1977 broda

Upload: bewareofdwarves

Post on 03-Jun-2018

251 views

Category:

Documents


0 download

TRANSCRIPT

  • 8/11/2019 1977 Broda

    1/3

    I

    eitschrift

    fur

    Allg. Mikrobiologie 1 7

    6

    I

    1977

    I

    491-493

    Institut fiir Physikalische Chemie,

    Universitit Wien)

    Two

    kinds

    of lithotrophs missing in nature

    E.

    BRODA

    Eingegangen a m 14.9.1976

    Two groups of lithotrophic bacteria, the existence of which may be expected on evolutionary

    and thermodynamical grounds, have not yet been detected: (A) photosynthetic, anaerobic, am-

    monia bacteria, analogous to coloured sulphur bacteria, and

    B)

    chemosynthetic bacteria that

    oxidize ammonia to nitrogen with

    0

    or nitrate as oxidant.

    The versatility of the prokaryotes in their energy metabolism has long astonished

    microbiologists. The bacteria have developed processes, i.e., enzymes, for the utili-

    zation of a wide range indeed of exergonic reactions. Attention is now drawn to further

    processes in energy metabolism which on the basis of considerations on the evolution

    of the bioenergetic processes

    BRODA

    975a) may be expected to have existed, or

    to exist, but which have not yet been found. Two kinds of lithotrophic bacteria

    with such mechanisms will now be predicted. Lithotrophs are bacteria that use in-

    organic reductants in their energy metabolism FROMAOEOTnd SENEZ 960); all

    autotrophs must belithotrophs, though the reverse need not be true. The two bac-

    teria here predicted would generate dinitrogen (N,).

    The nitrifying bacteria make adenosine triphosphate, ATP, through oxidative phos-

    phorylation coupled to the aerobic oxidation of ammonia, a highly exergonic process.

    Thus, in nitrification Nitrosomonas produces nitrite, and Nitrobacter makes nitrate.

    The redox reactions are :

    NH + 1.5

    0

    = H,O + NO; +

    2

    H + ; AG; = 5 kcal

    NO;

    +

    0.5

    0

    =NO;;

    AG;

    = 18kcal

    (1)

    2)

    The negativity of the free enthalpy change, AG;, is th e precondition for the produc-

    tion of ATP and, consequently, for the endergonic reduction of

    CO,

    to biomass. The

    reduction occurs, as in plants, through t h e

    CALVIN

    cycle; the reductant, NADH, is

    obtained by reverse electron flow forced by ATP.

    Clearly, the nitrificants, one main group of the chemolithotrophic bacteria, could

    evolve only after the biosphere began to contain,

    as

    a consequence of the photo-

    synthetic activity of the blue-green algae, free oxygen BRODA975a b). The tran-

    sition to the oxidizing biosphere took place about 2 giga-years (Gy) ago

    RUTTEN

    971)

    Similarly the free oxygen made possible th e rise of the second important class of

    chemolithotrophs, the colourless (white) sulphur bacteria. These thiobacilli make

    ATP on the basis of reactions

    of

    the overall types:

    HS-

    +

    0 5

    0 +

    H+ = H,O S; AG; = 51 kcal

    S

    +

    1.5

    0

    +

    H,O

    =

    S O 2

    +

    2

    H + ;

    AG;

    =

    139 kcal

    3)

    4)

    The thiobacilli presumably descended from coloured, photosynthetic, sulphur bac-

    teria, i.e., the photolithotrophs gave, after the advent of O,, rise t o the chemoli-

    thotrophs. In other words, oxidative phosphorylation evolved from photosynthetic

  • 8/11/2019 1977 Broda

    2/3

    492 E.

    BRODA

    phosphorylation. This is indicated by the conversion hypothesis for the origin of

    respiration from photosynthesis (BRODA

    175a).

    The basic processes in the energy metabolism of the photosynthetic sulphur bac-

    teria are

    2

    HS- + 2 Hf

    +

    CO, = (CH20)+ H 2 0

    + 2

    S; AG; = 11 kcal

    5 )

    0.5

    HS-

    +

    CO,

    +

    H,O

    = (CH,O)

    +

    0.5

    H+

    +

    0.5

    SO:-; AG;

    =

    18

    kcal

    Gj

    (CH,O) indicates unit quantity of biomass, no t formaldehyde. Reactions

    (5)

    and

    6),

    which are endergonic, are energized by light,

    Le . ,

    electrons are promoted photochemi-

    cally. A separation into exergonic and endergonic partial reactions would, in con-

    trast t o the position with the chemolithotrophs, not be meaningful with the photo-

    lithotrophs because

    CO,

    is indispensable as terminal (extracellular) electron acceptor.

    I n reactions (1) to 4) this role is played by 0 . Incidentally, for the processes 4) and

    (6) he term sulphurication might be introduced, in analogy to nitrification (processes

    Who, then, were the ancestors of the nitri ficants? Can they, in parallel to the

    evolution

    of

    the sulphur bacteria, have descended from photosynthetic ammonia

    bacteria ? Such (coloured)bacteria are not known. But apparently no search has ever

    been made for them. They may exist

    or

    else they may have existed, but

    died

    out.

