Deep Sea Thermal Vent Bacteria

The work of Holger Jannasch established that microbial activities occur at very low rates in the deep oceans; deep ocean sediments, in effect, are biological deserts because of their low temperatures, high pressures, and low inputs of organic matter. These rates are so low that bologna sandwiches accidentally submerged inside the submersible Alvin were not decomposed during several months of exposure to microorganisms of the deep sea.

What a surprise when investigators found an area of extremely high biological productivity at a depth of 2,550 m in a region of thermal vents (subsea volcanoes) off the Galapagos Islands. The thermal vents warmed the waters, but what was the source of food supporting the growth of worms several feet long and clams sev­eral feet across? There was no light to support photo­synthesis, and transport of organic matter from the sur­face was unlikely. The most likely explanation was that chemolithotrophic metabolism by autotrophic bacteria based on oxidation of hydrogen sulfide from the vents was providing the organic matter to support the growth of other organisms (see Figure). Establishing that bacte­rial chemolithotrophy was the source of organic matter would be difficult; to reach the vents in the Alvin would take hours, time on the bottom to carry out experiments would be extremely limited, and working Alvin's me­chanical arms would be difficult. Nevertheless, this was the task undertaken by Holger Jannasch and Carl Wirsen of the Woods Hole Oceanographic Institution.

Jannasch and his associates were able to collect sam­ples using specialized pressurized chambers and return living bacteria from the thermal vents to the laboratory for study. These investigators found that all surfaces in­termittently exposed to H₂S-containing hydrothermal fluid were covered with mats composed of layers of prokaryotic, Gram-negative cells interspersed with amorphous manganese-iron metal deposits. Enrich­ment cultures using thiosulfate as the energy source made from mat material resulted in isolations of differ­ent types of sulfur-oxidizing bacteria, including the ob-ligately chemolithotrophic genus Thiomicrospira. These studies established that chemolithotrophic bacteria sup­ported the productivity of the thermal rift region.

Jannasch and other scientists then asked about the maximal temperature at which bacteria in thermal vents could grow. Bacteria were observed in waters coming from the vents with temperatures well in excess of 100° C. Could bacteria actually grow there or had the bacte­ria grown elsewhere at lower temperatures? What was the upper temperature limit at which bacteria can re­produce? Some scientists hypothesized that, since there was liquid water because of the high pressures, bacteria could grow at temperatures of even 500° C.

Experiments were conducted by John Baross and Jody Deming who incubated bacterial samples from the thermal vents in chambers under very high pressures at temperatures of 250° C. Because the chambers had to re­main sealed under pressure to maintain the tempera-

A, Photograph of the deep sea submarine ALVIN.

300 CHAPTER 10 BACTERIAL REPRODUCTION AND GROWTH OF MICROORGANISMS

Date: 2015-02-28; view: 1167