Within a few metres of you, almost certainly, a vampire is feeding. It grabs its victim and punctures its skin, then drains its bodily fluids. The process takes just minutes. Afterwards the withered husk of the luckless prey is discarded, and the vampire settles down to digest its meal – and maybe make some new vampires.
But this is no Transylvanian immigrant, nor even a perpetual teenager that thinks lurking outside a girl's bedroom is acceptable behaviour. The vampires here are single-celled organisms called vampyrellids that prey on all manner of other single-celled organisms – and even on bigger animals such as worms. They are voracious and ancient, and according to two recent studies, they are everywhere.
"Vampire amoebas" were first described in 1865 by the Russian biologist Leon Semenowitj Cienkowski, one of the founders of microbiology. He discovered bright red single-celled creatures, rather like amoebas, which attacked algae by perforating their cell walls and extracting their contents. Evidently conscious of the similarity to vampire folklore, Cienkowski called the microbes Vampyrella.
Several more species followed, which are collectively known as vampyrellids. Nowadays the vampyrellids are thought to belong to an enormously diverse group of single-celled organisms called Rhizaria.
Their macabre feeding style has fascinated microbiologists for 150 years. A 1926 study describes how Vampyrella lateritia "spreads partly around the doomed cell" and "within a minute or so the transverse walls of the attacked cell begin to bend gradually inward". When they finally buckle, the vampire amoeba "suddenly swells" due to "the injection of algal cell contents into the animal through an oval opening".
We now know that they do not just attack algae. Some species can tackle fungi, or even multicellular animals – specifically, nematode worms. If there is not enough food, the cells of some species can fuse together to form larger structures. These may travel further, allowing them to seek out more distant sources of food.
After they have eaten their fill, vampyrellids build a hard wall around themselves called a cyst. "They stay in an immobile state and digest their food," says Sebastian Hess of the University of Cologne in Germany. This takes a day or two, and at the same time the cell divides. As a result, when the cyst reopens there may be two vampire amoebas where previously there was just one.
Hess and his colleagues have tried to find out how the many vampyrellid species are related. He grew eight kinds of vampyrellid in the lab and sequenced their DNA.
Although the organisms Hess looked at had all been labelled as vampyrellids, nobody knew if they were truly related. In many ways they are quite different. For instance, while Vampyrella species tend to be roughly circular, Leptophrys vorax is much more changeable, able to extend several long arms or stretch itself out like silly putty.
But the DNA confirmed that they all belong to the same group. Hess found that there are at least two related groups within the vampyrellids: one is made up of the various Vampyrella species, while the other contains organisms such as L. vorax. They seem to have divided based on where they live: Vampyrella all live in ponds and puddles, whereas members of the other group live in soil.
A year later, Cédric Berney of the Natural History Museum in London and his colleagues took things a stage further. They discovered eight new types of vampyrellid that live either in the sea or in brackish water, as well as 454 DNA sequences that clearly belong to vampyrellids and which have turned up in samples from around the world.
They are widespread and ubiquitous, but they usually don't occur in high cell numbers
That means there are far more vampire amoebas than anyone guessed – especially in the sea, where until now no one had really looked. In Berney's analysis, the two groups Hess found were part of a larger one – and then there seemed to be two more such big groups, one of which was mostly made up of marine species.
Most of these species are only known from the traces of DNA they have left behind in water or sediment, so we don't know what they look like. Berney's analysis of the DNA suggests that vampyrellids contain more genetic diversity than all the world's fungi, although Hess is sceptical because the analysis is only based on one gene.
These new species may be tricky to find. "[Vampyrellids] are widespread and ubiquitous," says Hess. "But they usually don't occur in high cell numbers." That might be because they are predators that each need lots of space to hunt, or it could be that something is eating them.
But 150 years after they were discovered, the big mystery about vampire amoebas remains unsolved. How do they break through the tough cell wall of a bacterium, or a fungus, to suck out its innards? These cell walls are hard to get through: fungal ones are made of chitin, the same stuff as lobster shells.
We know it happens quickly, says Hess. "The perforation of the cell wall… can be done in five to ten minutes." You could imagine some sort of scissors or club, but Hess thinks it is probably chemical. "They must have a set of enzymes which can digest plant cell walls," he says. "I'm starting to look at that."
It's not just an academic question. Many companies and scientists are interested in making biofuels from algae. These would be an almost zero-carbon source of fuel for vehicles. But it is very difficult to break down the outer walls of the algae to obtain the energy-rich sugars within. Hess says the enzymes of vampire amoebas could turn out to be very handy indeed.