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Hydrostatic pressure and prokaryotic activity in the deep sea

Periodic Reporting for period 1 - HYADES (Hydrostatic pressure and prokaryotic activity in the deep sea)

Reporting period: 2016-04-15 to 2018-04-14

The deep-sea realm (depth > 200 m) is characterized by low temperature (0–4°C), absence of sunlight, and high hydrostatic pressure, which increases by 1 atm every 10 m depth. It is a key site for degrading organic matter back to carbon dioxide and into other inorganic compounds. This is a crucial process for driving the carbon cycle in the deep-sea mediated largely by microbes, especially prokaryotes. Recently, global carbon budget estimates indicated an unresolved mismatch between organic carbon supply and prokaryotic carbon demand in the deep-sea. Hence measured prokaryotic activity in the deep-sea exceeds the input of sedimenting organic carbon from the sunlit euphotic layer. Without realistic measurements on prokaryotic activity, we cannot advance our understanding on the ocean carbon cycle. Thus, one of the most important issues discussed for some decades remains still unresolved as is the role of hydrostatic pressure on deep-sea prokaryotic activity. Because prokaryotic activity is traditionally measured on board of research vessels under atmospheric pressure conditions, the results obtained hitherto on deep sea heterotrophic microbial activity might be biased if hydrostatic pressure does influence heterotrophic microbial activity. While it has been shown already in the 1940s, that there are some piezophilic (‘pressure-loving’) bacteria present in the deep-sea preferentially growing under high-pressure conditions, their ecological role is still not clear. Overall, the main objectives of HYADES project are to understand deep-sea prokaryotic activity and metabolism under in situ hydrostatic pressure and reveal the contribution of piezophilic prokaryotes to the total prokaryotic community in the dark ocean.
To test the influence of hydrostatic pressure on prokaryotic activity in the deep-sea, we adopted an in situ microbial incubator (ISMI) which is a custom-made underwater device developed in the lab of the MSCA-fellow’s PhD supervisor in Japan. The ISMI, as the name suggests, is an instrument to incubate deep-sea microorganisms at the depth of sampling and hence, without change of the hydrostatic pressure. During the project, the ISMI was deployed at a depth between 175 and 4000 m in the Pacific Ocean, Caribbean Sea, Atlantic Ocean, and Southern Ocean during three research cruises. Fourteen deployments of the ISMI revealed lower prokaryotic activity in the in situ incubations with ambient pressure conditions than under atmospheric pressure. Using fluorescence in situ hybridization coupled with autoradiography and microscopic observations, we found that only a few highly active Bacteria contributed considerably to the increase in prokaryotic activity under atmospheric pressure conditions, suggesting that decompression activates these bacterial groups and piezophilic prokaryotes are only a minor fraction in the deep-sea.
The results were presented at international conferences, symposia, and regular laboratory meetings, the latter to reflect on the progress in the work and discuss the interpretation of the results. To inform the general public how researchers work on research vessels, activities on board during the cruises were posted on blogs at University of Vienna (http://medienportal.univie.ac.at/uniview/dossiers/dossiers-list/kategorie/1177/?no_cache=1) laboratory webpage (http://www.microbial-oceanography.eu) and MSCA-fellow’s individual research blog page (https://researchmap.jp/yutdein/?lang=english). Information about the work carried out in the project is also given in the lab and MSCA-fellow’s webpages.
The in situ incubation using the ISMI represents a major challenge. Although there were some problems detected in the operation of the ISMI during the project in the early stage of the project, these problems were fixed with some modifications. The ISMI worked reliably in the deep-sea. Thus, we could firmly establish that in situ incubations represent a powerful tool for studying hydrostatic pressure on deep-sea prokaryotic community. Our results carried out during the project revealed that decompression stimulates activity of deep-sea prokaryotes, suggesting that traditional prokaryotic activity measurements conducted under atmospheric pressure overestimate their carbon demand especially in bathypelagic (1000–4000 m) waters. Due to scarcity of data that makes it impossible to evaluate geochemical and biological models for the deep-sea (the IPCC Fifth Assessment Report), the results allow now to re-assess geochemical and biological models on deep-sea carbon cycle. Currently, there are considerable efforts underway to make economic use of the resources in the deep-sea, particularly the rare earth and metals. This deep-sea mining has to be guided by solid scientific knowledge on the ecology of the deep-sea. The results of this project will contribute to reconciling the deep ocean carbon flux. The new insight on the ecology of deep-sea prokaryotes under the hydrostatic pressure condition has potential to assist in culturing approaches of deep-sea microbes which might be further contributing to biotechnological and biomedical applications.
Procedure for deploying a in situ microbial incubator (ISMI) with a winch from a research vessel.