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Electrogenic NItrate Reduction In marine Sediments

Periodic Reporting for period 1 - ENIRIS (Electrogenic NItrate Reduction In marine Sediments)

Reporting period: 2016-04-01 to 2018-03-31

Recently, a novel type of long, filamentous bacteria has been discovered, which are capable of guiding electrical currents over centimeter distances. These so-called “cable bacteria” were first isolated from marine sediments, and laboratory experiments demonstrate that electrons are passed on from cell to cell along the longitudinal axis of the filaments, hence electrically coupling sulphide oxidation at depth with oxygen reduction at the sediment surface. This long-distance electron transfer (LDET) radically extended the known physiological capacities of prokaryotes, and profoundly changed our understanding of biogeochemical transformations in marine sediments. In addition to oxygen, these sulphur oxidizing bacteria can use nitrate as an electron acceptor. However, the product of this “electrogenic” nitrate reduction, and thus the effect of this newly described process on the global nitrogen cycle, are still unknown. ENIRIS aimed to: 1) investigate the nitrate reducing metabolism of cable bacteria and 2) elucidate the environmental factors that regulate the occurrence of cable bacteria in the environment.

The elevated input of nitrogen into coastal waters due to fertilizer use and intensive farming remains a problem of concern within Europe, and now also affects the coastal zone of rapidly developing countries (China, India, Brazil). Eutrophication results in the degradation of water quality with negative effects on important economic sectors such as fisheries and tourism as well as on public health. Still, our current understanding on nitrogen dynamics in the environment is far from being exhaustive, and new knowledge is needed to support policy-makers in generating innovative and effective actions to contrast Eutrophication on a European scale. Cable bacteria have been recently reported from a wide range of aquatic environments across the globe, which indicates that “electrogenic” nitrate reduction has a potential global dimension. This new form of nitrate reduction could have important implications for the removal (if dinitrogen is the end-product of the reduction) or recycling (if ammonium is the end product) of bio-available nitrogen in marine ecosystems.
The aim of ENIRIS is to provide crucial insights on this novel nitrate reducing processes that will significantly increase our comprehension of nitrogen dynamics in coastal waters, sustaining Europe in the vanguard of this research field.

The goal of this project is to identify the end products of the “electrogenic” nitrate reduction, the overall impact of cable bacteria on nitrogen cycling, and the environmental factors that regulate cable bacteria development in natural environments.
In laboratory incubation we investigated nitrogen transformations in sediment enriched with cable bacteria, and clearly identified the product of cable bacteria-mediated nitrate reduction. Our conclusions are supported by two independet lines of evidence based on geochemical and genetic analysis. To obtain a more comprehensive understanding on the impact of LDET on the sediment nitrogen cycling we also addressed the potential occurrence of secondary impacts, such as the ones induced by the alteration of the porewater pH stimulated by cable bacteria activity.
Beside the above mentioned studies we investigated the natural occurrence and physiological limits of cable bacteria. During two successive cruises in the Eastern Gotland Basin (Baltic Sea) on board of the Research Vessel “Skagerak” of Gothenburg University, we studied if transient availability of oxygen stimulates cable bacteria development. Our findings indicate that cable bacteria can develop at very low oxygen levels (in the low end of the hypoxic interval) thus expanding our understanding on their potential distribution in natural oxygen-depleted environments.
A series of manuscripts on these results obtained by the above mentioned studies are in preparation or submitted to peer-reviewed scientific journals and have partially been presented at international conferences and workshops.
Studies conducted within ENIRIS utilized a combination of state-of-the-art techniques including electrochemical, geochemical, molecular biology, and bioinformatics methodologies. Our research involved scientists and students from Belgium, The Netherlands, Denmark, Sweden, and Australia.
Scientific work conducted throughout ENIRIS enhanced our knowledge on various aspects of cable bacteria metabolism, on their impact on nitrogen and sulphur cycling, and on the factors that regulate their occurrence in natural sediments. All in all, ENIRIS gave a substantial contribution to the characterization of the impacts of an important, and so far overlooked, player in regulating nutrients and energy fluxes in highly reactive coastal environments, which are particularly threatened by anthropogenic activities.
Cruise Gotland Basin 1
Cruise Gotland Basin 2