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Biology

Abundant bacterial vesicles found in seawater

Biller, S. J., Schubotz, F., Roggensack, S. E., Thompson, A. W., Summons, R. E., & Chisholm, S. W. (2014). Bacterial Vesicles in Marine Ecosystems. science, 343(6167), 183-186. Link

Prochlorococcus, a marine cyanobacterium, is one of the smallest photosynthetic microorganisms (organisms that produce food and energy from sunlight).  They are also very abundant, accounting for 30 to 60% of chlorophyll in low nutrient regions of the open ocean in the mid-latitudes.  Researchers were actually working on another experiment using cultures of Prochlorococcus when they happened upon these very abundant vesicles.  Being naturally curious, they took a closer look (Fig. 1).

The vesicles contained lipids, proteins, nutrient transporters, and various other components including portions of the genome sequence.  Vesicles were continually produced during periods of growth under both light and dark conditions and production rates varied from two to five vesicles per cell per generation.

 Figure 1. (A) Scanning electron micrograph of Prochlorococcus strain MIT9313 shows the presence of numerous small spherical features (vesicles, indicated by arrows) near the cells. Scale bar, 1 μm. (B) Purified Prochlorococcus vesicles as seen by negative-stain TEM. Scale bar, 100 nm. (C) Thin-section electron micrographs confirm that Prochlorococcus vesicles are circular, membrane-enclosed features lacking notable internal structure. Scale bar, 100 nm. (D) Particle size distribution of purified Prochlorococcus vesicles as determined by nanoparticle tracking analysis. (E) Particle size analysis of an unperturbed culture confirms the presence of particles of two broad size classes consistent with whole cells and vesicles. (F) Vesicle production during growth of MIT9313, comparing the number of vesicles (open circles) with cells (squares) in the culture. Values represent means ± SD of three biological replicates. Vesicle limit of detection was ~107 ml−1.

Figure 1. (A) Scanning electron micrograph of Prochlorococcus strain MIT9313 shows the presence of numerous small spherical features (vesicles, indicated by arrows) near the cells. Scale bar, 1 μm. (B) Purified Prochlorococcus vesicles as seen by negative-stain image. Scale bar, 100 nm. (C) Thin-section electron micrographs confirm that Prochlorococcus vesicles are circular, membrane-enclosed features lacking notable internal structure. Scale bar, 100 nm. (D) Particle size distribution of purified Prochlorococcus vesicles as determined by nanoparticle tracking analysis. (E) Particle size analysis of an unperturbed culture confirms the presence of particles of two broad size classes consistent with whole cells and vesicles. (F) Vesicle production during growth of MIT9313, comparing the number of vesicles (open circles) with cells (squares) in the culture. Values represent means ± SD of three biological replicates. Vesicle limit of detection was ~107 ml−1.(Biller et al. 2014).

We’re not sure why Prochlorococcus release vesicles although there are a few running hypotheses the authors propose:

1) DNA transfer. The vesicles contained Prochlorococcus RNA and DNA fragments covering a broad range of the genome (Fig. 2).  This suggests that the vesicles may be a mechanism of horizontal gene transfer between organisms.

Figure 2. Prochlorococcus membrane vesicles contain DNA sequences covering a wide range of the MED4 chromosome position.

Figure 2. Prochlorococcus membrane vesicles contain DNA sequences covering a wide range of the MED4 chromosome position.

2) Stimulate heterotrophic bacteria growth. Prochlorococcus grows better in the vicinity of other bacteria. It relies on other bacteria to relieve them of toxins in the water.  By releasing carbon in the form of vesicles into the water, the cyanobacteria provide food for heterotrophic bacteria species to grow and break down the toxins Prochlorococcus can’t (Fig. 3a).

3) Decoys for viruses. These vesicles could serve as a decoy for viruses by principle of dilution.  With more vesicles in the water, viruses are more likely to attack vesicles than actual Prochlorococcus (Fig. 3b).

Figure 3. Potential roles for vesicles in marine ecosystems. (A) Purified Prochlorococcus vesicles can support the growth of a heterotrophic marine Alteromonas. Alteromonas growth patterns are shown in a seawater-based minimal medium supplemented with media only (control), vesicles (+vesicles), or a defined mixture of organic carbon compounds (+organic carbon mix) as the only added carbon source. Growth curves represent the mean ± SEM of three replicates. The optical density at 600 nm (OD600) increase of the “+vesicles” and “+organic carbon” trials both corresponded with a significant increase in Alteromonas cell concentration (measured by plate counts) as compared with the control after 48 hours (t test; P < 0.05). (B) Marine phage-vesicle interactions. TEM micrograph of cyanophage PHM-2 bound to a vesicle from Prochlorococcus MED4 (see also fig. S12). The shortened tail indicates that this phage has “infected” the vesicle and is therefore unable to infect a bacterial cell. Scale bar, 100 nm.

Figure 3. Potential roles for vesicles in marine ecosystems. (A) Purified Prochlorococcus vesicles can support the growth of a heterotrophic marine bacteria. Heterotroph growth patterns are shown in a seawater-based control, supplemented with vesicles (+vesicles), or a mixture of organic carbon compounds (+organic carbon mix) as the only added carbon source. Growth curves represent the mean ± SEM of three replicates. The optical density at 600 nm (OD600) increase of the “+vesicles” and “+organic carbon” trials both corresponded with a significant increase in heterotroph cells. (B) Marine virus-vesicle interactions. We see by the shortened tail of the virus that it has “infected” the vesicle rather than a bacterial cell.

Despite relatively small concentrations in seawater, the Prochlorococcus vesicles contain numerous components vital to ecosystem processes.  Prochlorococcus vesicles likely influence many of the interactions between microogranisms and their environment in mid-latitude open oceans.  Additionally, the vesicles are small enough to compose a potentially significant portion of previously documented concentrations of dissolved organic carbon.  We can expect to read and hear many exciting new developments as scientists continue to unravel the mysteries of these vesicles!

Science Magazine also released a podcast interview of the primary author, Steve Biller.

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