What a mysterious green sludge discovered in lava caves tells us about life on other planets

Deep within the lava caves of Hawaii, microbial life thrives. In fact, a recent study has found that the life teeming within the caves is made up almost entirely of unknown species.

More surprisingly, the researchers found that the microbial life was structured into complex networks of species that relied on each other. Within these networks were ‘hub species’, with so many links to other species that if they were removed, it could cause ecological collapse.

We spoke to Dr Rebecca Prescott and Dr Stuart Donachie, microbiologists at the University of Hawaii at Mānoa and authors of the study.

How did you search for life in these lava caves?

Dr Rebecca Prescott: We had 70 samples of microbial mats [layers of microbes that live on surfaces] that we looked at from a variety of volcanic environments of different ages. The environments included lava tubes, geothermal caves and steam vents.

In order to identify the microbes, we looked at a gene called the ‘16S rRNA gene’. It’s like a little tag that helps us with identification. We also looked at what microbes were ‘hanging out’ together, to try to better understand the structure of these communities.

Tell us more about this technique.

Dr Stuart Donachie: Prior to 1986, microbiologists could only identify microbes that they’d cultured in the lab. Everything we knew about microbial diversity – meaning how many species there are – was based only on what we could grow in Petri dishes. That gave us a rather narrow view, but it was the best we could do at that time.

In 1986, there was a method developed that involved sequencing [the 16S rRNA gene]. This method based on extracting as much DNA as possible from the environmental sample, and then making copies of this particular gene. Once this method was applied in the environment, we detected bacteria that had never been seen before.

RP: There are lots of microbes in the environment that have never been cultivated. The majority of them haven’t been. We saw a lot of microbial groups [in the volcanic samples] that are what we call ‘hub species’, meaning that they have a lot of connections to other species of bacteria in these networks. If you were to remove them, you might see a lot of those connections collapse. So they may have important ecological roles.

What sort of things do these hub microbes do?

RP: I can only speculate here. I want to stress that we don’t know what their function is. But one example of a hub species would be the Chloroflexi bacteria that we found in the volcanic environments. Some Chloroflexi can photosynthesise at low light levels. They may be able to take carbon into the system. Other organisms around them may not have that ability to photosynthesise in really low light.

SD: Anywhere you get these photosynthesising organisms [such as Chloroflexi], they take inorganic carbon and they make organic molecules from that. But they’re kind of like leaky cells – they excrete other biological molecules into the environment. They also die. Their cells break down and it releases the contents of the cells into the environment, and that allows other organisms to grow, which need pre-formed organic molecules.

RP: I study ‘quorum sensing’, which is bacteria talking to each other through chemicals. When they do this, they can respond to something in the environment as a group.

Part of the reason we wanted to understand these network structures is because we see really high levels of quorum-sensing genes in a lot of caves, and I don’t have a good explanation as to why you would see that. So that’s another possibility for why you may get a particular organism showing up as a hub species in a network – it could be that it’s doing something and then is communicating with others.

Could this tell us more about microbes in other extreme environments?

SD: I don’t know that we necessarily addressed this in this study, but as a personal observation, [research like this demonstrates] the importance of water. I went into probably 20 caves in the Kīlauea caldera between 2006 and 2009. And some of them are completely dry and a regular temperature. It was a challenge to identify anything biological, except for the plant roots coming through the ceiling.

But in others, the cave was hot and extremely humid. Relative humidity was about, I think, 102 per cent. It’s like being in a sauna. There was rainwater from the ground above dripping through the ceiling, then flowing over the walls of the cave and dripping onto the floor. The floor was hot – we put a temperature probe into the ground and it was something like 90°C a couple of inches down.

The water that was coming from the ceiling was dripping onto the floor and being converted into steam. So you get this kind of circulation of rainwater becoming groundwater, and then being converted into steam. And that’s where we saw the richest, thick, green microbial mats, which kind of illustrates the importance of water.

That’s why we always say where there’s water, there’s life. Life, at least as we know it, needs water. Hence we’re looking for liquid water on other planets or any other body, because that’s the first thing we know of that’s needed for life.

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