Forams: Nature’s masons and its storytellers

Limestone is composed largely of crystallised calcium carbonate. Some of it comes from the skeletal remains of well-known creatures like corals,but much of the rest comes from less appreciated but truly remarkable organisms called foraminifera,or forams for short.

Forams have been called “nature’s masons” and deservedly so. Most of the 6,000 species of these single-celled protists construct surprisingly complex,ornate and beautiful shells to protect their bodies. After forams die,their shells settle in ocean sediments—and may eventually become rocks that can be used to shelter our bodies.

While tiny relative to ourselves and most familiar marine creatures,forams are extremely large for single-celled organisms,often reaching a third of a millimeter in size. That is 100 times larger than most bacteria and three times as large as a human egg cell,one of the largest cells in our bodies.

The largest forams can reach a few centimeters. The most impressive of all,now extinct,were flat,disk-shaped species called nummulites (from the Latin meaning “small coins”) that were abundant in the limestone used to build the great pyramids of Egypt.

In more recent times,forams have served an altogether different service—to science. Because they produce shells that make good fossils,and have long been abundant and widespread in the oceans,forams are particularly valuable to geologists and paleontologists in telling us about Earth’s history. The forams in the limestone just outside Gubbio provided the first clues to one of the most exciting scientific discoveries in the past century.

In the 1970s,the geologist Walter Alvarez was studying the exquisite limestone formations around this town. Because different species with different shell shapes evolved at different times,foram fossils have been widely used to date rocks. Alvarez was trying to figure out the ages of the rocks around Gubbio. He learned that the topmost layer of the rocks from the Cretaceous period always contained a diverse array of large fossil forams. But the layer of rock just above it,which signalled the beginning of the later Tertiary period,lacked most of those Cretaceous species and contained only a few,much smaller species of forams. Separating the two rock layers was a thin layer of clay that appeared to lack fossils altogether.

Geologist Jan Smit,at VU University in Amsterdam,discovered a similar pattern in rocks in southern Spain. The abrupt disappearance of forams in these layers of rock indicated that something had happened at the boundary between the Cretaceous and Tertiary periods. The end of the Cretaceous period also coincided with the extinction of the dinosaurs,as well as once-abundant marine animals such as ammonites.

Alvarez,Smit and their colleagues wondered: What on earth could have caused the disappearance of widespread,tiny organisms like forams,as well as much larger creatures?

As it turned out,it wasn’t something on earth but something from space. Chemical analyses of the clay marking the boundary of the two periods,carried out by these geologists and their collaborators,revealed that it contained extraordinary levels of the element iridium,a material rare on Earth but more abundant in certain kinds of asteroids. The scientists proposed that the iridium was fallout from an asteroid that struck Earth at the end of the Cretaceous period,65 million years ago.

It took a while to locate where that happened,but the impact crater was ultimately identified underneath the Mexican village of Chicxulub on the Yucatan Peninsula. The asteroid was about the size of Mount Everest and traveling at about 50,000 mph when it hit the Earth,drilling a 120-mile-wide crater and ejecting so much material into the atmosphere that food chains on land and in the oceans were disrupted for thousands of years. The impact caused one of the greatest mass extinctions in history,from the largest animals to tiny forams.

Eventually,forams and the oceans rebounded,and new species evolved. But today,forams are warning us of a new threat,for they are not merely witnesses to Earth’s history,but critical participants in it. Forams are a vital part of a “biological pump” that removes carbon dioxide from the atmosphere. When carbon dioxide dissolves in seawater,one reaction product is carbonate. In making their calcium carbonate shells,the large mass of so-called planktonic forams floating in the upper levels of the oceans sequester about one quarter of all carbonate produced in the oceans each year.

The increasing levels of carbon dioxide in our planet’s atmosphere threaten to overwhelm this biological pump by inhibiting the formation of calcium carbonate shells. As more carbon dioxide dissolves in the ocean,the waters acidify,decreasing the concentration of carbonate and making it more difficult for these organisms to form calcium carbonate shells.

The ocean surface is now about one-tenth of a pH unit more acidic than in preindustrial times. And indeed,a recent study of forams in the Antarctic Ocean found that their shells are now 30 per cent to 35 per cent thinner than in the preindustrial era.

As terrestrial animals,we are most focused on how climate change affects our immediate habitat. But ocean acidification may be a sort of stealth asteroid of environmental change. At current rates of carbon dioxide production,ocean acidity is projected to increase by another three- to four-tenths of a pH unit by the end of the century with potentially catastrophic effects on shell-forming creatures and food chains. And in the present geologic period,guess who is at the top of those chains?