Through The Sandglass

USGS beach health advice - but let's hear it for meiofauna

The US Geological Survey newsroom has just put out an advisory on the importance of washing your hands after playing in the sand on the beach:

By washing your hands after digging in beach sand, you could greatly reduce your risk of ingesting bacteria that could make you sick. In new research, scientists have determined that, although beach sand is a potential source of bacteria and viruses, hand rinsing may effectively reduce exposure to microbes that cause gastrointestinal illnesses.

Yes, indeed, my mother - and mothers in general - were right: "wash your hands!".

Beach pollution is a significant problem, but things could be a lot worse.

I learned many things in the course of writing the book, but perhaps the most extraordinary revelation was the staggering diversity of minute creatures that creatively and tenaciously make their livelihoods in the miniature worlds between the grains of sand on our beaches. The general term for this microscopic zoo is meiofauna, the "lesser animals." Representatives are shown in the image at the head of this post (photos by M.D. Hooge), and it is their world that Rachel Carson was referring to in The Edge of the Sea, when she wrote "Walking back across the flats of that Georgia beach, I was always aware that I was treading on the thin rooftops of an underground city." And the meiofauna do a heroic job of keeping our beaches clean. Here are a couple of extracts from Sand on the interstitial jungle:

In the great hierarchy of living things on our planet, it is generally accepted that, at the head, there are five kingdoms, one of which—animalia—contains all multicelled animals. Within each kingdom are a number of phyla: all mammals, birds, reptiles, amphibians, and fish belong to the phylum chordata. Depending on which biologist you speak to, between thirty-six and forty total phyla are recognized on our planet. Rainforests, our flagships of biodiversity, are known to contain sixteen phyla. The spaces in between sand grains are home to twenty-two.
..........................................................................................................

The dark, watery world beneath the surface of the intertidal zone of most beaches is home to many kinds of bacteria—and a jungle of other minute creatures. Silica, or quartz-based sands, are the most hospitable, since the calcium carbonate in sands composed of shell fragments makes the water too alkaline for many creatures to flourish. Pick up a handful of wet quartz sand on the beach and you are holding a miniature zoo with thousands of inhabitants. Some of these were first observed by Antony van Leeuwenhoek as he peered down his microscope at sand grains and his animalcules, but we owe our understanding of the incredible diversity and importance of this community to the work of Robert Higgins, who since the 1960s has devoted his life to identifying and describing its members. Now retired from the Smithsonian Institution, Higgins is the pioneer of work on meiofauna (“lesser animals”), creatures whose giants are 1 millimeter long; he continues to contribute to the identification of new species, which crop up at the rate of a dozen or so every year. In a 2001 symposium that paid tribute to his discoveries, Higgins modestly described his work as “how the ‘lesser-knowns’ became better-known.”

Life between the sand grains is not an easy one. The grains move and settle under the pressure of the waves and the incessant flushing of the tides, water occasionally drains out completely, and the ecosystem contains a variety of predators. Life has had to adapt, and it has done so in strange and extraordinary ways that reflect an intimacy with the behavior of granular materials. Many of these creatures have armored or padded bodies designed to withstand abrasion by moving sand grains; their shape is often flat or elongated to enable squeezing between the grains; and they have developed a variety of ways of attaching themselves, using glue or suction, to individual sand grains, clinging on for dear life. Rotifers, nematodes, mystacocarids, tardigrades, gastrotrichs, turbellarians, and kinorhynchs—it’s tempting to view these little animals as rejoicing in their exotic names. Tardigrades, which live in both marine water and freshwater, can’t swim, so hanging on to a sand grain is vital; some use mechanical suction toes, some claws, some both. A gastrotrich can glue and unglue itself in an instant. A kinorhynch is ungainly, described by Higgins as an umbrella in a canister, but it moves effectively, if slowly, exploring one sand grain at a time. Rotifers, so named because they look like rotating hairy wheels, are represented by 2,500 different species, most of which are freshwater dwellers. Tardigrades have the remarkable ability to suspend operations if the water disappears, remaining dormant and dehydrated for a hundred years, only to spring back to life when rewetted. They seem to do this by replacing the water in their cells with sugar, which renders them immune to freezing and radiation, a talent that is of considerable interest in the worlds of medicine and extraterrestrial biology.

In the entire living world, only three new phyla were identified in the twentieth century, and one of them came from the strange world of meiofauna. In the 1970s, Reinhardt Kristensen, a professor at the University of Copenhagen and an old student of Higgins, showed him a collection of creatures from the coast of Brittany that he could not identify. “Oh,” responded Higgins, “I have one of those too.” The animals fit with no known group of living things, and it took several years of meticulous collaborative work to define the character of the creature as a new phylum. It looks rather like a classical amphora with snakes emerging from it, and they named it loricifera, “corset-bearing,” reflecting the appearance of the rings that sheathe the animal. More than seventy species have now been identified, but we know next to nothing about their behavior, since, sadly, they die before reaching the laboratory. One remarkable thing we do know is that loricifera have the smallest cells of any known animal.

