Bacteria-based technology to degrade toxic compounds of mercury in marine sediments

The MER-CLUB Project, in which the Instituto de Ciencias del Mar (ICM-CSIC) is involved, aims to develop a marine bacteria-based technology to degrade a toxic mercury compound and thus to improve the quality of the marine environment. This compound, monomethylmercury, accumulates in marine sediments and enters the food chain.

First MER-CLUB meeting in AZTI, Sukarrieta. Top row from left to right: Anders Lanzen (AZTI), Ulrich Soltmann (GMBU), Stefan Bertilsson (SLU), Olga Sánchez (UAB), Silvia G. Acinas (ICM-CSIC), Andrea G. Bravo (ICM-CSIC) , David Amoroux (UPPA) and Mónica Estupiñán (AZTI). Bottom, from left to right: Ibon Lekue (AFESA Medio Ambiente S.A.), Laura Alonso-Sáez (AZTI), Eduardo Alzola (AFESA Medio Ambiente S.A.).Sediments play a key role in marine ecosystems, providing nutrients that feed aquatic species and mineral resources that guarantee richness and biological diversity. However, pollution from industrial effluents and wastewater accumulates in the sediments. It was not until the 1990s that a waste discharge law was implemented. Until then, large amounts of metals, including mercury,  accumulated in the sediments.

Now, an international consortium led by Dra. Laura Alonso-Sáez from AZTI, and composed of the Institute of Marine Sciences (ICM-CSIC, IP: Silvia G. Acinas), the Autonomous University of Barcelona (UAB, IP: Olga Sánchez), the Swedish University of Agricultural Sciences (SLU, IP: Stefan Bertilsson), the University of Pau and Pays de l'Adour (UPPA, IP: David Amoroux), the Association of Innovative Medical, Bio- and Environmental Technologies (GMBU, IP: Ulrich Soltmann) and a small company, AFESA Medio Ambiente (AFESA, IP: Eduardo Alzola), is developing the MER-CLUB research project for the bioremediation of these ecosystems.

Reducing mercury accumulated in the marine environment

“The goal of MER-CLUB is to reduce the levels of two chemical forms of mercury, monomethylmercury (MMHg) and inorganic divalent mercury (Hg2 +),” says Silvia G. Acinas (ICM-CSIC), one of the researchers. The scientist explains that the first form of mercury “bioaccumulates in organisms and biomagnifies in food chain”, while divalent mercury “accumulates to a lesser extent in organisms, in spite of the fact that it is the chemical form that predominates in water and sediments”.

Bioremediation is a biotechnological process to restore an environment altered by contaminants

Bioremediation is a biotechnological process that uses microorganisms to restore an environment altered by contaminants. MER-CLUB’s main objective is to develop a biotechnological solution based on detoxifying bacteria to reduce the presence of mercury in marine sediments.

“Adjusting a chemical method to remove mercury bound to each of these chemical forms is a long and expensive process,” explains Andrea G. Bravo at ICM-CSIC. The researcher adds that “the main line of the project is to describe the functional diversity of native bacteria and bacterial communities, which already live in contaminated sediments, in order to enhance their activity and simultaneously eliminate chemical forms of mercury.”

Marine bacteria for bioremediation

Culture of bacteria isolated from marine sediment.A new bacterial strain was used, isolated from the deep ocean at more than 4,000 metres depth and taxonomically associated with the genus Alteromonas.

“We will analyse the potential of this strain and other new strains isolated from contaminated marine sediments. We aim at developing a technology capable of reducing by 50 to 70% the amount of mercury in the sediments,” explains Laura Alonso Sáez (AZTI). “So far, as a result of our research, we already have several strains of Alteromonas isolated from seawater, which have a great capacity for mercury degradation,” she continues.

Bacteria in the Alteromonadaceae family are large, fast-growing cells with relatively large genomes compared to other bacteria. Many of these genomes contain the mer operon, made up of a set of genes involved in the metabolic pathways of mercury transformation and degradation, such as merB and merA.

The accumulation of mercury in marine sediments after centuries of industrial activity

Taking into account data from the past three decades, it has been estimated that mercury accumulated in coastal environments adds a total of 50,000 and 100,000 tonnes.

The researchers pinpoint cases such as the Japanese Bay of Minamata, where an acetaldehyde-producing factory caused an increase of up to 2,000 mg / kg in the concentration of mercury in the sediments; or minery activities in the Idrija mine in Slovenia, which caused an accumulation of more than 1,800 tonnes of mercury in the sediments for 500 years. Also, events such as the California gold rush, which left more than 140 tonnes accumulated in the San Francisco Bay at the end of XIX century.

“It is estimated that the Mediterranean sea accumulates about 1,000 tonnes of mercury, 238 tonnes of them in the form of monomethylmercury, which enters the food chain,” point out the researchers of the project. This is confirmed by the fact that high levels of mercury have been detected in biomarker species (mussels) and explains the accumulation of this compound in marine sediments, which constitutes “a potential global risk for coastal systems and food chains,” the scientists agree.

Application of the technology developed by MER-CLUB

The place chosen for the pilot study to be carried out in the final stages of MER-CLUB, is the Nervión estuary, in the Basque Country. It is a relevant environmental site because the mercury concentrations were above the threshold of “strong contamination”. The application of bioremediation technologies in real conditions will be possible with the participation of the company AFESA Medio Ambiente.

MER-CLUB will monitor the decontamination of mercury in marine sediments in a controlled environment

“The first step will be dredging of contaminated marine sediments to carry out the experiment in a closed system, on a small scale (1-4 kg), in which the conditions will be optimised to enhance the activity of selected bacteria. Subsequently, we will scale the experiment up to 40 or 50 Kg of sediments,” indicates the team of researchers.

MER-CLUB will monitor the decontamination of mercury in marine sediments in a controlled environment, studying the chemical cycles of mercury and the interactions between natural bacterial communities and detoxifying bacteria introduced in the system. The experiment will last for a month to determine the decontamination efficiency in these complex systems.

This will allow estimating the time and viability of this new bioremediation strategy and, during the last phase of the project, determining the potential for transferring this sustainable technology to the market.