<p>Submarine groundwater discharge (SGD) is a globally important process</p><p>supplying nutrients and trace elements to the coastal environment, thus</p><p>playing a pivotal role in sustaining marine primary productivity. Along with</p><p>nutrients, groundwater also contains allochthonous microbes that are</p><p>discharged from the terrestrial subsurface into the sea. Currently, little is</p><p>known about the interactions between groundwater-borne and coastal</p><p>seawater microbial populations, and groundwater microbes' role upon</p><p>introduction to coastal seawater populations. In the current study we</p><p>investigated seawater microbial abundance, activity and diversity in a site</p><p>strongly influenced by SGD. In addition, through laboratory-controlled</p><p>bottle incubations, we mimicked different mixing scenarios between</p><p>groundwater and seawater. Our results demonstrate that the addition of</p><p>0.1 μm filtered groundwater stimulated heterotrophic activity and</p><p>increased microbial abundance compared to control coastal seawater,</p><p>whereas 0.22 μm filtration treatments induced primary productivity and</p><p>Synechococcus growth. 16S rRNA gene sequencing showed a strong</p><p>shift from a SAR11-rich community in the control samples to</p><p>Rhodobacteraceae dominance in the <0.1 μm treatment, in agreement</p><p>with Rhodobacteraceae enrichment in the SGD field site. These results</p><p>suggest that microbes delivered by SGD may affect the abundance,</p><p>activity and diversity of intrinsic microbes in coastal seawater, highlighting</p><p>the cryptic interplay between groundwater and seawater microbes in</p><p>coastal environments, which has important implications for carbon</p><p>cycling.</p>
