Diamond Goat Early-Morning Show

September 5, 2023 Interview with Dr. S. DasSarma;

DG: Today it is my pleasure to introduce Dr. DasSarma, Professor at the University of Maryland, Baltimore. So Dr. DasSarma, thank you for being part of this episode. In hindsight of such a successful career, can you start with you taking us back in time to tell us about your background?

SD: Surely, so I immigrated to the US with my Family when I was in primary school, I was in the third grade. Then I studied at Indiana University, in Bloomington, Chemistry. Then I went on to graduate school at MIT, in Cambridge. My PhD mentor was Har Gobind Khorana, who was well known for having helped to crack the genetic code. I was an NSF graduate fellow there for five years. So that is my general education background.

DG: You know what, it seems like I knew weren't born in America, 'cause I have interviewed some Indians and you don't have that thick accent, you know...

SD: Yeah, well, you know, when I was in India, as I said, I was in the third grade, I actually studied in an English-medium school. So in India you have a choice; in those days, you had a choice of being instructed in either the language in your State or in English, which was at that time, the national, sort of, common language. And so, I knew English before I came here, of course, you know, my accent probably changed over the years, it has been quite some time....

DG: So the next question is: In the 1980s, you discovered the mobile genetic elements in Halophilic Archaea as a graduate student with Dr. Khorana, as I think you mentioned before, at from MIT. So I think Dr. Khorana was a Nobel laureate, so has Dr Khorana's mentoring affected your career, like right now?

SD: Well, he was certainly an inspiration. My father who was a chemist, had mentioned Dr. Khorana to me from an early age. He was one of a small number of Asians who had won a Nobel Prize. In fact, they had come to the United States at about the same time, in the 1950s, and my father went back to India, whereas Dr. Khorana stayed on and of course had this illustrious career. So, you know, I think that he was quite an inspiration, because he was so dedicated to his research, and you know, he spent over six decades doing research, and his whole commitment was to doing cutting research. He actually didn't teach any classes at all, even though he was employed at the University of Wisconsin in Madison, and then at MIT for multiple decades. He devoted a hundred percent of his time to research. And so that kind of dedication, sort of instilled in me a path forward, in life. I would say that was probably his most significant influence in my thinking. Research is it!

DG: So what got you interest in the microbiology field?

SD: So I had done some research at Indiana University, prior to Khorana's lab, with a fellow by the name of S R Jaskunas. And he was a molecular biologist working on microorganisms in the Chemistry Department. And that's how I sort of became more interested in molecular biology, as opposed to more strictly chemical topics. And he had set up a new lab at IU at the time, and he had a nice group of graduate students and undergraduates. And so he taught me how to work with E. coli, how to do plating and how to do some genetic work with microorganisms. So I had already had about two years of experience working with microorganisms when I started in Dr. Khorana's lab. When I went to Khorana's lab, he handed me a culture, an Erlenmeyer culture of a microorganims, which was deep purple. And that's when I was introduced to the organism that I am still working with, which is a Haloarchaeaon, called Halobacterium. And so that's where I got or how I got exposed to Microbiology, through these two gentlemen.

DG: I mean, let's go back to your time at the Khorana, Dr Khorana's lab. So what mobile, I think you discovered mobile genetic elements in Archaea. So what is the difference between like Archaea and Bacteria, during your time there?