    The photochemical promotion of electrons from

    NH:

    t o reduce CO,, the fundamental

    feature of such hypothetical bacteria, would from the point of view

    of

    energetics not

    be too difficult

    1.3 NHZ + CO, = (CH,O) + 0.65 N, + H,O + 1.3 H + ; AG; = 12 kcal (7)

    This would involve a direct biotic oxidation of

    NH2,

    i.e. of NH,, to N,. Such a

    reaction is unknown.

    In contrast to the anaerobic and endergonic reaction

    (7) ,

    an aerobic and exergonic

    oxidation of NH, to

    N,

    could, like th at to NO, or NO;, occur only after the appe-

    arance of

    0

    in the biosphere:

    8)

    1 2) .

    NH?

    0.75

    0 = 0.5 N, 1.5 H,O + H+ ; AG; =

    5

    kcal

    (The exergonicity of NH, oxidation by 0 is,

    of

    course, also evident from the fact

    that NH, is considered as a commercial fuel). Chemolithotrophs capable of reaction

    (8) would compete with the nitrificants, responsible for reactions (1) and

    (2).

    They

    would likewise be colourless,

    i .e . ,

    white. But, like reaction

    (7),

    reaction (8) has never

    been observed.

    .

    In reaction

    8), 0

    could be replaced as an oxidant by

    NO,

    or NO;:

    NHZ + NO, =N, + 2 H,O; AG; = 86 kcal

    9)

    The resulting reaction, here written down only for the stoichiometrically simpler

    case

    of NO;,

    could also be considered as a variant of denitrification, i . e . , of nitrate

    or nitrite dissimilation, or, in the terms of

    EGAMI

    TAKAHASHTt at . 1963), of nitrate

    or nitrite respiration.

    Thus the missing photolithotrophs and chemolithotrophs would both produce

    N, from

    NH,. So

    far only

    NO;

    or NO, are known as important biotic sources

    of

    N,.

    This is set free in denitrification:

    NO,

    0.75

    (CH,O)

    H+

    = 0.5N,

    +

    0.75

    CO,

    +

    1.25 H,O; AG; = -95 kcal

    The only exception is the production, of uncertain quantitative importance, of

    N,O

    from NH, by some aerobic chemoorganotrophs

    YOSITIDA

    nd ALEXANDER

    970)

    ;

    the N20 further yields, abiotically,

    N, JOHNSTON972).

    Apart from this N O by-

  • 8/11/2019 1977 Broda

    3/3

    Two missing lithotrophs

    493

    pass, the biotic pathway from NH, to N,, reversing the fixation of atmospheric N,,

    must take the detour via nitrification.

    This

    s,

    or

    was, not true if the missing litho-

    trophs here put forward exist, or existed.

    Acknowledgement

    I like to thank Dr. G. A. PESCHEKor discussions.

    Addi t ion in proof

    An extensive survey of the role of

    N,O

    in the atmosphere has now been given by

    HUN

    and JUNQE 1977).

    Refe rences

    BRODA,.,

    1975a.

    The Evolution of the Bioenergetic Processes. Pergamon Press Oxford.

    BRODA,

    .,

    1975b.

    The history of inorganic nitrogen in the biosphere. J. Mol. Evol.,

    T,87-100.

    FROMAQEOT,

    . and SENEZ,

    . C., 1960.

    Aerobic and anaerobic reactions of inorganic substances.

    In: Comparative Biochemistry, Vol. 1, 347-409

    (M. FLORKIN

    nd

    H. S. MASON,

    Editors). Aca-

    demic Press New York.

    HAHN,J. and

    JUNQE., 1977.

    Atmospherous nitrous oxide:

    a

    critical review.

    Z.

    Naturforsch.,

    828. 190-214.

    JOHNSTON

    .,

    1972.

    Newly recognized vital nitrogen cycle. Proc. net. Aced. Sci. Wash.,

    69,

    2369-2372.

    RUTTEN, . G., 1971. The Origin of Life by Natural Causes. Elsevier Amsterdam.

    TAKAHASHI,., TANIQUCHI,

    .

    and EQAMI, F.,

    1963.

    Inorganic nitrogen compounds: Distribution

    and metabolism. In : Comparative Biochemistry,

    Vol. 5,92-202

    M.

    ITLORKIN

    nd H.

    S.MASON

    Editors). Academic Press New York.

    YOSHIDA,. and

    ALEXA NDER, ., 1970.

    Nitrous oxide formation by

    N i t r o s omoms

    europea

    and

    heterotrophic organisms. Soil Science Amer. Proc.,

    34

    880-882.

    Mailing address: Prof. Dr. E. BRODA

    Institute of Physical Chemistry,

    University

    Wiihringer StraDe 42

    A-1090

    Wien, Austria,