The ability of freshwater aquatic vascular plants to accumulate heavy metals was examined in some detail during a five year study. Differences in uptake rate were found to depend on the species of plant, the seasonal growth rate changes and the metal ion being absorbed. Lead and mercury were concentrated to a greater extent than the lighter nickel and copper. Laboratory experiments were designed to establish uptake rate constants which were used to calculate water concentrations of mercury from the analyses of plant samples from the river. ‘Background’ levels of mercury in aquatic plants of 35–50 ng g-1 dry weight corresponded to a water concentration near 15 ng L-1 of total mercury of which 25–30% was methylmercury. Higher concentrations of mercury in the plants indicated a proportional increase in the mercury level in the water.

Comparative rates of absorption of copper, lead, cadmium, nickel, tin, and inorganic and methylmercury ions from water by Elodea densa were measured. Methylmercury, at relatively low tissue concentrations, was the only ion that quenched the laser-induced fluorescence.

Elodea densa plants were grown in flowing water for 25 days at methylmercury (MMC) concentrations of 7.5 × 10−10, 7.5 × 10−9, and 7.5 × 10−8 M. Toxicity development was different within and among the three types of meristematic tissues—apex, root, and bud. Apical cells were most sensitive to MMC and developed aberrant nuclear and mitotic characteristics at lower concentrations than did roots. Root meristems had a total inhibition of mitotic activity at 7.5 × 10−9 M with no aberrant symptoms at lower MMC levels. Conversely, the mitotic activity in bud meristems was zero in the control and increased in the presence of MMC. However, the divisions were abnormal. Higher concentrations of MMC (up to 2.5 × 10−6 M) had the unusual effect of stimulating the development of additional buds. The formation and development of root and bud initials were inhibited at levels of 7.5 × 10−8 and 0.25 × 10−6 M MMC in the water.

Elodea densa plants continuously exposed to low levels of methylmercury (7.5 × 10−10 to 7.5 × 10−8 M) in a flowing water system over a 25-day period concentrated methylmercury in young tissues. Toxicity effects in apical meristems were not linear with dose and time but were interactive in response. Microscopic studies of plants treated at 7.5 × 10−10 M showed a predominance of "spaghetti" prophases, a gradual inhibition of mitosis, and imbalance of mitotic stages. Higher concentrations of methylmercury produced different effects: bi- and multi-nucleate cells, polyploidy, aneuploidy, and altered or incomplete cell walls. At the highest concentrations (7.5 × 10−8 M) total pycnosis developed in the apex within 11 days, followed by a disintegration of nuclear and cell contents. Demethylation in tissues was directly related to methylmercury concentrations in the tissues and to tissue age.

Elodea densa plants exposed to low levels of methylmercury in water (7.5 × 10−10 M) accumulate sufficient methylmercury within 25 days to severely damage the surface membrane and the internal structure of the leaf chloroplast.

Rats were dosed once per os with either plant-incorporated or solution-radioactive methylmercury (MeHg). While whole-body retention did not change, the incorporation into some organs was approx. 2.5 X higher when administered Hg was in the plant-incorporated rather than in solution form. The only noticeable change in demethylation occurred in the cerebrum where MeHg:Hg ratios were 79:10 (plant-incorporated MeHg) and 65:24 (solution-MeHg). Between 5 to 7% of the total mercury was excreted in faeces and urine either in the inorganic (faeces) or organic (urine) forms. Mercury levels in mitochondrial and soluble fractions of cerebrum were noticeably lower with plant-MeHg than with solution-MeHg.

A successful method was developed for the quantitative separation of biologically incorporated inorganic- and methyl-mercury from animal, fish or higher plant tissues. The differences in animal and plant tissue composition determine the initial treatment of tissues for the highest efficiency of extraction and the subsequent quantitative separation of the mercury species by thin layer chromatography. Thus, animal or fish tissue was treated initially with ethanolic potassium hydroxide solution whereas plant tissue was treated initially with a strong nitric:perchloric acid mixture. Recoveries of both inorganic- and methyl-mercury exceeded 96%.

