Lemonade from broken amber: scientists pick up the pieces and move on to termite discoveries. (Cover Story).
When the University of Massachusetts at Amherst scientist and her colleagues took a look at the pieces under a microscope, they were pleasantly surprised. The fracture had cleaved the termite's abdomen, exposing fossils of the microbes that had helped the animal digest its woody meals. The finding stimulated Margulis and her colleagues to break more termite-bearing pieces of amber.
In those specimens, among partially digested chunks of wood, was a mixture of single-celled organisms remarkably similar to the microbial melange that infests the guts of a primitive termite species still alive today. The discovery provides a welcome link between ancient and modern termite groups and may help scientists better understand the emergence of symbiosis between termites and their gut biota.
PLASTIC GEM Amber is the fossilized resin of ancient woody plants. That resin, a viscous substance that forms in canals or ducts in the wood, often oozes into channels inside the plant or seeps out of holes or cracks in the plant's surface. Once exposed to the air, volatile chemicals in the sticky liquid begin to evaporate, and tough chemical bonds form between the long molecules left behind. The result is a natural plastic that has become a favorite substance for jewelers. If buried in sediments and subjected to heat and pressure for millions of years, this solidified tree gunk can harden even further and eventually become an organic gem that's light enough to float on salt water.
A treasure on its own, amber becomes even more valuable if it contains remnants of prehistoric life. Anything that happened to fly, walk, fall, or drift into the tacky resin before the substance fully congealed would become immortalized. Many animals or objects that don't often show up in the fossil record--from midges and mosquitoes to feathers and fur--are preserved in fine detail when they're locked in amber.
Termites are prominent members of that list. The insects live below the ground, in mounds, or inside logs or trees, and they must maintain a humid environment to avoid desiccation. Typically, the only parts of the insects to be preserved in sediments as fossils are their wings, says David A. Grimaldi, a paleontologist at the American Museum of Natural History in New York. When trapped in amber, however, even a termite's antennae and delicate leg structures can remain intact.
The eight amber-encased termites that Grimaldi, Margulis, and their colleagues recently studied belong to an extinct species dubbed Mastotermes electrodominicus. The insect's specific name comes from a combination of electron, the Greek word for amber, and a geographical reference to the Dominican Republic, where the 15-to-20-million-year-old amber was unearthed.
Scientists believe the resin that eventually became Dominican amber came from Hymenaea protera, a tree closely related to today's West Indian locust tree, Hymenaea courbaril. Grimaldi and his colleagues say that tiny pieces of undigested wood found in the termites' guts can't be definitely linked to H. protera, but the researchers speculate that the insects fed upon the dead or dying roots of the ancient ambermaker
Worldwide, the fossil record for 20 termite species closely related to M. electrodominicus stretches back about 40 million years. However, some isolated wings found in sediments as many as 130 million years old may belong to the Mastotermes group as well.
Even if that's true, those remnants probably don't reflect the true antiquity of the genus, says Grimaldi, because all of the species are morphologically very primitive. So-called lower termites rely on gut biota to help them digest the cellulose in wood, and they retain many features also found in cockroaches, the termite's closest cousins. Cockroaches first appeared in the fossil record at least 300 million years ago.
THEN THERE WAS ONE All but one species of the Mastotermes termites are now extinct. The lone holdout, the soil-dwelling species Mastotermes darwiniensis, lives primarily in the arid tropics of northern Australia. Known as giant northern termites, these are voracious critters. Besides wood, Grimaldi says, they gnaw the insulation off buried electrical cables and chew through the tires of vehicles that are left sitting on dirt too long.
Don't dismiss those stories as just tall tales from the Outback, says Stuart Smith, an entomologist with the Northern Territory Department of Primary Industry and Fisheries. When the termites destroy polyvinyl chloride pipes or electrical insulation, they don't really eat it, Smith notes. They're just looking for cellulose for food. They've even been known to chew the surfaces of billiard balls.
M. darwiniensis can be found across about 40 percent of the continent and is the most serious horticultural pest in the north and northwest, says Smith. The termites attack homes, fence posts, fruit trees, and timber. Far larger than individuals in any of the other 104 termite species in the Northern Territory, the 12-mm-long insects destroy about 10 percent of the territory's mango trees each year. Railroads don't use wooden ties and utilities don't depend on wooden telephone poles in those areas. In the absence of pesticides, the termites can ravage a home in a matter of months.
"How could something with an appetite like that go extinct?" asks Grimaldi, referring to the 19 other Mastotermes species that have fallen by the evolutionary wayside.
Of course, it's possible that undiscovered species of Mastotermes remain alive in the jungles of Central America or elsewhere. Although termite tallies of Mexico and Panama have been fairly thorough, insect surveys in the five Central American countries that lie between these two nations aren't as complete, says Timothy G. Myles, an entomologist at the University of Toronto.
