After a rainstorm, you step outside to a world transformed. Colors have become darker and more vibrant while everything else seems muted – the normal chitter and chatter of wildlife becomes conspicuous in its absence. Earthworms, normally hidden safe in the soil, suddenly litter the ground, lying bloated and still on the sidewalks. But the most striking transformation is the smell – that earthy, pungent scent is as pervasive as it is unmistakable.
If you look at this picture, you can almost smell it:
The smell itself actually has a name: petrichor, which is a combination of “petri-,” a Greek base meaning “stone,” and “ichor,” an ethereal fluid that served as blood for Greek gods and immortals. But where does this inimitable smell come from?
It’s a combination of plant oils and the waste product of a soil-loving bacteria in the Streptomyces genus. The oil in question is produced by plants during dry periods – ostensibly as a sort of birth control until more water is available, as it postpones seed germination.
The other culprit, the bacterial waste, is a compound called geosmin. Streptomyces bacteria are important decomposers; as they break down rotting vegetation and other organic material in soil they produce geosmin as a by-product. Geosmin is also behind the earthy taste of beets and the muddy smell of catfish.
During rainfall, the plant oil and geosmin are bled out of their hiding places and released into the air, creating that wonderful distinctive scent.
When English poet John Donne claimed that “no man is an island,” he probably did not anticipate how closely his philosophical musings would align with biological theory four centuries later. As genetic research progresses, scientists are realizing that evolution may be more about cooperation among organisms than competition – truly, no organism is an island unto itself.
Biologists increasingly can pinpoint instances of interdependence among species in all kingdoms of life – leading some to believe it is time for traditional Darwinian theory to evolve. Mounting evidence of cooperation among diverse creatures and their respective microbial communities provides tantalizing hints of a more comprehensive view of life – one that challenges the definition of an organism. For decades many microbiologists have believed that no organism evolves alone, but rather as a joint effort with the millions of microscopic creatures teeming with them – fungal, bacterial, and protist. Now, evolutionary biologists are catching on: some think that natural selection acts on “super organisms,” the creature plus its microbes, rather than an organism itself.
Charles Darwin proposed the theory of natural selection in 1859, and it remains a hallmark of evolutionary biology today. Natural selection’s basic tenet is that traits that prove beneficial to an organism will become more common over successive generations. While this basic premise seems almost obvious in its simplicity, many evolutionary puzzles are left unaddressed. For example, the level of organization on which natural selection acts remained an enigma. Do evolutionary pressures act on cells themselves, or whole organisms… or even groups of organisms?
Biologists Eugene Rosenberg and Ilana Zilber-Rosenberg think they have the answer. In 2007 they proposed that organisms adapt to their environments with and because of their microbial communities. They noted that a change in the makeup of species in Mediterranean corals’ microbe population, prompted by changing sea temperatures, enabled the coral to fight off a devastating bleaching virus. The coral, which lacks an adaptive immune system, overcame a viral threat in one generation. The microbial community of the coral successfully fought off the lethal threat, ensuring its survival into another generation. Their observation led the team to develop the hypothesis that natural selection acts not just on one set of genes, but on all of the genes within (and on) an individual, including those of the micro-occupants.
All lifeforms possess robust microbial communities that are linked to physiologic function – humans, for example, rely on hundreds of species within our gut to digest our food and absorb nutrients. Hyenas have unique microbe collections in their anal glands, the distinctive scent of which acts as a badge of pack membership. The mixture of intestinal microbes in the common fruit fly influences with whom they choose to mate. Rosenberg believes these facts justify extending his team’s hypothesis to encompass all life, rather than just this specific Mediterranean coral.
Biologists have long accepted the importance of microbes to the lives of larger creatures – for example, the mitochondria in your own cells originated from a once free-living bacterium that was engulfed by a larger cell – yet many hesitate to agree with Rosenberg’s broad generalization of cooperative evolution in larger creatures.
Roberto Iglesias Prieto of the National Autonomous University of Mexico does not believe Rosenberg and his team proved that the Mediterranean coral was suffering from the viral perpetrator they identified. He, among other marine biologists, calls for a more rigorous examination of Rosenberg’s claim. Iglesias Prieto also cautions that an organism’s fitness might not rely on its entire set of microorganisms, but probably only its beneficial microbes.
Other biologists like John R. Finnerty, director of Boston University’s marine program echo this caveat. Finnerty does not question Rosenberg’s basic claim, but suggests the primary coral research does not support the larger hypothesis that natural selection acts on super-organisms. In some cases, a creature may need a very specific species of microbe to fill a role, while in others the co-occupancy is more of an incidental arrangement between the microbe and host, Finnerty says. The relationship between host and microbe can be very flexible – a fact that Rosenberg’s hypothesis does not address.
