Polymer networks produced by marine diatoms in the northern Adriatic sea.

Using high resolution molecular technique of atomic force microscopy, we address the extracellular polymer production of Adriatic diatom Cylindrotheca closterium analyzed at the single cell level and the supramolecular organization of gel phase isolated from the Northern Adriatic macroaggregates. Our results revealed that extracellular polysaccharides freshly produced by marine diatoms can self-assemble directly to form gel network characteristics of the macroscopic gel phase in the natural aquatorium. Based on the experiments performed with isolated polysaccharide fractions of C. closterium and of macroaggregates gel phase, we demonstrated that the polysaccharide self-assembly into gel network can proceed independent of any bacterial mediation or interaction with inorganic particles.

AFM imaging of extracellular polymer release by marine diatom Cylindrotheca closterium (Ehrenberg) Reiman & J.C. Lewin.

Extracellular polysaccharide production by marine diatoms is a significant route by which photosynthetically produced organic carbon enters the trophic web and may influence the physical environment in the sea. This study highlights the capacity of atomic force microscopy (AFM) for investigating diatom extracellular polysaccharides with a subnanometer resolution. Here we address a ubiquitous marine diatom Cylindrotheca closterium, isolated from the northern Adriatic Sea, and its extracellular polymeric substance (EPS) at a single cell level. We applied a simple procedure for AFM imaging of diatom cells on mica under ambient conditions (in air) to achieve visualization of their EPS with molecular resolution. The EPS represents a web of polysaccharide fibrils with two types of cross-linking: fibrils association forming junction zones and fibril-globule interconnections with globules connecting two or more fibrils. The fibril heights were 0.4-2.6 nm while globules height was in the range of 3-12 nm. Polymer networks of native gel samples from the Northern Adriatic and the network formed by polysaccharides extracted from the C. closterium culture share the same features regarding the fibril heights, pore openings and the mode of fibril association, proving that the macroscopic gel phase in the Northern Adriatic can be formed directly by the self-assembly of diatom released polysaccharide fibrils.

Biophysical scenario of giant gel formation in the northern Adriatic sea.

The macroscopic gel phase (mucilage) appears episodically in the northern Adriatic Sea covering tens of square kilometers of sea surface. Current views leave no doubt on phytoplankton production as a proximal source of polymers constituting the gel network, but the mechanism leading to its rapid production remains unknown. We introduced electrochemical sensing of marine microparticles and atomic force microscopy to image supramolecular organization of marine gel network. Our biophysical scenario of mucilage event features self-organization of biopolymers into microparticles ("marine vesicles") that under specific conditions transform to giant gel by a fast vesicle-to-gel transition.