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[1] The eastern Mediterranean Sea (EMS) is a nutrient-poor ocean with unusually high nitrate-phosphate ratios (up to 28:1) and relatively depleted 15 N/ 14 N ratios (expressed as d 15 N) in deepwater nitrate (d 15 NO 3 À of 2.5%) and sediments (<3.5%) that have been attributed to extensive N 2 fixation. To quantify the role of atmospheric NO 3 À deposition in the N cycle in the EMS, we analyzed NO 3 À and its d 15 N in samples of dry and wet deposition on biweekly and per event basis from April 2006 to September 2007 on the island of Crete. Both dry and wet deposition samples have consistently negative d 15 N compared to air N 2 , implying a strongly depleted atmospheric source calculated to be (weighted annual estimate) À3.1%. The low d 15 N of wet deposition is in agreement with data from other environments, but the consistently depleted nature of dry deposition is unusual and supports the view of an origin from association of atmospheric NO 3 À mainly with dust and sea salt. This situation is due to high levels of sulphate compared to ammonium, sea salt, and lithogenic material. Mass balance and isotope-mixing calculations show that the present-day inputs of nitrate from external sources into the surface water layer of the eastern Mediterranean Sea have a weighted mean d 15 N-NO 3 À of 0.5% to 1.5% and that particle flux to and mineralization in the deepwater pool over the last 40-50 years can account for the unusually low d 15 N ratios found in deepwater NO 3 À without the need of any significant N 2 fixation.

Hadal trenches are the deepest and most remote regions of the ocean. The 11-kilometer deep Challenger Deep is the least explored due to the technical challenges of sampling hadal depths. It receives organic matter and heavy metals from the overlying water column that accumulate differently across its V-shaped topography. Here, we collected sediments across the slope and bottom-axis of the Challenger Deep that enable insights into its in situ microbial communities. Analyses of 586 metagenome-assembled genomes retrieved from 37 metagenomes show distinct diversity and metabolic capacities between bottom-axis and slope sites. 26% of prokaryotic 16S rDNA reads in metagenomes were novel, with novelty increasing with water and sediment depths. These predominantly heterotrophic microbes can recycle macromolecules and utilize simple and complex hydrocarbons as carbon sources. Metagenome and metatranscriptome data support reduction and biotransformation of arsenate for energy gain in sediments that present a two-fold greater accumulation of arsenic compared to non-hadal sites. Complete pathways for anaerobic ammonia oxidation are predominantly identified in genomes recovered from bottom-axis sediments compared to slope sites. Our results expand knowledge of microbially-mediated elemental cycling in hadal sediments, and reveal differences in distribution of processes involved in nitrogen loss across the trench.

Little is known about viruses in oxygen-deficient water columns (ODWCs). In surface ocean waters, viruses are known to act as gene vectors among susceptible hosts. Some of these genes may have metabolic functions and are thus termed auxiliary metabolic genes (AMGs). AMGs introduced to new hosts by viruses can enhance viral replication and/or potentially affect biogeochemical cycles by modulating key microbial pathways. Here we identify 748 viral populations that cluster into 94 genera along a vertical geochemical gradient in the Cariaco Basin, a permanently stratified and euxinic ocean basin. The viral communities in this ODWC appear to be relatively novel as 80 of these viral genera contained no reference viral sequences, likely due to the isolation and unique features of this system. We identify viral elements that encode AMGs implicated in distinctive processes, such as sulfur cycling, acetate fermentation, signal transduction, [Fe–S] formation, and N-glycosylation. These AMG-encoding viruses include two putative Mu-like viruses, and viral-like regions that may constitute degraded prophages that have been modified by transposable elements. Our results provide an insight into the ecological and biogeochemical impact of viruses oxygen-depleted and euxinic habitats.