Astrobiology Revealed #16: Nita Sahai

On how iron-eating microbes could thrive in Europa’s ocean

by Aubrey Zerkle

In this Q&A we asked Nita Sahai about her paper “Bioenergetics of iron snow fueling life on Europa.” Nita is a full Professor and endowed chair in the Department of Polymer Science, within the School of Polymer Science and Polymer Engineering at the University of Akron in Ohio, where she also has joint appointments with the Departments of Biology and Geosciences, as well as the Integrated Bioscience Program. Nita describes why Europa is an exciting place to look for life in our solar system, and explains how she and colleagues calculated which microbes could thrive in different environmental niches of its ocean. She also offers some amazing advice for early-career astrobiologists! (This interview has been edited for length and clarity.)

I like to start these Q&As by asking people how they got involved in the field of astrobiology, since it’s such a diverse field and everyone’s path is different. How did you go from a PhD in electrolyte adsorption to proposing new metabolic pathways for extraterrestrial life?

Probably quite typical of an astrobiologist pathway, my PhD title talks about electrolytes adsorbing onto mineral surfaces, but at the bottom of it all is physical chemistry of the mineral-water interface. The PhD was focused on inorganic ions, but as soon as I got into a post-doc I immediately started working on organic molecules and mineral surfaces. I’m trained as a low-temperature geochemist, and [during the PhD] I was working on environmental issues. But since my post-doc, I was also working on organic interactions on mineral surfaces within the body, like bone mineralization. Biomineralization was just getting heated up in the late 90s and early 2000s, everyone was looking at bacterial biomineralization. My research was starting to look at membrane and mineral interactions in the body, from the silicosis point of view. This is a respiratory disease in people who work in blasting rock surfaces and such. My CAREER grant, which I got in my second year as an Assistant Professor at the University of Wisconsin-Madison, was precisely to look at membrane-mineral interactions in the origins of life, which is ultimately part of what I brought into astrobiology as well. Along the way, I’ve been working continuously on both the medical and the geochemical aspects of organic-mineral interface reactions, whether in Earth’s environment or some other planet’s environment.

In this series, we’ve talked a lot about potentially habitable places in our solar system like Mars and Enceladus, but we haven’t heard much about Europa. Can you tell us why Europa is an attractive place for astrobiologists to search for life?

Europa is one of the moons of the gas giants, it’s a moon of Jupiter while Enceladus is a moon of Saturn. The Galileo spacecraft took a lot of magnetometric data and other sorts of information [on Europa] that were detected by remote sensing. Models that were developed for it suggested it has a liquid ocean surrounded by a cover of frozen ice on the surface, like Enceladus. Further modeling of both these [moons] showed that there might still be active volcanism at the seafloor, or at least enough temperature being generated in the deep interior to keep seafloor basalt alteration going on, which generates hydrogen through serpentinization reactions. That set everybody looking at methanogenesis, because serpentinization produces hydrogen, and there’s CO2 also being generated from seafloor volcanic activity. So if you’ve got CO2 and you’ve got hydrogen, you could potentially have methane-based metabolisms [on Europa’s seafloor].

Then Cassini flew by Enceladus and actually imaged and sampled its jets of liquid water, that might be one of the most exciting images that NASA has ever captured in our solar system. And they found phosphorus, as well as nitrogen-bearing compounds in the Cassini plume. So the question has always been, can Europa have a similar sort of biogeochemistry? The two moons are very different in size, Enceladus is a lot smaller than Europa on average. So, whether there’s enough primordial heat still there to keep the oceans hot and keep the reactions going, that’s a key part of the general planetary model.

It’s exciting because there’s also the Europa2020 mission that is supposed to launch this fall, so in a few years we’ll have a mission that’s going to get us much more specific information about Europa. Cassini was this amazing magic mission, but it was over almost the entire outer solar system. Europa2020 is going to collect a lot more information that is Europa-specific. And so I think this is a great time to think about what sorts of things we could plan for in a future mission, or how we could interpret what we discover in this one. 

That’s exciting! In your recent PNAS paper you proposed iron could potentially fuel life in Europa’s ocean. You mentioned that people in the past who were thinking about metabolic pathways for Europan life largely ignored iron – why is that? And how did you decide to start thinking about iron?

Initially people who modelled the data on Europa suggested that there was possibly methanogenesis and methanotrophy happening at the seafloor where the water interacts with the rock. And that’s totally right, it could still be happening if there’s enough heat from the interior coming up to fuel any seafloor activity. Reducing agents are being delivered from seafloor interaction with the seawater, which would be ferrous iron, hydrogen, and methane. But to get an iron-reducing metabolism, it was not clear how the reduced iron coming up from seafloor-water interactions would get oxidized in the first place, for it to be reduced [by the bacteria].

