Marine colloids, agents of the self-cleansing capacity of aquatic systems: Historical perspective and new discoveries
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2018 The Author Colloids in the ocean are mainly composed of natural organic matter (NOM), with trace amounts of metals, metalloids, radionuclides, pollutants, and variable amounts of anthropogenically produced engineered nanoparticles and nanoplastics. State-of-the-art sampling and instrumental techniques were required to unravel both size spectra and chemical composition. Chemical components of colloidal macromolecular organic matter (COM) include aquagenic substances (composed of microbially produced Exopolymeric Substances (EPS), gels, transparent exopolymer particles (TEP) and TEP precursors), and pedogenic substances (mostly humic and fulvic matter). COM can be bioavailable and biodegradable. EPS in aquatic systems can form flocs of Marine Snow (MS) or Marine Oil Snow (MOS). EPS is thus important in initiating and maintaining aggregation, flocculation, and sedimentation processes of particles and microorganisms that act also as sorbents of pollutants, a process that contributes in a major way to the 'self-cleansing capacity of aquatic systems. COM contains a diverse array of functionalities which can make them good metal chelating agents, amphiphilic and surface active pH-sensitive and redox-active. Due to numerous electrostatic and hydrophobic interactions between different moieties and ligand groups within a macromolecule, equilibrium constants, Ki, will become distribution functions for most reactions rather than fixed constants, as is common for low molecular weight organic molecules. Colloidal pumping of trace substances bound to EPS and other reactive macromolecular NOM compounds is counteracting the prevailing pathway of degradation of the bulk macromolecular NOM. Due to strong sorption capabilities for stable and radioactive metal ions, particle- and colloid-reactive radionuclides are useful to trace particle and organic carbon fluxes in the ocean. Open research questions include the role of different mechanisms that affect size spectra and composition of EPS in response to changing conditions, the role of macromolecular organic carrier molecules of radioactive and other trace substances that are used as oceanographic tracers; the potential use of the protein/carbohydrate ratio as a predictor of colloid or particle stickiness, aggregation or emulsification processes, and the importance of reactive oxygen species (ROS) mediated polymerization reactions of proteins and other molecules, which are part of EPS, for MS and MOS formation.
