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A.C.A. da Costa
and -sensitive cells, showing that, as expected, the resistance is related not only to
the concentration of mercury in the medium but also to the type of mercury
compound. These observations were also described in a more recent paper
discussing the relation between mercury speciation and biotoxicity (Farrel et al.
1990). For instance, Bacillus sp. is resistant to a 10 mg/l HgCl2 solution, while
Pseudomonas sp. is resistant to a 6 mg/l HgCl2 solution. For the bacterium
Pseudomonas aeruginosa, a 10 ~lg/ml HgCl2 solution, a 200 nmol HgH solution
and 50 nmol methylmercury and ethylmercury chloride solutions, all inhibited
the cells, indicating that the mercurial species also have a marked effect on their
metabolic activities, as previously mentioned. Mercury resistance cannot be
generalized, because the biochemical effects produced in different microbial cells
are dependent on the characteristics of that particular genus, or even that
particular strain, or even variety inside the same microbial strain; it is closely
related to the enzymatic apparatus of the cells, as well as their availability to
incorporate the compound. Even associations with mineral particles, as well as
some organic materials, can markedly alter this figure.
Mercury is an element with a high affinity for sulphur compounds, presenting
a high affinity for biomolecules such as amino acids, proteins (amino acid
sequences), pyrimidines and nucleic acids. Its reactiviness is closely related to
-SH and S-S groups from the biomolecules (Steel 1960). Taking this information
into account, it is not difficult to realize that the affinity/resistance of different
microbial cells for mercury will depend on the structure of their composing
proteins and cells, and could easily be revealed by the sequencing of proteins
from different microbial cells. However, this kind of amino acid mapping is not
readily available for all kinds of microorganisms; it is better to estimate the
extent of the interaction between mercury and microbial cells through the basic
composition of the chemical constituents of microbial cells.
Valle and Ulmer (1972), studying the biochemical effects of heavy metals,
properly described this sort of interaction regarding certain biomolecules,
emphasizing their observations in the reactions involving synthesis and
degradation of methylmercury by microbial cells. The authors mention that
methylmercury and dimethylmercury are not strongly adsorbed to sediments,
being somewhat soluble in water, thus, available to be assimilated by microbial
cells. The authors stress that the basis for methylmercury production comes from
the reaction between HgH and methylpentacyanocobaltate, and also that three
different agents can transfer methyl groups in biological systems, S-adenosylmethionine, derivatives from N-methyltetrahydrofolate and methykorrinoids.
The first two can transfer methyl as carbonium ion (CH;), unable to transfer
methyl to HgH; and the latter can transfer groups as carbanion (CH~), CH;, or as
CH 3 radical, being able to methylate mercury salts of elemental mercury with CH 3
radical.
Apart from these considerations, HgH can markedly alter the permeability of
the cellular membrane and organomercurials can affect the permeability of
chloroplast membranes. Bharathi et al. (1990) also observed changes in the
permeability of cell membrane in a Desulfosarcina-like bacterium as a function of
mercury uptake. The HgH ion presents low affinity for phospholipids while alkyl
mercuric chlorides promptly interact with phospholipid layers. From these
A.C.A. da Costa
and -sensitive cells, showing that, as expected, the resistance is related not only to
the concentration of mercury in the medium but also to the type of mercury
compound. These observations were also described in a more recent paper
discussing the relation between mercury speciation and biotoxicity (Farrel et al.
1990). For instance, Bacillus sp. is resistant to a 10 mg/l HgCl2 solution, while
Pseudomonas sp. is resistant to a 6 mg/l HgCl2 solution. For the bacterium
Pseudomonas aeruginosa, a 10 ~lg/ml HgCl2 solution, a 200 nmol HgH solution
and 50 nmol methylmercury and ethylmercury chloride solutions, all inhibited
the cells, indicating that the mercurial species also have a marked effect on their
metabolic activities, as previously mentioned. Mercury resistance cannot be
generalized, because the biochemical effects produced in different microbial cells
are dependent on the characteristics of that particular genus, or even that
particular strain, or even variety inside the same microbial strain; it is closely
related to the enzymatic apparatus of the cells, as well as their availability to
incorporate the compound. Even associations with mineral particles, as well as
some organic materials, can markedly alter this figure.
Mercury is an element with a high affinity for sulphur compounds, presenting
a high affinity for biomolecules such as amino acids, proteins (amino acid
sequences), pyrimidines and nucleic acids. Its reactiviness is closely related to
-SH and S-S groups from the biomolecules (Steel 1960). Taking this information
into account, it is not difficult to realize that the affinity/resistance of different
microbial cells for mercury will depend on the structure of their composing
proteins and cells, and could easily be revealed by the sequencing of proteins
from different microbial cells. However, this kind of amino acid mapping is not
readily available for all kinds of microorganisms; it is better to estimate the
extent of the interaction between mercury and microbial cells through the basic
composition of the chemical constituents of microbial cells.
Valle and Ulmer (1972), studying the biochemical effects of heavy metals,
properly described this sort of interaction regarding certain biomolecules,
emphasizing their observations in the reactions involving synthesis and
degradation of methylmercury by microbial cells. The authors mention that
methylmercury and dimethylmercury are not strongly adsorbed to sediments,
being somewhat soluble in water, thus, available to be assimilated by microbial
cells. The authors stress that the basis for methylmercury production comes from
the reaction between HgH and methylpentacyanocobaltate, and also that three
different agents can transfer methyl groups in biological systems, S-adenosylmethionine, derivatives from N-methyltetrahydrofolate and methykorrinoids.
The first two can transfer methyl as carbonium ion (CH;), unable to transfer
methyl to HgH; and the latter can transfer groups as carbanion (CH~), CH;, or as
CH 3 radical, being able to methylate mercury salts of elemental mercury with CH 3
radical.
Apart from these considerations, HgH can markedly alter the permeability of
the cellular membrane and organomercurials can affect the permeability of
chloroplast membranes. Bharathi et al. (1990) also observed changes in the
permeability of cell membrane in a Desulfosarcina-like bacterium as a function of
mercury uptake. The HgH ion presents low affinity for phospholipids while alkyl
mercuric chlorides promptly interact with phospholipid layers. From these