8. REACTIONS OF INORGANIC SUBSTANCES
365
dium butyrate and by higher concentrations of urine, broth, or sodium
acetate. This inhibitory effect of organic substances gave rise to the
classical postulate of a sort of antinomy between the autotrophic and
heterotrophic systems. However, van Niel (3) stated that the importance of these facts had been greatly exaggerated. Meyerhof (41)
had observed earlier that Nitrosomonas and Nitrobacter, though inhibited by amino acids and amines, tolerated relatively high concentrations of organic nitrogen-free compounds, particularly sugars. Jensen
(42) confirmed that pure cultures of Nitrosomonas are not inhibited
by 0.3-0.6 M concentrations of organic acids, polyalcohols, and carbohydrates and established that the inhibition by glucose reported by
earlier authors was in reality caused by toxic decomposition products
formed during heat sterilization of the sugar. According to KingmaBoltjes (43) and others (44-46), growth of Nitrosomonas is even enhanced by certain organic compounds, especially carbohydrates; this
would explain the well-known fact that nitrification is more rapid on
natural soils rich in organic matter than on poor soils or in artificial
culture on purely inorganic media.
Lees (47) attributes the inhibition of growth in nitrifying bacteria
by low concentrations of amino acids to the formation in the culture
medium of chelates between the amino acids and certain oligo-elements
indispensable for the growth of the organisms. Nitrogen compounds may
also inhibit growth by other mechanisms. Thus methionine, which inhibits nitrification strongly in soil (48) and in artificial cultures during
growth (49, 50), may act indirectly by giving rise to —SH groups,
which are very toxic to nitrifying bacteria.
With the exception of two facultative autotrophs, one identified as
Thiobacillus novellus by Starkey (51) and the other described by
Tyulpanova-Mosevich (52), the thiobacilli are obligatory autotrophs
which cannot utilize any organic compound. This behavior cannot be
explained by the lack of appropriate enzyme systems, since—as we
have seen—these organisms and particularly T. thioparus and T. denitrificans possess the same enzymes for carbon metabolism as do the
heterotrophic bacteria. The postulated absence of "permeases," i.e., of
active systems which would provide for the endocellular penetration
of organic substrates (53), constitutes a very reasonable hypothesis,
but experimental verification has not yet been possible.
In fact, chemo-lithotrophic organisms are distinguished from heterotrophs not so much by any fundamental differences in their biochemical
activities as by the remarkable extent of biosynthetic capability in
chemo-lithotrophs, which enables them to produce from carbon dioxide,
ammonia, and a few inorganic salts the entire range of their cellular
constituents and the growth factors necessary for their development.
365
dium butyrate and by higher concentrations of urine, broth, or sodium
acetate. This inhibitory effect of organic substances gave rise to the
classical postulate of a sort of antinomy between the autotrophic and
heterotrophic systems. However, van Niel (3) stated that the importance of these facts had been greatly exaggerated. Meyerhof (41)
had observed earlier that Nitrosomonas and Nitrobacter, though inhibited by amino acids and amines, tolerated relatively high concentrations of organic nitrogen-free compounds, particularly sugars. Jensen
(42) confirmed that pure cultures of Nitrosomonas are not inhibited
by 0.3-0.6 M concentrations of organic acids, polyalcohols, and carbohydrates and established that the inhibition by glucose reported by
earlier authors was in reality caused by toxic decomposition products
formed during heat sterilization of the sugar. According to KingmaBoltjes (43) and others (44-46), growth of Nitrosomonas is even enhanced by certain organic compounds, especially carbohydrates; this
would explain the well-known fact that nitrification is more rapid on
natural soils rich in organic matter than on poor soils or in artificial
culture on purely inorganic media.
Lees (47) attributes the inhibition of growth in nitrifying bacteria
by low concentrations of amino acids to the formation in the culture
medium of chelates between the amino acids and certain oligo-elements
indispensable for the growth of the organisms. Nitrogen compounds may
also inhibit growth by other mechanisms. Thus methionine, which inhibits nitrification strongly in soil (48) and in artificial cultures during
growth (49, 50), may act indirectly by giving rise to —SH groups,
which are very toxic to nitrifying bacteria.
With the exception of two facultative autotrophs, one identified as
Thiobacillus novellus by Starkey (51) and the other described by
Tyulpanova-Mosevich (52), the thiobacilli are obligatory autotrophs
which cannot utilize any organic compound. This behavior cannot be
explained by the lack of appropriate enzyme systems, since—as we
have seen—these organisms and particularly T. thioparus and T. denitrificans possess the same enzymes for carbon metabolism as do the
heterotrophic bacteria. The postulated absence of "permeases," i.e., of
active systems which would provide for the endocellular penetration
of organic substrates (53), constitutes a very reasonable hypothesis,
but experimental verification has not yet been possible.
In fact, chemo-lithotrophic organisms are distinguished from heterotrophs not so much by any fundamental differences in their biochemical
activities as by the remarkable extent of biosynthetic capability in
chemo-lithotrophs, which enables them to produce from carbon dioxide,
ammonia, and a few inorganic salts the entire range of their cellular
constituents and the growth factors necessary for their development.