Here’s something new under the sun… cyanobacteria shed in the ocean.
Most people are well aware of the fact that dogs and cats shed, but did you know that oceanic cyanobacteria also shed?* OK, so the cyanobacteria don’t shed hair, and you won’t need a lint brush. In a report in this week’s Science, researchers from the Chisholm lab show that cyanobacteria living in the world’s open oceans are shedding zillions of membrane vesicles. Before you accuse them of pollution, allow me to explain why they are the most important bacteria you’ve never heard of– the Prochlorococci. Don’t worry about being too late to this fountain of knowledge, scientists only discovered them about 25 years ago.
Prochlorococcus species are cyanobacteria that live in the oligotrophic ocean, areas of the world’s open oceans that are nutrient poor when it comes to sources of nitrogen, phosphorous and bioavailable forms of essential metals like iron. These bacteria dominate these otherwise ocean deserts and are possibly the Earth’s most populous species by sheer numbers of individuals. Remember about half of the world’s primary productivity comes from aquatic sources. Prochlorococci are a major contributor to global nutrient cycles and a significant number of the oxygen molecules you are constantly breathing in and out were produced by a Prochlorococcus.
You may be curious as to why scientists have only recently discovered these numerous and critically important photosynthetic organisms. The answer is because of their other trademark- their size. Measuring in with widths and lengths well under a micrometer, they are the world’s smallest photosynthesizers. Prochlorococci pass through many of the standard filters used for trapping microbes from aquatic samples. They were detected using a cell-sorting technique that could just detect their faint chlorophyll fluorescence signal. These fascinating microbes have been studied in the ocean as well as the laboratory for the last couple of decades, but it is a labor of love. Since Prochlorococci live in such a nutrient-limiting environment, they don’t have much competition. Consequently, these cyanobacteria don’t have to grow very fast, and their small size is another way of conserving their resources. In the laboratory, they don’t grow as fast as the other model cyanobacteria and their extremely small size means scientists have to wait quite a while before significant biomass is achieved for experiments.
So, it is quite a remarkable discovery that something so tiny that has toiled so diligently to fix carbon dioxide into biological molecules would just shed them as tiny vesicles into the ocean. The vesicles released by the Prochlorococci and Synechococci** are small membrane-closed vesicles that contain proteins and even small fragments of DNA and RNA. Surely, the bacteria are not just throwing away precious resources. It’s unlikely that science has stumbled upon the first eating disorder among primary producers, so the obvious question arising from this observation is why the bacteria shed these vesicles. The authors offer some hypotheses to justify this seemingly wasteful phenomenon.
It could be an example of the cyanobacteria ‘casting their bread upon the waters.’ Preliminary results suggest that- as the saying goes- they find it again. The open ocean environment is a hard place to live, but cyanobacteria are not the only organisms living there. Heterotrophic microbes are also part of the biological community and previous reports have demonstrated that they can stimulate the growth of Prochlorococcus strains. Perhaps the vesicles are a way for the cyanobacteria to feed their beneficial heterotrophic neighbors without putting their cells on the menu.
The vesicles could also serve as bacterial countermeasures against cyanophages, viruses that infect cyanobacteria. For all intents and purposes, the vesicles look like miniature bacteria complete with receptor proteins the phages use to recognize their bacterial victims. The authors were able to observe ‘fooled phages’ that had bound and injected their genetic material into vesicles. In this way, the shed vesicles would be a good defense against these oceanic pathogens. From the phage perspective, an ocean filled with zillions of vesicles, each appearing like a potential host would seem like a lottery they can’t lose. However, the cyanobacteria have ensured the odds are in their favor by shedding many, many more vesicles into the environment relative to the susceptible cells.
Finally, the vesicles could just be the clichéd ‘message in bottles’ of seafaring lore. Even an ocean teeming with trillions and trillions of other Prochlorococci is isolating when reproduction is asexual. For microbes, setting your genetic material adrift on the ocean swells is a hopelessly romantic gesture. That DNA may eventually make its way to another cyanobacterial cell that will find it useful and incorporate the DNA into its genome. It is also possible that the messages are not necessarily genetic. The vesicles could also be used to store and transport specialized metabolites as a defensive or offensive strategy. For now, only the cyanobacteria seem to be able to decipher the messages.
* Hopefully, you weren’t unwittingly gifted any pet cyanobacteria for Christmas.
**larger cyanobacteria than the Prochlorococci, but still not in a position to freely give away hard won nutrients
Fun fact: Dr. Chisholm has also co-authored a couple of children’s books on photosynthesis. Check them out here.