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ScienceWeek
HISTORY OF OCEAN SCIENCE: ON DEEP-SEA BIOLOGY
The following points are made by Robert Kunzig (Science 2003 302:991):
1) In the summer of 1833, Edward Forbes (1815-1854) set sail from his home on the Isle of Man for Norway, where he and a friend intended to do some "botanizing" and rock-collecting. Years later he would become known as the father of deep-sea biology; but then, he was just a budding naturalist, 18 years of age, loose of limb, long of hair, and very eager. In Bergen, Forbes checked into a rooming house and found that he did not even need to go outside to start exploring [(1), p. viii]:
2) Forbes wrote: "Amongst my Bergen treasures I especially value a quantity of sand, which I found in a spitting box in my lodgings. As yet I have only examined a small portion; but I expect many minute curiosities in the shell way from it. Several species hitherto only found in Britain have rewarded my search already."
3) Natural history was all the rage at the time. The beaches of Britain were being combed for shells and seaweeds by legions of passionate amateurs. Forbes made himself into something more: by 1843, 10 years after his trip to Norway, he had published learned treatises on starfish and mollusks. In May of that year he became Professor of Botany at Kings College in London. "Much, very much remains to be done," he said in his inaugural lecture, "and there is no fresher field for original research and the development of a grand philosophy than that of Natural History." [(1), p. xi].
4) Yet, 3 months later, this open-minded, ambitious young man told a meeting of the British Association for the Advancement of Science that there was probably nothing alive to study on the whole of the deep-sea floor: below 300 fathoms lay a vast "azoic zone". Having marveled at the contents of a spittoonful of sand, Forbes was ready to dismiss more than half the planet as uninteresting.
5) Forbes and his followers (who were more set on his hypothesis than he was) had a plausible argument. How could anything live in the freezing, crushing dark of the deep sea? Forbes even had some pioneering data. During more than a year in the eastern Mediterranean on H.M.S. Beacon, a Royal Navy survey ship, he had dragged a small dredge along the bottom at a hundred different places. Going as deep as 230 fathoms, he noticed that the number of organisms he caught got smaller the deeper he dredged. Extrapolating that trend, he arrived at a "zero of animal life" at 300 fathoms, and extrapolating from the tiny Aegean to the whole world ocean, whose average depth is around ten times the depth reached by Forbes, he arrived at a general principle. Forbes was interested in general principles. The "grand philosophy" he was after was no less than an explanation for the distribution of all life on Earth. So he extrapolated.
6) We now know that the extrapolation was horribly wrong --Forbes's dredge let most organisms escape, and the Aegean is much poorer in sea-floor life than most areas of the deep ocean -- but there is no denying that it was fruitful. By claiming that deep-sea biology did not exist, Forbes helped bring it into existence. The azoic zone hypothesis was an irresistible challenge. It was disproved by a series of three British expeditions, on ships called Lightning, Porcupine, and Challenger. They were all led by the same man, Charles Wyville Thomson (1830-1882), who believed, even before he had collected proof, that the deep-sea floor was "the land of promise for the naturalist, the only remaining region where there were endless novelties of extraordinary interest ready to hand."(2-4).
References (abridged):
1. Literary Papers of the Late Professor Edward Forbes, F.R.S. (Lovell Reeve, London, 1855)
2. C. W. Thomson, The Depths of the Sea (Macmillan, London, 1873), p. 49
3. C. W. Thomson, quoted in E. Linklater, The Voyage of the Challenger (John Murray Ltd., London, 1972), p. 276
4. T. H. Huxley, Autobiography and Selected Essays, A. L. F. Snell, Ed. (Houghton Mifflin, Boston, 1909); http://human-nature.com/darwin/huxley/autobiography.html
Science http://www.sciencemag.org
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POTENTIAL IMPACT OF CARBON DIOXIDE INJECTION ON DEEP-SEA LIFE
The following points are made by B.A. Seibel and P.J. Walsh (Science 2001 294:319):
1) The potential for global warming has spurred the development of various strategies to control the concentrations of greenhouse gases, particularly carbon dioxide, in the atmosphere. Technologies for carbon capture, storage, and sequestration to reduce greenhouse gas concentrations are receiving increasing attention. Because of its enormous volume, the ocean is an attractive site for possible storage of carbon dioxide. First proposed 25 years ago by C. Marchetti, disposal in the ocean is now being actively explored.
