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What are hadal zones and ocean trenches?

In 2017 Professor Ronnie N. Glud led an expedition to the Mariana Trench and retrieved sediments from the deepest spot on Earth, The Challenger Deep (almost 11,000 m deep).

He found a surprisingly high microbial activity in the hostile high-pressure environment (pressure is almost 1,100 times higher than at sea level).

The microbes found were communities of bacteria thriving in the sediment of the trench. The trench sediments revealed themselves to be inhabited by almost 7 times more bacteria than in the sediments of the surrounding abyssal plain at a much shallower depth of 6 km.

This was described in Nature Geoscience

The microbial communities

The structure of the hadal microbial communities that convey the processes to be investigated by us is essentially unexplored. Benthic microbial communities at shallower depths are characterized by a great phylogenetic and functional diversity, which is related to a combination of the general functional diversity of microbial food webs, microheterogeneity, and geochemical gradients.

A detailed understanding of the relationship between benthic microbial community structure and function has not yet been established, and a major fraction of the microbes identified in community surveys have no cultured relatives and their physiology is not known.

Virus

The abundance of viruses in marine sediments exceeds microbial abundance by an order of magnitude, representing a diverse and dynamic component of benthic marine ecosystems, with significant impact on microbial mortality.

In the process of host lysis, viruses generate dissolved organic matter which supports microbial activity and stimulates biogeochemical cycling.

Moreover, viruses likely represent the largest pool of unexplored genetic diversity on the planet and are key drivers of microbial diversity and community dynamics through their selective infection pressure and as mediators of horizontal gene transfer.

However, the diversity and specific adaptations of hadal viral communities and their potential role in structuring benthic microbial communities have not been investigated.

A previous expedition to the Mariana Trench by professor Ronnie N. Glud has confirmed that there are very few large animals in these zones.

How do the organisms get food?

Deep-sea benthic communities are mainly sustained by the sedimentation of organic material produced in the photic zone.

Traditionally, the organic material reaching the deep sea has been considered to be relatively refractory. While this may be generally true, there is growing evidence that still fresh and nutritious phytodetrital material can be deposited at great depth, including deep-sea trenches.

Organic material gets funneled down

Recent advances also document the importance of topographic seascapes and their interaction with fluid dynamics for focusing or winnowing sedimentary material. Hadal trenches represent extreme sea-scapes with intensified downslope funneling of material forming local depocenters.

Organic carbon reaching the seabed is degraded through a complex, but relatively well understood, microbial food web.

Extracellular hydrolysis and fermentation provide substrates for respiratory organisms that oxidize the material releasing CO2 and nutrients.

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