FUN TO BE BAD: The Depths of the Abyss...The Mariana Trench

Wednesday, April 17, 2013

The Depths of the Abyss...The Mariana Trench

The Mariana Trench or Marianas Trench is the deepest part of the world’s oceans. It is located in the western Pacific Ocean, to the east of the Mariana Islands. The trench is about 2,550 kilometres (1,580 mi) long but has an average width of only 69 kilometres (43 mi). It reaches a maximum-known depth of 10.911 km or 6.831 mi at the Challenger Deep, a small slot-shaped valley in its floor, at its southern end, although some unrepeated measurements place the deepest portion at 11.03 kilometres (6.85 mi).

At the bottom of the trench the water column above exerts a pressure of 1,086 bars (15,750 psi), over one thousand times the standard atmospheric pressure at sea level. At this pressure the density of water is increased by 4.96%, making ninety-five litres of water under the pressure of the Challenger Deep contain the same mass as a hundred litres at the surface. The temperature at the bottom is 1 to 4 ⁰C.

Countless amazing species thrive there in the harshest imaginable conditions. At the bottom of Challenger Deep, calcium carbonate shells are not an option because the intense pressure, over 1,000 times sea-level — dissolves the mineral. Not a problem. The microscopic forams down there make soft shells, presumably of protein or other organic polymers. There are over 400 species of soft-shelled forams living in the sediment of Challenger Deep.

A xenphyophore, A large single celled organism

Xenophyophores have soft shells which, though pressure proof, are often not particularly sturdy, thwarting most collection attempts thus far. As a result, we know a lot less about their biology than we would like. We do know they live all over the world at great depth, and seemingly, the deeper and more hostile the environment, the better.

Xenophyophores trap and eat tiny particles by engulfment, as true “amoebas” do. They were not distributed evenly in the trench — some areas have more and some less. The giant protists, are probably choosing their homes, as the sea cucumbers do, to maximize current-borne food extraction. As the currents encounter their lumpy dwellings, the water slows and particles drop out. Inside the the shell is a branched collection of tubular filaments that has many nuclei and no cell partitions, leading some to describe them as among the largest “single-celled” organisms on Earth.

Xenophyophores seem to be claiming a marine niche similar to the terrestrial niche occupied by lichens. That is, they specialize in taking spots that are so harsh that they have almost no competition for their home. There could be 50-100 species of xenophyophores in the Challenger Deep.

Xenophyophores are single celled organisms. The single cell branches and splits into hundreds of tubes which ramify and interconnect into a complex network. The test builds up as it proliferates, secreting a slimy organic cement. These are the largest structures produced by a single cell.

The Real Challenger Deep Scene is Microbial

Bacteria and archaea were not the focus of the study, but the fact is that bacteria thrive in the sediments of Challenger Deep and are found there in even greater abundance than on the surrounding abyssal plain. Bacteria aren’t the only microbes, hundreds of microscopic foraminifera species are also found there. The authors of this study at the University of Southern Denmark hypothesize that the canyon walls act as a nutrient funnel that concentrates bacteria chow in the trench. Why that nutrient enrichment doesn’t seem to extend to the top of the food chain, I do not know.

Astrobiologist Kevin Hand described what he called “an astonishingly bizarre microbial ecosystem” on talus blocks in the Sirena Deep (where the drop-cam video of the xenophyophores was taken). There, one of the remote landers filmed dark brown, shag carpet-like bacterial mats sprouting from rocks on the floor.

These bacteria seem to be living off of the products of a spontaneous, ambient-temperature chemical reaction between rock and seawater. The mats are composed primarily of bacteria called Paracoccus denitificans, which seem to be the primary producers in the system, feeding on hydrogen and methane released by seawater-induced serpentinization of the rocks on which they’re growing.