2) Recent modeling studies indicate that carbon dioxide must be released at great depths to avoid substantial outgassing. Direct studies of the biological consequences of carbon dioxide injection are in their infancy, but a large literature on the physiology of deep-living animals indicates that they are highly susceptible to the carbon dioxide and pH excursions likely to accompany deep-sea carbon dioxide sequestration. Microbial populations may be highly susceptible as well. The impacts of ocean sequestration on deep-sea biota and the biogeochemical cycles dependent on their metabolism are therefore of great concern. Increased carbon dioxide results in decreases in seawater pH. Primary responses of organisms to the consequent internal acid-base imbalance include metabolic production and consumption of acid-base equivalents, passive chemical buffering of intra- and extracellular fluids, and active ion transport.
Science http://www.sciencemag.org
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ON THE PUZZLES OF HYPERTHERMOPHILES
In this context, the term "geothermal" refers to the escape of heat from the interior of the Earth into the Earth's surface, and the term "hydrothermal" refers to hot solutions rising from cooling molten rock (magma). "Hydrothermal vents" are hot springs occurring in volcanic regions of the ocean floor. The heavy-metal ions and hydrogen sulfide dissolved in the overheated vent fluid precipitate as metal sulfides as soon as contact with seawater cools the fluid. This reaction produces the characteristic underwater black "smoke" plume, with a vent "smoker chimney" building up from precipitated materials, mostly gypsum and sulfides.
"Plate tectonics" is the modern theory that unifies many of the features and characteristics of continental drift and sea floor spreading into a coherent model. Continental drift is the slow movement of the Earth's land masses, a shifting across the underlying molten material. Sea-floor spreading is the process whereby sea floor is continuously created as the crustal plates move apart, and continuously destroyed where the plates push against each other. The term "mid-ocean(ic) ridge" refers to a topographic feature of a tectonic spreading center between diverging oceanic plates. New crustal material is formed by upswelling magma (molten material from which rock forms) as the plates diverge.
In general, bacteria have adapted to a wide range of temperatures, with the range of temperature over which optimal growth can occur in any one species spanning approximately 20 degrees centigrade; the range over which any growth at all takes place usually spans approximately 40 to 50 degrees centigrade.
Bacteria that grow at temperatures of less than 15 degrees centigrade are called "psychrophiles". Obligate psychrophiles, which have been isolated from Arctic and Antarctic ocean waters and sediments, have optimum growth temperatures of approximately 10 degrees centigrade and do not survive if exposed to 20 degrees centigrade.
The term "mesophilic bacteria" refers to those bacteria in which optimum growth occurs between 20 and 45 degrees centigrade; such bacteria can usually grow in or survive temperatures between 10 and 50 degrees centigrade, and all animal pathogens are in this group.
So-called "thermophilic bacteria" are the only organisms that can grow at temperatures higher than 60 degrees centigrade. Such temperatures are encountered in rotting compost piles, hot springs, and oceanic geothermal vents. In the runoff of a hot spring, various thermophiles are found near the source where the temperature has fallen to approximately 70 degrees centigrade. An example is the species Thermus aquaticus, which has an optimum temperature for growth of 70 degrees centigrade, and a maximum temperature for growth of 79 degrees centigrade.
In the mid-1980s, researchers discovered bacteria in nutrient-rich, extremely hot hydrothermal vents in the deep sea floor. For example, the bacteria in the genus Pyrodictium thrive in the temperature range 80 to 110 degrees centigrade, temperatures at which the water remains liquid only because of the extremely high pressure.
