Some bacteria, such as Prochlorococcus and Synechococcus – more commonly known as blue-green algae, can produce and release thin-walled membrane sacs, known as vesicles. Since they are two of the most important contributors to CO2 fixation on Earth, vesicle release could play an important role in carbon cycles. Yet, studies have been mostly limited to observations in cultures. The Marie Skłodowska-Curie Actions supported VESYNECH project has gained insights into the ecology and function of Synechococcus vesicles, one of the most abundant forms of cyanobacteria in the oceans. “We got amazing results, specifically about the effect of both light, and nitrogen and phosphorus limitation, on both the release and content of vesicles,” says José Manuel García Fernández from the University of Cordoba, the project host.
It is thought that in other bacteria, vesicles perform diverse functions including promoting the growth of ‘helper’ bacteria, reducing viral attacks on cells or even as vehicles to transfer genes. To understand why and how these vesicles are produced, the first objective was to characterise them in cultures of Synechococcus after being exposed to different stressors. Cultures of Synechococcus were grown for several days under the stress conditions of light shock and nitrogen and phosphorus starvation. Samples were collected for flow cytometry to measure cell abundance, for nanoparticle tracking analysis to measure vesicle concentration, and for absorbance to measure the growth curve. Vesicles were also isolated, concentrated and studied with transmission electron microscopy before culture death, to verify that what was being measured was indeed vesicles and not other nanoparticles like viruses. Protein concentration and composition were also studied. The team demonstrated that light shock strongly stimulates vesiculation. Furthermore, it more than doubles the protein concentration inside vesicles, with some involved in stress response and photosynthesis. “These results provide evidence that membrane vesicles might help bacteria to survive stressful conditions,” explains García Fernández. The team also demonstrated that vesiculation increases during phosphorus starvation. However, nitrogen starvation limits vesicles and decreases their protein concentration. “This makes sense since nitrogen is considered the primary nutrient crucial to cyanobacteria growth, and as Synechococcus is abundant in nutrient-poor open oceans, shedding nitrogen-rich vesicles would be wasteful,” adds research fellow María del Carmen Muñoz Marín.
A vital phytoplankton community
VESYNECH provides a better understanding of the phytoplankton community’s metabolism and also how marine phytoplankton will be affected by different nutrient and light stresses caused by climate change. VESYNECH’s results could also support radically changing current patterns of production, consumption, recycling and waste disposal. “Cyanobacteria could be an alternative source of food and even an alternative source of energy with Synechococcus being one of the best-characterised organisms for biofuel production. Synechococcus can also biosynthesise polyhydroxyalkanoate polymers, offering an alternative source for plastic, a key problem in our oceans,” notes García Fernández. Now working as a principal investigator at the University of Cordoba, Muñoz Marín in her new project is focusing on the possible movement of genetic material between organisms. To test for this, in collaboration with researchers Ignacio Luque and Rocío López Igual from the Institute of Plant Biochemistry and Photosynthesis in Seville, Spain, she is working on three constructions with the green fluorescent protein (GFP) combined with a signal peptide to send the protein to the periplasm – a space in the membrane of cyanobacteria. “We will see if vesicles are involved in horizontal gene transfer – moving genes between organisms, but crucially also how vesiculation works at the genetic level,” says Muñoz Marín.
VESYNECH, nitrogen, cyanobacteria, cell, carbon cycles, membrane, climate change, gene transfer, protein, Synechococcus, blue-green algae, vesicles, phosphorus