Hawaii’s ancient lava caves teem with mysterious life forms

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

On the island of Hawaii, scientists 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 contain toxic minerals and gases. Yet microbial mats are a common feature of Hawaii’s lava caves.

Samples of these pads, taken between 2006 and 2009 and then again between 2017 and 2019, revealed even more unique life forms than expected. When the researchers sequenced 70 samples for a single RNA gene commonly used to identify microbial diversity and abundance, they couldn’t match any results to known genera or species, at least not with a high degree of certainty.

“This suggests that caves and fumaroles are understudied distinct ecosystems,” the study authors wrote.

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

In recent years, subterranean microbes have gained more interest because they exist in an environment very similar to that of Mars. But there is still a long way to go.

Recent estimates suggest that 99.999 percent of all types of microbes remain unknown, leading some to call them “dark matter”.

New research from Hawaii highlights just how obscure these life forms are.

Diversity varies between sites. Older lava tubes, those between 500 and 800 years old, contained more diverse microbial populations than geothermally active sites or were less than 400 years old.

While these older sites were more diverse, younger and more active sample sites had more complex microbial interactions, possibly due to lower diversity. Microbes may need to work together to survive better.

The 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 cooler caves, for example, Proteobacteria and Actinobacteria are more abundant.

“This raises the question, do extreme environments help create more interactive microbial communities, with microorganisms more dependent on each other?” wonders microbiologist Rebecca Prescott of the University of Hawaii at Manoa.

“And if so, what is it about extreme environments that helps create that?”

In younger lava caves, microbes tend to be more distantly related. This suggests that competition is a stronger force in harsher environments, one that reduces the chance of closely related species living side by side.

Stalactite formation in Hawaiian cave with white microbial colonies. (Kenneth Ingham)

Several classes of bacteria, such as Chloroflexi and Acidobacteria, exist in almost all places, regardless of age.

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

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

But for now, this is just speculation. Because only one gene was partially sequenced in the study, Prescott and her colleagues can’t say what a particular microbe’s role is in their underground community.

“Overall, this study helps illustrate the importance of studying microbes in co-culture rather than growing them alone (as isolates),” says Prescott.

“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 gene expression, affecting their performance in the community.”

The study was published in Frontiers in Microbiology.

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