Archaeal Web

by Elio
 

A venting black smoker emitting sulfide-rich fluids. Courtesy of Univ. of Wash­ing­ton

Deep sea hydrothermal vents are characterized by steep physical and chemical gradients and by the presence of unusual and fascinating life forms. Varied organisms grow there: giant tube worms that exploit the metabo­lism of sulfide-oxidizing bacteria, clams, crabs, shrimps, and more. These like their water tepid. But some archaea prefer the hotter neighborhood near the vents that emit water at around 400°C. These microbes are hyperther­mo­philes, thriving at temperatures around 100°C. Many have a specialized life style, and one them, Pyrodictium, is particularly interesting.
 

First discovered in 1979 by Karl Stetter’s group, these organisms are strictly anaerobic chemoautotrophs that get their energy from oxidizing hydrogen (and reducing elemental sulfur) by this reaction: H₂ + S° → H₂S. Of interest to us here, these ar­chaea associate to form web-like flakes that are visible with the naked eye. Pyrodictiums are flat, irregularly shaped, disk-like cells 0.3 to 2.5 μm across and up to 0.3 μm thick, sometimes thinning out to a mere 0.08 μm thick.
 

Pyrodictium occultum cells enmeshed in their sticky, glycoprotein matrix. Source: Reinhard Rachel

Individual cells are enmeshed in a unique, extracellular matrix made up of bundles of hollow tubules, the can­nulae (Latin: “little reeds”). Cannulae have an outer diameter of only 25 nm but can be up to 40 mm long. They are built from three (and perhaps more) glyco­protein subunits ordered in a helical array – an ar­chi­tecture that can withstand temperatures up to 135°C without denaturation or loss of secondary structure. Cannulae make up a large portion of the biomass of Pyrodictium cultures, an investment that bespeaks their importance for these organisms.
 

Growth and division of these organisms was observed at 90°C under anoxic conditions using a dark-field light microscope (which takes quite a set-up). The researchers observed that growth of the cannulae is directly linked to cell division. When the progeny cells separate afterwards, they remain connected by their cannulae. Repetition of this process over and over results in a dense network, with each cell having multiple connections with its neighbors. Using electron tomography, the cannulae were seen to penetrate into the periplas­mic space, which looks like a sturdy way to anchor the cells to the cannulae. Thus, each cannula must pass through an outer membrane twice, which seems like a good trick. Is such a thing seen in any other prokaryotic cell?
 

A reconstruction from an electron tomo­graphic scan in false color showing two cannulae (yellow), one inside the peri­plasm (blue) of a cell (magenta). Source

Why do these archaea go to the trouble to construct such elaborate and expensive structures? Not having tools for genetic studies at hand, one can only guess. But this me­chanism must be vitally important; cultures maintained in the laboratory for over 20 years have never produced a cannula-less mutant. Cannulae are hollow, suggesting they may play some role in transport, in addition to cre­ating Pyrodictium's macro-architecture. As is the case with many archaea, these organisms invite many more studies about their innovative way of life.
 

 
 

 
 

Comments

[elio schaechter]

Peter,

You are right, and various authors have pointed out that Pyrodictium should be considered to be a multicellular organism. The concept of multicellularity among prokaryotes is gaining momentum.

Elio