Biology of the Mariana Trench

Life in the Mariana Trench?

Think about it. The oceans cover, say, 75% of the earth's surface. More than 75% of the ocean's waters is deep sea. That's, say, between 1000 and 6000 meters deep.

Pressure increases with depth. Temperature decreases with depth. Nutrient levels tend to be low in the open ocean. Light only penetrates into the upper 100 - 200 m of the oceans (depending on how clear the water is). It is completely dark below this upper zone - called the "photic zone". Therefore, photosynthesis only occurs in the photic zone.

The Japanese submersible Kaiko, sampling mud at the bottom of the Challenger Deep in the Mariana Trench
(Photograph courtesy and copyright Dr. Hideto Takami, JAMSTEC.)

So, is there life on the ocean floor? You bet! Is there life on the bottom of the Mariana Trench? Plenty! But it is not the kind of life you see in the jungle or in the pond in the local park.

First of all, most life forms at that depth are microorganisms or microbes. Tiny organisms you cannot see with the eye.

Below a depth of about 100 meter, the temperature is roughly 2 or 3 °C. All bacteria from these depths are called psychrophilic. They love the cold. That is all it means.

Pressure increases with one atmosphere for about every 10 meters (one atmosphere is 101,325 Pa; one bar is exactly 100,000 Pa). While some microbes (microsp merely are able to tolerate high pressures (barotolerant), others actually depend on it (barophilic).

Barotolerant microbes usually don't grow at pressures higher than 500 atmospheres and grow best at lower pressures. Barophilic microorganisms grow best at high pressures although they still grow at 1 atm as well.

So-called extreme barophiles really depend on high pressure.

Microbes retrieved from the bottom of the Mariana Trench are often extreme (or obligate = means they need the pressure) barophiles. Their growth rate is much lower than that of barotolerant and barophilic microbes, but it is optimal at about 700 to 800 atm. Not surprisingly, barotolerant and barophilic microbes also tend to be psychrophilic. This is particularly true of extreme barophiles, which don't even fare well at higher temperatures.

What does pressure mean to a microbe? First of all, pressure tends to decrease the binding capacity of enzymes. Enzymes are proteins, large moleucules that are folded in a very complicated manner, a bit like origami. (Enzymes are not just proteins, they are special kinds of proteins: they affect chemical reaction rates. They are catalysts: they speed up formation or breakdown of certain chemical compounds. Sometimes, such a reaction does not even occur without the enzymes. They are helper molecules.) The enzymes of extreme barophiles are often folded differently, in a way so that the pressure has less effect on them.

Membranes can also be affected by pressure. The cell wall outer membranes of barophiles tend to have a different protein composition compared to regular microbes. The porins (diffusion channels in membranes) of a barophile can be made up by a specific outer membrane protein. Its production is caused by a specific gene, which is switched on by high pressure.

The cytoplasmic membranes of barotolerant or barophilic organisms tend to have more of certain chemical compounds: (poly)unsaturated fatty acids and phosphatidylglycerol (a phospholipid). When this page was written, scientists thought that these compounds ensure the membrane’s flexibility at high pressure. It's possible that new research has lead to different conclusions in the meantime (more later).

Geochemical implications
These compounds also end up in the sand and clay (sediments). Scientists used to think that if they found polyunsaturated fatty acids in marine sediments, this meant that phytoplankton or zooplankton had been present (scientists describe this as polyunsaturated fatty acids being biomarkers for phytoplankton and zooplankton). They thought that bacteria were not able to produce this stuff. Later research suggested that barophilic bacteria may also attribute to the presence of these fatty acids in marine sediments. Looking at the chemical composition of geological material is called geochemistry (and so is that particular kind of chemistry: organic geochemistry).

Links:

• information about extreme eukaryotes. Eukaryotes means something like: not bacteria and not viruses; fungi are eukaryotes and so are we.
• Paul Yancey's deep sea pages. Paul Yancey is Professor of Biology at Whitman College, Walla Walla, Washington, USA. He specializes in the physiology and biochemistry of environmental adaptation, particularly stresses (troubles) related to water and osmoregulation (to do with seawater being salty) from humans to deep-sea animals.