If the 19 missing species truly are extinct, they may have simply eaten themselves out of house and home. However, David A. Nickle, a research entomologist at the Agricultural Research Service (ARS) in Beltsville, Md., speculates that other termite species could have supplanted the extinct Mastotermes species. Some present-day termites have more effective techniques for protecting their colonies, including chemical defenses, sticky sprays, or elaborate mounds or nests. But M. darwiniensis soldiers who defend the colony against invaders have none of that. Nickle notes, "They've just got their jaws"
What M. darwiniensis lacks in defense mechanisms might be compensated by its numbers. These termites live in colonies of up to 3 million individuals. Each colony--which, as in other termite species, is actually just one big extended family--can stretch across 400 meters and delve about three-quarters of a meter below ground. As a comparison, termites native to the United States have colony populations that range up to only about 200,000, says Guadalupe Rojas, a research entomologist at ARS's Southern Regional Research Center in New Orleans.
Competition from other termites does seem to keep M. darwiniensis in check, says Smith. Human suppression of those rival species, which are easier to control than Mastotermes, has led to a population explosion among M. darwiniensis. Mirex, the most effective insecticide against the primitive pest, is now banned in most parts of the world. The chemical, which is a possible human carcinogen, also was marketed as a fire retardant under the trade name Dechlorane. It hasn't been manufactured or used for either purpose in the United States since 1978, and it's soon due to be phased out in Australia.
Smith and his colleagues have looked for more eco-friendly ways to control the pest--including killer fungi and parasitic mites--but those tested so far either haven't been effective or have had detrimental effects on other species as well. M. darwiniensis' natural predators include ants, birds, echidnas, bandicoots, lizards, and other reptiles.
LIFE'S A GAS Unlike other amber-entombed insects, even termites, specimens of M. electrodominicus are almost always preserved with bubbles of gas emanating from the spiracles, or breathing holes, on their bodies. Sometimes the bubbles are up to 10 times the size of the termite itself, Margulis and her colleagues note.
Before cracking open some of their specimens to examine the insides, the researchers drilled into the bubbles and analyzed the gases there. They found elevated concentrations of methane and carbon dioxide. Normal atmospheric concentrations of these gases are 0.0002 percent and 0.03 percent, respectively. The air in the bubbles contained as much as 26 percent methane and 11.6 percent carbon dioxide, the researchers say. The scientists attribute these extraordinary concentrations to the termites' gut biota, which they say continued to digest the insects' last meal and produce gases even after their host died. Margulis and her colleagues report their findings in the Feb. 5 Proceedings of the National Academy of Sciences.
These elevated concentrations of carbon dioxide and methane in the 20-million-year-old bubbles are typical of those produced by the lower termites. The gases are a natural byproduct of what's being digested in the termite's gut, says ARS' Nickle. They're generated by the microbes in the termite's gut that digest the cellulose in wood and other plant matter. Although the particular species of symbionts differ among the various groups of termite species, and even among species within the same group, almost all of them produce methane and carbon dioxide.
The rate of a termite's gas emission varies from species to species and depends on its diet and the temperature of its environment, among other factors. Numerous researchers have conducted detailed measurements of the insects' production of carbon dioxide and methane because those gases are planet-warming greenhouse gases. Some of those studies blame termites--or, more accurately, their gut biota--for as much as 5 percent of the methane in the present-day atmosphere.
But those studies almost certainly overestimate the effect of termites, says Paul Eggleton, head of the termite research group at the Natural History Museum in London. Hardly any of the methane produced by soil-dwelling termites reaches the atmosphere, he contends, because methane-eating bacteria in the soil scavenge the gas for their own nutrition and belch out carbon dioxide as waste. Only methane that diffuses from the colonies of mound-building termites would reach the atmosphere, and that probably accounts for only 1 percent of the atmosphere's methane.
The fact that the soil bacteria convert the methane from termite emissions into carbon dioxide shouldn't be cause for worry, either. If the wood simply rotted naturally, rather than being eaten by the termites, the same amount of carbon dioxide would return to the atmosphere.
The resin that embalmed the M. electrodominicus termites long ago preserved details smaller than 1 micrometer across, including the multilayered walls of cells in small bits of undigested wood in the insects' gut contents. The microscope also revealed cell nuclei, the cell membranes of spiral-shaped bacteria and the spore coats of other microbes. In all, Margulis and her colleagues found at least three different size classes of single-celled microorganisms that were similar to the symbionts that live today's lower termites.
Eggleton says it's interesting but not surprising, that Margulis, Grimaldi, and their colleagues found microbes within the gut of 20-million-year-old termites locked within amber. What's amazing, Eggleton notes, is the "extraordinary extent to which those gut biota and their microscopic features are preserved" in the Dominican fossils.
There's a long connection between termites and such microbes. Genetic evidence suggests that this type of symbiosis was in place in insects at least as far back as 250 million years ago (SN: 5/19/01, p. 314). Teasing out the details of this long evolutionary drama may take, among other things, a lot more broken amber.
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|Date:||Mar 30, 2002|
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