BOOKSHELF 



By Laurence A. Marschall 



Aglow in the Dark: 

 The Revolutionary Science 

 of Biofinorescence 



by I 'incent Pieribone 

 and David F. Gruber 

 Harvard University Press, 2005; 

 $24. 95 



Wombats don't normally glow in 

 the dark; neither do albatrosses. 

 Except for a small assortment of insects 

 such as fireflies and glowworms, crea- 

 tures on land or in the air seldom need 

 to make their own light. Even when it's 

 cloudy, the Sun, Moon, and stars pro- 

 vide ample illumination for finding 

 food and signaling companions. 



In the ocean, though — particularly 

 at depths beyond the reach of sun- 

 light — bioluminescence is the rule. 



Colorless jellyfish (Aequorea victoria) was the 

 source of the first bioluminescent protein to be 

 chemically characterized. The jellyfish in this 

 photograph is lit to show its morphology; it emits 

 its own light only in discrete dots around the 

 margin of the bell. 



The female humpback anglerfish 

 (Melanocetus johnsoni) dangles a glim- 

 mering lure from a stalk on its snout to 

 beckon its prey. The luminescent ed- 

 ible clam (Pholas dactylus) signals dis- 

 tress by squirting a glowing blue liquid 

 from its siphon. The male sea firefly 

 (Cypridina hilgendorfi) rises toward the 

 surface when ready to mate, spitting 

 out punctuated streams of liquid light 



as he goes. His female counterpart pur- 

 sues those luminescent strings to their 

 source, homing in on her sweetheart 

 like a pilot following runway lights 

 safely into an airport. 



What all these creatures have in com- 

 mon are proteins closely associated with 

 light-emitting molecules called lu- 

 ciferins. In the presence of triggering 

 chemicals, luciferins convert chemical 

 energy directly into light without heat- 

 ing, a cold luminosity that is both strange 

 and mysterious. Light-producing reac- 

 tions were first probed in 1887, but only 

 in the late decades of the twentieth cen- 

 tury were they understood as more than 

 incidental curiosities. 



Even in luminescent organisms, only 

 traces of bioluminescent proteins are 

 present. That scarcity long kept chem- 

 ists in the dark about their molecular 

 structure. Then, in 196 1 , the Japan- 

 ese biochemist Osamu Shimomura 

 collected and dissected more than 

 9,000 jellyfish of the species Ae- 

 quorea victoria. It took a year for Shi- 

 momura to purify a macroscopic 

 quantity of a protein that he called 

 aequorin, the bioluminescent sub- 

 stance from the mashed jellyfish. 

 But with the stuff in hand, he 

 showed that the jellyfish turned its 

 light on and off by regulating the 

 amount of calcium in its cells. 



S hiniomura's discovery marked 

 the beginning of a boom in re- 

 search about light emitted by bio- 

 chemicals. Aequorin's ability to 

 signal calcium levels made it an 

 ideal substance for probing the 

 chemistry of living organisms: in- 

 ject a little aequorin into the cells 

 of another creature, from a lowly 

 barnacle to a tool-making baboon, and 

 given the appropriate level of calcium, 

 the aequorin would turn on like a pi- 

 lot light. Experimenters could know at 

 a glance when calcium was flowing 

 into a muscle or a nerve. 



Vincent Pieribone, a neurophysiol- 

 ogist at Yale University, and David E 

 Gruber, a doctoral candidate in 

 oceanography at Rutgers University in 



New Brunswick, New Jersey, are 

 among the many scientific beneficia- 

 ries of that pioneering work. New 

 molecular techniques make it possible 

 to manufacture large batches of light- 

 emitting proteins in fermentation vats. 

 You no longer have to decimate ma- 

 rine populations to discover new light- 

 emitting molecules. Even more im- 

 portant, thanks to the genetic revolu- 

 tion, DNA sequences that code for 

 light-emitting molecules can be at- 

 tached to specific genes of virtually any 

 creature, giving rise to cells that glow 

 only when those genes are expressed. 



Thus, by tagging genes with mole- 

 cules that literally flash their presence 

 at the microscopic level, biologists can 

 pinpoint the sites of tumors, observe 

 the development of degenerative dis- 

 eases such as Alzheimer's in living crea- 

 tures, trace nerve action, and the like, 

 all in cell-level detail. 



The Rock from Mars: 

 A Detective Story on Two Planets 



by Kathy Sawyer 

 Random House, 2006; $25. 95 



ALH84001 is its name, a fist-size 

 lump of rock discovered near the 

 Allan Hills in Antarctica back in 1984. 

 At NASA's Johnson Space Center in 

 Houston, where the rock was sent for 

 study, meteoritic experts determined 

 that it had been ejected from the sur- 

 face of Mars during the planet's colli- 

 sion with a small comet or asteroid. Af- 

 ter wandering the inner solar system for 

 about 16 million years, ALH84001 col- 

 lided with the Earth, landing on the 

 Antarctic ice sheet some 13,000 years 

 ago. Several dozen meteorites of simi- 

 lar composition have been discovered 

 over the years, and investigators agree 

 that they all originated on Mars. 



When it comes to ALH8400 1 , how- 

 ever, Martian citizenship is virtually 

 the only point of expert agreement. In 

 the early 1990s David S. McKay, a geo- 

 chemist at NASA, and a group of col- 

 leagues studying the rock became con- 

 vinced that it held evidence of biolog- 



NATUKAl HISTORY March 2006 