Hydrothermal vent communities
The surroundings of the Mariana Trench are just as fascinating. The submersible Alvin has made many dives into the Mariana Trench. In 1987, scientists found chimneys and hydrothermal vent communities in the Mariana Trench back arc basin (mussels, crabs, tube worms etc.). These hydrothermal vents (black and white smokers) are a heat source for the cold water. The Mariana system also contains many large mud volcanoes.

Links:

• The Ecology of Deep-Sea Hydrothermal Vents. Book for scientists and students - by Cindy Lee Van Dover, at Princeton University Press.
• USGS introduction to black smokers
• Extremobiosphere Research Centre (part of JAMSTEC)
• Deep sea research photography (JAMSTEC)
• DEEPSTAR: Deep Sea Microorganisms Research at JAMSTEC: oil-eating bacteria that can live in the deep sea
• In June 2006, Elsevier Science published the book "Extremophiles" (Methods in Microbiology, Volume 35). It costs approximately 50 euros, 35 pounds or 60 dollars.
• By the way, they have also found very interesting life forms in whale carcasses. The chemistry in whale carcasses and hydrothermal vents is very different from normal seawater and that is why such very different - not so say strange - organisms develop there.

The deepest mud sample
On March 2, 1996, the Japanese submersible Kaiko (which went missing in 2003) scooped out some mud from the bottom of the Challenger Deep, at a depth of 10,897 m. This sample was taken to the surface without being contaminated. A diluted suspension of the sample (that is just a bit of muddy water) was used for microbe cultures. Thousands of organisms were isolated and cultured (grown), but none were barophilic (loving pressure), halophilic (loving salt) or acidophilic (loving acid) bacteria. Surprise, surprise, alkaliphiles (loving alkaline conditions) and thermophiles (loving heat) were found.

The number of facultative psychrophiles (means those that can but do not need to live in cold water) was lower than that of the alkaliphiles and thermophiles and almost equal to the number of filamentous fungi and ascomycetes (another kind of fungus). The scientists thought that many of the microbes living at the bottom of the Mariana Trench were transported there by sinking marine snow (big particles, mainly made up of dead material stuck together with goo).

The deepest living microbes
Samples scooped out a few days earlier, on February 28, 1996 did result in the successful isolation of several (extreme) barophiles, closely related to the genera Shewanella, Moritella and Colwellia.

Two barophilic strains were chosen for further studies. They had higher percentages of certain unsaturated fatty acids relative to normal strains. The next step was a structural and quantitative analysis of the membrane lipids of two of the barophilic isolates.

The deepest living shrimp
But not just microbes live there. Japanese scientists visiting the Mariana Trench in Kaiko in 1995 made detailed observations of shrimps, a scale worm and a sea cucumber at a depth of 10,911 m (35,800 ft). During the summer of 1998, Kaiko captured an amphipod (Hirondellea gigas, a Crustacean, subclass Malacostraca) at the Challenger Deep. A kind of shrimp.

In fact, one had already been retrieved by a trap in 1978, from a depth of 10,476 m. An obligate barophilic bacterium strain was isolated from this amphipod (yes, most organisms have bacteria and other organisms living in and on them; this goes for human beings too). It could only grow at pressures of more than 518 bars (about 50MPa). More amphipods were caught later in 1998. This particular amphipod - if you're interested - appears to be phylogenetically related to the Cirripedia (a different subclass of the Crustaceans - Crustaceans, that's organisms like lobsters and shrimps).

The deepest found fish
Of course, the Deepest Recorded Fish was also found in the Mariana Trench (at 27,460 ft - 5.2 miles - below the ocean surface).

Protecting the ecosystem
Should we - or can we - protect the ecosystem on the bottom of the Mariana Trench? Well, not really. If you know something about plate tectonics, you will realize that eventually, the Mariana Trench will disappear anyway. If you go to the Alps in Europe, you can find back pieces of ancient oceans. Oceans tend to disappear, because ocean plates are constantly being destroyed - at trenches - and being created - at ridges.

Whether mankind is able to influence life at the bottom of the Mariana Trench much is difficult to say. It does not seem very likely - certainly if we stick to research only - but then, what do we really know? Not so long ago, scientists believed that the deep sea was devoid of all life...

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Most recent update: January 16, 2008