TLDR; A strange consortium begins its life by becoming an aquarium for a swimming community of microbes, transitioning into a living greenhouse, and ends its life cycle by turning into stone.
(Above) Screenshot from www.microbiological-garden.net
As I continued exploring the website, I came across a few obscure topics that I'm hoping to discuss in future posts, but the one that fascinated me the most was the story titled "magic microbial spheres". Wondering what on Earth that could be, I opened it up and I certainly wasn't disappointed.
Out of all of the microbial consortia I've read about thus far, this might take the cake as being one of the most unusual. To begin, let's start by discussing how these "magic spheres" form and go through their life cycle.
In the beginning, a number of different heterotrophic bacteria aggregate and self-assemble into a hollow sphere which is supported with the aid of extracellular matrix. Once formed, the cells forming the sphere fill the space inside with hundreds of their flagellated progeny, which swarm within it like shoals of fish swimming within an aquarium.
While this behavior is incredibly bizarre, another microbe (Vibrio splendidus 12B01) has also been shown to form a similar structure. In both cases, the movement of these bacteria appears to mix the contents of the water inside so that nutrients permeating the sphere are evenly dispersed throughout. In contrast, a colony of bacteria that is comprised of a solid mass of sedentary cells would gradually become so large that nutrients couldn't diffuse far enough to reach the interior, causing those within the deepest parts of the colony to starve and suffocate. So, by mixing the water in this manner, these magic spheres are able to push past that upper size limit and get significantly larger than normally-structured communities of bacteria.
(Top) An image depicting the flow of nutrients within the spheres of Vibrio splendidus 12B01.
(Bottom) Alternatively, some prokaryotes (such as the Pseudomonas Aeruginosa colony shown here) create biofilms with intricate channels that allow water to flow throughout the interior and supply the cells with oxygen and other essential metabolites.
However, unlike the spheres of Vibrio splendidus 12B01, the mixed-species spheres appear to attract filamentous cyanobacteria and the diamond-shaped diatom Navicula which are able to sense it over large distances (presumably through sensing some sort of pheromone the spheres release). Once in contact, the diatoms and cyanobacteria pierce the membrane and enter the sphere, joining the community of swimming bacteria inside. Much like the tiny swarming heterotrophs, the photosynthesizers also continuously move within the sphere, perhaps to aid in circulating nutrients.
(Above) Navicula sp.
While the benefits of this relationship are difficult to determine, it's possible that living within the sphere could offer protection against phagocytotic predators and antimicrobials emitted by other competitors. Secondly, microbial photosynthesizers and heterotrophic bacteria have long since been known to engage in syntrophy (exchanging metabolites) with one another.
During this process, the photosynthesizers offer the heterotrophs organic byproducts of their light-independent metabolic reactions that would be too energy intensive to recycle back into the Calvin cycle. In exchange for this waste, the heterotrophic bacteria may provide some other metabolite (such as amino acids) in order to promote the health and growth of their partner. Since this behavior is widespread amongst communities containing algae and cyanobacteria, it's reasonable to assume that they same might be occurring within the magic spheres as well.
Regardless, as the communities mature, the spheres begin to undergo some extraordinary changes. At first, the cyanobacteria and diatoms make up a relatively small portion of the community, but as they photosynthesize within it, they gradually begin to fill the sphere with their progeny, turning it into a living, microbial greenhouse. Eventually, after a few months have passed, the spheres begin to darken as the cyanobacteria become more densely packed.
As this occurs, the growth of the cyanobacteria and diatoms begins to interfere with the circulation of nutrients, causing the symbiosis within the sphere to gradually fall apart. In response to the environmental stress, the free-swimming bacteria and photosynthesizers break free from the sphere in order to migrate to new habitats. During which, the swarming heterotrophic bacteria are able to once again consolidate, giving rise to the next generation of magic spheres.
Meanwhile, the punctured remains of the membrane are left abandoned. The polysaccharides that were left behind as the cyanobacteria and diatoms glided across its inner surface gradually drive the formation of calcite crystals which expand to fill the interior.
Sometime later, the sphere's membrane begins to break down, peeling back like a mold and revealing the sphere of calcite within, known as an "ooid". In a sense, the magic sphere concludes its lifecycle by turning into stone. A befitting end for such an other-worldly and ethereal structure.
(Above) A diagram depicting the complete life-cycle of the magic spheres.
Apparently, after doing some further researcher, I found that these magic spheres typically reside within the lamina of microbial mats that grow along tidal flats and salt marshes. Definitely something I'll have to look out for as I collect and film more samples this summer.
Side Note : Ooids have been found along beaches all over the world and are generally assumed to form as grains of detritus or calcium carbonate are held aloft by the churning of tides and accumulate calcite as they tumble throughout the water column. However, this process may also play an important role in generating ooids. This is especially interesting, given the role ooids play in serving as habitats for "euendolithic" microbes (those that bore into solid rock) and may have had in providing a home for microbes on the early Earth.
(Above) Euendolithic cyanobacteria growing within
ooids (collected in the Bahamas)
Citations :
https://www.youtube.com/watch?v=krKBQ-j_oeI&t=120s
https://royalsocietypublishing.org/doi/10.1098/rstb.2019.0077
https://www.researchgate.net/publication/23792051_Molecular_and_morphological_characterization_of_cyanobacterial_diversity_in_the_stromatolites_of_Highborne_Cay_Bahamas
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