Biological Generation of Methylmercury
in Anaerobic Pond Sediment
PAUL L. BISHOP
University of New Hampshire
Durham, New Hampshire
EDWIN J. KIRSCH, Professor
School of Civil Engineering
Purdue University
West Lafayette, Indiana
INTRODUCTION
Mercury contamination of the environment has become a major source of concern in
the United States within the last two years. U.S. waterways in thirty-three states have been
shown to contain elevated levels of mercury (1). In many cases, levels considered dangerous
for human consumption have been reached. Analyses offish and other aquatic organisms
have shown that they concentrate the mercury internally (2,3). Wood (1) and Jensen and
Jernelov (4) reported that mercury found in fish is almost entirely in the form of
methylmercury. This is the case even when mercury metal or inorganic mercury salts are the
only forms of mercury entering the environment. Should mercury be methylated prior to
ingestion, the likely site for active transformation is in the bottom sediment where elemental
mercury or inorganic mercury tends to accumulate as insoluble precipitates.
Jensen and Jernelov (4) were the first to report that both mono- and dimethylmercury
can be produced in the lake bottom sediments. There has been much discussion, however,
concerning the conditions under which mercury methlyation can occur. Researchers in
Sweden reported that aerobic conditions were necessary for methylmercury generation (5).
Howe (6) has stated that mercury methlyation cannot occur under anaerobic conditions.
Shiboko and Nelson (7) reported that mercury was methylated under anaerobic conditions
but that methylation was much more efficient under aerobic conditions. This finding was
supported by Van Den Berg (8). On the other hand. Grant (9) stated that biological
conversion of inorganic mercury to methylmercury required the absence of oxygen. Wood,
et al (10) have suggested that inorganic mercury is transformed to methylmercury in cell-
free extracts of methanogenic bacteria but have not established that this reaction occurs as a
biological process mediated by whole viable cells in situ.
This study was designed to establish the role of anaerobic bacteria, particularly
methane-producing bacteria, taken directly from natural sources in the generation of highly
toxic methylmercury compounds from inorganic mercury. In addition, the role of organic
pollution in enhancing this reaction was studied.
METHODS AND MATERIALS
Cultures
Two cultures were used in the experiments reported. Most of the studies employed
anaerobic pond sediment taken from a sewage treatment plant tertiary lagoon. The second
culture consisted of sediment enriched with sodium acetate to enhance the growth of
methanogenic bacteria.
Sediment samples were obtained from the Green Meadows sewage treatment plant
tertiary lagoon located in West Lafayette, Indiana, one day prior to initiating an
experiment. The sediment was blended in the laboratory and the total volatile solids
concentration was adjusted to 10,000 mg/1. The sediment was maintained under anaerobic
conditions until needed.
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