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