SD: So the concept of Archaea was actually discovered right about the time I joined Khorana's lab, actually a couple of years before. And there is another scientist by the name of Carl Woese, who is the primary scientist who came up with the Archaeal hypothesis. So what he did was, he compared the RNA sequences, a specific one, the 16S ribosomal RNA, across the full spectrum of the single-cell, simple, otherwise, very simple, organisms, and at that time, people thought that all of the organisms that were single-celled were all related to one another. And by comparing this particular 16S molecule, he found that in fact there was this profound division between the Bacteria and Archaea. And in the past, everything was "Bacterium" or they were all "Bacteria" but once he found this fundamental division, he initially said, well this particular group is the Eubacteria, or the true Bacteria, and the other group was the Archaebacteria, or the old, let's say Bacteria, and then later on, he felt that the division between these two groups was so deep that he took the "bacteria" out of the name Archaebacteria, and he changed it to just Archaea, and instead of Eubacteria, he started calling them Bacteria. And so that's the nomenclature that we now have. It's a division of Life at the highest level, which he called Domains. There are three Domains of Life: the Bacteria, the Archaea, and then the higher organisms which are the Eukarya, or Eukaryotes. So, on the one hand, the Bacteria and the Archaea look very similar when you look at them in the microscope. They are all single-celled organisms, that divide by binary fission and things like that, but when you start looking at them in more detail at their molecular level, then you start seeing a lot of fundamental differences. And in fact, one of the most interesting things about the Archaea is that a number of their molecular properties are very similar to the higher organisms. That discovery led to deeper studies on how life evolved on Earth.

DG: So what mobile genetic elements did you discover in the halophilic Archaea that is different from the Bacteria?

SD: Right, so that was your original question! I had to sort of preface my thoughts in terms of what is the difference between Bacteria and the Archaea. The genomes of these two types of organisms are quite similar, generally they are circular, and they are small. They have about two million base pairs, or two-to-five million base pairs in their genomes. And these genomes are evolving very rapidly, and one of the characteristics of genome evolution is the ability to recombine and scramble genes. And the basis for this kind of gene scrambling that goes on in Bacteria and the Archaea, are these mobile genetic elements, or transposable elements, or some people call them jumping genes. They are literally genes in the genome that code for an enzyme or a protein that catalyzes their own movement. They are an agent in the evolution of genomes. And because we didn't really know, what was the difference between Bacteria and Archaea across the board, the discovery of these same types of genetic mobile elements in Archaea was quite interesting. And what it said is that these genomes in Archaea, are evolving using similar mechanisms as in Bacteria. However, the elements themselves are quite different because they divided or were separated in evolutionary history for a very long time. The finding of transposable genetic elements in Bacteria and Archaea said that these genomes are probably evolving in a similar manner. Now, higher organisms also have transposable genetic elements, and those were discovered by a scientist named Barbara McClintock back in the early or the mid-century, last century. And she was the first person to discover transposable elements, and she did that in maize, corn. If you have ever looked at Indian corn and you've seen that they have this variegated appearance - the kernels have a lot of sectors, a lot of different colors.

DG: Oh, Indian corn? Indian corn?

SD: Yes.

DG: I have never seen what Indian corn is. Is there a difference between normal corn and Indian corn?

SD: Yes, because the Indians were growing corn in America, before the settlers came. The corn that they were growing, were, they had different colors of their kernels, they weren't necessarily the kind you'd find in the grocery store, where they are all the same color. If you look it up, you will see. So what she did was, she did genetic crosses between different types of corn, and she discovered the basis for this kind of variability. And if you think about it, jumping genes or mobile elements are naturally pre-disposed to causing genetic variation, like in color. And we see the same thing in our Archaea as well.

DG: So you organized the team that set up the first genome sequence and the genetic code for a halophilic microbe, Halobacterium sp. NRC-1. So my question is, why did you choose to decode this specific microbe? Because I know there are probably a lot.

SD: Yeah, so that's a great question. So, I had made a decision to work on this organism after I left Dr. Khorana's lab. The organism, because of the discovery of Archaea, there were lots of interesting questions to be addressed during that time. And actually, one of the most interesting questions was about its genetic code. Now I had mentioned that Khorana had won the Nobel prize for his part in deciphering the genetic code. And one of the thoughts was that perhaps there are organisms on Earth that use a different genetic code. He had mostly worked with E. coli. And then there were other people who worked on a few other, different organisms, including yeast and also plants, wheat germ, and also some animals - the reticulocyte system. And what people found, is that the genetic code is common across the board. It seemed like all organisms used the same genetic code. And that was interpreted by Francis Crick as the "Frozen Accident", in other words, the genetic code had evolved to a particular code, and that all organisms that are extant, all life on Earth uses the same genetic code. Now it's very interesting to think that well, maybe there are other, there were some very primitive organisms that survived, that were using a different genetic code? That would say that maybe Life originated and survived in several different lineages. And the Archaeal lineage was one of those. And no-one really had studied the genetic code in Archaea. So that's why I wanted to study the detailed genetics of Halobacterium, because, even in spite it's name being -bacterium, it's an Archaeon. So I wanted to study it in great detail to determine whether it's genetic code was the same, or different. That is why I focused on this one organism.

