TLDR; Cyanobacteria can form complex supracellular structures called "ropes" that act as protective safe-havens for other bacteria. Here, we'll be discussing their function, evolutionary history, and how they might have aided life in conquering the land.
Recently, I've become fascinated with this cyanobacteria called "Microcoleus". Unlike most of the filamentous cyanobacteria that show up in my samples, their trichomes are housed within a communal sheath and glide back and fourth, traveling along its length like an underground subway tunnel. This behavior is known as "rope-building" and seems to have evolved in multiple lineages of cyanobacteria, but is most well studied in Microcoleus.
At first glace, it seems like such a behavior would be fairly maladaptive, since grouping together in tight bundles would lead to competition for light and metabolites amongst the trichomes. However, it seems that Microcoleus uses its thick, gelatinous sheaths to stabilize loose sediments by binding sand grains together. By doing so, it is able to monopolize environments that are often too difficult for other cyanobacteria to colonize.
(Top) Microcoleus chthonoplastes binding sand grains together within an intertidal mat. (Bottom) M. chthonoplastes alongside other cyanobacteria and diatoms.
For this reason, Microcoleus and other rope-builders are often the first organisms to take root in such habitats and serve as a stable foundation for other organisms to attach to. The sheaths can also serve a number of other functions as well, such as water retention and UV absorption. Because of this, these cyanobacteria are especially good at forming biocrusts (terrestrial microbial mats) in dry, inland habitats and in forming the scaffolding for more diverse communities.
However, there are also cases in which Microcoleus live as solitary individuals and only bundle together in response to the low nitrogen availability (a common problem faced by bacteria in oligotrophic environments), such as in the case of M. vaginatus. While M. vaginatus are not capable of nitrogen fixation on their own, their aggregation seems to attract a "cyanosphere"; a tiny community of heterotrophic bacteria which feed upon the photosynthate (carbohydrates produced via photosynthesis) excreted into the environment by cyanobacteria. In exchange, the heterotrophic bacteria are able to fix nitrogen and gather phosphorous from the environment and share them with their tube-building partners.
The reason for this selective bundling is still unknown, since plenty of cyanobacteria form cyanospheres without sorting themselves into aggregates. Multiple proposals have been put forward, but perhaps the most interesting is the idea that M. vaginatus congregate in an effort to locally deplete the oxygen in various parts of the biocrust where they live, thereby providing their nitrogen-fixing partners with protection from oxygen (This makes sense, given that previous studies have shown that biocrusts develop anoxic microhabitats at night when photosynthetic activity is temporarily shut-down.)
Regardless, their ability to work together with nitrogen-fixers enables them to thrive in inhospitable environments where nitrogen is the primary limiting nutrient. For that reason, I've been very interested in whether similar organisms could have colonized dry, nitrogen-poor continents on the early Earth (or perhaps the similar conditions on Leeuwenhoek.)
However, their fossil record is fairly spotty, with the oldest instances occurring roughly 1.0 BYA in what is now Northwestern Siberia. The late emergence of rope-builders might have occurred in response to the gradual diversification of predatory eukaryotes, poor preservation, or perhaps the expansion of oxygenated, terrestrial habitats where the localized depletion of oxygen would have come in handy. For now, they represent a small glimpse into what life could have been like on the early Earth, back when the land had remained largely unconquered and cooperation was necessary for survival.
Citations :
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0007801
https://www.researchgate.net/publication/344383451_A_symbiotic_nutrient_exchange_within_the_cyanosphere_microbiome_of_the_biocrust_cyanobacterium_Microcoleus_vaginatus
https://www.paleoitalia.it/wp-content/uploads/2023/09/13_Sergeev.pdf
https://paleobotany.ru/palynodata/species/113445
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