Hydrothermal Vents and Light-independent living Systems S. Krishnamurthy Tuskegee Institute, Tuskegee Institute, AL 36088 The central dogma of biology is that life is founded on the enerev of the sun that is caotured in ohotosvnthesis bv the &nK and bacteria. This sit Lation has'heen changed recently hv the discoverv of Weissel. et al. ( 1 ) and Corlisq. et al. ( 2 )of deep-sea hydrdthermal v e k s a t ~ i l a ~ a g which o s house an exotic colony of clams, mussels, fish, crabs, and giant tube worms. In the freezingdarkness, about 2.4 km below thesurface of the ocean, enjoying the warmth of the vents, there thrives an entire self-contained ecosystem drawing the energy not from the sun but from bacteria that metabolize the hydrogen sulfide dissolved in the water. The RISE Project Group (3)discovered another hydrothermal vent system jetting out water a t about 380°C on the axis of the East Pacific Raise, 21°N, off the tip of Mexico's Baja California. The hottest waters there are blackened by sulfide precipitates. The cooler springs are clear or milky white and support exotic benthic communities of giant tube worms, clams, and crahs similar to those a t Galapagos. Hekinian, et al. (4) have discovered massive sulfide deposits from the diving saucer CYANA in the East Pacific Raise off the tip of Baja California. The two hydrothermal vents (more may be discovered later) show that the bacteria on the ocean ridges use H2S as an enerev source and these hacteria serve as food for the colonies thyt exist without light. These discoveriesare hound to have fwrearhing influence on the theories of the origin of life, since the range ol'conditions around rhr vents could imitste the verv esrlv conditiuns on Karth. The bacteria count for the Galap. ago; system was 106-108/cc and that for Baja colony was 10SIcc. It appears that the hacteria are characterized by very rapid growth. Another finding of interest is the occurrence of helium-3 in these waters. The isotope helium-3, unlike helium-4 which is contaminated by alpha decay of heavier elements, is almost comoletelv orimordial. left over from the formation of the &k. As ;he'hydrother&al watersextract heat from therocks. they also extract helium-3. The ratio of heat to helium-3 for the Galnpagus samples turned out to he identical with ihe rario fur the samoles from Haia California (51.These new facffi will bring fresh insight and &derstanding& the problem of ocean dynamics, biological theories, and the mechanism of sulfide and other ore formation, as has been pointed out by Waldrop (6).
Prokaryotic organisms like the blue-green algae are known to live on C07 as the *ole source of cnrhon. but thev are ~ h o tosynthetic, ihat is they use light. purple bacteria sueh as Chromatium are anaerobic, and they require H2S. Blum (71, with great prescience, pointed out that all the photosynthetic reactions carried out by the photo-autotrophic hacteria with the aid of light in the absence of oxygen may, by some species of these hacteria, be carried out in darkness, hut only if oxygen is present. Light and 0 2 seem to be interchangeable in these reactions. Gaffron (8) found that if the green algae Scenedesmus a d a ~ t e dto anaerohic conditions it can utilize H2 in photosynthetic reduction of CO?, whereas nurmallg it carries on meen ulantlike ohotosvnthesis. Further. Scenrdvsmus mav he made t o carry out thereduction of CO; in the dark, in thk presence of 0 2 .
The photosynthetic bacteria carry out chemical reactions that have a positive free energy change, AG, and hence the need for an energy source, light. The hacteria in the hydrothermal vents carry out a reaction that proceeds with a large decrease in free energy, as the following calculations show. For the photosynthetic reaction, For thermodynamic purposes, the reaction can he rewritten after division by 6 as follows: C02 + HzO
-
[CH20] + 02; AG = 114.3 kcal
In eqn. (1) [CH20]is a hypothetical compound representing the smallest unit of a carbohydrate and not formaldehyde. Inserting the value AGG02 = -94.3 and AG.fH20 = -56.7 kcal in eqn. (I), the AG.f[CHzO] comes out to he -36.7 kcal. COz+4HPS+O~-[CHzO]+3H20+4S
(2)
and inserting the value AG.r = -36.7 kcal., gives the AG.for the reaction of eqn. (2) to he -80.4 kcal. This free energy change is negative and large and hence the reaction is spontaneous. A similar calculation for the following photochemical reactions gives the corresponding AG' values, as shown below: L'iht
COz + 2 HzS -+[CHzO] + 2 S + HzO;AG Light
COz + 2 Hz+
[CHzO]+ H20;AG
= +16.9
kcal, (3)
= t0.9 kcal
(4)
From these calculations, it is obvious that the hydrothermal vent bacteria are working on sound thermodynamic grounds! The discoveries of the hydrothermal vents and the associated life systems have opened up exciting possibilities in several disciplines. We seem to he on the threshold of a new era. Future work will undoubtedly highlight the profound influence these discoveries will have on science. One mav mention in passing the possibility of aquaculture of these barteria. feedine them with industrial waste hvdrown sulfide and reaping clams, fish, and crahs to feed the po&ation of the earth. Literature Clted 11) Weiss, R. R.Lonsd.de, P. F.. Lupton, J. E., B a i n b r i d ~A. . E., and Craig, H..Nofure lLondon),267. -3
(1977).
(2) Corlisa, J. B., Diamond. J., Lyle, E., and Crane, K., Eorrh & PIonal. Sci. Loft.. 40.12
(1978).
(3) RISE ProjedGroup, Science, 207,1421 119801. (4) Hekin-, R., Fevcrier, M., Birchoff, J. L., Picof
P.. and Shanks. W. C., RISE P m W Group, Science, 207.1433 (19801. (5) Edmon, J., Earth & Plansf Sri. Latf., 48, I(1979l. (6) Wsldrop, P, M., Chom. & Engn. Neua, Mmeh 10,1980,pp. 31-33; March 24,1980,PP. 23-25. p. 116.
(7) B l m , H.F.,"TimeaAnwandEvolution,"PrinrrtonUnh.Presa,Princeton, NJ.1955, (8) Gaffmn. H.,"Photmwthpais,photorduductionanddarkreduction of Ozin certain algae.' B i d Roo., 19.1-20, 11944).
Volume 58 Number I2 December 1981
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