Inevitably, the community of organisms growing on or living in sand has its own name: psammon. Among the psammon are psammophiles (or arenophiles, if you prefer Latin to Greek), psammobionts, and psammoxenes. The names are weighty, but the members of the community are not; what they are is wondrous. Most of us don’t know they exist, but we should be grateful for them. Without meiofauna, the sands of our beaches and lakeshores would be stinking, toxic places, with organic debris rotting unconsumed and dangerous bacteria rampant. The microscopic creatures of the meiofauna feed off this debris: they keep our beaches clean.

[for more information on these fascinating creatures, see the site of the network of 94 European marine institutes, http://www.marbef.org/wiki/Meiofauna_of_Sandy_Beaches, the International Association of Meiobenthologists at http://www.meiofauna.org/, http://discovermagazine.com/1995/apr/lifeonagrainofsa491, and http://whyfiles.org/022critters/meiofauna.html, the latter particularly on Higgins.]

"Giant sand worms lived in Torbay"

 

What a headline - how could I resist? It's from the UK newspaper, The Telegraph, earlier this year - it's not the paper I read, so I only just came across it on the web. Torbay is on England's southwest coast, a popular holiday resort boasting sun, sand, and surf, but also, reflecting its diverse and important geology, the area has been designated "The English Riviera Geopark." Geologist Kevin Page, of Plymouth University, is deeply involved in the preservation and recognition of the region's geodiversity; it's one of his projects that led to the headline and reports such as "Traces of an unknown lifeform have been found in rocks in a secret location in Torbay" and "Scientists have found evidence of a giant prehistoric sand worm in an English seaside resort" (but the image above is purely my own fabrication).

Torbay is in Devon, the only county in Britain to lend its name to a geological period, and Devon’s geological fame is in part linked to its inspiration for the establishment of the original Devonian System in 1836 by Sir Roderick Murchison and Professor Adam Sedgwick, two of Europe’s great geological pioneers. 400 million years ago, Devon lay in balmy tropical latitudes, the legacy being great tracts of limestones containing abundant reefs, corals, trilobites and other key critters of the times. But Torbay gradually drifted into desert latitudes and this, combined with general climatic deterioration, resulted in the environment changing dramatically. Flash floods hurtled down desert wadis, scree blanketed the flanks of newly formed mountains, and seas of dunes invaded the landscape; the rocks, from the Permian period, are almost uniformly red sandstones, gravels and conglomerates, recording the dire and extreme conditions that ultimately presaged one of the planet's greatest extinctions (these rocks form the cliffs in the background of the image above).  But life, often struggling, there was, and these rocks preserve traces of it - often, literally trace fossils, tracks, trails, and burrows, footprints in the sands of time,  rather than the creatures themselves whose remains were battered and decomposed into oblivion. And Dr. Page's discovery, that led to the dramatic reporting, is an example of such trace fossils - giant burrows. Now "giant" may be a slight exaggeration, but they are certainly big, and the worms, if indeed they were responsible, were apparently up to a meter long and fifteen centimeters wide. Dr. Page (shown with burrowed sandstone) reports:

It really is quite extraordinary. Nothing like this has ever been found before. The underground area is peppered with these burrows. There is no supporting evidence to suggest they were made by creatures we know about, so what were are looking at is an entirely new life form. It is very, very strange. They were made at the end of the Paleozoic period before dinosaurs came along when the earth teemed with creatures which are now extinct. We have found the holes but as yet we haven't found the animals which made them. They would have looked like the worms from the film Dune. It is science fiction meeting science fact. We know about giant millipedes at the time but this is something quite different. They are unknown to science and a completely new species. It is life, but not as we know it.

The obvious reference to Frank Herbert's book and the classic science fiction movie (not to mention the cult classic, Tremors) has provided fodder for some of the more lunatic fringes of the internet, with sites devoted to the paranormal and the "unexplained" enjoying fantastic speculation. The science, once the analysis is complete, will be published later this year.

Trace fossils, while providing only a vicarious view of life, are, nevertheless, immensely important; they often the only view we have of certain life forms at certain times and they yield insights into lifestyles and locomotion. The recent issue of Earth magazine carries a short piece illustrating this from the Cambrian (also named by Sedgwick) of Wisconsin, in sandstones from perhaps 500 million years ago. Adolf Seilacher, now in his eighties and perhaps the greatest reader of the runes of trace fossils, and James Hagadorn, report how early visits to the land may have been possible.

 

The trace fossil they describe (above) could be the track of a small 12-legged marine critter that, like a hermit crab, kept its gills moist by coiling itself into a shell, and set out onto the beach. If this is the case, then "hermit-like" behaviour originated 290 million years earlier than had been previously thought.

And then, of course, there are the most popular and evocative trace fossils - below, the longest dinosaur tracks in the world, in Bolivia - strolls in the lake, side-by-side, 70 million years ago.

