Chemical composition and 234Th (IV) binding of extracellular polymeric substances (EPS) produced by the marine diatom Amphora sp.

Abstract

In order to chemically characterize strongly Th (IV)-binding exopolymeric substances (EPS) from a single organism, “particulate” (i.e., p, attached, or capsular) and “dissolved” (i.e., d, non-attached) EPS from a lab culture of the marine diatom Amphora sp. were isolated by centrifugation, followed by alcohol precipitation or ultrafiltration. Both dissoved EPS and particulate EPS were mainly composed of carbohydrates, with only a small fraction of proteins. Individual fractions that were further separated by anion exchange (AEC) contained EPS with significantly different compositional characteristics and molecular weights. The particulate fraction was composed of two different glucans, p1 and p2, but with the same molecular weight of 25 kDa, while more than 90% of the dissolved fraction was composed of two different acidic polysaccharides, f1 and f2, with a similar molecular weight of 1000 kDa While both f1 and f2 fractions contained the neutral monosaccharides fucose and galactose, as well as glucuronic acid, they were in a different mole ratio: f1 in a ratio of 1:1.1:1.6, and f2 as 1:0.8:2.8. In addition to glucuronic acid, both f1 and f2 fractions contained relatively high concentrations of sulfated polysaccharides, with a sulfate content of 9.7% in f1, and 18.2% in f2. The difference between total acid polysaccharide concentrations, as determined by the Alcian blue method, and the actual concentrations of EPS can be explained by the relative amounts of sulfate and glucuronic acid. Dissolved EPS, and fractions f1 and f2 that were labeled with 234Th (IV), all showed peaks at isoelectric points (pH IEP) of about pH 3 during isoelectric focusing, indicating that Th (IV) binding by EPS was dominated by the acidic polysaccharides in f1 and f2. The strong binding of 234Th (IV) to these acidic polysaccharide-rich EPS compounds enables us to locate and closely look at the agents who are responsible for binding of 234Th (IV), which is relevant for a better understanding of the oceanographic applications of POC/ 234Th ratios to particle and organic carbon dynamics in marine systems.