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November-December 2007
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< previous | 1 | 2 | 3 | 4 “Ann had the idea,” Losick explains, “that maybe we could learn something from similar molecules found in contemporary microbes.” It turned out that the bacterium that Losick’s lab studies has genes similar to those known to produce a certain type of these molecular fossils. Their shared MSI-sponsored postdoctoral fellow, Tanja Bosak (now an assistant professor at MIT), learned the microbial genetics techniques Losick’s lab uses and obtained evidence that in the contemporary bacterium, these molecules help to protect its spores from damage by oxygen. (Although many life forms need oxygen, it can also cause a lot of damage.) Bacillus subtilis, Losick explains, creates a “particularly macho type of spore, maybe the most sturdy kind of dormant cell on the planet. It can survive extremes of time and temperature and radiation.” What might this molecule have been doing two billion years ago? “The exciting implication,” Losick replies, “is that these molecules may have appeared as a response to the rise of oxygen in the atmosphere, so the timing of their appearance in the rock record may bear on the issue of when oxygen levels first rose.” “The planet is about 4.5 billion years old,” elaborates another of Pearson’s collaborators, paleontologist Andrew Knoll, Fisher professor of natural history and professor of earth and planetary sciences. “The oldest rocks we can look at are 3.8 billion years old.” Chemical evidence suggests that life was already present then, but certainly “by 3.5 billion years ago there were active microbial communities on the earth’s surface.” These microbes were a little different from those we know today because there was still no oxygen. (Even today, he says, oxygen is just a “veneer on the surface of the planet. If you put your shovel in the mud of a marsh and dig a centimeter beneath the surface you are down to anaerobic microbial ecosystems, which remain critical for most of the biologically important element cycles on the earth’s surface.”) By 2.4 billion years ago, the chemistry of sedimentary rocks suggests that there was “at least a little bit of oxygen in the atmosphere and surface ocean.” Then, “for a period of about eighteen hundred million years, oxygen levels hovered at maybe a couple of percent of today’s levels. Only in the last 600 million years or so do we have environments with enough oxygen to support the biology of large animals, and only in that interval do large animals actually appear,” Knoll says. That is also when the algae first become ecologically important, joining and eventually displacing photosynthetic bacteria as the basis of early food chains in the world’s oceans. “So there is a time coincidence,” he says, “between major events in the history of life and major events in the history of the planet.” Following those threads might also suggest how life could arise, and what it might look like, elsewhere in the universe.
Bacteria fertilize plants through the formation of specialized symbiotic “tissues” on the roots, known as “root nodules.” Bacteria in these nodules form “factories” able to capture atmospheric nitrogen gas and convert it into ammonia-containing compounds that the plants can use for growth. By themselves, plants cannot effect this transformation, which is why they need “fixed” nitrogen (not gas) as fertilizer. “Everybody is interested in finding the perfect molecule to trace cyanobacteria,” Pearson reports, in order to trace the origins of oxygenic primary production (cyanobacteria use a type of photosynthesis that releases oxygen, and this process is responsible for our oxygen-rich atmosphere). More broadly, she hopes to “figure out how to relate the incredible diversity of microbes that are out there to the kinds of preservable organic molecules—or biomarkers—that they make...and that we can find in the sedimentary record.” Because individual species of microbes are mutable (“They evolve quickly and we have no idea if there is phylogenetic integrity over billions of years”), she is trying to relate particular preserved molecules to their functions in the environment, rather than to species, in order to try to get an idea of what the ecosystem looked like as life evolved on Earth and, in turn, shaped Earth’s environment—oxygenating the atmosphere, detoxifying the oceans, breaking down rock and organic matter to form soils. “MSI has been absolutely essential to building the bridges that I have needed to other research groups,” Pearson says. “Although my questions are geology- and earth-history-driven, the work that I do in order to answer those questions is all biology and chemistry. I don’t practice field geology,” she says, “so building bridges with other parts of the University has been critical.”
Photosynthetic cyanobacteria form colorful mats in the warm springs at Yellowstone National Park, a place that plays a central role in microbial-diversity studies. Historically, says Knoll, “there have been a few of us in FAS who have been interested in microorganisms, but there wasn’t anything as tangible as a Museum of Comparative Zoology, which has provided a focal point for people interested in animals for 150 years, [or] the Herbarium, which has done the same thing for people interested in plants” (which Knoll calls, half-joking, “green algae with a graduate degree”). He continues, “Because of Harvard’s good fortune in the nineteenth and early twentieth centuries, we built up these really quite remarkable facilities for studying plants and animals...and, not surprisingly, we then populated our faculty with people who study [them]. These areas are not unimportant now,” he explains, “but so many horizons have opened up for studying microorganisms that the simple identification through the MSI of a community of people concerned about microbiology has been tremendously important. Many of the basic biological processes that underlie the nature of changes in the environment are microbial, so in a world where the environment may be changing faster than our ability to understand it, having a better process-oriented idea of what microbial communities are actually doing in nature has practical importance as well.”
Left: These cave mineral deposits were difficult to explain through purely geologic processes. Spelunker scientists in the 1970s discovered that microbes play a role in their formation. Right: The recognition that microbes can respire insoluble metals has led to a complete reassessment of processes, such as rusting, that used to be considered largely abiotic (nonliving) and are now known to have a microbial component. Scientists refer to this as “microbial-assisted corrosion.” For his part, geochemist Daniel Schrag, professor of earth and planetary sciences and a member of the MSI steering committee, emphasizes that “almost all the reactions on the earth’s surface are catalyzed by microbes—in soils, in waters, in swamps. If you don’t know what microbes are doing, you don’t really know what is going on.”
Photograph by Colleen Cavanaugh Other kinds of “extremophile” microbes can live without light or oxygen near hydrothermal vent chimneys. The ecosystems around these vents are entirely supported by both the free-living and the symbiotic bacteria. “It is clear that this field will not reach its maturity, in the sense that it becomes less exciting, during my research lifetime or probably the research lifetimes of my students,” Knoll says. “We have whole new horizons in the nature and treatment of disease; whole new horizons in simply understanding the diversity of life as it actually exists—not what we thought existed because we could see it; whole new horizons in the nature of the relationship between organisms and the environment both today and in the future, and on the timescale of the whole development of the planet. These are wonderfully large, exciting questions and MSI gives us a forum to discuss them. You do find, every once in a while, someone who has actually thought about the same problem in a very different way”—and that can be the most important sort of catalyst: the kind that leads to new discoveries. Jonathan Shaw ’89 is managing editor of this magazine.  
 
 
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