BREAK

DG: So I think in this experiment you found that the proteins are highly acidic, so through this experiment - how do proteins function at such high salinity and low water activity conditions? Because I know that they can denature and under such extreme conditions.

SD: Yea, so we made these really fundamental discoveries about how this organism is able to survive in its very extreme, extremely salty environment. These organisms can survive and have been isolated from salt mines, where the salt was deposited hundreds of millions of years ago, during the Permian period from two hundred and fifty million years ago. So, they are very, very, very resistant to salt. And that was one of the reasons why we thought that well, you know, salt has very profound effects on molecules and biomolecules, and that maybe there would be a genetic code difference. Well, it turns out that it uses the same code as E. coli and Humans, so we didn't discover a difference in the genetic code. But when we sequenced the genome, we found that the proteins were all highly acidic. And that means that under the neutral pH inside the cells, the proteins are all negatively charged. Whereas in every other kind of organism that you can imagine, they are always neutral proteomes. On average the proteomes are neutral. So, there are some acidic proteins, and there some basic proteins, but if you average all of them, what's called the isoelectric point of the proteome, which is the average PI of the proteins, that is always around seven. In this case the PIs were around four! And so that was a very fundamental difference that we discovered when we sequenced the genome. And the rational was that, and this is still not, I mean I think people accept it, but I think there is still some more work to be done to establish this more carefully - and that is that the negative charges are found mostly on the surface of the proteins. Most proteins are pretty hydrophobic in the middle, so there really isn't space for a lot of charges, so the Haloarchaeal proteins are negatively charged on the surface of the protein. What that does - it does two things: one is that, because all the proteins are negatively charged, they repel one another, in other words, they stay in solution, under conditions which would otherwise result in their precipitation. So, if you take proteins from any other organism, and you add a lot of salt to it, what happens first is that the proteins aggregate, because the salt will take away the water, so the proteins become crowded. And then, when they get close -some are negatively charged, some of them are positively charged - they clump or aggregate and then they precipitate out. But if that happened in the halophiles, because they have a very high salinity inside of the cell, they would not be able to function. So, the way the organisms evolved was by making their proteins very negatively charged, or acidic, and so they could stay in solution, and they could compete with the ions in the medium for solvation, for water. You need water, because, you know, in order for enzymes to work, they need to have the right substrate and things like that, so they need to be in solution. They can't be just one big aggregate. So that's what we found from sequencing the genome, and that was really, really very exciting!

DG: I know that you mentioned that this Bacteria, or this microbe was found in a salt mine. And so my question is: did you have to go to the salt mine to get the Bacteria? Or did someone else do it for you?

SD: Well, that's a great question. Well, this is sort of a field of environmental microbiology - there are a lot of people who travel to very exotic places, to find organisms, and discover organisms, and you know, over the years, I have developed a liking for that as well. But really, I am a laboratory scientist, so most of our work has been done with organisms that other people have isolated. For instance, Halobacterium sp. NRC-1 was isolated by a fellow, by the name of Walther Stoeckenius, in San Fransisco, and if you've ever flown into San Fransisco... You live in San Fransisco, right?

DG: Oh, I live near it, yes.

SD: So you've flown in to the San Fransisco airport. When you fly, you fly over some salt ponds. And you can see the colors. And actually, that organism, I believe is from there. You know they are very salty ponds. They are producing salt from the sea and so very early on, people isolated the organisms that were producing the color in those ponds. So, but it is very to isolate these organisms, in one sense, because people need salt, so there are salterns everywhere in the world. You know, in India, in China, in Europe, everywhere, Africa, Australia.... You can get salt from different places and isolate organisms that are very similar, in terms of their molecular biology, but in terms of their pedigree they are all different because they have all been isolated in different places and at different times. And I have some wonderful collaborators who have sent me salt. And in fact, one gentleman is coming from Cochabamba, Bolivia. And we are collaborating on isolating organisms from the largest salt flat on the planet, which is in the Andes, about three thousand meters up. It is called the Salar de Uyuni. So, this is a salt flat that is larger than the State of Delaware, I think. So, there are tremendous resources out there, for this kind of work, but in answer to your original question, I prefer just to stay put, and do the work in the laboratory, or on the computer, rather than flying off to different places.