Aquatic plants exposed to 488-nm laser light emit a bright, red–orange fluorescence. This fluorescence is brightest in young tissues, especially meristematic tissue of both shoots and roots. Older tissues of shoots and root hair areas both emit a pink and a white fluorescence respectively. The quality and intensity of fluorescence are decreased by the age of the tissue.

Elodea densa plants containing either methylmercury or inorganic mercury-203 were transferred to a flowing water system along with untreated plants for a period of 6 months. Although little or no radioactivity was measurable in the water throughout the experimental period, untreated plants accumulated some methylmercury (MMC) and to a lesser extent inorganic mercury (MC) within the 1st month. Treated plants lost more MC than MMC within the 1st month. The two species of mercury move in opposite directions within the plant with time: MMC moves toward young tissue whereas MC moves toward older tissue. Methylmercury remains stable in younger plant tissues almost indefinitely, but converts partly to inorganic forms in old and decomposed tissue. Conversion of inorganic mercury to methylmercury was not detected.

Mercury transfer from bed sediments to freshwater fish was observed 20 days in the laboratory. Organic-rich sediments from the Ottawa River were contaminated with mercuric chloride to produce a total mercury concentration of 1·023 ppm.

- Suspended sediments (solids) contribute 58% (982 kg per year) of all mercury transported downstream in the study section of the Ottawa River. Although filtered water contains a low mercury concentration (13 ng/1 or 0.013 ppb), it accounts for 41% (689 kg per year) of all mercury transported downstream. The role of bed sediment movements on the mercury transport is very small, only 1%, in the study section. Most of the mercury (96.7% of total mercury and 97.8% of methylmercury) is in bed sediments. Biomass contains an insignificant portion of mercury (0.2% of total mercury and 1.7% of methylmercury), though it has a higher ratio of methylmercury to total mercury; plants (20%), invertebrates (40%) and fish (85%). Methylmercury production and destruction are in equilibrium in water and sediments without biological agents, such as higher aquatic plants, invertebrate and fish.

MOST of the mercury (up to 97%) found in aquatic systems is associated with bed sediments1. Fish, invertebrates and plants contain only 0.02% of the total, though they contain a high proportion of organic mercury. Field surveys show that the proportion of organic mercury in the sediments range from 0.1 to 2.1%2–6. Jacobs and Keeney placed river sediments mixed with inorganic mercury in the Wisconsin and Fox Rivers7. After 12 weeks exposure to the natural environment the concentration of methylmercury in the sediments was only 3%. This concentration of methylmercury was attained within 4 weeks after sediments were placed in the bottom of the rivers. This indicated that the processes of methylation and demethylation of mercury were in equilibrium in these environmental conditions within 4 weeks. But no experiments were conducted to show the degradation of methylmercury in sediments. After the methylation study by Jensen and Jernelöv8, many investigations into methylmercury production by microorganisms in bed sediments have been carried out. Little attention, however, has been paid to the ratio of methyl-mercury to the total mercury existing in the sediments. Spangler et al. observed degradation of methylmercury in mixed cultures from sediments9. The degradation reached 50% within 5 d, they also found methylmercury degradation in sediments. But detailed information about the sediments was not available and results fluctuated wildly. We do not know the total amount of methylmercury existing in all components of the systems, including fish, invertebrates and plants as well as various types of bed sediments. Here we use methylmercury equilibrium concentration levels in the Ottawa River sediments to estimate the amount of methylmercury in various types of bed sediment where detailed information of total mercury concentrations (total mercury estimation 31 kg) and types of sediment is available. Two ways (methylmercury production and degradation) of reaching equilibrium were observed during 50 d in identical environmental conditions.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTIsotopic organic and inorganic mercury exchange in river waterAkira Kudo, Hirokatsu Akagi, D. C. Mortimer, and Donald R. MillerCite this: Environ. Sci. Technol. 1977, 11, 9, 907–908Publication Date (Print):September 1, 1977Publication History Published online1 May 2002Published inissue 1 September 1977https://pubs.acs.org/doi/10.1021/es60132a011https://doi.org/10.1021/es60132a011research-articleACS PublicationsRequest reuse permissionsArticle Views57Altmetric-Citations13LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts

