Hawai’i’s ancient lava caves are teeming with mysterious lifeforms

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The microbes are the smallest known living organisms on Earth and can be found almost everywhere, even in cold, Mars-like conditions of lava caves.

On the island of Hawai’i, scientists have recently discovered a wonderful assortment of new microbes thriving in geothermal caves, lava tubes and volcanic vents.

These underground structures were formed between 65 and 800 years ago and receive little or no sunlight. They can also harbor minerals and toxic gases. Yet microbial mats are a common feature of Hawaiian lava caves.

Samples of these mats, taken between 2006 and 2009, and again between 2017 and 2019, reveal even more unique lifeforms than expected. When the researchers sequenced 70 samples for a single RNA gene, commonly used to identify microbial diversity and abundancethey could not match any results to known genera or species, at least not with a high level of confidence.

“This suggests that the caves and fumaroles are under-explored diverse ecosystems,” write the authors of the study.

Microbes, after plants, make up most of the biomass on our planet and nearly all of Earth’s deep subsoil biomass. Yet because these organisms are so small and live in such extreme environments, scientists have historically ignored them.

In recent years, subterranean microbes have gained more interest because they exist in environments very similar to those found on Mars. But there is still a long way to go.

Recent estimates suggest 99.999% of all species of microbes remain unknownfirst some to refer to as “black matter”.

New research from Hawai’i underscores just how obscure these life forms are.

Diversity between sites varied. Older lava tubes, those between 500 and 800 years old, harbored more diverse populations of microbes than geothermally active sites or were less than 400 years old.

While these older sites were more diverse, the younger, more active sampling sites exhibited more complex microbial interactions, likely due to lower diversity. Microbes may need to work together to survive better.

Researchers suspect that it takes time for microbes to colonize volcanic basalts, and as the environment around them changes, so does their community structure. In colder caves, for example, proteobacteria and actinobacteria are more prevalent.

“This brings us to the next question: do extreme environments contribute to creating more interactive microbial communities, with microorganisms more dependent on each other?” wonders microbiologist Rebecca Prescott of the University of Hawaii at Mānoa.

“And if so, what is it about the extreme environments that contributes to creating this?”

In younger lava caves, the microbes tended to be further apart. This suggests that competition is a stronger force in harsher environments, a force that reduces the chances of closely related species living side by side.

A stalactite formation in a Hawaiian cave with white microbial colonies. (Kenneth Ingham)

Several classes of bacteria, such as Chloroflexi and Acidobacteria, existed at nearly all sites, regardless of age.

These microbes seem to be key players in their communities. The authors call them “hub” species because they bring together other microbes.

It is possible that Chloroflexi microbes provide carbon sources in the ecosystem by harnessing light energy in relatively dark conditions.

But for now, that’s just speculation. Because only one gene was partially sequenced in the study, Prescott and his colleagues cannot say what role a particular microbe plays in their underground community.

“Overall, this study helps illustrate how important it is to study microbes in co-culture, rather than growing them alone (as isolates),” said Precott.

“In the natural world, microbes do not grow in isolation. Instead, they grow, live, and interact with many other microorganisms in a sea of ​​chemical signals from those other microbes. This can then alter their expression genetics, affecting their work in the community.”

The study was published in Frontiers in microbiology.

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