DG: I see. Well, that answers my question - I always fly to San Fransisco, I see these and I just assumed there was something else, maybe pollution from the city, but, yea, thank you for answering that question, and finally, I know there is some microbe living there. I mean your lab found that certain genes are acquired through horizontal gene transfers. So why are certain genes only able to be acquired through these horizontal gene transfers?

SD: Yea, so that goes back to the mobile genetic elements question. Because mobile genetic elements can assist in the movement of genes between different organisms, so I would say that horizontal gene transfers are happening all the time, in Nature. Because when you think about it, unlike higher organisms, with microorganisms, they are very fragile, in the sense that if you break the cells open, the DNA spills out, and then organisms can actually eat the DNA, so there are ways of different organisms eating the DNA or consuming the DNA, the DNA can go into a different cell, and then it can through these mobile elements doing recombination, they can get integrated into the genome. So that is something that has been going on since from beginning of evolution. The genes that became fixed are then discoverable as horizontal gene transfers. So there has got to be something special about a particular gene that confers an advantage in the cell in which its moved in to, and became fixed. So really, horizontal gene transfers are going on all the time. Most of them are not preserved. They are lost, because they are not conferring any advantage. I'll give you a really good example in the medical field: there are transposable elements that confer resistance to antibiotics, and these are just naturally occurring genes that are found in environmental organisms. If a gene like that gets into a pathogenic organism that maybe it is causing a disease, maybe in humans, then suddenly that organism becomes resistant, and the antibiotic no longer is able to kill that organism, and then it becomes a medical problem. And because people have been using antibiotics for a long time - this over the course of decades - this has become a pretty serious problem in hospitals, because mobile genetic elements are involved in movement of these resistance genes, and you are selecting for organisms that are capable of surviving even with antibiotic. And then once they acquire one antibiotic resistance, if you then select with a different antibiotic, it can become resistant, potentially to a second antibiotic. And this can go on until you get what is called a multiply antibiotic resistant microbe, and then you can get what is called a "superbug", that can be a real scourge in a hospital. And so, this is actually a pretty serious problem out there. But the natural phenomenon, I mean, microorganisms have been around for billions of years, they have been exchanging their genes, and they have been learning new tricks from each other by horizontal gene transfer for a long time. We humans did not know about it until fairly recently, so now we know and so we are studying the basis for those types of rearrangements.

DG: Yea, it is sad that we humans can't do that as well. I mean that would be amazing if we could do that, but…

SD: Well, there are ways in which we are doing gene therapies, with CRISPRs, and I think you have interviewed some of the discoverers of that. We can do that in some microorganisms, in fact we developed some methods of doing essentially that, we can do specific deletions of genes to find out what the function of genes are. So that's done using a particular set of genetic methods. But there's also this new method, using CRISPRs to actually insert genes into genomes.

DG: Yea!

SD: Yea, so that has huge promise for gene therapy. So yes, we are doing it, but we are just doing it in a much more precision oriented, you know, thoughtful way, hopefully!

DG: Yes, I remember yes, I did interview Dr. Marraffini, but he never specifically said it was a gene transfer. So. I don't know, maybe in human effort, maybe they don't mention it is a gene transfer. Maybe they want to say it is something separate? So. I mean, your recent...

SD: Gene transfer, sorry, sometimes might be considered a natural phenomenon. In the case of genetic engineering it's human-engineered gene transfer. It is still gene transfer, though: if you are taking a gene from one organism and putting it into another, or you are gene out from one organism and mutating it and tailoring it, and then putting it back in, that's still a gene transfer.

