Microbes are the smallest known living organisms on Earth and can be found virtually anywhere, even in the cold, Mars-like conditions of lava caves.
On the island of Hawai’i, scientists recently discovered a wonderful assortment of new microbes that thrive in geothermal caves, lava tubes and volcanic vents.
These underground structures were formed 65 and 800 years ago and receive little or no sunlight. They can also harbor toxic minerals and gases. Yet microbial mats are a common feature of Hawaiian lava caves.
Samples of these mats, taken between 2006 and 2009 and then again between 2017 and 2019, reveal 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 failed to match any results to known genera or species, at least not with high confidence.
“This suggests that caves and fumaroles are different, little-explored ecosystems,” the study authors write.
Microbes, after plants, account for most of our planet’s biomass and almost all of the biomass in the Earth’s deep subsoil. However, because these organisms are so small and live in such extreme environments, scientists have historically overlooked them.
In recent years, underground microbes have received increased 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 that 99.999% of all microbe species remain unknown, leading some to call them “dark matter”.
New research from Hawai’i underscores just how obscure these life forms are.
The diversity between the sites varied. The oldest lava tubes, those between 500 and 800 years old, housed more diverse microbial populations than geothermally active sites or were less than 400 years old.
While these older sites were more diverse, the younger and more active sample sites had more complex microbial interactions, possibly due to less diversity. Microbes may have to work together to survive better.
Researchers suspect that it takes some time for microbes to colonize volcanic basalts, and as the environment around them changes, so does the structure of their community. In cooler caves, for example, Proteobacteria and Actinobacteria are more common.
“This leads to the question, do extreme environments help create more interactive microbial communities, with microorganisms more dependent on each other?” asks microbiologist Rebecca Prescott of the University of Hawaii in Mānoa.
“And if so, what is it in extreme environments that helps create this?”
In the younger lava caves, microbes tended to be more distantly related. This suggests that competition is a stronger force in the harshest environments, reducing the chances of closely related species living side by side.
Different classes of bacteria, such as Chloroflexi and Acidobacteria, existed at almost all sites, regardless of age.
These microbes appear to be key players in their communities. The authors refer to them as “hub” species because they bring together other microbes.
It is possible that Chloroflexi microbes can provide carbon sources in the ecosystem by harnessing light energy in relatively dark conditions.
But for now, this is just speculation. Since only a single gene was sequenced in the study, Prescott and his colleagues cannot say what the role of a particular microbe in their underground community is.
“Overall, this study helps illustrate how important it is to study microbes in co-culture, rather than growing them on their own (as isolates),” says Prescott.
“In the natural world, microbes don’t 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 work in the community. “
The study was published in Frontiers in microbiology.