Amphiphilic exopolymers from Sagittula stellata induce DOM self-assembly and formation of marine microgels

The reversible self-assembly of dissolved organic matter (DOM) yields Ca-bonded microscopic gels containing an estimated one thousand step increase of organic matter concentration compared to bulk seawater. Field studies indicate that Ca-bonded microgels concentration in seawater range from 106 to 1012 microgels × L− 1 reaching a corresponding estimated global mass of 1–100 gigatons (Gt) of organic matter. Although this huge gel pool has far reaching implications for the cycling of carbon and other elements in the World Ocean it still remains largely unexplored. A critical pending question is the role of crosslinkers other than Ca-bonds in DOM assembly. Marine bacteria release amphiphilic exopolymer substances (EPS) that are essential for attachment and that could serve as models to investigate if hydrophobic bonds could also be involved in DOM network formation. Here we show that DOM assembly can be readily induced by nanomolar concentrations (20 µg × L− 1) of hydrophobic exopolymer released by Sagittula stellata (SEP). Consistent with previous studies on hydrophobic properties of SEP our results indicate that SEP-induced DOM network formation exhibit characteristic features of hydrophobic interactions. Although the significance of gel formation by bacterial exopolymer in global carbon balance remains unknown, it offers intriguing hints about foraging strategies of marine bacteria. Bacterial exopolymer could be vital for their survival in oligotrophic environments often containing only micromolar levels of substrate. Release of minute quantities of exopolymer may facilitate the capture and concentration of substrate by forming nutrient-rich DOM networks in the bacteria immediate neighborhood. These studies complement and give further support to the hypothesis that low energy physical interactions could play a pivotal role in DOM assembly further emphasizing the urgent need to investigate the mechanism underlying DOM/gel mass transfer in carbon flux dynamics.