Tuesday 3 April 2012

Small Cells, Big Trouble for Toxic Mercury Bioaccumulation

Many trace metals are utilised by microbes in metabolic reactions. Mercury is one of these elements. Mercury is used in demethylation reactions of substrates and results in the formation of methylmercury for which there are two forms, mono-methylmercury and di-methylmercury, both are toxic to humans. Methylmercury concentrations have been steadily increasing in coastal waters as increasing amounts of mercury are deposited into the sea either by polluted rivers or contamination from coal-fired power stations. This in turn leads to increased methylmercury formation by marine bacteria.

Higher marine methylmercury levels are a major concern because it biomagnifies in the marine food webs. Organisms consume microbes contaminated with methylmercury, which is easily absorbed but very difficult to excrete due to it being more soluble in lipids than water. The result is an increasing build up of methylmercury as it moves through the food web, from bacteria to piscivorous fish. If contaminated fish (tuna for example) are consumed by humans it can have a number of detrimental effects to health. The developing nervous systems of unborn babies are at particular risk.

Methylmercury levels in coastal regions have been well reported. However, a number of studies have concentrated on methylmercury concentration in the open oceans. Even though such areas are far away from potential mercury contamination sources, oceanic levels are also increasing. Heimburger et al (2010) conducted a 20 month study in the North-western Mediterranean, at a site called DYFAMED to investigate why this could be happening. This site was chosen because it is separated from coastal waters by the Ligurian current while being relatively close to the shore (~ 50 km). The hypothesis of this study was that methylmercury production in such an environment is seasonally dependent on the type of primary production. This was examined by measuring the methylmercury concentrations and the distribution of phytoplankton throughout the water column, reaching down to the sea floor (2350m).

Methylmercury concentrations showed a double peak of 0.22 pM and 0.82 pM at a depth of approximately 50m (euphotic zone) and 400m (aphotic zone) respectively. The upper euphotic zone (0-10m) showed the lowest concentrations (< 0.10 pM). The levels seasonally changed with the highest peaks in the autumn months. This correlated with blooms of nano- and picophytoplankton in the euphotic zone during oligotrophic conditions.

The authors concluded that the high methylmercury concentrations at 50m in the autumn months were caused by the larger surface to volume ratio of the nano- and picophytoplankton compared to larger photosynthetic microbes. This enables a higher rate of mercury intake and mercury methylation. The even higher concentrations in the aphotic zone was attributed to the sinking of POM from the nano- and picophytoplankton reaching the deeper water were hetrotrophic microbes feed upon them, starting the methylmercury accumulation. The small size of the nano- and picophytoplankton POM aggregates also sink more slowly and are more easily fed from, increasing activity in the microbial loop and further increasing biomagnification. Despite having less input of mercury (mainly atmospheric) than coastal areas, the oligotrophic conditions of the open ocean seem to favour mercury methylation as nano- and picophytoplankton are more common. Although it must be said that some of this is still just speculation.

I reviewed this article because it possibly has a link with my first blog from last year which talks about a shift in marine phytoplankton to smaller sized species as a result of global warming. This coupled with high amounts of atmospheric release of mercury from power stations, could increase the amount of methylmercury biomagnification in marine organisms caught for human consumption.

A Review of:
Heimburger L, Cossa D, Marty J, Migon C, Averty B, Dufour A and Ras J (2010) Methyl mercury distribution in relation to the presence of nano- and picophytoplankton in an oceanic water column (Ligurian Sea, North-western Mediterranean). Geochimica et Cosmochimica Acta. 74: 5549-5559.

2 comments:

Samantha Bowgen said...

It wouldn't be as realistic in the open water, but maybe if mercury pollution was treated in coastal waters, this would reflect well on the amount dispersed out to sea.
Do you know if there are any current methods in place to treat mercury contaminated water? I have read about successful small scale treatments. E.g. where doses of Stannous chloride were used, successfully removing >94% of mercury. Although the solution may then cause problems of its own..

Chemicals can be used to render the contaminating mercury insoluble. Or even are there treatments to reduce the activity of the microbes involved..

Most likely not sensible ideas, but would be interesting to see how mercury pollution is currently being tackled.

Lee Hutt said...

Hi Sami

As far as I am aware there are not any activley used methods in treating this problem, in the ocean anyway. There seems to be several studies looking into this but most seem to focus on fresh water or soil.

Zhuang et al (2003) used lignin (derivative from trees) to remove mercury from soil and water. Because it contains many polar functional groups it can remove metal ions by binding with them and creating large macromolecule lignin-metal complexes, eliminating the metal availability. I think the problem with this however is what other metals does it eliminate from the environment? Could cause more harm than good. Other techniques I have read about involve electrodes but I doubt this would be realistic in an ocean setting.

Zhuang JM, Walsh T and Lam T (2003) A new technology for the treatment of mercury contaminated water and soils. Environmental Technology. 24: 897-902.