Investigation of the role of cell hydrophobicity and EPS production in the aggregation of the marine diatom Cylindrotheca closterium under hypo-saline conditions.

Aggregation of diatoms is of global importance to understand settling of particulate organic carbon in aquatic systems. In this study, we investigate the aggregation of the marine diatom Cylindrotheca closterium during the exponential growth phase under hypo-saline conditions. The results of the flocculation/flotation experiments show that the aggregation of the diatom depends on the salinity. In favorable growth conditions for marine diatoms (salinity of 35), the highest aggregation is achieved. To explain these observations, we used a surface approach combining atomic force microscopy (AFM) and electrochemical methods to characterize both the cell surface properties and the structure of the extracellular polymeric substances (EPS) cell produce, and to quantify the amount of surface-active organic matter released. At a salinity of 35, the results showed that diatoms are soft, hydrophobic and release only small amounts of EPS organized into individual short fibrils. In contrast, diatoms adapt to a salinity of 5 by becoming much stiffer and more hydrophilic, producing larger amounts of EPS that structurally form an EPS network. Both adaptation responses of diatoms, the hydrophobic properties of diatoms and the release of EPS, appear to play an important role in diatom aggregation and explain the behavior observed at different salinities. This biophysical study provides important evidence allowing to get a deep insight into diatom interactions at the nanoscale, which may contribute to a better understanding of large-scale aggregation phenomena in aquatic systems.

Reconstructed membrane vesicles from the microalga Dunaliella as a potential drug delivery system.

The aim of this biophysical study is to characterize reconstructed membrane vesicles obtained from microalgae in terms of their morphology, properties, composition, and ability to transport a model drug. The reconstructed vesicles were either emptied or non-emptied and exhibited a non-uniform distribution of spherical surface structures that could be associated with surface coat proteins, while in between there were pore-like structures of up to 10 nm that could contribute to permeability. The reconstructed vesicles were very soft and hydrophilic, which could be attributed to their composition. The vesicles were rich in proteins and were mostly derived from the cytoplasm and chloroplasts. We demonstrated that all lipid classes of D. tertiolecta are involved in the formation of the reconstructed membrane vesicles, where they play fundamental role to maintain the vesicle structure. The vesicles appeared to be permeable to calcein, impermeable to FITC-ovalbumin, and semipermeable to FITC-concanavalin A, which may be due to a specific surface interaction with glucose/mannose units that could serve as a basis for the development of drug carriers. Finally, the reconstructed membrane vesicles could pave a new way as sustainable and environmentally friendly marine bioinspired carriers and serve for studies on microtransport of materials and membrane-related processes contributing to advances in life sciences and biotechnology.