BREAK

DG: So, you also have an interest in the Astrobiology field. So, my question is: So, what got you interested in becoming an Astrobiologist?

SD: Yes, now that has been an interesting transition. So, after we finished the genome sequence of Halobacterium sp. NRC-1, it became clear to me that the ability of this organism to be able to survive in really extreme conditions was of interest from the standpoint of astrobiology. And before then, the term 'astrobiology' really hadn't been used. So that was a new term, around 2000, I would say, when it was popularized by a number of people and so, when you study. Initially, you know, people were skeptical about the field because, really it was a field that didn't have a specific organism to study, because we only knew about Life on Earth. We did not know about Life anywhere else. So, when we thought about this, we looked at the properties of halophilic Archaea, and we found that they had a lot of the properties that one would expect for an organism that might be able to survive elsewhere, say on Mars, or even in space. And so that's how I put the two and two together, because I was interested in this organism, and I was also interested in space and NASA-types of studies. You know I'd grown up in the 1960s, when the Apollo mission was going very strong, and there was a lot of excitement. You know, in 1969, the US landed astronauts on the moon. So, I had that experience as a part of my youth. So, when NASA started talking about bringing samples back from Mars, and potentially looking to see if there might be Life on Mars, then the idea of becoming an Astrobiologist became pretty attractive! And specifically, because I felt like, until we do get samples back, which was probably going to happen in the 2030's, we really have to, we only have the organisms on Earth to try to understand what Life might be like in a place like Mars. And one of the best organisms that could potentially survive on Mars, is the organism that we sequenced, Halobacterium sp. NRC-1. So it was really a natural fit.

DG: You also did research with the Antarctic halophilic microorganism, Halo, I can't, sorry, Halorubrum lacusprofundi. First of all, do you know why these names are so long? So, I mean is there a difference between the NRC-1 and this microorganism?

SD: Right, yes, there are some differences. And just in terms of the name - the names, these are mostly from Latin. Actually, Halo is salt, actually that is from Greek, rubrum is color, so it is a colored organism, and lacus is lake, and profundi is deep, and so it is actually isolated from an Antarctic Lake called Deep Lake. So it's a salt-loving-colored-microorganism from Deep Lake. So that's what Halorubrum lacusprofundi stands for. And that was isolated a while back by some Australian scientists. Deep lake is located in East Antarctica, so it is close to Australia, it is in the Australian part of Antarctica. So they are the ones who isolated it. And so, once we finished the work on Halobacterium NRC-1, we decided to compare it to something that was similar but different. So, we chose Halorubrum lacusprofundi because it could grow in equally high salinity, but it could also grow in cold temperatures. You know, which is natural, because it was growing in Antarctica. Initially, we studied how cold could it grow? And it could actually grow down to about minus two degrees. It grew slowly. It grew pretty well in a refrigerator on a stirplate, and just stirred the medium. It would turn a KoolAid orange, so that's why we decided to compare those two organisms.

DG: Yea, that is interesting I never knew Deep Lake. I think, is there Life on Antarctica, like other than these microorganisms, I don't know, I don't think there are, like big organisms there, right?

SD: Uh, well, penguins, mainly, right?

DG: Yea, except for penguins?

SD: And they are having problems now, right? Because of the climate change down there, but...

DG: but is that specific? The lacusprofundi - are they being affected by climate change? Cause I know you said they can live in cold conditions, right?

SD: Right, right, so I am very concerned with respect to the climate change issue. But with respect to the microorganisms, they have been around for billions of years. They have seen all the changes that you could possibly imagine. And so there really isn't any concern about them, except for the fact that if the temperature changes, you will select one type of organism over another type. Now there is a concern from the standpoint of infectious disease. So as the temperatures increase, many of the vectors for transmitting diseases, are also changing their habitats. So like, for instance, malaria, which was eradicated from the United States, which is, of course spread by mosquitoes, those were endemic to Florida, for instance, and as climate changes, it is likely that they are going to come back. But the microorganisms themselves, they are just going to evolve into something that can tolerate the higher temperatures.

DG: Hm. I mean, so this research on the Antarctic microbes, do you think Life is possible on Mars?

