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ABYSS - Assessment of bacterial life and matter cycling in deep-sea surface sediments

Final Report Summary - ABYSS (ABYSS - Assessment of bacterial life and matter cycling in deep-sea surface sediments)

The deep seafloor covers more than 60% of the Earth’s surface, but remains one of the least explored regions on our planet. Organisms in this environment largely depend on the input of organic matter produced by photosynthesis in upper water layers, but only a small fraction of this organic matter ultimately reaches the seafloor. At the seafloor, the benthic bacteria dominate deep-sea life in terms of abundance and biomass. These bacteria play important roles in carbon and nutrient cycling. By remineralization they recycle nutrients back into the water column, keeping oceans productive. While some bacteria also return a significant amount of sedimentary carbon in the form of CO2, others fix that CO2 and keep it in the deep ocean, slowly accumulating detrital organic matter that gets buried in subsurface sediments. This research project investigated the diversity and function of bacterial communities as a giant biological reactor in the deep sea. We deciphered the global diversity and composition of the deep-sea microbiome and assessed major drivers of deep-sea bacterial community structure. At the beginning of the ERC project, we showed that thousands of different types of bacteria can be discovered per gram of deep-sea sediment, but that very few are known and hardly any are yet isolated for laboratory studies. We applied state-of-the-art and novel technologies to explore the diversity and function of deep-sea microbial communities, in combination with a range of environmental data. We identified a few bacterial groups that are abundant and widely distributed in deep-sea surface sediments at a global scale. Among these, one specific group stood out called JTB255, and by separating single bacterial cells from the complex mixture of organisms in deep-sea sediments and analyzing their genomes, we were able to reveal their metabolic potential and putative role in deep-sea ecosystems. JTB255 are able to grow on proteins and lipids that may be derived from cell wall material and other biogenic compounds in marine sediments, which may explain their ecological success in this environment. They account for a major proportion of cells in deep-sea sediments at a global scale, with on average 10 million cells per ml of deep-sea sediment (a milliliter of deep-sea sediment contains on average a total of 1 billion cells). At the end of the ERC project, we have novel tools at hand to specifically sample and identify a major proportion of the bacterial community in Arctic deep-sea sediments. These include substantial innovations in deep-sea technologies such as combined camera and bathymetry systems, and robotic samplers. We have also revealed substantial microbial community dynamics in deep-sea sediments across different environmental gradients, combining field studies and experiments. Seafloor bacteria respond directly to surface ocean dynamics (e.g. primary production, particle flux) because they change in activity and composition when their food source, the sedimenting plankton changes. Hence they are sentinels of climate change and other anthropogenic factors. These results are of specific interest in our main study area, the Arctic Ocean. The response of microbial deep-sea communities to changing environmental conditions will be key to understanding potential ecological feedback mechanisms in future climate scenarios, and based on our results, we have contributed to developing an innovative deep-sea long-term observatory in the Arctic.