[On Europa] you’ve got about 5 to 20 kilometer-thick surface ice and about 80 to 100 kilometer-thick seawater. UV radiation and cosmic rays hit the surface of the ice, and through radiolysis split up the water and make reactive oxygen species (ROS). Now, how do you get [ROS from the surface] through 5-kilometer-thick ice, in the best case scenario? There are various mechanisms that Greenberg and others have worked out by which you can get the ROS delivered into the top of the ocean, right below the ice-ocean interface. So what Greenberg and Pasek did already was to say if you assume different rates of delivery of oxidants into the ocean by these mechanisms, and rates of reductants coming up from the bottom, then depending on the oxidant delivery versus the reductant delivery, you would have different amounts of iron oxidation. The whole ocean could basically be like an acid mine drainage system, because you’re also oxidizing lots of sulfide to sulfate [which produces sulfuric acid in acid mine drainage], etc.

Then they left it at that, and said that this is not a very healthy situation for life, so they were equivocal about whether life could exist in such an ocean or not. I have a geomicrobiologist colleague in the geosciences department here at Akron [John Senko] who has worked in acid mine drainage systems his whole life. So when I read that paper, I talked with him about this for a couple of years and we came up with the “iron snow” hypothesis.

Can you describe the “iron snow” hypothesis, and how it might overcome some of the roadblocks to life on icy moons?

One of the problems with the former model of Greenberg and Pasek is that it requires ROS to be delivered all the way down the water column to the bottom of the ocean. And if there is any life in this water column, then that ROS will oxidize the iron, but it’s also going to damage the cell membranes and any other biological molecules. So this is a major roadblock we identified - how do you even get the first molecules to form, forget about life, if you’ve got ROS destroying things like proto-cell membranes, proteins, and RNA? We needed a way to prevent that free delivery of ROS to the bottom of the ocean. The mechanism we’ve proposed is that the ROS gets turned over by whatever mechanisms Greenberg came up with, but the iron(II) in the ocean gets directly oxidized [to iron(III)] right near the ice-water interface. Iron (III) is insoluble, so it precipitates out and forms iron oxyhydroxide. This is the “iron snow,” which is the terminology from the acid mine drainage literature, where they have exactly the same thing.

So this was the hypothesis we had, that it should be possible for these iron oxide nanoparticles falling to the bottom [of Europa’s ocean] to serve as your oxidizing equivalents [for microbial life]. You’ve managed to get the iron oxide to the bottom and now you’ve got iron-reducing bacteria that can chomp on this, and they’ve got hydrogen as a potential reductant coming from the seafloor. Similarly, sulfate reducers can now survive because you don’t have the ROS, so you can also do calculations of sulfate being reduced by hydrogen coming from the seafloor. And methanogens were always a possibility.

That’s the iron snow model, and the major roadblock was the ROS. The other one was that the original model of Greenberg and Pasek requires whole volumes of ocean to turn over and mix. Here we’re saying you just get [iron snow] to fall out of the so-called sky, off the bottom of the ice, so you’re basically agnostic about the rate of turnover. All you need is to know the rate of ROS delivery, and the rate at which iron(II) is being produced by seafloor-water interactions.

Then the question becomes, okay, great model, but can you show whether it actually works?  All these different types of metabolisms deliver different amounts of free energy for the cell to live, depending on the temperature, the pH, and the net solution chemistry. So you need to know how much iron is available, how much sulfate is available, how much bicarbonate is available, how much hydrogen is available, how much CO2 is available, etc., to calculate that energy.

So you calculated the energy yield for these different microbial metabolisms using thermodynamic models?

We brought in Doug LaRowe who is a really good thermodynamics modeler, and Doug did all the calculations. Knowing what we already know about previous estimates of Europan seawater, which is slightly oxidizing, and the seafloor basalt, which everybody assumes is the same as an average ocean basalt on Earth, [we asked the question] what solution chemistry, pH, and temperature are you going to come up with when you mix the two? The seawater is cold, the rock is hot, and of course the answer depends on how much of each you mix. So there’s a mixing ratio issue, and we don’t know how much mixing is going on, it depends on the permeability and on the scale you look at. Some areas of the rock might be more fractured than others, so there might be more water flow going through there.

