The Fate of Marine Bacterial Exopolysaccharide in Natural Marine Microbial Communities

Abstract

Most marine bacteria produce exopolysaccharides (EPS), and bacterial EPS represent an important source of dissolved organic carbon in marine ecosystems. It was proposed that bacterial EPS rich in uronic acid is resistant to mineralization by microbes and thus has a long residence time in global oceans. To confirm this hypothesis, bacterial EPS rich in galacturonic acid was isolated from Alteromonas sp. JL2810. The EPS was used to amend natural seawater to investigate the bioavailability of this EPS by native populations, in the presence and absence of ammonium and phosphate amendment. The data indicated that the bacterial EPS could not be completely consumed during the cultivation period and that the bioavailability of EPS was not only determined by its intrinsic properties, but was also determined by other factors such as the availability of inorganic nutrients. During the experiment, the humic-like component of fluorescent dissolved organic matter (FDOM) was freshly produced. Bacterial community structure analysis indicated that the class Flavobacteria of the phylum Bacteroidetes was the major contributor for the utilization of EPS. This report is the first to indicate that Flavobacteria are a major contributor to bacterial EPS degradation. The fraction of EPS that could not be completely utilized and the FDOM (e.g., humic acid-like substances) produced de novo may be refractory and may contribute to the carbon storage in the oceans.

Fig 1. EPS production by JL2810.

Time course of bacterial growth (OD600), EPS production, and Glc consumption during the cultivation of JL2810.

Fig 2. Effect of EPS and Glc on the microbial growth in seawater.

(A) Growth curves. (B) Consumption of sugar during microbial cultivation.

Fig 3. Three fluorescence components identified by the PARAFAC model.

(A) Fluorescence graphs of the three components (component 1–3). (B) Fluorescence intensities of the three components after cultivation of seawater for 14 days.

Fig 4. Bacterial community composition analysis.

Phylogenetic compositions of the bacteria in the 2-d (2D) and 14-d (14D) cultures were analyzed. The relative abundance of each class is expressed as the mean percentage of total sequences obtained in the treatment. The major classes (A) and genus belonging to Flavobacteria class (B) and Gammaproteobacteria class (C) were indicated.