In May, a study was published in journal Nature Astronomy, conducted by Massachusetts Institute of Technology professor Dr Sara Seager and other researchers, showing single-celled organisms like Escherichia coli (E. coli) and yeast can thrive in both 100% hydrogen gas and helium atmospheres.  Wikinews discussed the findings with Dr Seager to know more about her research.
Life has not been observed in any habitat other than Earth, which has an oxygen-rich environment.  While Earth's atmosphere is dominated by nitrogen gas, oxygen is essential for advanced living organisms.  Some species of microorganisms do not require oxygen for metabolism, called anærobic organisms, such as methanogens which rely on carbon-dioxide while releasing methane.
Researchers used Escherichia coli strain K-12 and Saccharomyces cerevisiæ strain S288C for this experiment.  The two microorganisms were kept in four different environments: one being 100% air, and the other three being anærobic environments: 100% H2, 100% He, and 80%-20% N2-CO2.  The environments were kept in at 28° Celsius.  The researchers made sure the anaerobic experiment environments were anoxic, and had installed oxygen sensors to report fluctuation in the oxygen level.  They monitored growth of E. coli using optical density measurement, and they used a hæmocytometer for yeast.
The study reported the organisms were reproducing normally in both 100% H2 and 100% He environment.  However, the sigmoid-shaped growth curve was not on par with 100% air.  E. coli and yeast switch from ærobic respiration, which uses oxygen, to anærobic respiration and fermentation.  Both processes are less efficient and do not produce as much energy as ærobic respiration.
E. coli in an 80%-20% N2-CO2 environment had slower growth rate as CO2 dissolves and makes the liquid medium acidic.  Such reduction in growth rate was not observed for yeast cultures, which can thrive in acidic environments.  However, yeast's growth rate in 100% air was far greater than in the other three media.  The likely reason for this significant difference was lack of oxygen for non-respiratory purposes, the research reported.  Oxygen is essential for synthesis of biochemicals such as heme and sterols, which are important for yeast.  In atmospheres lacking oxygen to produce these chemicals, yeast fungi have stunted growth rate.
With this discovery, Dr Seager said scientists can now observe even more planets to study for habitable life.
"There's a diversity of habitable worlds out there, and we have confirmed that Earth-based life can survive in hydrogen-rich atmospheres.  We should definitely add those kinds of planets to the menu of options when thinking of life on other worlds, and actually trying to find it", Professor Seager said.
A rocky planet with expanded hydrogen-rich atmosphere should be easy to detect using the emerging technologies.  Hydrogen and helium gas have very low density.  Dr Seager said, "It's kind of hard to get your head around, but that light gas just makes the atmosphere more expansive [...] And for telescopes, the bigger the atmosphere is compared to the backdrop of a planet's star, the easier it is to detect."
The research paper noted rocky planets which have radius below 1.7 times Earth's radius (Earth's radius is roughly 6360 km) can support a hydrogen-rich atmosphere, if water were to react with Iron.
The research paper reported E. coli releases a number of gases when it lives in hydrogen-based atmosphere including ammonia, methanethiol, dimethylsulfide, carbonyl sulfide, carbonyl disulfide and nitrous oxide.  These gases can serve as biosignature gases which can help astronomers detect and study potential life on exoplanets.
Confirming life can thrive in atmospheres that do not have oxygen, Seager said "Astronomers should keep an open mind as to which planets are worth searching for life".
With NASA's James Webb Telescope scheduled to be deployed next year, the paper suggests researchers could observe smaller exoplanets that orbit small red-dwarf stars.
What prompted your interest in studying the survival of single-celled organisms in hydrogen gas environments?
((Sara Seager))  We wanted to show to astronomers that life can [survive] in hydrogen gas environments because planet atmospheres dominated by hydrogen are the easiest to study with current techniques (as opposed to N2 or CO2-dominated atmospheres).
((WN))  What led you to conduct this research?
((Sara Seager))  It's one of my life's goals to find signs of life on exoplanets.  We'd hate to have missed something just because we were too narrow minded.
When I and others write proposals to use the upcoming telescopes (including the James Webb Space Telescope and the extremely large telescopes now under construction: ELT, TMT, GMT) we want to be able to point to evidence that hydrogen atmospheres shouldn't kill or destroy life.
((WN))  When did the team start working on this study?
((Sara Seager))  About three years ago.  However we started working in earnest about 2 years ago.
((WN))  Could you explain — in very simple terms — what you've done and what you saw?