 

 

[Torbay worms http://www.telegraph.co.uk/science/science-news/4995386/Giant-sand-worms-lived-in-Torbay-scientists-claim.html; http://www.thisissouthdevon.co.uk/news/280m-year-old-worm-new-species/article-769879-detail/article.html. Bolivia tracks images, Jerry Daykin, Cambridge, Creative Commons Attribution License, http://images.google.co.uk/imgres?imgurl=http://upload.wikimedia.org/wikipedia/commons/0/03/Dinosaur_tracks_in_Bolivia_1.jpg]

The sandfish - "like a man diving into water"

My aspiration for the book and this blog, beyond the hope that they would be read at all, has always been that readers would be surprised. This arose simply because of the extent to which I surprised myself - clearly, I had a good idea of the diversity of journeys and connections, and the fascinating questions, answered and unanswered, that the topic would lead to, but the more I looked, the more surprised I became. And it doesn't stop. I've lost track of all the ideas for posts that have simply gone on to the "to do" pile, shouldered aside by something immediate that comes up. I recently wrote about locomotion in sand, robotics, the intimate relationships small critters have with granular materials, and whether sand is a solid or a fluid or something else entirely. And just a couple of days ago, browsing through the Science Daily online news headlines, there was a report on research that intersects with all of these. 

I've mentioned Scincus scincus, otherwise known as the sand skink, sandfish, or sand swimmer, before. This little desert lizard has a remarkable ability to disappear, and then "swim," beneath the surface of the Saharan sand, rather like its blind Australian relative, the itjaritjari, the local description of whose skill is the subtitle to this post. Now new research at the Georgia Institute of Technology has shed amazing light on how these lizards do it. The sandfish demonstrates distinctly interdisciplinary skills, and it took collaboration between the School of Physics and the Interdisciplinary Bioengineering Program to analyse them. Daniel Goldman and his colleagues built a small tank filled with glass beads (the sand) in which the density, the packing, could be controlled by pulses of air from below, a sort of fluidised bed. They then let a sandfish demonstrate its skills and used high-speed X-ray imaging to reveal its activities. It was already clear that the shape of the lizard's body and limbs was well-suited to sand locomotion, and there were some suggestions that the nano-scale character of its smooth scales also contributed to its skills. But does it use its limbs to "swim"? The Georgia Tech study suggests not - that, once below the surface, the sandfish tucks its legs alongside its body and propels itself with a snake-like wave motion, a single-period sinusoidal wave that travels from the head to the tail. The news release on the study discusses the kinematics in more detail, and contains links to several short videos of Scincus scincus in action; these make for compulsive viewing, and I've taken the liberty of using screenshots from them to illustrate this post.

 

It turned out that the density of the sand made little difference for the lizard - it simply changed the frequency of its undulation and maintained speeds of up to six inches a second; its methodology is unique, as Goldman describes:

The large amplitude waves over the entire body are unlike the kinematics of other undulatory swimming organisms that are the same size as the sandfish, like eels, which propagate waves that start with a small amplitude that gets larger toward the tail....The results demonstrate that burrowing and swimming in complex media like sand can have intricacy similar to that of movement in air or water, and that organisms can exploit the solid and fluid-like properties of these media to move effectively within them

The team then developed an empirical model of the drag forces faced by the lizard, and, indeed, any object moving through granular material:

Although viscous hydrodynamics can predict swimming speed in fluids such as water, an equivalent theory for granular drag is not available. To predict sandfish swimming speed, we developed an empirical model by measuring granular drag force on a small cylinder oriented at different angles relative to the displacement direction and summing these forces over the animal movement profile. The agreement between model and experiment implies that the noninertial swimming occurs in a frictional fluid.

And, as always with research of this kind, the implications and potential applications for handling of granular materials go far beyond the skills of a clever lizard:

In addition to having a biological impact, this study’s results also have ecological significance, according to Goldman. Understanding the mechanics of subsurface movement could reveal how the actions of small burrowing organisms like worms, scorpions, snakes and lizards can transform landscapes by their burrowing actions. This research may also help engineers build sandfish-like robots that can travel through complex environments.

“If something nasty was buried in unconsolidated material, such as rubble, debris or sand, and you wanted to find it, you would need a device that could scamper on the surface, but also swim underneath the surface,” Goldman said. “Since our work aims to fundamentally understand how the best animals in nature move in these complex unstructured environments, it could be very valuable information for this type of research.”

And, as always with our attempts to better understand the intricacies of the natural world, it's probably more complex. Research conducted recently in Germany and also reported in Science Daily last year, describes specifically how the sandfish uses its limbs to move "in a way very similar to the crawl stroke in swimming." Ah well, it just makes the little lizard all the more interesting - watch this space!

[The Georgia Tech study is described at http://www.sciencedaily.com/releases/2009/07/090716141140.htm and http://www.gatech.edu/newsroom/release.html?id=3161, and the abstract in Science is available at http://www.sciencemag.org/cgi/content/abstract/325/5938/314 - full version for subscribers.] 

