Counting up microbes

The Science News top story of 2013 was reported in “Your body is mostly microbes” by Tina Hesman Saey (SN: 12/28/13, p. 18). The article noted that “only about 10 percent of a person’s cells are human; microbes make up the other 90 percent.”

Several readers noted that although microbes make up 90 percent of a person’s total number of cells, most of a person’s mass is quite human. “The unsuspecting reader of this article may be led to believe that the human body consists of 90 percent bacteria by weight,” Dietrich Marcuse e-mailed. But microbial cells are smaller, and lighter, than human cells on average.

Calvino Raben isuggested online that the percentage of a human’s biomass composed of microbes would be a more telling number, to which Scott Linford replied, “More telling in what way? True, our little friends are little. So is the 90/10 ratio misleading, just hype? No. Because those little guys have millions of active genes. Millions of distinct proteins inside us and yet not human. And that ratio will blow 90/10 out of the water.”

Hesman Saey responds: “While it’s true that microbes account for only 1 to 3 percent (perhaps a couple kilograms) of an adult human’s body weight, there are at least 150 bacterial genes for each of the 22,000 or so human genes in the body. A 2010 study in Nature found 3.3 million different bacterial genes in human intestines alone, many of which enable microbes to make nutrients and break down foods that humans can’t on their own.”

Wonders of engineered parts

The year’s No. 2 story was “Bioengineers make headway on human body parts” by Meghan Rosen (SN: 12/28/13, p. 20).

Scientists have more work to do to make artificial organs truly functional, noted online. “To think, after all the work getting the structures, the researchers will need to consider re-creating any beneficial epigenetic changes in a natural organ. They also might need to consider bathing these organs in the same organic stew real ones grow in, not just the nutrients, but immunological factors and others. These are exciting times, with much progress to be made.”

How to map the universe

In the No. 3 story of the year, “Planck refines cosmic history” (SN: 12/28/13,p. 21), Andrew Grant wrote about measurements of radiation left over from the Big Bang. Science Visualized in the same issue showed data collected by the Planck space telescope at a variety of microwave and infrared frequencies.

Jesse Stoner e-mailed to ask about the layout of such maps. “These so-called maps never have any reference as to what direction the radiation is coming from,” Stoner writes. “What causes the maps to be oval? If they represent a view from Earth in all directions, shouldn’t they be round? And what is the band across the center of the oval?”

Grant says that the maps’ oval shape is a way to depict a 3-D universe in two dimensions, just as atlases include various 2-D projections of the spherical Earth. The band across the center of each map is the plane of the Milky Way, including the galaxy’s dust.

Online reader John Landwehr had another question spurred by Planck’s view of the universe. If, as stated in Grant’s story, “the universe began as a smooth ball of energy that then expanded uniformly in all directions,” Landwehr wrote, why haven’t scientists found the origin point in the universe where the Big Bang occurred?

Grant replies that it’s important to remember that there is no space outside the universe. “The universe is expanding because the distance between objects within it is increasing,” he says. “Space itself is getting stretched out. Extrapolate backward and those distances go to zero. That brings us to the moment of the Big Bang. Since then each point in the universe has spread apart from the others, with no central origin point. No matter your location in the universe, distant objects appear to be moving away in all directions.”

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