
• Microbial regulation of N-cycling in these groundwater-seawater interfaces is not well understood.

Purpose of the Study:

• To investigate spatio-temporal variations in microbial communities and N-cycling potential in a Mediterranean STE.
• To understand how microbial abundance, activity, and N-cycling genes change with location and season.

Main Methods:

• Collected groundwater samples at various depths and distances from the shore during winter and summer.
• Analyzed microbial abundance, heterotrophic activity, taxonomic composition, and N-cycling gene abundance.
• Utilized quantitative PCR and multiple linear modeling to assess microbial and environmental influences.

Main Results:

• Microbial abundance, activity, and diversity showed strong spatial variation across hydrochemical zones and aquifer depths, with limited seasonal differences.
• Functional gene predictions indicated complex, spatially structured N-cycling pathways.
• Niche partitioning was observed between ammonia-oxidizing archaea in freshwater zones and bacterial denitrifiers in saline zones.
• Microbial properties better predicted nitrogen concentrations (NO2-, NH4+) than environmental variables.

Conclusions:

• The STE microbiome's functional potential is complex and spatially structured, driving N-cycling variations.
• Microbial communities, rather than solely environmental factors, are key drivers of nitrogen cycling in subterranean estuaries.
• Understanding microbial niche partitioning is essential for predicting nitrogen dynamics in coastal ecosystems.