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

Deep Sea Thermal Vent Bacteria—cont'd

В, Colorized micrograph of deep sea thermal vent bacterial community; the filaments of Beggiatoa are abundant.

ture and prevent water from turning to steam, it was impossible to sample the chambers and culture bacteria. Deming and Baross therefore measured protein and nu­cleic acid content at the end of the experiment, both of which appeared to increase. Based on these observa­tions they reported that bacterial growth occurred in the chambers incubated at 250° C. Their results were imme­diately questioned by many scientists. Holger Jannasch could repeatedly grow some of the bacteria from the thermal vents at temperatures of 100° to 110° C, but not at higher temperatures. No one was able to repeat the

experiments that purportedly demonstrated bacterial growth at 250° C. Independent confirmation is critical in science. Eventually it was shown by Art Yayanos at the Scripps Institute of Oceanography that the results re­ported by Deming and Baross could be explained by abiotic changes that occur at high temperature and pressure. Bacterial growth apparently had not occurred at 250° C. The initial report had not met the essential test of the scientific method—that of repeatability by others. The upper demonstrated growth temperature remains about 110° С

isms grow best at low temperatures. Such organisms, known as psychrophiles, have optimal growth tem­peratures of under 20° C. As long as liquid water is available, some psychrophilic microorganisms are ca­pable of growing below 0° C. Psychrophilic microor­ganisms are commonly found in the world's oceans and are also capable of growing in a household re­frigerator, where they are important agents of food spoilage.

Mesophiles are microorganisms that have optimal growth temperatures in the middle temperature range between 20° and 45° С Most of the bacte­ria grown in introductory microbiology laboratory courses are mesophilic. Many mesophiles have an op­timal temperature of about 37° C. Many of the normal resident microorganisms of the human body, such as Eschericia coli, are mesophiles. Similarly, most human pathogens are mesophiles and thus grow rapidly and establish an infection within the human body.

Thermophilic microorganisms are organist¹ with high optimal growth temperatures. The» philes, such as Bacillus stearothermophilus, growatm atively high temperatures, often growing only abova 40° C. The upper growth temperature for extern thermophilic microorganisms, such as those foundii deep thermal rift regions of the areas where volcaul activity heats the ocean water under very high pre; sure, is about 110° C. Water will remain in a liqiii state at temperatures above 100° С when it is such jected to high pressure. Thermophiles have optiis growth temperatures above 45° С and manyfe-mophilic microorganisms have optimal growth tef peratures of about 55° to 60° С One finds thai philic microorganisms in such exotic places as he springs and effluents from laundromats. Howe» many thermophiles can survive very low tempi tures, and viable thermophilic bacteria are route found in frozen antarctic soils.

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