An example of a well written post from 2011

Small Cells, Big Trouble. Toxic Mercury Bioaccumulation

Mercury is used by marine bacteria in demethylation metabolic reactions and results in the formation of two forms of methylmercury (MeHg), both toxic to humans. MeHg concentrations have been rising in coastal waters as increasing amounts of mercury are deposited into the sea either by polluted rivers or atmospheric contamination from coal-fired power stations. Organisms consume microbes contaminated with MeHg, which is easily absorbed but difficult to excrete due to it being more soluble in lipids than water. The result is an increasing build-up of MeHg as it moves through the food web, from bacteria to fish (biomagnification). If contaminated fish (e.g. top predators like tuna) 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.MeHg levels in coastal regions have been well reported. However, a number of recent studies have shown that oceanic levels are also increasing, even though such areas are far away from potential mercury contamination sources.

Heimburger et al (2010) conducted a 20 month study at a site in the North-western Mediterranean, chosen because it is separated from coastal waters by the Ligurian current whilst being relatively close to the shore (~ 50 km). They measured the MeHg concentrations and the distribution of phytoplankton throughout the water column, down to the sea floor (2350 m). MeHg concentrations showed a double peak of 0.22 pM and 0.82 pM at depths of approximately 50 m (euphotic zone) and 400m (aphotic zone) respectively. The upper euphotic zone (0-10m) showed the lowest concentrations (< 0.10 pM). The levels changed seasonally, with the highest peaks in the autumn months correlated with blooms of nano- and picophytoplankton in the euphotic zone during oligotrophic conditions.

The authors concluded that the high MeHg concentrations at 50 m in the autumn were caused by the higher surface to volume ratio of the small nano- and picophytoplankton cells compared to larger photosynthetic microbes found nearer the surface. Thet speculated that this enabled a higher rate of mercury intake and mercury methylation. The even higher concentration in the aphotic zone was attributed to the slow sinking of small POM aggregates from the nano- and picophytoplankton reaching the deeper water where heterotrophic microbes feed upon them, starting the MeHg accumulation. ,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.

I reviewed this article because it possibly has a link with my earlier blog 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 MeHg biomagnification in marine organisms caught for human consumption.

Source: 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.

(Edited from a post by Lee Hutt, 2011)

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.

4 April 2012 13:48

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.

7 April 2012 09:46