Sand-carrying bearded ladies

Carrying a handful of sand is never easy. But imagine a change of scale - I have a pile of river pebbles and cobbles (garden, ornamentation, for the purpose of) that I need to move, and fortunately, I have a wheelbarrow. But if all I had were my own limbs, carrying an armful of cobbles is nigh-on impossible; now imagine that you're an ant - every cobble is a sand grain and it needs to be moved up and out of the deep subterranean nest that you have been busy excavating. Now of course if the sand is damp, then the grains tend to stick together and carrying a clump of grains is feasible, but, if the sand is dry, then the problem is that of an armful of cobbles, and a conscientious ant is not going to be satisfied with one grain at a time. And the luxuriously-named pogonomyrmex is an ant that's solved the problem.

I am constantly fascinated by the inventiveness and adaptability of creatures, particularly very small ones, in their relationships with sand - endless examples of nature's creativity. Not being in any sense an expert on such things, I just keep encountering more extraordinary examples, of which pogonomyrmex is one. This encounter is thanks to the blog by Alex Wild, a biologist at the University of Illinois with a particular enthusiasm for ants (his website,  http://www.myrmecos.net/index.html, includes two categories listed as "ants" and "not ants"); furthermore, Alex is a superb photographer of his subjects - the images above are from his site and used here with his kind (and instantly granted) permission.

The Greek word pogon means beard, and pogonomyrmex is a genus of generally desert-dwelling harvester ants - bearded ones. The beard is a basket-like structure of stiff hairs below the head that is extremely handy for carrying seeds, eggs - and sand grains; it's called a psammophore, "sand-carrier," and can clearly be seen in the portrait of pogonomyrmex rugosus, below (also courtesy of, and copyright, Alex Wild).

 

It was the psammophore of pogonomyrmex desertorum in action that first struck me - these are the images at the top of this post; the hairs can be seen clutching a load of fine sand grains, and, as the ant cleans herself off after dumping them (right), the whole structure becomes clear. And yes, we know it's a female because males don't dig - it's the females, the bearded ladies (a phrase I have unashamedly borrowed from Alex) who do the work. And, as Alex remarks,

What most intrigues me about psammophores is that lots of completely unrelated groups of desert ants have them. Apparently psammophores evolve at the drop of a hat, although this is perhaps not too surprising. Most ants already have hairs of some sort on the underside of the head.

The excavating efforts of the pogos (as, it seems, these ants are affectionately known) are monumental - these ladies can dig. Research at Florida State University used innovative techniques to reveal the subterranean dimensions and architecture of nests of pogonomyrmex badius, the Florida harvester ant. The sheer size of an excavated pile of sand next to a nest entrance is a fairly good indication of the extent of the underground workings, but their design is difficult to see. Walter R. Tschinkel developed a method of filling a nest with a thin slurry of orthodontal plaster (not to mention molten zinc and aluminum) and then excavating the cast; the sandy Florida soils are sufficiently permeable that air is easily forced out of the small structures of the nest, allowing them to be completely filled with the plaster. The results are incredible, and a full report is available at the BioOne site here. Large nests can be three meters deep, and the architecture is exquisite (photo below).

Shafts descend in a regular helix form, with chambers excavated at varying intervals off them. The number of chambers at shallow levels is much greater than at depth, with different populations of the colony occupying different depths and performing different functions. It would seem that the senior citizens live and work in the relatively less crowded upper levels, with the youths consigned to the depths. The demography and dynamics of the colony are complex, the details - and mysteries - revealed in the article fascinating. As Tschinkel comments,

The nest's architecture represents a considerable investment of energy and time, and provides shelter, microclimate choice, and defendability. In addition to these obvious services, nest architecture is likely also to be a mechanism for integrating the colony members into an efficient functioning whole, the superorganism. The large amounts of time and effort invested in nest construction surely must pay dividends in fitness, for this energy could have been invested in other fitness-enhancing functions instead. An as yet unmet challenge is to identify how particular features of the architecture serve particular colony functions, and how this service contributes to colony fitness.....

The functional significance of the top-heaviness is mysterious. This large space seems under-used, typically harboring very low worker density. Perhaps this area is important for regulating brood development rate during the cooler months, for these chambers are exposed to the greatest diurnal variation in temperature. Alternately, a unit volume of upper chamber is cheaper to excavate than deeper ones because the soil is brought up over smaller vertical distances, but if this space is not heavily used, why create it? If top-heaviness is not functional, but merely a non-adaptive epiphenomenon of some other behavior, it is an expensive one.  

This is but an initial excavation of the extraordinary world of pogos and their relatives - there's more to come.

