Bacterial Vesicles in Marine Ecosystems

Vesicles are known to be released from many heterotrophic organisms, gram negative bacteria release vesicles containing lipids, DNA, RNA and proteins during growth. Though the mechanism for this are not yet understood, though it can be influenced by many external factors. Vesicles have been shown to influence model organisms in many ways, but research up to this point has not uncovered their functions and abundance in marine ecosystems.

Biller et al. (2014) discovered what they deem to be a vesicle produced from the photoautotroph Prochlorochoccus  (and a strain of Synechoccus), and confirmed this finding using Negative stain and thin section micrographs and independently with nanoparticle tracking analysis of unperturbed cultures.

Prochlorochoccus are dominant photoautotrophs world wide with a global population of ~1027 cells and are the most abundant photoautotroph in ogliotrophic oceans (Synechoccus is the second).

Biller et al.’s experiments with the vesicles found that vesicles were produced during light and dark periods, with production consistent with other vesicle producing microbes. And the initial data predicts Prochlorochoccus produces 10^27 – 10^28 vesicles per day, though this is likely an overestimation.

Similarly to other vesicles Prochlorochoccuss’ contain lipopolysaccharides as well as normal cyanonbacterial lipids. They have rigid membranes, many proteins (though not known if all are functional), and importantly DNA.

Bacterial membrane vesicle release may account in part, for the relative abundance of membrane proteins among all dissolved proteins in seawater.

Supporting their findings with former field and lab data, observations of vesicle structures from coastal and ogliotrophic (Sagrasso sea) sites were classified as similar vesicles, though the origin was not confirmed through further analysis.

In the Sargasso Sea such vesicles were also found in and below the euphotic zone. Mechanisms dictating their distribution are unknown, but as vesicles and bacterial abundance correlate, they lean to the likelihood of microbial origin.

Prochlorochoccus are thought to produce large fractions of dissolved organic carbon (DOC) for oligotrophic regions, and these vesicles may be a contributing mechanism. Lower estimations of the initial data from this study predict Prochlorochoccus  supplies 10^4 – 10^5 tonnes of fixed C exported into the oceans per day. Also vesicle lipids have structural similarities to, and may account for a proportion of lipid DOC.

Biller et al. (2014) examined whether the Prochlorochoccus vesicles could support heterotrophic bacteria in culture and found that Alteromonas and Halomonas could both grow in sea water supplemented by Prochlorochoccus vesicles as the only carbon source. They contain about 1/100^th of a cells carbon and likely influence carbon movements in the microbial food web. The presence of proteins and nucleic acids infer that they influence N & P cycles as well.

Prochlorochoccus has adapted to have very low phosphorus and nitrogen demands, though why an organism would supply vital materials in harsh oligotrophic environments is puzzling. Vesicle functions could be that the growth of Prochlorochoccus is positively influenced by the presence of heterotrophs,, so supporting them increases Prochlorochoccus fitness. Vesicles could be diversions for cyanophages – their use as such confirmed by Biller et al.s’ experiments.

As a purpose or side affect, they will also influence microbes through horizontal gene transfer. Vesicles have been shown to facilitate horizontal gene transfer before in Escherichia coli and Acinetobacter. Metagenomics of “wild vesicles found DNA pool of significant homology to 33 species phyla from the 3 domains. These sequences could reflect the export of prophage sequences within vesicles or DNA arising from phage infection of vesicles in the field. These vesicles could even assist microbes by providing a reactive surface!

It seems most likely that photoautotroph vesicles are important in many biotic and abiotic microbial processes in the sparse environment of ogliotrophic seas. I look forward to further research on this subject, and how this will fit in and help shape our knowledge of ocean processes.

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.