High-throughput sequencing identifies degraders of dDNA. Seasonal-scale dynamics of dissolved deoxyribonucleic acid (dDNA) and active bacterioplankton community response to dDNA in surface coastal waters

Seawater contains cell-free dissolved DNA (dDNA) that is a potential source of carbon, nitrogen and phosphorus for heterotrophic bacterioplankton. Previous studies have shown that seawater dDNA concentrations can have seasonal-scale cycles that are potentially linked to primary production. This study characterised the seasonal variation in dDNA concentrations at a coastal marine sampling station in the Western English Channel, linking changes in concentration to cognate physicochemical and biological factors. We show that there is variation in the dDNA concentration that is linked to the different prevailing phytoplankton groups, specifically dinoflagellates. The impact of phytoplankton-derived dDNA on active bacterioplankton communities was also assessed in dDNA amended seawater microcosms using 16S rRNA high-throughput sequencing and RNA Stable Isotope Probing using 13C- labelled dDNA. Rhodobacterales increased in abundance in response to dDNA additions confirming previous studies that suggest that the major bacterioplankton taxa are able to utilise dDNA as a growth substrate. 13C- labelled dDNA uptake showed that two Rhodobacterales taxa were able to assimilate labelled carbon. This study shows that coastal seawater dDNA concentrations are dynamic, with variations linked to specific high-DNA content phytoplankton. We also show dDNA is a source of dissolved organic nutrients for bacterioplankton members of the Rhodobacterales and potentially other bacterioplankton groups. This study represents a change in our understanding of organic nutrient cycling in coastal watess and it highlights that nucleic acids are not a refractory stable pool within coastal systems, and instead a dynamically changing and readily utilzied source of DOC, DON and DOP for bacterioplankton, particularly during the summer when nutrients are limiting.