[All pogo photos courtesy of and © Alex Wild, http://myrmecos.wordpress.com/2008/03/19/the-bearded-ladies/; nest cast photo from The nest architecture of the Florida harvester ant, Pogonomyrmex badius: Walter R. Tschinkel, Department of Biological Science, Florida State University, Tallahassee, FL 32306-4370]

Of reptiles and robots - locomotion in sand

Anyone who clambers up the slip face of a dune becomes rapidly, and agonizingly, aware that locomotion in sand is a challenge. Even walking on the beach often requires a different kind of gait. Granular materials just make life difficult for humans, other critters, and vehicles (see the problems for Spirit, bogged down in the sands of Mars, Humphrey Bogart's encounter with the angle of repose, or the travails of participants in the Marathon des Sables). For us, simply walking on sand requires around two-and-a-half times as much energy as on a hard surface - we're actually better off running, which only takes one-an-a-half times as much effort (although my knees are reluctant to put this to the test). Yet there are large numbers of our companions in the world that are exquisitely adapted to locomotion on and in granular materials - at least under the right conditions. The complex networks of tracks and trails in the sands of the Kelso dunes reminded me of this (as I paused for breath), together with the impossibility of photographing fleet-footed lizards dashing effortlessly for cover at speeds that the eye can barely follow. 

The ghost crab, so-named because of its ability to disappear instantly into the sand, and whose genus, Ocypode means "swift-footed," is one of the record-holders in the granular olympics; in the right conditions, it can scoot across the surface at speeds in excess of five miles an hour, its small body becoming a blur. However, if the conditions are not just right and the sand is soft, the ghost crab runs into trouble and is swiftly overtaken by the zebra-tailed lizard who, although surprisingly does not spend a great deal of time in the sand, nevertheless can cope with widely varying granular conditions, even quicksand. It would seem that the lizard's talents derive from clever foot-design and actions, but the fact is that we're not entirely sure (see http://www.sciencemag.org/cgi/content/full/315/5810/325). The variety of critters who are good at moving around in sand is equalled by the variety of ways in which they do it. Think of the snakes, the sidewinder for example, whose waving, looping motion leaves such unique and beautiful designs; or another lizard, the sand skink, also known as the sand swimmer or sand fish, which seems to have taken a kind of nanotechnology approach, the microscopic design of its scales reducing friction and minimizing abrasion as it swims effortlessly beneath the surface of the sand for much of its existence. The sand skink's motion is also unique, and enables it to treat the sand as a viscous fluid through which it can swim (http://www.plosone.org/article/info:doi/10.1371/journal.pone.0003309). Or consider the charmingly named itjaritjari, a strange little marsupial mole that lives throughout Australia’s deserts. It seems to have no eyes or ears, but put it down on the sand and it will disappear, as the Aboriginal people say, “like a man diving into water.” No one knows how the itjaritjari navigates or senses, but it has a hard nose and front feet well adapted to excavating, the back ones being webbed to help push through the sand; the young are carried in a pouch that cleverly faces backward so as not to fill up with sand. And then there are the insects.

Trying to understand the skills of these expert critters is the focus of research around the world, for efficient locomotion in granular materials is critical in the realm of robotics, particularly for extraterrestrial explorers (as Spirit's problems so dramatically illustrate).  Daniel Goldman at the Georgia Institute of Technology in Atlanta was one of the researchers chasing ghost crabs and zebra-tailed lizards across laboratory sands in his pioneering work in the field of biomechanics, and he has now turned his attention to robotics. As reported earlier this year, Goldman, together with colleagues from the University of Pennsylvania and Paul Umbanhowar at Northwestern (who was extremely helpful to me as I was writing the book, trying to comprehend his extraordinary work on oscillons - no doubt the subject of a later post), has made real progress on designing a robot that doesn't get bogged down. Their laboratory test bed track consisted of an eight-foot-long container filled with poppy seeds and with holes in the base so that granular material could be aerated and turned into variable types of fluidized bed with different densities. "We used poppy seeds as the granular material because they were large enough not to get into the SandBot motors but light enough to be manipulated with our air blowers," explained Goldman. "We have done experiments with small glass beads, which more closely approximate desert sand, and found no qualitative change in the results." The photo below shows the SandBot at rest and churning through the poppy seeds. The robot's "legs" are C-shaped and their rotation characteristics variable. The ability of the robot to keep moving is highly sensitive to the density (and therefore the packing) of the sand and the frequency with which the wheels rotate. It will hardly be a surprise to any of us who have buried the wheels of a 4WD in the sand that the looser the material and the faster the wheels are rotating, the more easily will the robot bog down. As Goldman said, "changes in volume fraction [packing and density] of less than one percent resulted in either rapid motion or slower swimming. We saw similar sensitivity when we changed the limb rotation frequency."

The Georgia Tech news release concludes with the following: Goldman "also plans to use the information to help roboticists design devices with the appropriate feet and limb motion to move well in complex terrain - including sand. Future robots may have the ability to sense the type of material they are walking across, allowing them to adjust their limb motion accordingly. Such smart robots would advance the exploration of other planets, as well as search-and-rescue missions in disaster settings." This is important - and fascinating - stuff, worth contemplating next time you're struggling up the face of a sand dune, gazing enviously at that lizard.