SD: So that is what we were trying to explore with comparing these two organisms. You know, what makes, let's call it Hla, for short, different from NRC-1, for short, for the Halobacterium? So, we did something called comparative genomics: we compared the complete genomes of both. And by the way, the Hla genome work was done with Carl Woese, who was the original discoverer of Archaea, so that we actually published a paper together with him, on that genome. So, once we finished that genome, we could compare the details of their genes, and what we found was that their proteins were a little different. They were a little less negatively charged in Hla, and there were also some other, subtle differences in their proteins, which we have followed up, by taking a single model protein and doing what's called mutagenesis, so we change specific amino acid residues, or building blocks of the protein, and we could show that there were only a handful of amino acids in the protein, that were responsible for the ability of those proteins to function at low temperature. So we have spent a fair amount of time to work on that. So, the fact that there are these differences in Hla, that allow these proteins to work in, not just high salinity, but also in cold temperatures, makes it more likely to be able to survive on Mars, because, as you know, the surface of Mars is very cold. The warmest it gets, is probably like, around the equator, around 70 degrees [Fahrenheit], but most of the time, and most of the surface, is well below freezing. So if an organism is going to do well on Mars, it has to be what is called a psychrophile or psychrotolerant organism, which is a cold-tolerant organism, and I would classify Hla as a psychrotolerant organism.

DG: So if you had the opportunity to go to Mars, would you shift your research focus on investigating Martian microbes?

SD: Well, of course, yea, I mean, if I could do that... Now, you know, I am at the age, where I am not really planning to go to Mars, and I know, that younger people, like yourself, are thinking about that. It's an exceedingly difficult trip. And so, you know, it would take months, sitting in a spaceship, to get there. But NASA is proceeding with robotic missions, so they are in the process of collecting samples right now, and they will then, at some point, send them back here. And then we will know if there actually are Martian organisms. We don't know that there are any organisms out there. There is no reason to believe that there are organisms there, that are alive at the present time. However, no one has ruled out that possibility either. And it would be absolutely fascinating to know if they are using the same kind of molecular biology and chemistry? If they have DNA? If they have proteins? If they have DNA, do they use the same genetic code? Which was the question which we asked of our halophilic Archaea here on Earth. So there are going to be very profound and fundamental, sort of philosophical questions, you know about Life in the Universe, Life in the solar system, etc. So yea, I would love to do that! And I hope that NASA is listening because I would like to have some of those samples when they bring, return them back, to see if there might be some interesting microorganisms there!

DG: So do you think that the Purple Earth Hypothesis is true?

