Calcium and iron promote reversible self‑assembly of dissolved organic matter into particles

Abstract

Dissolved organic matter (DOM) consists of dissolved molecules, biopolymers, and aggregates with a broad range of molecular sizes. In freshwater and seawater environments, spontaneous self-assembly of DOM forms hydrated particulate organic matter (POM) networks. This conversion from DOM to POM affects carbon transfer through microbial and particle food webs and export to sediments. Particle food webs are based on the direct POM ingestion by zooplankton species. This DOM assembly occurs when the inter-polymer or inter-colloid distances allow chemical (covalent) or physical (e.g., electrostatic, hydrogen) bonds. Here we explore the underlying mechanisms of self-assembly using lake waters with different concentrations of polyanionic DOM with humic content and calcium (Ca) and iron (Fe) crosslinking. We experimentally adjusted the cations by chelating and/or increasing the concentration of calcium or iron. To monitor the self-assembly of DOM, we employed homodynamic laser scattering. Results indicate that DOM self-assembly and physical gel-particle formation result from low-energy Ca+2 and Fe+3 counterion bonding. It can be readily reversed by Ca and Fe chelators, resulting in the disassembly of the network and dispersion of DOM polymers. Calcium cations appear to promote a higher level of self-assembly, reaching larger hydrodynamic sizes during stabilization, compared to iron cations. Our results indicate that the chemical environmental context critically affects the formation of POM from DOM, influencing ecosystem processes such as carbon sedimentation and storage, and providing alternative pathways for heterotrophic consumers (i.e., food webs based on particles).