[The report on Goldman's work linked above also contains some videos of the SandBot's efforts, enjoyable viewing. Ghost crab image via Creative Commons license at mmhaffie's flickr site, zebra-tailed lizard image, Madalyn Berns and ScienceMag, sand skink image credit Matt Gunner at flickr, sidewinder and cricket images, National Geographic]

More adventures of bacillus pasteurii - mending concrete

The efficiency with which bacillus pasteurii manufactures calcium carbonate and glues things together makes for ways in which our microbial friend can assist us beyond just preventing earthquake damage and sculpting deserts (see the previous post) and here's, literally, a concrete example. But I should point out here that the image above is quite unrealistic, simply a Sandglass photoshop special.

The recipe for basic concrete is simple and has been around for a long time. The ancient Egyptians knew how to make it (there is a lively debate as to whether the pyramids, at least in part, are made of concrete), and the Romans perfected the formula. The fundamental ingredients are around 75 percent sand and gravel, 15 percent water, and 10 percent cement. The cement, cooked from materials such as limestone and clay, is the chemical glue; the hardening of concrete is not simply due to drying, but involves complex chemical reactions. The physical characteristics of the sand, its size and shape, influence the properties of the concrete, but because of the importance of chemistry, the composition of the sand and the other ingredients is critical. The wrong impurities will ruin the quality of the concrete.

The global demand for concrete is massive: after water, concrete is the most consumed material on Earth. Every year, the equivalent of more than 400 million dump trucks of concrete is transported to construction sites. Every man, woman, and child on the planet “consumes” around forty times their own weight in concrete per year. Which is, of course, an average—for residents of the Western world, it’s much more, despite the fact that around half the world’s concrete production and consumption today is accounted for by China.

But concrete is vulnerable to deterioration, corrosion, and cracks, and the consequent damage and loss of strength requires immensely expensive remediation and repair. So, is there a way in which incipient microscopic fractures in concrete can self-heal? The answer, thanks to our microbial friend, would seem to be yes, through the process referred to as biomineralisation, microbiologically induced calcite precipitation (MICP). Around the world, research groups are working and collaborating on different approaches to turning this into reality, but the principle is quite simple. The chemistry of concrete is alkaline, but fortunately, bacillus pasteurii tolerates this - it simply needs nutrition, space to work, and a little encouragement. A team led by Henk Jonkers at the Delft University of Technology in the Netherlands are well on the way to accomplishing this, through actually mixing living bacteria, along with calcium lactate, an organic compound that such bacteria convert to calcium carbonate, into the concrete to begin with. They found that, as the concrete cured, incipient cracks were effectively sealed with calcium carbonate. There remain problems of keeping the microbes alive for a long period of time, and retaining sufficient microscopic space in the concrete for them to have room to maneuver, but progress is being made (the addition of clay to the mix provides a safe haven). And, importantly, the more effective this process proves, the less concrete we will have to manufacture and the lower the associated carbon dioxide emissions from making cement.

As a recent article in The Economist concluded, "If the process can be scaled up, it may be prove that the best way to preserve concrete is to infect it."

[for details on the work of Henk Jonkers, see http://www.tudelft.nl/live/pagina.jsp?id=8691221d-ebab-4841-97cb-1cfacad3a4bc&lang=nl, a recent conference presentation, a link to the abstract of the group's most recent paper here, and The Economist at http://www.economist.com/science/displaystory.cfm?story_id=13570058. For work at the South Dakota School of Mines and Technology, see http://adsabs.harvard.edu/abs/2001SPIE.4234..168R, at the S. V. National Institute of Technology in India, http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1285418, and for non-bacterial approaches, http://www.eurekalert.org/pub_releases/2009-04/uom-scf042209.php.]

From Bogart to Bugs - the Angle of Repose

I try to make a point of watching all movies that feature sand in a significant role (probably the subject of a future post), and my 88-year-old father-in-law who lives in Philadelphia recently told me of one that I'd missed - Humphrey Bogart's Sahara. "Watch out for the great lesson in the angle of repose," he told me (he's an engineer). And he was right. The movie was made in 1943 and, while Bogart, of course, accomplishes amazing and rather unrealistic feats of survival and military strategy (it was, after all, a wartime release), it features some remarkable scenes of sediment movement. It was filmed in what is now the Anza-Borrego State Park and the dunes beside the Salton Sea, the area that was then the army's California Desert Training Center. In the scene that my father-in-law was referring to, a lone and desperate soldier attempts in vain to climb the steep face of a dune, the sand cascading, sliding, and avalanching constantly around him - just before, miraculously, a British patrol arrives out of, literally, nowhere.

So there I was, back in the strange world of granular materials and piles of sand, any one of which has its own natural angle of repose - the maximum slope at which the grains are stable. Steepen or undermine the slope and grains will tumble down the flank of the pile, reestablishing the angle of repose. Strictly speaking, there are two key angles for a pile of any particular material: the angle of repose is its slope after avalanching and coming to rest; the angle of stability is the slightly steeper slope that it can adopt as grains are added before avalanching (and there's also the angle of internal friction, reflecting the fact that the angle of repose is governed by the friction between the particles). But we'll stick with the first, simple, definition. The angle can vary enormously, depending on the size of the grains, the range in size, how much moisture there is, and so on. You can demonstrate this easlily in your kitchen (like so many geological phenomena) by making piles of the different granular materials which are easily to hand - normal fine-grained salt makes a much shallower pile than coarse salt crystals. The angle of repose is of interest to physicists and sedimentologists - whether it's the means by which dunes move (see the banner image on this blog), small-scale features on the beach, landslides, or the determining factor in the internal structure of sand ripples and banks - cross bedding - that tells us so much about chapters in the Earth's history.

