Using biomarkers to trace sources, sinks, reactivity, and budgets of dissolved organic matter in aquatic systems  Major vegetation around the world is on the move as global climatic patterns shift, altering terrestrial sources of dissolved organic matter (DOM) entering water from rivers to the ocean. Changes in the sources of DOM will affect its bioavailability to native aquatic microbial communities. In Harfmann et al. (2019), despite unique initial optical dissolved organic matter signatures, vascular plant leachates lose their distinctive chemical characteristics (as indicated by optical proxies and biomarkers trends) upon microbial and combined microbial-photochemical degradation. In freshwater or estuarine systems, observed DOM compositional convergence may act as a natural buffer to provide stability of aquatic DOM cycling in the face of future landscape changes in vegetation inputs.

Role of organic biopolymers in the fractionation and scavenging of radionuclides in aquatic systems  Natural radionuclides, e.g., thorium and protactinium isotopes, have long been used as tracers of oceanic processes. In particular, the ratio of the two longer-lived 231Pa and 230Th, have been used to assess boundary scavenging, ocean circulation, and paleo-productivity. Nonetheless, different radionuclides ratios in suspended or sinking particles can vary as a function of chemical composition, location, depth, size, and etc. Many field studies have been shown that the bulk organic content in the particles is not a good predictor for the scavenging, even though most likely, sorption to biopolymers has the potential to control the extent of radionuclide scavenging. This is likely due to the fact that these biopolymers are minor components in the particle flux, and co-produced with biomineral phase in the organisms, e.g., biogenic silica and CaCO3 shells, thus hiding their role when one only determines major components in particle assemblage. Evidenced from our field data in Chuang et al. (2013, 2015a), hydroxamate siderophoric moieties are major classes of biopolymers that have a role in binding Th, Pa and Po radionuclides in the sinking particles as well as in the colloidal organic matters; CaCO3 and opal (Si) are important in predicting removal and fractionation of Th and Be in the ocean. Consistently, our laboratory data strongly support that biopolymers in diatom frustules of Phaedoctylum triconutum greatly enhance Kd values of 7Be, 234Th, 233Pa and 210Po (Chuang et al., 2014, 2015b).