((Sara Seager))  We grew microorganisms E. coli (a 'Prokaryote') and yeast (a more complex 'Eukaryote') in atmospheres other than air, including molecular hydrogen (and also helium and a carbon dioxide/nitrogen mixture).  We saw that life could grow and thrive in these other atmosphere types.  Although, the microbes grew more slowly than they would in air, i.e.  with oxygen.
((WN))  What activities did this study involve?
((Sara Seager))  Custom-made sealed bottles with E. coli and yeast growing in them.
((WN))  Which activity took the most time and attention?
((Sara Seager))  The custom equipment was the most time consuming.
((WN))  Were there any challenges the team faced while conducting this experiment?
((Sara Seager))  Hydrogen gas is considered unsafe in large quantities.  Although we only used small amounts there were safety regulations that made our work challenging.
((WN))  What is the most fascinating aspect of this discovery?  How is it going to affect future space exploration?
((Sara Seager))  That E. coli, a simple single-celled organism, can produce so many different gases.  Proving to the astronomy community that life can survive and thrive in a hydrogen-dominated environment will enable astronomers to jump-start the search for life because hydrogen-dominated planet atmospheres are easiest to study.
((WN))  Were you surprised by the results?
((Sara Seager))  No, and the biologists on my team and elsewhere are not surprised either.  Hydrogen is not known to be toxic to life.  And, the life was not gaining energy from the atmosphere but rather from the "broth", the liquid culture the microbes were living in.
One thing I was personally surprised about is the diversity of gases produced by E. coli; this lowly life form can give us scientific hope that a range of interesting gases might also be produced by simple microbial-type life on exoplanets. 
((WN))  How would you define what a 'biosignature gas' is and what is its advantage in looking for life in exoplanets compared to other forms[?]
((Sara Seager))  A "biosignature gas" is one that is produced by life, can accumulate in a planetary atmosphere, and be detected with remote space telescopes.  It is a "sign" of life, not foolproof evidence of life.
Exoplanets are so so so far away.  We can barely study their atmospheres at all.  Looking for gases that don't belong in a given atmosphere is the main way we can hope to search for signs of life beyond our solar system.
An example is oxygen on Earth.  Oxygen (O2) fills our atmosphere to 20% by volume.  Without plants and photosynthetic bacteria, there would be no oxygen.  If there is an alien civilization with the kind of telescopes we are hoping to build, and they can see oxygen in our atmosphere, they will know oxygen is a highly reactive gas and shouldn't be present.  The aliens might infer there is life producing the oxygen.
The alternative is SETI — search for extra terrestrial signatures by way of mostly radio signals.  That requires intelligent life intentionally sending us a signal.  In the search for biosignature gases we can rely on simple single-celled life.
((WN))  Did your team also investigate the growth pattern of the bacteria in other biosignature gases?
((Sara Seager))  No.
((WN))  There are a few microbes on Earth known to thrive in hydrogen-rich environments.  Why did you decide to investigate yeast and E. coli for this experiment?
((Sara Seager))  Hydrogen-rich environments on Earth are incredibly rare and not well known.  We decided to do a simple experiment to communicate clearly to astronomers that live can survive and thrive in hydrogen environments.
((WN))  If there are planets with hydrogen-dominant atmosphere which can host life — will they have ultra-violet protection layer like the ozone gas?  If not, how will it affect the evolution of the life of these single-celled bacteria?
((Sara Seager))   No.  The organisms may have to develop UV resistance like some life on Earth has.
((WN))  In the experiment, the study reported yeast had "a substantially lower maximal cell concentration in the pure hydrogen-gas environment".  What could be the reason for that?
((Sara Seager))  Even though life can survive without oxygen, life reproduces more with oxygen.
((WN))  The research paper talks about systems with hydrogen gas in abundance in the atmosphere.  Do you suspect microorganisms might be living in some of those systems?
((Sara Seager))  On Earth, yes, in niches.
((WN))  Are there habitats on Earth that might hint at what life on a hydrogen-rich atmosphere planet might look like?
((Sara Seager))  Not really.
((WN))  Should this change our search for life on other planets?  If so, how?
((Sara Seager))  This should open up — continue to push — astronomers on what kinds of planets might be habitable.  We will have so few planets to search for life around, even with our upcoming sophisticated telescopes, that we want to keep options open.
((WN))  What are some of the ways to detect microscopic alien life on an exoplanet?  Or is it beyond the limitations of physical instruments?
