Microbial community composition and mercury cycling in sediments of tagus estuary

Tese de doutoramento, Farmácia (Toxicologia), Universidade de Lisboa, Faculdade de Farmácia, 2016

Mercury is a pervasive pollutant well known to cause several disorders in humans and wildlife. The major concern related with mercury pollution is the neurotoxicity associated to methylmercury and its presence in aquatic systems, as it undergoes bioaccumulation and biomagnification in the food chain. In aquatic ecosystems, mercury-resistant microorganisms are the main responsible for methylation of Hg2+ and also for processes of detoxification (reduction of Hg2+ and demethylation of methylmercury). High levels of mercury, including methylmercury, have been shown to exist in the Tagus Estuary. This study aims to give an insight about the involvement of microorganisms in the cycle of mercury in the Tagus Estuary, based on their phenotypic and genetic characterization. To achieve this, mercury-resistant microorganisms were isolated from sediments of four mercury-polluted areas of the Tagus Estuary (Barreiro, Cala do Norte, Rosário and Alcochete) and, after their characterization their potential to transform mercury compounds was evaluated. The isolates encompassed aerobic microorganisms, such as Bacillus sp., Vibrio sp., Aeromonas sp. and Enterobacteriacea sp., and anaerobic microorganisms, such as Clostridium sp., Enterobacteriaceae sp. and the Archaea sulfate-reducing bacteria (e.g. Desulfovibrio desulfuricans). Their resistance to mercury compounds ranged from 0.41-140 μg/mL for Hg2+ and 0.04-50.1 μg/mL for CH3Hg. The genetic system conferring detoxification ability (mer operon genes) was found only in 7% of the isolates, being all aerobes. This set of data indicated the involvement of these microorganisms in the processes of methylation and detoxification of mercury in the Tagus Estuary. To evaluate this hypothesis, isolated microorganisms and microbial communities were incubated with HgCl2. The results showed that these microorganisms are able to reduce Hg2+ into Hg0, resulting in the removal of around 50% of the total added mercury. The highest removal rates were observed among isolates of high contaminated areas (Barreiro and Cala do Norte). It was also observed the formation of organomercurials, including methylmercury. The rate of methylation among the isolates ranged between 1-8%. Moreover, it was found that bacteria isolated from salt marsh are influenced by plants species such as Sacocornia fruticosa and Spartina maritima since the kinetics of Mercury mobility between plant’s roots and the surrounding environment affects mercury-resistant microorganisms’ selection. Thus, these results are the first evidence of the relevance of interaction between bacteria and plants in Hg cycling in the Tagus Estuary. To understand better the conditions promoting methylation and demethylation, three microbial communities (aerobic, anaerobic and sulphate-reducing bacteria communities) were incubated with isotope enriched mercury species (199HgCl and CH3201HgCl). The results showed that microbial communities are actively involved in methylation and demethylation processes, being the methylation directly related with sulphate-reducing bacteria communities with rates up to 0.07% (after 48h), while the demethylation process is strongly promoted (rates up to 100%) by aerobic community. To obtain optimal conditions for mercury reduction, the effects of ambient factors, such as organic matter (glucose), sulphate, iron and chloride, on microbial reduction were evaluated by factorial design methodology. The results revealed that sulphate enhances microbial reduction, while chloride inhibits it. Overall, the results showed that microorganisms of Tagus Estuary are involved in processes that change mercury speciation through reduction and demethylation and formation of methylmercury. The removal is a pathway for detoxification and can be used on the bioremediation strategies. Meanwhile, the formation of methylmercury represents a risk for human health. Thus, this study’s set of data is useful for both risk assessment and bioremediation purposes.