Allochthonous groundwater microorganisms affect coastal seawater microbial abundance, activity and diversity

Abstract

AbstractSubmarine 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-situobservations), 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 andSynechococcusgrowth. Amplicon sequencing of the 16S rRNA gene showed a strong shift from a SAR11-rich community in the reference SGD-unaffected coastal samples to aRhodobacteraceae-dominated one in the <0.1 µm treatment, in agreement with theirin-situenrichment 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 and climate.Key Points:Groundwater discharge into the coastal zone delivers both nutrients and allochthonous microbes.Groundwater microbes interact with seawater populations, by which affecting the delicate autotroph-heterotroph balance.Subterranean microbial processes are key drivers of food webs, potentially affecting biogenic carbon fluxes in the ocean and climate.