Despite these concerns, biologists are becoming more interested in the role our resident microbes fill. In 1998 microbiologist Lynn Margulis wrote that “the full impact of the symbiotic view of evolution has yet to be felt.” Her foresight anticipated Rosenberg’s ambitious, broadened concept of how to define an organism and a concept of evolution that stresses cooperation, rather than competition, as the main catalyst for change. Researchers are currently working toward teasing out the exact roles microbes play in the production of life, but there is a general-consensus that we literally are more than the sum of our parts.
As you wait in line for fast food, it’s not only your taste buds that are looking forward to the juicy burger. Deep in your intestines a hundred trillion bacteria cells are licking their proverbial bacteria lips in anticipation—worst of all though, these gut microbes could be making you fat.
According to new study published in Science, your fatty diet could be sustaining a population of obesity-promoting bacteria. In a collaborative effort, researchers from University of Washington and University of Colorado discovered that the type of bacteria living in the human gut may contribute to obesity. Furthermore, the composition of these microbial populations depends on their host’s diet.
Researchers began by culturing gut-bacteria from twins, one obese and the other healthy. Despite the genetic similarities between twins, it became evident that the genetic makeup of their gut bacteria could vary dramatically. In order to test whether these variations could be a contributing factor in obesity, Jeff Gordon and his team at University of Washington transplanted samples of bacteria from each twin into germ-free mouse pairs. As suspected, the mice receiving the microbes from the obese twin gained significantly more fat mass and began to display other biological markers of obesity.
The researchers then housed both the obese and lean mice together to allow their gut bacteria to interact. If the mice were fed a diet low in fat, the lean profile bacteria replaced their obesity-linked brethren. However, if the mice were fed high fat diets, this protective effect was lost; the bacteria cultured from the obese twin continued to survive in the mouse’s guts.
It seems that the specific profile of bacteria living in the GI tract play at least some role in the development of obesity. Furthermore, the profile of these bacteria can be altered by changes in diet. While this certainly isn’t a magic cure to the obesity epidemic, it certainly sheds light on just how complex this issue is. “We’re very optimistic but it is hard to say how long before [these findings] can be applied in humans, especially due to regulatory obstacles,” cautions Knight, lead researcher from University of Colorado.
Jacques Izard, an instructor in immunology at Harvard’s Forsyth institute echoed Knight. “At this point in time, only very small numbers of subjects have been included in the microbiome/obesity investigations,” Izard said. “The results are however so significant that a new field is being created as we speak. It opens new doors of treatment that go beyond probiotics and prebiotics.”
The bacteria in your body out number your own cells about ten to one. The food we eat is the food they eat, and these results suggest we can alter their population for our benefit… if we can just manage to resist that double cheeseburger. So maybe the bacteria in your gut are making you fat, but if they are, it’s because you’re enabling them.
To mark the boundaries of our yards, most people plant hedges or construct fences. Hyenas, on the other hand, use paste – an oily, waxy, yellowish substance secreted from their anal glands.
Last fall I spoke with evolutionary ecologist Kevin Theis about his fascination with hyenas and his time spent tracking their various cliques as they roamed about the Kenyan Masai Mara National Reserve. During his time there, Theis became particularly interested in the hyenas’ scent-marking behavior.
Many mammalian species take advantage of their odiferous excretions – usually glandular goop, urine, or feces – to stake a claim to their territory. This behavior is known to biologists as “scent marking.” Hyenas mark their clans’ territory by extruding their anal pouch and dragging it along the ground, leaving a pungent paste trail behind them. Theis also suspects they use paste to communicate more nuanced information like fertility or advertising social status.
To determine the true nature of paste messaging, Theis first needed to identify what paste is exactly.Through a chemical analysis of the anal paste of hyenas from four different clans, Theis found that each group had a distinct “perfume” – allowing individuals to rapidly recognize if they were in a friendly or rival clan’s territory.
He also found that the waste products of microbial communities living within the hyenas’ anal pouches are responsible for paste’s distinctive odors. Each clan’s signature scent results from the unique composition of microbial species shared among that social group, meaning hyenas rely heavily on their resident cooperative microbe species for social communication.
Theis is continuing his work from the department of microbiology and molecular genetics at Michigan State University, where he aims to “elucidate the mechanistic roles bacteria play in the scent marking systems, and thus social lives, of solitary and social hyena species.” Read his blog here.