((Sara Seager))  Currently beyond limitation.  Future telescopes will be able to search for gases that might be linked to microbes.
((WN))  Are there any candidate exoplanets or satellites which meet the above description to be potential hosts for life?
((Sara Seager))  There is a list, but we do not know enough about the exoplanets, we need more information.
((WN))  Are some categories of exoplanets more likely to support hydrogen-based lifeforms?
((Sara Seager))  Super Earths.  We know hot Jupiters and giant exoplanets have hydrogen/helium atmospheres but they are too hot beneath the atmospheres for life to survive.
((WN))  Can you describe the kind of planet that would likely have such an atmosphere?
((Sara Seager))  Planets are born with hydrogen in their atmospheres; even Earth had a tiny amount in its atmosphere.  Past calculations (that is theory, not observation) show the scenarios under which a planet can have a hydrogen-dominated atmosphere.  It can outgas if it is made of building blocks that have H-containing minerals.  Or some planets can capture an H/He from the planetary nebula while the planet is forming, then even if they lose some H and He it might be maintained from outgassing from the interior.
((WN))  How massive do the exoplanets have to be, so there is enough atmosphere available to support hydrogen-based life?  I suspect below a threshold, the planet's gravitational pull might be too weak, the planet may lose its atmosphere.
((Sara Seager))  Correct.  We know very little about exoplanet atmospheres; forthcoming observations are needed to learn more.
((WN))  Will the yeast and E. coli survive in a very thin atmosphere or in vacuum?
((Sara Seager))  Probably not.
((WN))  Does E. coli or yeast consume hydrogen gas for their survival?
((Sara Seager))  No.
((WN))  How many hydrogen gas-dominant exoplanets or satellites have been discovered?  How common are they?
((Sara Seager))  Rocky planets with hydrogen-gas dominated atmospheres haven't been discovered yet because we can't yet observe and study rocky planet atmospheres.
((WN))  Can we predict the category of the host stars where the planets may support life?
((Sara Seager))  We cannot.
((WN))  What are some of the adverse effects of living in a high concentration of hydrogen gas for E. coli and yeast?
((Sara Seager))  To my knowledge, none.
((WN))  How did you ensure the gas concentration remained stable throughout the experiment?
((Sara Seager))  Sealed bottles.
((WN))  Were [the microbes] taken out of the bottles for [counting]?
((Sara Seager))  Yes, with a needle so that air would not enter the bottles.
((WN))  The growth curve of 100% helium and 100% hydrogen gas environments were nearly the same.  What does that suggest?
((Sara Seager))  That the life we studied can probably survive in any type of non-poisonous atmosphere if it has energy, nutrients, and liquid. 
((WN))  What's the significance of learning that microorganisms can survive and grow in a 100% hydrogen atmosphere?
((Sara Seager))  We used a pure 100% hydrogen atmosphere as a proxy for a hydrogen-dominated atmosphere (as atmosphere[s] are unlikely to be pure anything). 
The significance is for astronomers.  In our further search for life on exoplanets (once the next-generation telescopes are available to us) we will look for signs of life by way of gases in exoplanet atmospheres that might be produced by life. 
Rocky planets with hydrogen atmospheres will be easier to study than atmospheres with nitrogen (N2) or carbon-dioxide atmospheres because hydrogen is a light gas and creates an expansive atmosphere.
I should note that we don't have any rocky planets yet known with hydrogen atmospheres, but they are theorized.  Nor do we have a large number of rocky planets with accessible atmospheres at all.  So this whole sentiment is somewhat in the future.
((WN))  What can we infer from the growth curve of the bacteria in 80% nitrogen gas, 20% carbon dioxide environment?
((Sara Seager))  This atmosphere was used as a control and was not intended to have meaning on its own.
((WN))  How do you think next year's James Webb Telescope will set the course of astrobiology for years to come?
((Sara Seager))  Webb is poised to answer some of your above questions relating to rocky exoplanet atmospheres.  Webb has a small chance to find biosignature gases, but there are really only a small pool of planets available for study.
((WN))  Are there any future plans to follow up on this study?  Perhaps in different physical environments and with different microorganisms?
((Sara Seager))  My team is not planning on follow-up work.
((WN))  What lies ahead in this field of research?
((Sara Seager))  Observations with Webb, and with other telescopes now under construction.

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'Earth-based life can survive in hydrogen-rich atmospheres': MIT professor Dr Seager tells Wikinews about her research on organisms thriving in oxygen-less environment