The Best Australian Science Writing 2012 (26 page)

Waite is careful to note, however, that he's not attacking the liquid-water model. ‘We can't say that all of the things are con-sistent
with liquid water,' he says. ‘Some are and some aren't.' One possibility, he says, is that chemicals in the plumes might come from multiple processes, all operating at once.

Even if Enceladus has liquid water, that's not proof it has life. To answer that question, says Larry Esposito, a planetary scientist at the University of Colorado, scientists need to find biomarkers – chemicals that appear to have biological rather than geophysical origins.

That's not an easy quest in a world where organisms, if they exist at all, would live in underground lakes, ponds or oceans where sunlight never penetrates and photosynthesis isn't an option.

One possibility, says McKay, is a ‘methanogen system' in which microorganisms live by making both energy and biological building blocks by synthesising methane from carbon dioxide. He's particularly fond of this idea because it might be sustainable over very long time periods if geological processes can recycle the methane produced by the bacteria into zones where temperatures exceed 500°C. ‘That decomposes the methane,' he says. ‘You're turning geochemical heat into chemicals and the organisms are eating the chemicals.'

Ronald Oremland, a microbial biogeochemist with the US Geological Survey's office in California, believes an even better food source would be acetylene.

Acetylene-eating organisms exist on Earth, he points out. And, he notes, this chemical (used as welding-torch fuel on Earth) also occurs on comets. If there's enough on Enceladus, he says, it could be ‘fast food' for microbes – a primordial food source on which Enceladus bugs might still be nibbling away.

Acetylene has already been reported by Cassini's instruments. But to link it to acetylene-eating organisms, he says, scientists need to find by-products of acetylene metabolism, such as acetate and acetaldehyde.

Other possible biomarkers are amino acids, especially if
they can be tested for chirality – the relative amounts of mirrorimaged shapes known as D and L isomers. Abiotic processes tend to produce an even mix of the two. Biological ones favour one or the other. On Earth the L versions are favoured, but there's no reason extraterrestrial life couldn't do the reverse. ‘If we find amino acids and there's a strong chiral preference, that's persuasive evidence for a biological origin,' says McKay.

Another marker, McKay and Oremland agree, would be the ratio of carbon's two stable isotopes, 12C and 13C, in compounds emitted by the plume. That's because biological processes produce compounds with a slightly higher ratio of 12C to 13C than those produced by non-biological processes. Testing 12C/13C ratios, in fact, is one way sports authorities catch drug-cheating athletes, because synthetic hormones, produced in a lab, have different ratios from those produced by the athletes' own bodies.

None of this, however, can be done with Cassini's instruments. Such tests would require a return to Saturn, either with a specialised new Enceladus probe, or one piggybacked on a mission to Saturn's giant moon Titan, also on the wish-list for a return visit.

‘What we're being handed at Enceladus is a potential gift of looking at life in the outer Solar System,' Oremland says. ‘What's appealing about Enceladus is that you have some of the conditions for life. There's liquid water under the ice. It seems to have been around a long time. How long, nobody knows. One hundred million years? A billion? That's a long time for life to get going, provided there's something to eat.'

The search for ET

Remote places

The roach's secret

Wendy Zukerman

On a midnight foray into my kitchen, I flicked on the light and was confronted with a devil's playground. Cockroaches were fornicating on my pots and dancing on the cooker. They were grinding on my floor and scuttling around my fridge.

Disgusted, I reached for the light switch again. The snack I had hoped for was no longer enticing. But then I saw one critter climbing up the wall. I leaned over to take a look. ‘How have you come to live in my home?' I wondered briefly – before squishing it with an empty milk carton and going back to bed.

When I moved from Melbourne to Sydney to work for
New Scientist
, I was told only about the fabulous weather and the views over the harbour. No one mentioned the rampant roaches.

But my midnight encounter was just the first of many, and my question would not go away. So I decided to find out more about my unwanted guests and why they have been so successful in colonising our dwellings.

What I have discovered has taken me by surprise. Contrary to popular myth, these critters are not especially tough or radiation resistant. Indeed, they are pretty average as insects go. But in the past year, it has been shown that cockroaches do have one special power after all.

* * * * *

I began my search by heading to the vast collections of the Australian Museum in Sydney. The variety of cockroaches on display there is astonishing – big ones, small ones, green ones, striped ones, winged ones and wingless ones – but I soon noticed that they all conformed to the same basic pattern. While some of their cousins have branched out and evolved specialised features – such as the mantids, with their spiky legs for grabbing prey – roaches themselves have retained a rather plain body plan with no stings, pincers or other special traits.

It is estimated that only around 40 per cent of cockroach species can fly, for instance, and most are rather inept at it. They cannot fly very far. As for the notion that cockroaches would survive a nuclear apocalypse, think again: they are not especially tough.

A dose of radiation as low as 64 grays kills 93 per cent of immature German cockroaches. Sure, that is ten times as much as humans can take, but cockroaches do not look so tough compared with the humble fruit fly, which can survive exposure to more than 640 grays, or the parasitoid wasp,
Habrobracon
, which needs a colossal 1800 grays to kill it.

Continuing my search, I discover that cockroaches thrived long before there were kitchens. They are one of only a handful of insect orders whose fossil record goes back more than 300 million years. Their heyday was during the Carboniferous period, when around 40 per cent of all insects were roach-like, prompting some palaeontologists to describe this period as the Age of the Cockroach. Before turning to beetles, it seems that evolution once had an inordinate fondness for cockroaches.

