The Unusual Microbial Alliance Reveals the Evolution of Complex Life

Microorganisms in the remote bays of Western Australia are interconnected through tiny tubes, suggesting early stages of complex life evolution.

In Shark Bay, known by the Indigenous name Gathaagudu, microbes create slimy, multi-layered assemblages called microbial mats. This challenging environment, buffeted by tidal shifts and temperature fluctuations, has fostered bacterial communities alongside another single-celled organism known as Archaea, which have thrived here for tens of thousands of years. These microorganisms often coexist symbiotically, forming layered sedimentary structures known as stromatolites.

“The mats develop under hypersaline conditions with elevated UV levels. It withstands cyclones. Despite facing numerous threats, they persist,” comments Brendan Burns from the University of New South Wales in Sydney.

He posits that these contemporary microbial communities may resemble those that existed billions of years ago when complex life first emerged. This evolution might have been driven by a mutual dependence between bacteria and Archaea, leading to the formation of more complex cells known as eukaryotes.

Burns and his team returned some of these microbial mat communities to the lab to cultivate the organisms in high-salinity, low-oxygen conditions.

They successfully cultured only one type of bacterium, stromatodesulfovibrio nilemahensis, and a newly identified archaeon named Nearachaeum marumarumayae, a member of the Asgard Archaea group. These archaeal bacteria, named after the gods’ abode in Norse mythology, are regarded as the closest relatives to the eukaryotic cells that comprise the bodies of animals, plants, and humans.

According to team members, “These organisms seem to directly interact and share nutrients,” states Iain Duggin of the Sydney Institute of Technology. Although there is no direct evidence yet, the complete genomic sequence obtained allows for speculation regarding the metabolic processes of both organisms.

The genomic analysis indicated that bacteria synthesize amino acids and vitamins, while the Archaea produce hydrogen and various compounds, such as acetic and sulfuric acids. Both sets of products are unique, indicating a dependency on each other.

The researchers also observed indications of direct interaction between the two species. “We have observed what we refer to as nanotubes,” notes Duggin. “These microscopic tubes, seemingly produced by bacteria, establish direct connections to the surface of the Asgard cells.”

In addition to their interactions, the Archaeon cells generate vesicle chains that resemble SAC-like structures utilized for transporting molecules along extracellular fibers. Duggin notes that these nano-sized vesicles appear to engage with the nanotubes formed by the bacteria.

“While nanotubes may be too slender for conduits, they facilitate a type of multicellular binding that enhances resource sharing,” asserts Duggin.

The researchers identified a protein similar to human muscle proteins, a genomic sequence coding for a previously unknown protein, and a protein consisting of about 5,500 amino acids, which is substantial for ancient species. “While I can’t claim it’s directly connected to human muscle proteins, it suggests that their evolutionary origins may trace back much further,” says team member Kate Mischey from the University of New South Wales.

“What fascinates me most are the direct connections formed by nanotubes between bacteria and archaea,” comments purilópez-garcía from Parisa Clay University, France. “Such interactions have not been documented in prior cultures.”

However, discerning the exact behaviors of bacteria and Archaea is challenging, remarks Buzz Baum from the MRC Institute of Molecular Biology, Cambridge, UK. “It’s a complex relationship of conflict and cooperation,” he notes. “They interact, share, and sometimes clash, demonstrating a nuanced understanding of each other’s presence.”

Duggin believes the prevalent dynamic is more cooperative than combative. “These organisms coexisted in our culture for over four years, suggesting a level of harmony rather than contention,” he adds.

Burns and his colleagues propose that their findings may reflect an early stage in the evolution of eukaryotic cells within microbial mats. Roland Hatzenpichler at Montana State University aligns with this perspective.

“The study’s outcomes indicate that the newly identified Asgard Archaea engage directly with sulfate-reducing bacteria,” he remarks.

However, Lopez Garcia cautions that these interactions may not date back beyond 2 billion years. “While these archaeal and bacterial forms are modern, the microbial environments they inhabit may provide insights into ancient ecosystems,” he explains.

According to Hatzenpichler, we may be on the verge of better understanding the similarities between recent microorganisms and the cells they collaborate with to form primitive nucleated cells. “We’re now in an advantageous position to uncover deeper truths,” he concludes.

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