The following points are made by R.A. Zierenberg et al (Proc. Natl. Acad. Sci. US 21 Nov 00 97:12961):
1) The authors point out that eruption of volcanic rocks at mid-ocean ridges is the major mechanism by which heat is lost from the interior of the Earth. Approximately one-third of the heat is removed from the sea-floor spreading centers by convective circulation of sea water, and the magnitude of this heat loss requires that the entire volume of the oceans circulates through the mid-ocean ridges in approximately 10 million years. Seawater interaction with volcanic rocks at near 400 degrees centigrade results in substantial chemical flux and makes an important contribution to buffering the composition of some elements in sea water. Sea-floor hydrothermal vents support ecosystems with enormous biomass and productivity compared with that observed elsewhere in the deep oceans. What is the energy source that fuels these oases of life and what adaptations allow them to exist in these extreme environments?
2) Although there is a potential abundance of chemical energy at hydrothermal vents, deep-sea hydrothermal biological communities have had to adapt to extreme conditions to exploit this resource. Of particular interest are the hyperthermophiles, which are defined as microorganisms able to grow at 90 degrees centigrade and above. Approximately 20 different types of such organisms are now known. They have been found both within the walls of black smoker chimneys and where the hydrothermal vent fluids mix with the surrounding seawater. Classifications of the hyperthermophiles has provided new insights into the evolution and the origin of life. All but two of the hyperthermophilic genera are classified by *ribosomal RNA analysis as "Archaea" (formerly Archaebacteria), which are the second domain of prokaryotic life, in addition to the bacteria. By these phylogenetic analyses, the hyperthermophilic archaea types and the two hyperthermophilic bacteria types are the most slowly evolving within their domains, suggesting that life may have first evolved when the Earth was much hotter than it is now. Such a thesis is very controversial, the thesis suggesting that extant life forms are largely the result of temperature adaptations to lower (below hyperthermophilic) temperatures.
3) Evolution gives no clue, however, as to how life can thrive near and above 100 degrees centigrade. Most microbes, and all *eukaryotic cells, cannot survive at temperatures much above 50 degrees centigrade because of the general instability of biological molecules. The 3-dimensional structure of most enzymes and other proteins are lost at temperatures much above 70 degrees centigrade, and the double-helical structure of DNA has a comparable lack of stability in _in vitro_ studies. There are also a wide variety of ubiquitous metabolites that are rapidly hydrolyzed at temperatures above 90 degrees centigrade. How do hyperthermophilic cells circumvent these problems?
4) Although there are some examples of modified pathways and unusual enzymes in hyperthermophiles, in general the biochemistry of these organisms closely resembles that of the mesophilic world. Yet, most enzymes from hyperthermophiles are extremely stable at high temperatures, showing optimal catalytic activity above 100 degrees centigrade with virtually no activity at ambient temperature. These enzymes contain exactly the same 20 amino acids as enzymes from conventional organisms, so why are they so stable? Sequence comparisons of analogous proteins from hyperthermophilic and conventional organisms are essentially identical, so the enormous amount of sequence information now becoming available will be of little use in elucidating stabilizing mechanisms. Comparisons must be made at the level of 3-dimensional structures, yet even then, there are no gross structural differences between hyperthermophilic proteins and their mesophilic counterparts, and both forms are stabilized by the same noncovalent interactions. The number and extent of such interactions is generally only slightly higher in the hyperthermophilic versions, so extended protein stability at 100 degrees centigrade appears to be the result of very subtle, synergistic, and cooperative intramolecular interactions. Moreover, different types of hyperthermophilic proteins seem to have unique solutions to the problem. A general mechanism by which any conventional protein could be made stable and functional at temperatures above 100 degrees centigrade may not be forthcoming.
Proc. Nat. Acad. Sci. http://www.pnas.org
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Notes:
ribosomal RNA: A ribosome (not to be confused with riboZYME) is a small particle, a complex of various ribonucleic acid component subunits and proteins that functions as the site of protein synthesis. The tripartite kingdom proposal (Archaea, Bacteria, Eukarya) of Woese and others is primarily based on gene sequence analysis of particular ribosomal RNA fractions.
eukaryotic cells: In general, a eukaryotic cell is any biological cell containing internal membrane-bound organelles such as a nucleus.
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