These ancient ‘roachoids' were not that different from modern roaches. Females had a long ovipositor for laying eggs – it disappeared around 140 million years ago – but otherwise
they all look pretty similar to my kitchen guests.

Nowadays there are nearly 5000 species of cockroach and they live on every continent except Antarctica, finding homes in caves, woodlands and rainforests. Only a handful of species have adapted to living with humans, but it is these that have spread all around the world. ‘They give the rest a bad name,' says entomologist Nathan Lo at the University of Sydney. ‘While I am repulsed by the pests, not all cockroaches are nasty; some are quite beautiful.' Really?

Two of the most infamous pests are the large American cockroach and the smaller German cockroach. Their names are misleading, as it is not clear where they originated, nor when they spread around the world. It is the German variety, I learn, that infests my kitchen.

Which brings me back to my question: why have these critters been so successful? Martyn Robinson, an entomologist at the Australian Museum, thinks it is partly to do with their lack of specialised features. ‘Cockroaches have specialised in not specialising,' he says. In particular, most roaches are not fussy eaters. ‘Roaches are masters of cannibalism,' Lo says. ‘They'll eat everything and they'll eat each other.'

Sometimes they even eat us. There are many accounts of people sleeping in heavily infested buildings or ships being bitten by cockroaches. The varmints seem to have a special liking for calluses and nails, perhaps because they can nibble them without waking their victims. Sailors on some ships reportedly wore gloves while sleeping to protect their fingernails.

There is, however, something else special about cockroaches besides not specialising. When a cockroach is opened up – or even just squished – you can see a white mass that fills much of its abdomen. Known as the fat body, this consists of two types of cells: adipocytes, which are filled with fat globules, and mycetocytes, packed with bacteria.

Around a fifth of insects harbour mutualistic bacteria and some have mycetocytes similar to those of cockroaches. But the relationship between a cockroach and its resident endosymbiont, called
Blattabacterium
, is especially close. Kill the bacteria with antibiotics, and the roaches struggle to survive and often die. Kill the cockroaches and
Blattabacterium
definitely dies. While some insects acquire mutualistic bacteria from the environment, every cockroach hatches with
Blattabacterium
already inside it. When the eggs are developing, a few bacteria somehow move from the fat body to the ovaries, where they are taken up by every egg.

Last year, Zakee Sabree of the University of Arizona compared the genomes of bacterial strains living inside the American and German cockroach. He showed that the two strains diverged at least 140 million years ago – around the time the fossil record showed that the two cockroach lineages themselves split. Since they went their separate ways, neither bacterial lineage has changed much.

Lo thinks the relation between the ancestors of cockroaches and of
Blattabacterium
was so successful that there has been no need for the bacterium to change. ‘Once they developed a relationship, bam! They were able to dominate,' says Lo.

Even before Sabree's study,
Blattabacterium
had come to be seen as a defining feature of cockroaches. When it was found inside the Australian termite
Mastotermes darwiniensis
, it made people wonder whether termites were related to cockroaches. Sure enough, a genetic analysis by Lo in 2000 confirmed that termites are the descendants of a wood-eating cockroach. ‘At the time it was a little controversial, but now we know the termite is definitely a type of cockroach,' says Lo.

But what does
Blattabacterium
do? Most mutualistic bacteria in insects provide them with nutrients that they do not get in their diet and cannot make for themselves. The location of the bacteria provides another clue.

Nitrogen is a key component of amino acids, the building blocks of proteins, but animals usually excrete any excess nitrogen in the form of ammonia, urea or uric acid. By contrast, when cockroaches consume more nitrogen than they need, they store the excess in the fat cells of fat bodies, in the form of uric acid crystals. ‘Even when fed really high-nitrogen diets, they still don't void the excess nitrogen,' Lo says.

So it has long been suspected that
Blattabacterium
might help cockroaches recycle nitrogen. No one was able to prove it, though, because the bacterium will not grow outside roach cells. But nowadays there is a new way to find out what bacteria do: sequence their genome.

Even that is easier said than done. Just getting a pure sample of
Blattabacterium
DNA is tough. The cells containing the bacteria are sandwiched between fat cells, making it impossible to manually extract them. Plus, once the cells are collected, the bacteria have to be separated from the organelles inside the cell. Lo and his colleagues finally managed to isolate the bacterial DNA in 2008. Finally, it was time to sequence the genome.

But Sabree beat him to the punch. In 2009, his team published
Blattabacterium
's genome. The genomic analysis confirmed that the bacteria have all the enzymes needed to convert urea and ammonia into all ten essential amino acids. They can also produce several vitamins.

So the fat bodies not only provide a store of fat that enables many cockroach species to go for more than a month without food; they also allow it to survive on very poor, low-protein diets. ‘It has its own chef and refrigerator on its back,' says Sabree.

The partnership between the cockroaches and their mutualistic bacteria may well have been the key to their success over the past few hundred million years. Despite their dietary superpower, though, only a few cockroach species had all it takes to conquer human habitations. Entomologist David Rentz has counted no
fewer than 84 species of cockroaches around his home in the rainforests of northern Queensland. Yet only the German and American roaches actually live and breed in the house.

The species that have switched to living in human homes are not only able to eat just about anything – they are also able to go for long periods with little water. ‘Some other cockroaches come in at night when the lights are off,' Rentz says, ‘but usually you find them dead on the floor, because they get dehydrated [in the house].'