So we did this whole range of agnostic calculations across a temperature range where we know these bacteria can thrive, 0 to 100°C, and calculated how much water-to-rock ratio you need to get this temperature range. It’s an inverse problem – you’re saying you want to produce a target temperature, so how much initial seawater and initial hydrothermal fluid do you need to mix to get these target temperatures. Once you know that mixing ratio, then you can say okay, I know my initial seawater composition, I know my hydrothermal fluid composition, and I mix them in these ratios to get that target temperature, so what’s the average solution chemistry going to be.

You also have to estimate what the pressure is down there. It’s like 1.3 kbar or something at the bottom, because it’s such a thick column of water, 80 to 100 kilometers of water, that’s a lot of pressure. Once you know the target temperature, the pressure, the pH, and all the other solution parameters, now you can do your delta G calculation. Delta G of the overall reaction is how much free energy [microbes can] make a living on, using this metabolism. So you do this, and you make these isotherms and pH gradient lines that tell you at a given pH and temperature, how much free energy can a cell at most utilize for anabolic processes. If you assume every bit of energy in this delta G reaction is going into anabolism, then you can say this is the cell productivity, or how many cells per year this much energy can support.

And what did these productivity calculations tell you?

The cool thing we found was that with the iron snow mechanism, the iron-reducing bacteria produce on average approximately 4 times more energy than the methanogens and the sulfate reducers. Also, very importantly, there’s a wide range of pHs and temperatures where this statement holds. So from very low temperatures, like 2°C, all the way up to say 80°C, and from pH 2 to pH 8 or 10, in that whole range iron reducing is feasible and it’s more energy-yielding. That means more cells per year of primary productivity can be supported, compared to the other temperature ranges and pHs.

Methanogenesis works better than iron reducing at higher temperatures above, say 80°C, and at pHs more than 8.5 or 9, but it yields very little energy. And sulfate reduction is somewhere in between. So this tells us that there are different environmental pH-temperature regions depending on small variations in how much fluid-to-rock ratio you have, and that can vary widely. And so you can have different environmental niches in pressure, temperature, and solution composition where you have different energy-yielding metabolisms. So that was the other cool thing here.

You mentioned that there’s an upcoming mission to Europa – how could this or future missions look for iron-breathers and other signs of life in Europa’s ocean?

Well, of course ideally you would want a whole bacterial cell in a plume, or on a surface or something, like they did for Enceladus. Iron reduction can support many more cells per year, which means if there is ever a plume detected coming out of Europa, or if there’s some way that this water gets up to the surface through the ice, the more biomass you have, the higher the chances of detection. If you have more primary productivity and more suitable environments around the ocean, you’ve got more biomass, therefore your instruments don’t need to be that sensitive. You could have different types of primary productivity and presumably different types of biomolecules that these different organisms might be producing, so you don’t have to be sampling a jet directly from the seafloor-water interface where the methanogens live. You can sample something a little bit removed from that and maybe still capture signs of life, even under slightly different pH-temperature conditions than what those little methanogens prefer.

Ideally, you would want a cell or a fragment of a biomolecule of such organisms, but I think if one was to even find traces of phosphate compounds, reduced nitrogen, ammonium compounds – these sorts of things are not definitive biosignatures by any means. But I think if there were indications that it’s even possible to support life, if there is even enough reduced nitrogen and phosphate down there, that would be a good starter.

This has been really fascinating! Is there anything else you’d like to discuss that I haven’t asked you about?

Because this is for young astrobiologists, one thing I would say is, as we said before, this is such an interdisciplinary field, and it was not the field I did my PhD in. The one thing I was never afraid of was doing something new. I just had confidence in my skills as a geologist and geochemist, and the physical chemistry training I had as an undergrad and as a PhD student. I found that I can widen my horizons as long as I can follow through with hard work. So that’s really important. And don’t get disappointed with rejection. Because I applied two or three times to NASA and I never got funded, all this work is done completely unfunded. 

The other thing I would say is I’m an immigrant here, and the struggles that an immigrant goes through are very different and complex. I came here before the world of internet, cell phones, and free calling internationally, as many people like me before me. So when you look at people and think about them, think about the personal struggles they’ve gone through. There’s no family for support here, there’s nothing. You come with a few dollars in your pocket and a visa that depends on being in the good books of your PhD supervisor. And what that can do in a field in the past where there’s little accountability. These sorts of concepts that the young generation now is lucky enough to have to fall back on, to appreciate that. And to not always jump to conclusions about people’s motivations, they may not have the same life experiences as you. The true diversity is when you try to think the best of everyone and say “I don’t know what their circumstances are, and they probably are coming from a well-intentioned place.” That is my model for life!