The Role of Calcium in the Formation of Membrane Vesicles on Cyanobacteria Surfaces

Recent investigations have led to the conclusion that membrane vesicles (MVs) are a common feature of the matrix of gram-negative bacterial biofilms. Observations of biofilms from a variety of natural and artificial environments demonstrate the ubiquitous distribution of membrane vesicles. Their purpose is still not fully understood but significant progress in understanding their function has been made. For example, experiments with planktonic biofilms demonstrate that MVs bind antibiotics and increase the sorption capacity of biofilms. Because MVs mimic the bacterial cell surface, one can speculate that they have strong, adhesive properties, possibly with specific adhesions for attachment. It has also been shown that environmental conditions play a role in the expression of outer membrane of Helicobacter pyroli. However, in spite of the intense effort to understand the role of MV to biofilms properties, our knowledge of the factors responsible for their formations needs improvement.

What is the impact of calcium on membrane vesicles formation by cyanobacteria? We have performed experiments with three different strains of cyanobacteria, i.e., Synechococcus-type: Synechoccocus elongates, Synechococcus Green, containing phycocyanin, and Synechococcus Red, containing phycoerythrin. One set of samples were kept in the dark and the other was incubated under day- and night-light conditions. The surface topography of cyanobacteria strains were studied with super-resolution vertical scanning interferometry (SR-VSI), an imaging technique that resolves sample surface features as small as 50 nm. Our results show a  significant impact by calcium (0.6-4 mM) on the formation of MVs by Synechoccocus Green cells under light and dark conditions. In Synechoccocus elongates, MVs formed at concentrations > 1.5 mM. By comparison, cell surfaces of Synechococcus Red were not strongly impacted by dissolved calcium. Our results suggest that MV formation is strain-specific and triggered by environmental calcium concentrations.