SD: So that is a Hypothesis that I proposed, around 2005 or so. And that Hypothesis, is a hypothesis. OK? So it hasn't been proven, it hasn't been tested. But the reason that we proposed that, was because, first of all, that we worked on this purple organism, this organism that has a purple protein in it, for a long time, and then, people asked another question, and that was "why are plants green?" You might think that plants are green, because they are avoiding certain wavelengths. And it turns out that the Haloarchaea are purple because they are absorbing those same wavelengths. The purple ones are absorbing in the green, and the green ones are not absorbing in the green. That is why they are green. So that's what led to the idea that maybe these two types of organisms, and not plants, but really, microbial, photosynthetic organisms, called cyanobacteria, which are primitive versions of plants, if you will. They are not plants for sure, but they have some of the same photosynthetic systems. So those organisms might potentially have co-evolved, so that one organism was utilizing a portion of the spectrum, lets say the green part of the spectrum, and the photosynthetic organisms, containing chlorophyll, evolved in complementarity to those. So maybe the ones that were purple were above, and ones that were green were below. So, by the time the light reached the layer with the cyanobacteria, they didn't have the green portion of the wavelength to use for photosynthesis, so they used what was left over, which was the blue and the red. And if you look at the spectra, and you look at the Purple Earth Hypothesis and you look at the spectra, they look beautifully complementary. Soit's an idea, it's a hypothesis, because there is some rationale to believe it. But I think, what I do believe is that, competition between different types of microorganisms has been very important in evolution. And I can believe that competition for different wavelengths of light has been important. Whether specifically, the purple pigment, which is actually related to the pigment in our eyes, it has the same, very similar chromophore, the light absorbing portion, which is called retinal, or Vitamin A, it's a Vitamin A aldehyde form, that chromophore is very different from the chromophore that's used in photosynthesis, which is chlorophyll. And if you compare those two, you'll see that chlorophyll is much more complicated than retinal. Retinal is a much smaller molecule than chlorophyll. And so it makes sense from the Purple Earth Hypothesis, that on the Earth, the original organisms might have been using the energy of light, using retinal, and may have dominated. And in fact, they are still found all over the planet. They are found in the sea, they are part of the planktonic distribution of microorganisms, and that the more complicated, Oxygen evolving cyanobacteria evolved a bit later, and they might have had only a portion of the spectrum to work with. And that might be why they have, you know, green color. So that is a long-winded answer to your short question! You know, I personally don't feel like that scientists need to believe or not believe. This is not a faith-based subject-matter. This is something we have hypothesized, and then we test those hypotheses. So, I think that this is a testable hypothesis, the Purple Earth Hypothesis. Which we can, it's not easy to test, but it's something we can do experiments to address. So, I would say that I don't believe or disbelieve, but it's an interesting idea that could potentially suggest interesting experiments to do. And a lot of times, when you have a hypothesis, and you may do some experiments, and maybe the hypothesis is not validated, but you make a whole bunch of other discoveries, that opens a whole new set of doors. So hypotheses are really useful in science. So, I would put the Purple Earth Hypothesis in that category, I think that there are a lot of people interested. I personally am not pursuing, you know, testing of that hypothesis, but I think that others will do that, and I hope that they find some interesting results as a result of their work.

BREAK

DG: So let's say this hypothesis, it ends up being true, right? Do you think it will change human's views on finding Life on a remote, on Mars or some other planet?

SD: Yea, yes, so it does, because, you know, if we take the human perspective on Life in other planets, we would be looking for Oxygen or we would be looking for green light, right? Reflecting green light. But if we didn't find green light, does that mean that there is no Life there? And I would say, well maybe not, because Life there may be a different color. So, I think it is important for validating or deciding on whether there is the potential for Life in specific areas. You know, astronomy has advanced so rapidly, and there were no planets known outside of our solar system, until twenty, or twenty-five years ago. And now there are literally thousands, and people are, you know, of course, with the new space telescopes, the resolution that we can see objects which are increasingly more distant. So I don't think we can yet tell what color one of these extrasolar planets has, but I think in future we will be able to tell. And then we will be able to think about it from the prism of the Purple Earth, and you know, if we discover some planets out there, that are really absorbing green light, and looking purple, then you know, we have to ask ourselves, and we have to ask ourselves - well, maybe that's an Earth at an earlier stage in its evolution, right? - Before it became green from oxygenic photosynthesis. And you wouldn't find Oxygen there either, because you don't, these organisms don't use oxygen for photosynthesis. They are just using the retinal for phototrophy. They are just using the light from the Sun, to generate energy for growth!

DG: So why is your lab, you know, developed an interest in the study of buoyant gas vesicle nanoparticles or GVNPs, in Halobacterium?

SD: Yea, so we have been studying these as well, they are sort of related to the purple pigment, because, you know, in order to be phototrophic they have to absorb light, so they make these little buoyant nanoparticles that allow the cells then to float, so then they can photosynthesize more easily, or grow phototrophically. And these nanoparticles are really unusual, they are pretty unique, they are made of protein, they are full of the ambient gas, they are buoyant. And there is nothing else like them out there, so we were naturally gravitating towards them. So remind me again what your question is about the GVNPs?

DG: Like that why did you take an interest in it?