But it's also important in our daily lives - and not just in the kitchen.

Anywhere that granular materials are stored, whether it be in silos or in piles of aggregates, mining products, or cereals, the angle of repose - and its sensitivity to changing conditions - is something that needs to be carefully managed. Failure of the slope of a sand pile is a regular cause of tragedies on the beach and gold mining in placer deposits, not to mention in the sand mines of Long Island (previous post). The failure to properly manage the gigantic tips of refuse from the coal mines of Wales resulted in the appalling tragedy at Aberfan in 1966, where the collapse of one of those manmade mountains killed 144 people, including 116 children in the buried school.

But on a lighter note, the angle of repose would seem to be something well-understood by various critters, notably the antlion, sometimes referred to as the sand dragon. The antlion is the larval stage of a large family of insects, the  Myrmeleontidae, the adults of which are quite attractive four-winged creatures. But the larval stage - and they can remain larval for several years - are fearsome predators. The 2000 or so species occur around the world, known colloquially in North America as doodlebugs, since their rambling tracks in the sand resemble doodles on paper. Doodlebugs love the angle of repose and use if very effectively in obtaining their dinner. An antlionwill excavate, backwards, a pit in the sand and bury itself at the bottom, its voracious and venom-laden jaws poised for a visitor. The visitor - an ant or any other victim for the undiscriminating antlion - will fall into the pit and struggle in vain to escape against the angle of repose of the sides. Its struggles are further thwarted by the antlion throwing jawfuls of sand at it, precipitating further avalanches, until dinner is served (there's a great video of this drama here)

Oh, and when it's finally ready to move on in its development, the antlion will spin silk from its posterior and use it to glue together a cocoon of sand grains for its pupal stage. 

  

 [antlion and pit images, courtesy of Scott Robinson, pupa image,Jonathan Numer, Creative Commons Licenses; Aberfan image and detailed information from Iain McLean & Martin Johnes, http://www.nuffield.ox.ac.uk/politics/aberfan/home.htm]

Of Mice and Sand - Evolution in Action

This is the year of the great Darwinfest and, inevitably, sand makes its contribution. The immediate reference would be to the "sand walk" at Down House in the Kent countryside, Darwin's "thinking path" that he walked every day. A visit to Down House (appropriately under consideration as a World Heritage site) is, for many of us, an intellectually spiritual experience. Walk the sand walk, and the sense of place is resonant. What is arguably the greatest idea ever was formed here. It is the place where our understanding of ourselves and our place, and the place of every living thing in the natural world, was born.

The process of natural selection, illuminated today so brilliantly and so subtly by genetics, is a natural wonder, the place where the scenes of life's history, as revealed through geology, develop into the dramas of today's biosphere.  Sand, being the ubiquitous and hyperactive player in the games of our planet's past has, of course, always influenced evolution. Granular materials, whether in the desert or the surf zone, are not the easiest places to live, and the challenges have driven endless adaptations. But what I want to focus on here is an example of evolution in "real time," changes that are happening at a pace that we can comprehend on our timescale.

The coastline of Florida as we see it today is young. Quite apart from the fact that the coast today is not what it was yesterday, the major features, however they are constantly changing, were sculpted a few thousand years ago as sea levels rose after the last Ice Age. The barrier islands of the Gulf Coast and the sweeping beach and dune systems of the Atlantic side are works in progress. As are their populations of mice. Peromyscus is the most widespread mammal in North America, a species that has adapted to and enjoys a wide range of habitats, including the mainland of Florida, its Atlantic beaches, and its recently formed barrier islands off the Gulf Coast. But the mice that live on the light sand beaches and among the dunes have distinctly lighter fur than their relatives who inhabit the dark soils of the mainland, and  Hopi E. Hoekstra, John L. Loeb Associate Professor of the Natural Sciences in Harvard's Faculty of Arts and Sciences, together with her colleagues at the University of California, San Diego, and Leipzig in Germany, have demonstrated that this is evolution at a dramatic pace. A paper in this month's Molecular Biology and Evolution (http://mbe.oxfordjournals.org/cgi/content/abstract/26/1/35?etoc) reports the latest results from a project that has been going for some time. Not only does their work reveal some of the complexity of genetic switches, nucleotides, and amino acid mutations that choreograph evolutionary change, but it importantly sheds new light on the thorny and much-debated topic of convergent evolution, showing that, like so many things, it's not as simple as we might like to think. Now, when I say "we," I am not really referring to myself - as a geologist, the time it takes me to be out of my depth in the field of molecular biology can be measured on the nanoscale, so I shall return to the influence of sand.