The desorption rate of mercury from bed sediments was determined by a 10 week experiment. The rates ranged from 0.1 ng/cm2/day to 1.0 ng/cm2/day for Ottawa River bed sediments, depending on the environmental conditions. The rates decreased with an increase of exposure period to the water, but increased with an increase in the depth of bed sediments. The amount of mercury desorbed from bed sediments to overlying water was highly dependent on the volume (depth) of bed sediments. Calculations based on the experiment showed the half-lives of total mercury associated with bed sediments from as short as 2.1 years to as long as 1.8 × 102 years, depending on the depth of the bed sediments.

AbstractBed sediment from a known zone in the Ottawa River study area and cuttings of Elodea densa were set out in aquaria in a 24‐day controlled, flowing water experiment. Elodea was planted in sediment and in glass beads, and sediment was exposed with and without plants, all in the same aquaria for 7 days before the addition of mercury with 203Hg. Three concentrations each of mercuric chloride and methyl mercuric chloride were continuously metered into the input water to yield aquarium levels of 0.2, 2, and 10 µg/liter.There was no significant difference in the uptake rate between the two forms of mercury. Uptake was proportional to water concentration over the entire 17‐day exposure period in both plants and sediment. Methyl mercury was more toxic to plant growth in this time interval than inorganic mercury. Elodea growing in sediment absorbed significantly less mercury than Elodea growing in glass beads. Sediment from which the plants had been removed contained about twice as much mercury as plant‐free sediment. The aquarium walls were the largest absorbing compartment in the system and methyl mercury was absorbed more strongly than inorganic mercury. The total recovery of input mercury over the 17 days averaged 91%.

An increasing sugar gradient in the rib, a decreasing gradient in the upper region of the petiole, and a sharp increase in the base of the petiole were found when transverse segments of the rib and petiole of a sugar beet leaf or the isolated vascular bundles as a whole were analyzed. The sugar concentration and the gradient pattern varied considerably with the leaf position in the ontogenic sequence. The proportion of assimilated 14C exported as sucrose was highest for leaves of intermediate age, which were at the nearly mature stage. Mature and old leaves exported less 14C and young leaves, which had a high sugar concentration in the blade and a sharply decreasing gradient in the petiole, exported very little. Analysis of vascular bundles in the petiole which serve the tip and basal regions of the blade after 14CO2 incorporation into the blade indicated that a much larger amount of 14C-sucrose entered into and moved through the basal bundle than into and through the tip bundle.

The quantity and kind of radioactive compounds in the blade, main rib, and petiole of sugar beet leaves were determined at intervals up to 3 hours after a 1 minute period during which the blade photosynthetically assimilated C 14 O 2 . During the first 5 minutes, radioactive compounds were isolated from the main rib but these were due to the direct assimilation of C 14 O 2 by the rib and not to translocation into the rib. After 8 minutes, radioactive sucrose from the blade reached the rib and began to move down the petiole. This movement continued at a steady rate of about 1% of the assimilated C 14 per minute for about 1 hour. The concentration of C 14 per 2 cm section of petiole was a linear function of the distance, and this gradient moved down the petiole with an apparent velocity of 50 to 135 cm per hour. After 1 hour, the entire length of the petiole contained radioactive sucrose at a relatively uniform concentration per unit section. Although the blade still contained 40 to 50% of the assimilated C 14 in sucrose, movement out of the blade was reduced to a very low rate after 1 hour, but radioactive sucrose continued to leave the petiole for an additional hour. The absence of significant metabolism of the radioactive sucrose in the petiole, combined with the changes in gradient of that sucrose as a function of time, suggest that the petiole exerts a relatively passive role in the translocation process.