Examination of Prochlorococcus and their vesicles by transmission electron microscopy (TEM), micrographs and nanoparticle tracking, derived their structure and concentration respectively. They were found in greatest abundance during the exponential and stationary phases of growth - here they were around 10x that of Prochlorococcus cells. As stated by the authors, this represents vesicle production under lab conditions, the numbers produced naturally may vary. They hypothesised 10^27 - 10^28 to be produced per day. Responsible for the transport of molecules/substances, the contents of the vesicles were then analysed. Vesicles are supramolecular structures, consisting mainly of lipids, hence a number of lipids, fatty acids and lipopolysaccharides were identified. A diverse range of proteins and enzymes were also found following proteome analysis; nutrient transporters porins and hydrolases were among some of the identifiable proteins - are these another means by which cyano-bacteria can obtain nutrients in an oligotrophic environment? DNA and RNA were also encapsulated in these vesicles, amplification and sequencing showed that around 50% of the chromosomal sequence was bound in these vesicles. Transformation is one method of horizontal gene transfer (HGT), these vesicles may be the source of genetic material fueling HGT. Comparison to natural ocean samples was undertaken and similar findings to the lab study were found. Samples at varying depths were taken and prochlorococcal vesicles were present at each depth.
It has been hypothesised that Prochlorococcus may be responsible for the release of dissolved organic carbon (DOC) in to the oceans, owing to the diverse vesicle contents as determined by this study, a mechanism of DOC release could potentially be proposed. Additionally, the vesicles may act as a source of nitrogen and phosphorus. Heterotrophic bacteria (in culture) were successfully grown with Prochlorococcus-derived vesicles as the only carbon source, thus testing the above hypothesis. It is rather odd however, that Prochlorococcus should release these useful substances, particularly as it occupies a nutrient-poor environment. Further study revealed that these vesicles may function in defense against phages - could this be a primitive immune system?
It is evident that further study and consideration needs to be given to microbial 'organelles', as mechanisms underpinning defense, biogeochemistry and nutrient flow may not be as understood as initially thought. This study is particularly interesting from an evolutionary perspective.
Jack
It is evident that further study and consideration needs to be given to microbial 'organelles', as mechanisms underpinning defense, biogeochemistry and nutrient flow may not be as understood as initially thought. This study is particularly interesting from an evolutionary perspective.
Jack
Biller, S. J., F. Schubotz, S. E. Roggensack, A. W. Thompson, R. E. Summons, and S. W.Chisholm. (2014) Bacterial Vesicles in Marine Ecosystems, Science 343, : 183-186
Hi Jack
ReplyDeleteThanks for posting a very interesting review. I agree with your thoughts that this is fascinating from an evolutionary point of view. I wonder if these vesicles are an adaptation of Prochlorococcus living in an oligotrophic environment? Also just out of interest, the number of vesicles that are produced per day that you've given in the review, is that per cell or for the population of cells they were looking at?
Many thanks
Anita
Hi Anita - Thanks for your comment!
DeleteTaking into account the endosymbiosis theory, I wonder if these vesicles and their contents were produced, as a sort of 'payment' for the host cell? Or as I briefly mentioned, these vesicles have similar contents to some immunological cells such as neutrophils, monocytes and lymphocytes, and owing to the evolutionary significance of cyanobacteria did these form a primitive immune system?
With regards to your question: 10^27 - 10^28 were hypothesised values for the number of vesicles produced per day on a global scale, in the natural environment. They arrived at these values by deducing the rate of vesicle production of their lab-sample, which was 2-5 vesicles per cell, per generation. However, as stated in the article, these figures would likely change based on environmental conditions. They compared the number of vesicles at two sites; one nutrient-rich and one oligotrophic. Vesicles were found in greater numbers in the nutrient-rich site (6x10^6 per ml) compared to the oligotrophic (3x10^5 ml). So, I'm unsure if this is an adaption to oligotrophic waters?
Hope that answers your question! :D
Jack
Hi Jack
ReplyDeleteMany thanks for getting back to me. Yes this does answer my questions. I hadn't realized that the vesicles have morphological similarities to immunological cells, which is really interesting. The production of 2-5 vesicles per cell makes sense and sounds reasonable. The fact that more vesicles are produced an order of magnitude higher in nutrient rich waters suggests that there is another factor influencing this, such as defence against phages as you suggested. So probably not an adaptation to oligotrophic waters.
Thanks again, an interesting subject!
Anita
Hi Jack - thanks for the good review of this interesting paper. I think you may have overstretched the immune analogy a bit - I think the authors suggests a defensive role as a 'decoy' for viral infection, but I don't think you could say the vesicles behave like immune cells.
ReplyDeleteHi Colin!
DeleteAfter reading more into this area, I agree that the immunology idea is overstretched. Whilst I understand the authors thoughts on the vesicles and their potential role as a viral 'decoy', morphologically immunological cells and the vesicles describes aren't too dissimilar.
That said, after hearing and reading about the role some cyanobacterial species play in symbiotic relationships with marine microbial eukaryotes, I wonder if the extracellular vesicles play a role there?
Jack