Hoekstra's project has studied beach mice in diverse habitats, among them Santa Rosa Island, one of the longest barrier islands in North America, and the site of a Civil War battle. The mice forage on the white sands and among the dunes, eating mainly sea oats, and trying to avoid predators from the skies - owls, herons, and hawks. And, nature being red in tooth and claw, mice with lighter-coloured coats are less visible and more likely to survive. As natural selection, it's as simple as that (putting aside the complexity of the genetic mechanisms that actually allow it to happen). Completely distinctive subspecies have evolved and it's all happened in just a few thousand years - as Hoekstra says, “It’s a large effect mutation. And what it says is that adaptation does not always occur gradually, but may happen in these relatively large jumps.” The barrier island beach mice are genetically more similar to (but still different from) their Atlantic Coast relatives hundreds of kilometers away than they are to their nearby mainland cousins.

Unfortunately, avian predators are not the only threats that the beach mice have to deal with. Our lunatic obsession with beach front property development is destroying their habitats, and, barrier islands being where they are, hurricane damage is frequent and extensive; Hurricane Ivan excavated a completely new breach in Santa Rosa Island as seen in the image below - a great example for fans of storm-generated overwash sand deposits.

 

The beach mice are disappearing rapidly - since Hoekstra's project began, one subspecies has become extinct and six of the seven remaining are endangered.

There are other examples of sand habitats actively driving evolutionary change. The gloriously named bleached earless lizard lives among the stark white gypsum dunes of the White Sands National Monument, again formed only a few thousand years ago. The lizard is thus named because it is bleached - compared to its darker relatives beyond the dunes, it has lost its colour as a protective adaptation (it has been shown that this adaptation is not driven by temperature regulation). Spiders, scorpions, toads, and mammals living in the dazzling dunes are all paler than their brethren beyond the sand. 

[see Hopi Hoekstra's site: http://www.oeb.harvard.edu/faculty/hoekstra/Links/ProjectsPage.html. Beach mouse photo from

http://www.oeb.harvard.edu/faculty/hoekstra/Links/ProjectsPage.html, credit J.B. Miller, Santa Rosa hurricane damage photo from

http://coastal.er.usgs.gov/hurricanes/ivan/photos/florida.html. For more on evolution at White Sands, see http://www.nps.gov/whsa/naturescience/white-animals.htm; lizard photo from that site.]

Grain-by-Grain (1): worm superglue

We are not the only species that builds homes and condominium complexes from sand.  For example, some foraminifera construct shells from  sand grains, as does the Trumpet Worm - and excellent artisans they are. A particularly clever worm which has been in the news lately is the so-called  Sandcastle Worm, or Phragmatopoma californica to be precise, varieties of which live around the world, generally in the intertidal surf. To survive in this energetic environment, they build robust tube-shaped homes a few centimeters long from anything that's around - which is mostly sand - and congregate together in large colonial apartment buildings. The construction process is meticulous. The worm's tentacles select an individual sand grain and move it on to a remarkable appendage which puts two small dabs of glue on the grain and places it where the next piece of construction is needed. It holds it there for around 25 seconds, moving it a bit to ensure that it's snuggly in place, just as we do when gluing two things together. By the time the glue is set, the next grain is ready for the project.

  

The glue is amazing stuff - based on proteins, natural polymers which can be negatively or positively charged, plus calcium and magnesium ions, all structured into complex chains that link together and, through a chemical conspiracy similar to that used by mussels and other marine creatures that like to attach themselves to things, make a very strong glue. And, of necessity, one that works in the wet. Russell Stewart, Associate Professor of Bioengineering at the University of Utah, together with his colleagues, recognized the potential medical applications of such a natural superglue and set about understanding how it works and how it might be synthesized. The Sandcastle Worm is perfectly happy to work with whatever material is available and so, in the laboratory, Stewart provided the worms with sandgrain-sized glass beads which they proceeded to busy themselves with. The photo at right shows a worm tube started with natural sand and extended with the glass beads; below is a scanning electron microscope image of the extraordinary care and accuracy with which the beads are glued together.

Stewart used his detailed analysis of the glue to cook up a version that mimics the clever chemistry of the worm's secretion using synthetic polymers that are more practical to make. The synthetic glue has the potential to put small fragments of broken bone back together, to attach tissue scaffolds that encourage the growth of new bone, and to carry drugs to deliver antibiotics, pain killers and perhaps even stem cells with precision. As Patrick Tresco, associate dean for research at the University of Utah's College of Engineering, says in the press release: "Most current adhesives do not work when surfaces are wet so they are no good for holding together bone, which is wet and bloody. There is nothing like it [the synthetic worm glue] on the market today."

It may not be too long before many of us are profoundly grateful to the brilliant chemical engineering of the humble Sandcastle Worm.

(images from the University of Utah news release, http://www.unews.utah.edu/p/?r=111008-1, where there is a link to an amazing video of a Sandcastle Worm at work with fragments of silicon chips, and  http://education.cnsi.ucsb.edu/pages/ar/projects/projects2005/pdfs/Kristen_Mauricio.pdf.)