From Nano-Gels to Marine Snow: A Synthesis of Gel Formation Processes and Modeling Efforts Involved with Particle Flux in the Ocean

Abstract

Marine gels (nano-, micro-, macro-) and marine snow play important roles in regulating global and basin-scale ocean biogeochemical cycling. Exopolymeric substances (EPS) including transparent exopolymer particles (TEP) that form from nano-gel precursors are abundant materials in the ocean, accounting for an estimated 700 Gt of carbon in seawater. This supports local microbial communities that play a critical role in the cycling of carbon and other macro- and micro-elements in the ocean. Recent studies have furthered our understanding of the formation and properties of these materials, but the relationship between the microbial polymers released into the ocean and marine snow remains unclear. Recent studies suggest developing a (relatively) simple model that is tractable and related to the available data will enable us to step forward into new research by following marine snow formation under different conditions. In this review, we synthesize the chemical and physical processes. We emphasize where these connections may lead to a predictive, mechanistic understanding of the role of gels in marine snow formation and the biogeochemical functioning of the ocean.

Keywords: DOM; biopolymer self-assembly; marine microgels; marine snow; mathematical modeling; microbial loop; phytoplankton secretion; polymer networks theory; primary production.

Figure 1

A conceptual model of marine snow formation that requires the following steps: (1) microbial cell growth; (2) secretion of gels (i.e., biopolymers) aka polymeric substances from microbial cells; (3) formation of exopolymeric substances (EPS) which have a variety of forms including TEP and CSP; (4) reversible formation of nano-gels; (5) reversible formation of micro-gels; (5) reversible or irreversible formation of macro-gels; (6) apparent stickiness of particle population dependent on their protein content, i.e., their protein-to-carbohydrate (P/C) ratio; (7) irreversible chemical crosslinking of proteins in gels to form marine snow through hydrophobic or reactive oxygen species (ROS) mediated chemical crosslinking; (8) UV oxidation; (9) interactions of mineral surfaces with gels or marine snow; and (10) aggregation-disaggregation/fragmentation rates. Nanogels (100–150 nm) < microgels (~5 µm) < macrogels (100 µm) < marine snow (>500 µm to 10s of cm) occur on a size continuum.