Interstitial microbial communities of coastal sediments are dominated by Nanoarchaeota

Microbial communities in subsurface coastal sediments are highly diverse and play an important role in nutrient cycling. While the major fraction of microorganisms in sandy sediments live as epipsammon (attached to sand grains), only a small fraction thrives in the interstitial porewaters. So far, little is known about the composition of these free-living microbial communities. In this study, we compared the archaeal and bacterial community structures within coastal sediments and corresponding porewaters by 16S rRNA gene sequencing. We found that the free-living prokaryotes only had a ratio of about 0.2 to 2.3 % of the bulk communities, depending on the pore space. The interstitial communities were more diverse than the respective sediment-attached fraction and showed a much higher archaea-to-bacteria ratio. In the subsurface of the sandy beach investigated in this study, the archaea were mainly affiliated to Nanoarchaeota of the DPANN superphylum, with a relative abundance up to 50 % of the interstitial communities. The bacterial fraction included several species related to the Candidate Phyla Radiation (CPR). Both prokaryotic lineages are known to have small cell sizes, comprising not-yet cultured species with unidentified metabolic functions. Our findings were supported by the investigation of an adjacent tidal flat, showing a similar trend. Thus, our results indicate the presence of distinct interstitial microbial communities in the subsurface of coastal sediments. This natural enrichment of not-yet cultured Nanoarchaeota and members of the CPR provides the opportunity for targeted metagenomic analyses or even isolating members of these groups for further metabolic characterization.