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Benthic diagenesis and microbiology of hadal trenches

Periodic Reporting for period 4 - HADES (Benthic diagenesis and microbiology of hadal trenches)

Reporting period: 2020-07-01 to 2021-12-31

The hadal zone stretches from 6 to 11 km of depth and encompasses the greatest ocean depths and represents one of the most extreme and least explored marine environments on Earth. The extent by which microbial life in hadal settings differ from microbial communities in shallower - more well-studied oceanic environments. Finally, we aimed to understand how extreme pressure affect microbial life and biogeochemical processing by using novel, sophisticated pressure tank facilities.

Our findings documented that hadal trenches are depocenters for organic material and trench sediments are characterized by highly elevated biological activity. Our work showed that trench systems are unique deep-sea environments; with district lifeforms and biogeochemical function that cannot be understood by extrapolating findings from the ambient ocean – and that trenches are important for element cycling and carbon sequestration in the deep sea. The work has been presented in many different formats, including a long list of peer-reviewed articles - and more scientific papers are in the pipeline. Furthermore, HADES-ERC has been instrumental for the establishment of The Danish Center for Hadal Research. A Center of Excellence that is financially supported by the Danish National Research Foundation and will explore life and element cycling in the Hadal realm for the next 6+4 years.
During HADES-ERC, we have developed and applied new autonomous lander systems. The instruments have enabled the first detailed in situ assessments of early diagenesis in two Pacific trenches; the Atacama and the Kermadec trench. Our findings document highly intensified biological activity within the trench systems as compared to the adjacent and shallower ambient oceans. The work thus document that hadal trenches act as deep-sea hot spots for the mineralization of organic carbon and nitrogen. Surprisingly and despite the extreme depth, the mineralization involves the full suites of redox processes know from coastal sediments with active denitrification, iron, manganese, and sulfate respiration. Furthermore, our measurements demonstrate that hadal sediments act as sinks in the marine N cycle mainly through the activity of anammox bacteria and important sites for carbon sequestration in the deep sea.

The intensified microbial activity is sustained by high deposition rates of organic material partly through continuous downslope material focusing but mainly via seismic driven mass wasting events that place hadal systems among marine habitats with the highest encountered sediment accumulations rates. Laboratory-based investigations also revealed that increasing hydrostatic pressure might inhibit the microbial degradation of fast sinking particulate material ensuring a supply of nutritious organic carbon for the piezophilic communities active at hadal depth. The elevated microbial mineralization is mediated by largely unexplored bacteria and archaea. Detailed investigations of the microbial community structure show a very high diversity of the different functional groups – and distinct differences between communities between hadal trench systems and from hadal systems to the ambient ocean. Deposition rates of organics and thereby the overall levels of early diagenesis and redox zonation’s appeared to be an important driver for the microbial community structure. Clearly the anaerobic communities were seeded from diverse shallow environments, but also expressed specific hadal traits.

Phages were shown to be an important mortality factor for the microbial communities, but also a vector for horizontal gene transfer that in some cases enhanced the fitness of bacteria with specific traits such as chitinase degradation. Meiofauna communities dominated by nematodes grazed intensively on the microbial communities but were mainly confined to the very sediment surface which also had the highest abundance and activity of microbes. Several new nematodes species were identified and preliminary investigation hints toward specific hadal communities – but more biogeographic investigations are ongoing.

Laboratory-based experiments, using various novel pressure tank systems, revealed that hydrostatic has strong effects on the performance of microbial communities on sinking “marine snow”. Overall increasing pressure inhibited the degradation of the sinking particles which facilitated relatively high supply of nutritious organic material for hadal communities. This seems to be an important factor for food supply at great depth. Detailed investigations documented that microbial mediated enzymatic processes in sinking aggregates were differently affected by increasing pressure, which shaped the characteristics of organic material that ultimately was deposited at hadal depth. Distinct piezophilic enzymatic responses were observed in microbial extracts recovered from great depth. This demonstrates that specific adaptation to pressure is an important element for shaping benthic community in hadal settings.
Bathymetry of Kermadec Trench. From the HADES-ERC campaign in 2017. Courtesy: Heather A. Stewart