SD: So that is why we did it. And then when we started thinking about it, we realized that they were so special, that they might have a lot of practical uses. They are essentially just like a very small structure that's made entirely of protein. There were two main proteins, and one of them could be modified to display different foreign proteins on their surface. We were using them for displaying, say malarial proteins to try to make a malaria vaccine, and we also found a lot of other interesting properties, that we can mix and match various things, you can display multiple different proteins on their surface, and they are extremely stable; yet they are completely non-toxic. You can administer them by microneedles, and so they are a great platform for vaccine development and drug delivery. So we have, my appointment, is actually in the School of Medicine at the University of Maryland, so we are very interested in potential medical applications of these nanoparticles.

DG: Yea, you also developed an expression system to bioengineer GVNPs for biotechnology applications, so can you talk more about this expression system?

SD: Right, so these were done in Halophiles, so in the Haloarchaea, we had originally isolated the various different genes, and we found out what the promoter was, and we isolated plasmids that could be used for genetically engineering them. So it was a natural next step to put the gas vesicle nanoparticle (GVNP) genes, which is a pretty large cluster in a plasmid, and then we could express them, and we could insert in other proteins into this cluster, such as the malarial protein. We could express them and we could express them and produce GVNPs that essentially had the surface proteins from malaria or from Salmonella or from other viruses. So that's how we went about it. We basically collaborated with researchers who were experts in the pathogen. So they would say: "well, we should try and produce this protein on the surface". We did that, and then we would share our bioengineered GVNPs with them and then they would do experiments to show whether those GVNPs could be used therapeutically or for vaccine development and things of that nature. Those were, those are collaborative projects that are ongoing. Now one of my former students, who is a Professor in South Korea, in Busan, he has also found that you can take proteins that are produced in E. coli where it is a little easier to do the genetic engineering, that has a part of the GVNP protein fused to, let's say immune system proteins from humans, express those in E. coli and just incubate those with the GVNPs from Haloarchaea, and they will stick to it. That was a recent discovery that we are really excited about, because it would accelerate our ability to utilize these GVNPs for a variety of applications. There are other people working on these, people who have had other interesting ideas, such as, the fact is, that they refract light, so they can be used as a contrast agent, medically. And because they are nontoxic, so people are using GVNPs for, as a contrast agent. So you could combine those two, you could make contrast agents with certain, specific properties, so maybe you want to send the contrast agent to a specific part of the body, then you could attach the appropriate type of molecules to target it. Or you could potentially use it to deliver a drug to a particular, let's say you put a protein that will target a cancer. Then you could target these nanoparticles to the cancer cells. So these things are still being developed, and so I expect to hear about some really interesting applications in future.

DG: So what advice did you receive during your career that shaped your professional development or success?

SD: So that would go back to the original experiences in Dr. Khorana's lab, you know, where I would say that the devotion to science was probably the biggest influence in terms of motivating me in my own career. The scientists who do, I think, some of the most pioneering work are so completely devoted to the subject, so you have to be, you know, in order to really make a contribution to science, it's important to have that kind of devotion. Of course you have to have the originality, the ideas and the means to do that. But I think above all, I think the devotion is really important. If you are just doing science for, you know, for a little bit of fun, for a little while, and that's fine, and a lot of students do that. But the ones that do the best in their careers and in terms of contributions to science, are the ones who are truly devoted. And that is what I learned from, you know, my mentors.

DG: So is there anything else that we didn't touch upon, and that we should tell our audience?

SD: No, well, I mean, I guess I would just say that, you know, the World has quite a few problems, but we also have a lot science and technologies that can solve problems. And for young people, I would encourage them to think about careers where they can solve some of these problems, like some of our environmental problems. And one of the things we have been doing, or I have been doing recently, is to work with others, other like-minded scientists, other professionals, to address, you know, the climate issues. In particular, I have been working with other MIT alumni, and we have a group called the MIT Alumni for climate Action. So this group is very much devoted to educating and trying to find solutions to some of the bigger problems that the world seems to be facing at the moment. So, you know, I think the future is bright, so long as the young people seize the opportunities that are created by the tremendous amount of science that has been done, and the technology that has been developed, over the past fifty, seventy years, and really address some of the important problems, so I would probably leave it at that.

DG: Thank you, Dr. DasSarma, being part of this episode, for giving me the opportunity to ask you questions about Astrobiology and GVNPs.

SD: Terrific, thank you so much for the opportunity.