Louis Quichaud (M1, ENS de Lyon),
Ronnie N. Glud (HADAL center, University of Southern Denmark),
Expertise IT : Emmanuel Quémener (CBP, ENS-Lyon)\\**

Hadal trenches are some of the most unexplored areas on Earth and constitute the deepest parts of our oceans, ranging from 6 to almost 1l km of water depth. They are long but narrow V shaped depressions in the seafloor, that stretch along subduction zones. Despite their considerable depth they have been identified as depocenters for organic material with elevated microbial activity. Although, their bathymetry physically isolates them from their adjacent abyssal areas as well as from other trenches, recent studies indicate that some hadal communities appear connected. The oceanic currents ventilating the trenches could to some extent bond hadal trenches together and explain this connection. However, due to the difficulty of obtaining samples from the hadal zone, current studies are based on few locations within given trenches and trench–trench comparisons are few. Thus, our understanding of hadal benthic microbial communities is still in its infancy and many questions remain unanswered. This includes the variability of microbial diversity within and between trenches. Here, we propose to explore these patterns using data from 3 hadal trenches: the Atacama, Kermadec and Mariana trenches. To do so, we will re-analyse 16S rRNA gene amplicon data that were obtained in two different studies, infer amplicon sequencing variants, and combine the two datasets to gain comprehensive insights into the benthic microbial community structures of these environments. First, we will explore the variability of alpha diversity across trenches. We will test if the primary productivity in the overlying water column of each trench is reflected in microbial diversity in the sediment. We will also test if other factors such as water depth or sediment zonation could potentially drive alpha diversity in these systems. Second, we will investigate compositional dissimilarities between hadal microbial communities and test if trenches significantly explain these dissimilarities and to what degree the communities of different trenches are connected to each other. Lastly, we will focus on well-known functional groups to decipher possible biogeographical pattern and differences between trenches at the genus level.

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