Chemical and biological information in dissolved organic matter across the land-ocean continuum

Biogeochemical cycling of organic matter across the land-ocean continuum (LOC) is important in driving energy and material exchange between the fresh water and saline water ecosystems, including the direction and magnitude of the uptake of carbon dioxide. The “reactivity” of dissolved organic matter (DOM) can range from highly bioavailable (labile) to hardly bioavailable (recalcitrant). Originated in soil biogeochemistry, “priming” refers to when something (nutrients such as inorganic N and P) is added to soil, it affects the rate of decomposition of the soil organic matter (SOM). This concept has recently been introduced to OM cycling across the LOC, for example, the “recalcitrant” DOM can become more bioavailable in coastal waters affected by high anthropogenic inputs of inorganic N and P. This study examines a set of surface fresh water, groundwater and coastal water samples in Pearl River Delta region to understand how biological information (eDNA, usually known with a short degradation time) may be interpreted in the context of still unclear mechanisms of how fast dissolved organic matter degrades in the environment, which we also characterize at molecular level. Metagenomics analysis of eDNA samples collected from Dapeng Bay, Shenzhen has revealed a high range of diverse antibiotic resistance genes (ARG), as well as over 100 genera of eukaryotes. In this highly anthropogenically affected Bay with elevated inorganic N and P inputs by a local stream, ARGs and human pathogens are abundant in the influent of a waste water treatment plant, though a large number of them were efficiently removed with the combination of a wastewater treatment plant and an engineered wetland. However, some ARGs and human pathogens persisted in fresh water downstream of the WWTP, in sea water collected at the beach and in the bay. It is worth noting that the groundwater collected at the beach exhibited the lowest abundance of ARGs and human pathogens. However, minor traces of sulfonamide (a group of semi persistent antibiotics) resistance genes were detected. This study hopes to shed light on whether the highly labile eDNA can be used as a biomonitoring tool or not across the LOC. This will likely depend on the understanding of how the sources and reactivity of DOM affect eDNA degradation.